MALARIA IN THE NETHERLANDS







MALARIA IN THE NETHERLANDS
by
N. H. SWELLENGREBEL
and
A. DE BUCK
AMSTERDAM
SCHELTEMA & HOLKEMA Ltd.
19 3 8







PREFACE
The Netherlands is the only country in northwestern Europe where malaria has retained a firm foothold up to the present day and where, in former years, it was of more importance than anywhere else in those parts. In all other countries of northwestern Europe (except for East-Friesland in Germany, close to the Dutch frontier) it has completely disappeared or dwindled into insignificance. In the Netherlands the incidence of malaria has greatly diminished, no doubt, but it remains a serious problem, notably among the children of the malariainfested areas.
It is not for that reason, however, that we have ventured on writing this book. It is because we believe that malaria in this country may claim a wider interest, since it provides the investigator with, what might be called, a pocket-edition on the epidemiology of this disease. Conditions in the Netherlands are little complicated because of the existence of only one parasite and one insect-vector. Hence, wellknown and much discussed general epidemiological problems present themselves here in a shape so simplified as almost to partake of the nature of an experiment: Nobody doubts the transmission of



malaria by anopheles; but here we can actually tracé the path of the parasite from man to mosquito and back to man. Healthy parasite-carriers are universally considered a danger to their neighbours; but here we see them infecting mosquitoes in large numbers. Every one agrees that it is man, and not the mosquito, who is the host of the plasmodia during the inter-epidemic season; but here we are offered ocular demonstration of his acting that part and of anopheles failing to do so. Doubts are expressed whether malaria can always be eradicated by means of drugs; here conditions allow of analysing the circumstances which justify this doubt. There are those who believe that the human habitation, in certain circumstances, may act as a centre of malaria infection; here the house is seen to act that part and it becomes evident why it does so. Health officers observe that the well-established species of the genus Anopheles are not the only material for them to work with when they wish to apply the principle of speciessanitation; here they find a smaller unit, a race of an anopheline species, perfectly satisfying their requirements in this respect, both as an adult and as a larva.
This epidemiological analysis of malaria in the Netherlands, to which we have devoted most of our time since 1920, has been rendered possible first and foremost by the inspiring influence and guidance of the late nestor of the Dutch students of malaria, Dr. P.C. Korteweg; closely followed by a circle of intimate friends and collaborators: Professor W.A.P. Schüffner, Miss E. Schoute, Mr. H. Kraan, Dr. H. de Rook, and Mr. J. Nijkamp, among whom we feel proud



to count Lt. Col. S.P. James who, by his advice and example, has greatly stimulated malaria research in this country. Next come Dr. P.H. van Thiel and his collaborators who, by their investigations in the nonmalarious province of South-Holland, provided us with the indispensable counterpart of our observations in the malarious province of North-Holland.
The close association with the authorities of the Public-Health Department, notably with Dr. N. M. Josephus Jitta, chairman of the Sanitary Council, and with Dr. J.J.Th. Doyer, principal medical officer of Health in the province of North-Holland, was of great help to us. Of hardly less importance was the support we received from the physicians de Groot, Lampe, and Ris in Wormerveer, Beeker and Brugman in Uitgeest, Honig in Nieuwendam, Wagenaar in Marken, Piebenga and van Andel in Franeker, Dixhoorn in Medemblik, and many others.
Among foreign investigators who, by their researches, aided us to unravel our own problem, we first mention Sir Malcolm Watson whose "speciessanitation" is the keynote in this book; and next, Dr. L.W. Hackett, Professor A. Missiroli, and Professor E. Martini whose studies on egg-characters in anopheles we found of invaluable help. We are happy to mention in this place the authorities of the "International Health Division of the Rockefeller Foundation" who provided us with the financial means to carry on our investigations, during the years when lack of insight into the epidemiological problem rendered it impossible to command the necessary interest for the fight against malaria in this country.



Finally, we wish to thank Dr. Ch.W.F. Winckel for his valuable collaboration in the writing of the tenth chapter.
N. H. S. & A. d. B.
Department of Medical Zoology,
Institute of Tropical Hygiene,
Royal Colonial Institute,
Amsterdam 1938.



CONTENTS
Preface iii
List of illustrations viii
i. preliminary information ... i
II. Malaria in the nineteenth century. 9
III. Present-day malaria . . . .31
IV. The races of Anopheles maculipennis. 55
V. Behaviour of the shortwinged Anopheles maculipennis . . . .93
VI. Anopheline malaria . . . . i2y
VII. Transmission of the malaria parasite
from mosquito to man . . . .148
VIII. Transmission of the malaria parasite
from man to mosquito . . . .165
IX. Malaria control x8i
X. Malaria induced in patients suffering
from general paralysis of the insane. 219
Epilogue 250
Index .... oc.



LIST OF ILLUSTRATIONS
1. Map of the Netherlands PaSe 2
2. Incidence of malaria at Amsterdam in 1857 and 1922. „ 19
3. Geographical distribution of malaria in 1875 . ... 24
.. .» I9I9 • • 25
5. Annual variation of malaria incidence at Wormerveer. „ 36
6. Focal distribution of malaria at Wormerveer . . „ 45
7. Malaria morbidity in succeeding age-groups . 47
8. Geographical distribution of the long- and shortwinged 6g Anopheles maculipennis . • • • "
9. Distribution of fresh and brackish water through the ^ Netherlands
10. Anopheline density in the Netherlands . . ?r Plate I. Normal and hybrid sexual organs of K.
maculipennis type tac™s
11. Seasonal incidence of shortwings in an open shelter . Page 94
12. Seasonal incidence of shortwings in human habitations. „ 95 1, Annual fluctuations of the number of shortwings
during autumn in relation to temperature
14. Distribution of salty and fresh water in some coastal i2Q provinces
15. Distribution of shortwing and longwing larvae in the same provinces
16 Periodicity of anopheline infection
Plate II. Salivary glands of long- and shortwings dunng
hibernation and semi-hibernation. - Normal^and ^
degenerated sporozoites
17. Inverse relation of sporozoite-rate to rate of pregnant ^ ^ females
18. Sporozoite-rate and percentage of females having ^
imbibed hiiman blood •
19. Infectivity of malaria patients and healthy carriers . „ 171
20. Simple tertian turning duplicate .
21. Quotidian converted into tertian by 50 milligrammes ^ of neosalvarsan.
22. Quartan not susceptible to neosalvarsan 4
23. Triplicate quartan converted into simple quartan . „ 247



CHAPTER I
PRELIMINARY INFORMATION T opographical
This section serves to explain why this book is called "Malaria in the Netherlands" instead of "Malaria in Holland". The fact is that the present Kingdom of the Netherlands is the continuation of the "Republic of the United Provinces" born at the close of the sixteenth century. In that republic the province of Holland was the richest and most powerful, and so it was not unnatural that foreigners made the mistake of calling the whole country after that one province. Although the cases are far from similar, one might compare this mistake to another one, often made by foreigners, of using the name of England when referring to the United Kingdom as a whole.
At present the Kingdom of the Netherlands is composed of eleven provinces. The old province of Holland has been divided into two parts, NorthHolland and South-Holland. Nothing of its political supremacy remains in our days, but the two Hollands still are the site of the three most important cities, Amsterdam, Rotterdam, and the Hague.
The map on fig. i indicates the position of the eleven provinces and the site of the various cities, townships, and villages to which reference will be made in this book.
Malaria. r



Fig.i. Map of the Netherlands indicating ito Provinces; and the site of various localities to which reference is made in this book.
We have used the name "Netherlands" to indicate the whole country, and "Holland" to indicate the two provinces. Much against our inclination we have retained the adjective "Dutch", signifying pertaining to the Netherlands". It is an unpleasmg



word in our ears, but we know of no suitable substitute.
Geological
Geologists * hold that the Netherlands is nothing but the relic of a large delta area, joining England to the continent, through which the rivers Humber, Thames, and Rhine wound their way to the North Sea which, at that time (last glacial period), lay beyond Doggersbank. The sea-level was some 160-200 feet below the present one. Later on the melting of the ice, releasing huge quantities of water, caused this level to rise to twenty-three feet below the present one. Most of the delta area was submerged and England was severed from the continent by the straits of Dover. Large masses of sand, which had accumulated there during the preceding glacial period, were now transported in a north-easterly direction by the tidal current which made its way through the newly found opening. These sands were deposited along a line which, roughly, corresponds to the present coast-line of the Netherlands. At that time, however, the coast-line lay a long distance to the east of the present one, since the land now occupied by the northern portion of Friesland and Groningen, the two Hollands, and Zealand was submerged. The deposited sands gradually came to form a solid embankment, a so-called "bay-bar", which converted the sea enclosed by this embankment into a shallow
* e.g. P. Tesch, Tijdschr. Koninkl. Aardrijksk. Genootsch. 2nd Ser. Vol. 37. 1920, p. 163-175, 740-745; Vol. 39, 1922, p. 580-585; Vol. 45. 1927, p. 1-15; Vol. 47, 1930, p. 169-178. J. A. Baak, Regiondl petrology of the southern North Sea. Wageningen, 1936. H. Veenman & Zonen, p. 90-98.



lagoon, by protecting it from the inroads ol even the highest tides. The rivers Rhxne, Meuse, and Scheldt, discharging their silt-laden waters mto this lagoon, gradually converted it into a fresh-water swamp and, later on, into peaty land extending over the present provinces of Zealand, the Hollands, the greater part of Friesland and Groningen, and the whole of the inland-seas. In the centre was a lake» called lake Flevo. In this state the Romans found it when they reached here shortly before the beginning of our era, as evidenced by Roman remains
found in and on this peat.
In this way the Netherlands came into being, in the shape of a solid block of peat protected on the sea-side by a bar of sand-dunes. In the course of the first twelve centuries of our era much of this land was lost again. Owing to the sea gradually attainmg its present level the protecting bar of sand-dunes was broken to pieces, except along the coast of the present province of South-Holland. It was replaced by a new bar some distance farther inland. The peatland behind the dunes became submerged at high tide. One portion of it, however, was preserved. It lay between the higher grounds in the provinces of Utrecht and Guelders to the east, and that portion of the sandy ramparts which had successfully wi stood the onslaughts of the sea to the west. This peat-land was, moreover, charactenzed by the numerous intersecting tracts of fluviatile clay which the many arms of the Rhine and Meuse had deposited there together with abundant stores of subterranean fresh water The site of this resistant peat-land is the presen province of South-Holland, and the adjacent parts



of Utrecht and Guelders. We shall have to refer to it later on, when we come to treat of the geographical distribution of malaria, of fresh and brackish water, and of the races of Anopheles maculipennis in the Netherlands.
The banks of lake Flevo, now a salt-water bay, were scoured by every spring-tide. As a consequence, the bay gradually grew in size, finally to form what, seven years ago, was the Zuydersea. In our days this sea has been converted back into a fresh-water lake, called Yssel-lake.
The centuries, following the twelfth of our era, have witnessed the recovery of some of the lost ground. The soil was not redeemed in its original state, the peat was gone and marine clay had taken its place; but it was no less valuable for that change. Much, however, is irretrievably lost, because the sea has so utterly spoilt it that it is not worth reclaiming. The last stage of this struggle is now being enacted by the reclamation of the Zuydersea.
By land-reclamation the pro vinces of North-Holland and Zealand, a great portion of Friesland and Groningen, and the islands belonging to SouthHolland came into being. Often it was possible to profit by the natural extension of the coastal area caused by the accumulation of clayey deposits. As soon as these deposits rosé above sea-level at high tide they were protected, by embankments, from being swamped at spring-tide, the drainage being effected by sluice-gates operating at low tide. In North-Holland, however, land-reclamation mainly consisted of draining the numerous lakes, which



honeycombed the land, by means of pumps operated by windmills.
Land-reclamation in the Netherlands, and especially in North-Holland, has always been regarded by physicians as a procedure necessary, no doubt, from an economie point of view, but fraught with danger to the health of the inhabitants. The notion that the reclaiming of new land by means of windmills is a public-health measure was originated, we believe, by Lancisi * and since then it has often been repeated up to modern times t, but it has never been accepted here.
Modern investigations have made it abundantly clear that the local view was more correct than the foreign. Land-reclamations, especially those draining lakes, far from doing away with breedingplaces of anopheles, create them, by substituting for swamps and lakes the much more favourable drainage ditches. Moreover, they provoke the incursion of brackish water from subsoil reservoirs, and so the breedingplaces are often brackish, favouring the growth of the shortwinged Anopheles maculipennis.
So we conclude that the coastal areas of the Netherlands have been, and still are, malarious because of the land-reclamations which created them. The reason why there is much more malaria here than anywhere else in north-western Kurope is that more land has been reclaimed here than elsewhere.
* De noxiis paludum effluviis. Romae 1717, Lib.i, Pars poster, cap. ii, p. 86-91.
-j- A. Laveran, Traité du paludisrne. Paris I9°7» P- 3®> 37- 535> 537-



Medical
In the following chapters we shall mention more than once the fact that the less well-to-do in the malarious villages of this country can have their malaria treated free of charge and that, as a consequence, the costs of the treatment can never act as a deterrent from seeking medical advice. This is rather important, in view of the röle the so-called healthy carriers play in the transmission of malaria (p. 165). Without this point being made quite clear, we are afraid people might believe that there would exist no healthy carriers, if the patients were not reluctant to call in the physician because of the expense this would entail.
< There exist certain institutions in this country called sick-clubs. Their aim is to provide medical assistance for the population. Everyone whose income remains below a fixed limit can become a member of the sick-club. By paying a weekly contribution he becomes entitled to medical assistance, including medicines, free of charge for himself and his dependants. Urban sick-clubs have existed for a long time, but the rural variety is of more recent date. Sick-clubs have not yet spread to all rural areas, but the province of North-Holland is well provided with them.
Another form of medical assistance in rural areas is'organized by the so-called "cross-societies". They provide nursing facilities to their members, in the shape of district- and visiting nurses, and maintain depots of medical and surgical apparatus. These cross-societies play an important part in the organi-



zation of malaria control in this country. With the financial support of the government of North-Holland and of several local governments in that province, they have combined to organize the "Commission to stimulate the control of malaria by the population of North-Holland", with the principal medical officer of health for that province as chairman. This commission encourages the inhabitants to carry out certain measures, which will be mentioned later on in this book (chapter nine). Scientific investigations, as a preliminary to the execution of these measures, are undertaken by the members of a special commission of the Sanitary Council of the Netherlands, often with a grant-in-aid from the above mentioned provincial commission. A close collaboration between the two commissions is assured by the chairman of the provincial commission being a member of the Sanitary Council's commission and by the secretary of the latter being a member of the executive committee of the former.



CHAPTER II
Malaria in the nineteenth century
Difficulties in interftreting historical evidettce about
malaria
Some people hold that the medical history of this country gives evidence that malaria caused much more illness and death in former years than it does now. But we do not trust that evidence, because we have no means of identifying malaria among the diseases described in publications of the past. We cannot understand these descriptions; for although many of the technical terms are the same as those with which we are already acquainted, they have acquired an entirely different meaning in the course of the years.
The word "malaria" affords an excellent example of the change the meaning of technical terms has undergone, even in so relatively short a time as the nineteenth century *. In the literature of this country we meet it for the first time in 1846 in the composite "malaria diseases", a group of morbid conditions including fevers which may, or may not, show an intermittent character. Later on, in 1871, "malaria" means the poisonous exhalation of the swamps, and "malaria diseases" a state of ill-health caused by these exhalations. It commences in the guise of
* Nedcrl. Tijdschr. v. Geneesk, Vol. 70, 1926, 2nd half, p. 1105-1108.



diarrhoea, then changes into intermittent and remittent fevers, which finally lose their intermittent character and assume other properties not met with in pure intermittent fevers. Some years later, in 1875, "malaria diseases" are defined as a complex comprising pernicious intermittent fevers, remittent malarial fevers, indigenous cholera, asiatic cholera, and dysentery, i.e. anything but the malaria we know now. In 1892 a description * of an "acute malaria endemic" stresses the gastro-intestinal symptoms. Some patients only, out of seventy-seven, had fever (usually lasting for one day). All recovered within a few days, the majority without taking quinine. Finally, in 1894, the words "malaria" and "malaria diseases" became synonymous and were used in their modern sense. The writers, however, continued to refer to the records of 1871 and 1875 without being, apparently, aware that they were using the words of their predecessors in an entirely different sense.
In this particular instance it has been possible step by step to tracé the changes in the meaning of the word malaria, because they happened in fairly recent times. But the same process went on in earlier history and there we cannot tracé these changes. So it comes about that we are left in the belief that we understand the old writers, because we see them using familiar words, forgetting that these words conveyed a meaning to the minds of the authors which greatly differs from that which they convey to us.
* A. Couvée, Nederl. Tijdschr. v. Geneesk. Vol. 36, 1892, 2nd half, P- 573-577; see also P. C. Korteweg's comment upon it: Ibid. Vol. 72, 1928, ist half, p. 534.



The expression "cinchona bark, given in time, saves the patiënt" is another example of the same kind *. When meeting with it we are sure it can refer to nothing but subtertian malaria. After much reading we perceive that it does not mean —as it would in our days— that quinine saves the patiënt s life, but that, if the drug is not administered at the proper time, the fever is perverted into one which is no longer amenable to the action of the bark and so can no longer be prevented from working its worst on the patiënt for many weeks to come, without necessarily killing him.
But it is not often that the writers were so explicit. They had no need to be, for they understood each other as we understand our contemporaries. But we do not understand them. It is for this reason that we cannot conceive how people can talk so glibly about malaria which existed many centuries ago and about the damage it caused then, and how it disappeared and reappeared in regular periods. How can they know anything about it, they who live in our times and not in the days of the Romans?
And then the term "intermittent fevers": it has but one meaning now, but what a variety of meanings it had in former times! The disease bearing that name needed not to be intermittent nor was it required to be a fever. The doublé tertians of Walcheren in the year 1809 were rather of a continued kind, according to Davis t. The stage during which the patient's body feit hot was "of considerable
* Nederl. Tijdschr.v.Geneesk. Vol. 70, 1926, 2nd half, p. 1109-1111. f A scientific and popular view of the fever of Walcheren. London 1810, Samuel Tipper, pp. xxv + 200.



MALARIA IN THE NINETEENTH CENTURY
length, if not perpetual" as Wright * has it. Popken's t case, of an uneducated person whose intermittent consisted in a periodic writing of tolerably bad poetry, almost passes belief, but even without this extreme the term is applied so widely that it fails to convey any meaning to us.
Once one has become aware of these difficulties besetting the path of him who is looking for references to malaria in historical records, one feels very diffident in drafting epidemiological conclusions on the basis of seventeenth and eighteenth century wntings. We have the greatest respect for the knowledge of these old clinicians and we do not wish to find fault with them. But we venture to criticize the present-day tendency to identify the fevers they described with malaria in the modern conception of the word.
The Walcheren and Groningen epidemics
The same feeling of uncertainty arises regarding two great epidemics in the early nineteenth century, that of Walcheren in 1809 and of Groningen in 1826.
The famous Walcheren epidemie befell the illstarred military expedition to that island from July 3oth till the end of December of 1809. Melville § mentioned it as a lesson taught by history of, how
* History of the Walcheren remittents. London 1811, T. Bore pp
xlvui + 337. **
t Historia epidemiae malignae. Groningen 1827, J. Roemelingh, p. 48. Notabilis adfuit in versibus durante paroxysmo recitandis et conscnbendis facilitas, quos, etsi minime perfectos, tolerabiles tarnen, rn statu sano conficere nunquam potuisset homo literarum ceterum rudis et fere plebejus".
§ Sir Ronald Ross, Prevention of malaria. London iqio T Murrav
•n b^cSt j j '



malaria can frustrate the best military tactics. We do not believe malaria was wholly, or even mainly, responsible for a sick-rate of 12,863 out of 40,000 and a case-fatality reaching eighteen per cent in November.
It is probable that malaria predominated among the fevers which at first had been conspicuous by their absence, so that the soldiers already began to make light of the bogy of "Zealand fevers". They made their appearance in the second half of August. By August 25th 3,000 had fallen ill with the endemic intermittent which, though seldom fatal, was always tedious although it proved amenable to cinchona bark. But there are a number of facts which ought to make us doubtful whether malaria was responsible for the subsequent events.
First there is Dawson * who startles us by saying that it would exhibit the imbecility of human intellect to imagine that bark was of service in the disease, which offered the varied aspect of continuous or intermittent fevers, diarrhoea, and dysentery. Then there are the belated and overcrowded transport ships whose arrival in Flushing so closely preceded the turn for the worse events took in September. Davis tells us that the troops had been confined on these ships for a much longer time than had been anticipated. There was little room for water, provisions, and other comforts, with the result that the transports "had almost literally been converted into floating hospitals and thereby tended considerably to increase the number of sick".
* Observations on the Walcheren disease. Ipswich 1810, Battely, PP- 133-



Overcrowded ships, we may add in parentheses, were apt to breed typhus fever, as the East India Company knew to their cost *. Next we have Wright's testimony that he was unable to identify the Walcheren disease with the intermittent fevers he was familiar with in North-America and the West-Indies. Finally, there is the evidence of the forty-two postmortem examinations in the hospitals of Harwich and Ipswich recorded by Davis. They testify of the malarious substratum of the epidemie by the numerous findings of greatly enlarged spleens. But these records militate against the assumption that malaria was responsible for the catastrophe, by the of ten occurring abscesses and tubercles in the spleen, the liver and the lungs, and by the ulcerative processes located in the colon as well as in the small intestine.
In Walcheren the disease was very localized and affected neither the inhabitants of the island nor the Dutch army, which on October ist had a sickrate of six per cent t whereas that of the British army was fifty-seven per cent on the same day. The Groningen epidemie of 1826, on the contrary, was part of a pandemic spreading all along the coast of the North Sea from Denmark to the Flemish provinces. But the annual mortality in the town of Groningen was higher than anywhere else: 107 per thousand; in the preceding year it had been 28.
Uncomplicated intermittent fevers had been preva-
* M. A. van Andel, Nederl. Tijdschr. v. Geneesk. Vol. 81, 1937, 2nd half, p. 5429-5438.
■(• van Loo, Les fièvres zéelandaises. Gouda 1910, G. B. van Goor & Zn., pp. 57.



lent in 1825; they reappeared in 1826. In July, however, many children died of diarrhoea and convulsions, and numerous adults suffered from remittent fevers accompanied by vomiting and diarrhoea. Thuessink * believed that these pernicious fevers were contagious, but Bakker t doubted this. In September the epidemie reached its height with a monthly mortality of 276 per thousand. The patients died of "versatile nervous fevers" after two or three weeks' illness. Petechial eruptions were quite common and Galama § mentions that the pulse sometimes was but little accelerated. Up to this time the lower strata of the population, living in extreme poverty and squalor, had been the chief to suffer, but now the disease began to spread among the well-to-do. Thuessink holds the servants responsible for this as having intercourse with both groups. The same condition lasted through the whole month of October with a mortality of 245. In November the situation began to mend. The mortality dropped to 174 in that month and to 95 in December.
The records of post-mortem examinations are more numerous than those in Harwich and Ipswich referred to above, but they are very incomplete. Still they tend to show that malaria was less common here than it was in Walcheren. Among 134 cases Nijhoff 11 found an enlarged spleen in twenty-six
* Algemeen overzicht der epidemische ziekten in 1826 te Groningen.
Groningen 1827, J. Schierbeek, pp. 84.
t De volksziekte in 1826 te Groningen. Groningen 1827, W. v Boekeren, pp. 72 + 13.
§ Verhand. Holl. Mij. v. Wetensch. te Haarlem. Vol. 18, i8?o, p. 275-378-
II Observationes de epidemia Groningana. Utrecht 1827, O. J. van Paddenburg, pp. xii + 138.



only. Like in the Walcheren epidemie, other postmortem findings make us more than doubtful whether any of these people had died of malaria; the small intestine was inflamed and sometimes ulcerated in twenty-five cases, the colon in fourty-eight.
All this does not look like an epidemie of malaria, if for no other reason than that it was worse in the town than in rural areas. Still many have considered it as such, even to this day, because of Thuessink's contention that the pernicious fevers arose from the simple intermittents.
Any hypothesis, assuming that the Walcheren and Groningen epidemics were largely due to malaria, would immediately be called upon to explain the excessive mortality. This it could do only by the further assumption that subtertian malaria was not only present but actually predominated. We are not prepared to admit this, although we cannot exclude the possibility of subtertian having been present in the epidemics of 1809 and 1826. But it is highly unlikely. The greatly increased shipping of our days has not been able to transplant subtertian to this country, not even temporarily. The transmitting season, late summer and autumn, is distinctly unfavourable to the spread of this disease. Transmission during the height of the summer would only be possible if the habits of the insect vector of those days, and its distribution through human and animal habitations, had been entirely different from what we know them to be at present. All this, we repeat, is possible, but to admit it necessitates much speculation. We prefer to leave aside the Walcheren and Groningen epidemie and to turn to



more recent records which, although far from complete, allow of more reliable conclusions.
Evidence of a lasting reduction in the incidence of malaria
In the middle of the nineteenth century a number of Amsterdam physicians, including the Chief Surgeons of the more important hospitals, feit the need of reliable morbidity statistics. So they combined to make careful notes of the incidence of all the diseases they met with in their practice. Like their contemporaries, they were caught in the ideas of their time regarding the fundamental unity of the various "malaria" fevers. But their published records * have one feature, lacking in their predecessors': they teil us not only that the simple tertians were numerous, they teil us how many of them occurred each year. And these simple tertian fevers are the only ones which we may take as malaria, without the risk of committing egregious mistakes.
So it is their evidence only which we dare to accept as a reasonably reliable indication of how much malaria occurred in Amsterdam in those years. As we know the present incidence of the disease in that city, we are in a position to decide whether it has remained stationary or not.
Discarding all fevers termed bilious, pernicious tertian, quotidian, irregular, in short all but simple intermittents, we find, in the private practice of these physicians, that the annual incidence, of diseases which we venture to call malaria in the modern sense
* Verslagen, namens de 6' Commissie v. d. Geneesk. Kring te Amsterdam. Amsterdam 1850-1853, J. Noordendorp; 1854-1860, 1868, Metzier & Basting.
Malaria. 2



of the word, ranged from a minimum of 1,963 cases in 1861 to a maximum of 23,872 cases in 1857, between the years 1849 and 1866. The annual average was 5,286 cases. The cases occurring among the poor, entitled to treatment free of charge by the municipal surgeons, are known for the years 1856-1862 only. They ranged from a minimum of 3,665 in 1862 to a maximum of 15,001 in 1857, with an annual average of 8,904 cases.
Similar figures are known for Amsterdam from 1920-1936. Here again private practitioners and medical officers collaborated to register their cases of malaria and, moreover, to have them accurately diagnosed by blood examination. These figures, comparable to the combined private and poor-law practices quoted above, range from a maximum of 2,391 in 1922 to a minimum of 15 in 1932, with an annual average of 418 cases.
These figures are not representative of the total former and present incidence of malaria in this country, because they measure the incidence of urban malaria which is, undoubtedly, much below that of rural malaria. Nevertheless they allow, at least, of estimating how much the incidence of malaria has been reduced since the middle of the nineteenth century in one and the same place. Taking the maxima of 1857 and 1922 as a Standard, we count 38,873 cases in the former year, among a population of 250,000, and 2,391 in the latter, among a population of over 750,000, i.e. an incidence of a hundred and fifty-five per thousand in 1857 anc^ of three per thousand in 1922.
Beside these quantitative differences between mal-



aria in the middle of the nineteenth century and the disease as observed in our days, there exist qualitative differences. Part of the Amsterdam records allow of differentiating between simple tertian and quartan fevers. The following diagram (fig. 2) shows the monthly number of tertian and quartan cases in 1857 and of tertian cases in 1922.
Fig. 2. Malaria at Amsterdam. Comparison of the monthly incidence in 1857 and 1922. Continuous line: tertian 1857; dotted line: quartan 1857; broken line: tertian 1922.
This diagram shows that the monthly incidence of tertian malaria in the middle of the nineteenth century was entirely different from what it is now. In those days, tertian fevers attained their highest incidence between August and December, with a maximum in September and October. In addition to this peak of so-called "autumnal fevers", there was a second peak of so-called "vernal fevers" in



April and May. The vernal peak was often much better separated from the autumnal peak than is shown in fig. 2. As a rule it did not reach so high as the autumnal peak.
In our days, on the contrary, tertian malaria is most numerous between April and August. lts maximum occurs in May, June, or July; it roughly corresponds to the "vernal" peak of the past. In the diagram nothing remains of the autumnal peak of former times, except for a slight irregularity in the downward slope of the curve. Occasionally (p. 41) a true autumnal peak may be met with in our days, but always as a rarity, whereas it was the rule in old times *.
The curious part about the seasonal incidence of simple tertian in the past is, that it is in much better keeping with the incidence of anopheline infection as observed in our days (p. 129) than is the present monthly incidence of human tertian malaria. Anopheline infection likewise starts rising in August and also reaches its maximum in October. Consequently, the difference between past and present seasonal incidence of tertian malaria might be explained by supposing: (1) that anopheline transmission was autumnal, as it is now; (2) that the local tertian parasites of those days differed from the present ones by provoking primary fevers within a few weeks after invading their human hosts, instead of lapsing into prolonged latency on entering
* Similar records of the past have been quoted by Martini (Centr. Bit. ƒ. Bakter. i AU. Org. Vol. 96, 1925. P- 101-108). Missiroh s diagrams of the present monthly incidence of benign tertian in some parts of Italy likewise show two distinct peaks (Riv. dl Malar. Vol. 11, 1932, p. 1-24).



the human body, as is the habit of the strain of Plasmodium vivax with which we are now familiar.
The diagram likewise shows that quartan fevers were of common occurrence in those days. Like tertian fevers, they were most numerous in autumn, but they continued longer during the winter *. Quartan malaria is met with no longer in Amsterdam; elsewhere in the Netherlands it is too rare, in our days (p. 48), to allow of any reliable comparison being made between past and present conditions. It might even seem doubtful whether quartan of 1857 can be identified with the disease caused by Plasmodium malariae. So many fevers were called doublé or triple quartans which could not be distinguished from quotidians. However, in the days we are speaking of physicians were not so ready to label fevers as quartans. This we gather from case-records, like those left by Honkoop t, describing the patients' condition day by day for many days, leaving us in no doubt as to the true quartan course their fever ran.
* * *
* Dobreitzer (Le paludisme en Russie des Soviets, Moscou 1924, p. 40) records a slight rise of quartan during the winter in some parts of the Soviet territory.
f Nonnulla de cuta febrium intermittentium. Utrecht 1859, F. W. v. d. Weyer, p. 32-59. The same applies to the quartan fever Dr. Ch. W. F. Winckel's (see chpt.10) great-grandfather had, probably, acquired in his native town of Enkhuizen (North-Holland). He feil ill with it on September i6th 1810, in a South-Holland village, when, forced to enlist as a conscript, he was on his way to join his regiment at Versailles. The diary, he left, clearly shows that he had sixteen accesses of fever until October 3ist, each one separated from the next by an interval of two days. There is no indication of his being treated with cinchona bark. Except for a single entry, by the end of 1811, his diary makes no mention of relapses, in spite of the hardships necessarily attending a soldier's life of those days.



Mortality statistics in this country are of no service to confirm the conclusion arrived at above, viz. that the incidence of malaria has been greatly reduced since the middle of the nineteenth century. Statistical evidence since 1875 is unrestrictedly in favour of our contention. Deaths due to malaria were most numerous in the five coastal provinces, where malaria is known to occur in our time. Malaria mortality, which in 1875 was highest in Zealand (5.4 per ten thousand) and lowest in South-Holland (2.0 per ten thousand), had dropped to 0.1, or less, in all provinces by the beginning of the new century *. But the records of 1875-1900 include deaths caused by pernicious intermittent fevers. That addition robs them of all their value, since these pernicious intermittent fevers are the ones which we must regard with the gravest doubt. They were the fevers which were not intermittent at all and could not have been identified as malaria without blood examination, which was never practised in those early days. So we believe it advisable altogether to rejeet mortality figures as a source of evidence.
* * *
A question, closely related to the subject of this section, is whether historical records of recent date give us any clue as to the geographical distribution of the disease in the days to which the Amsterdam figures, quoted above, refer.
The only records at our disposal are the results of an unpublished inquiry among all local practi-
* Nederl. Tijdschr. v. Geneesk. Vol. 69, 1925. 2nd half, p. 247-257.



tioners regarding the occurrence of "malaria diseases" in 1875. Now this term, to indicate the object of the inquiry, was very badly chosen, as we know that many diseases which are certainly not malaria were considered to belong to this family. However, if we retain only references to localities where intermittent tertians were said to occur in great numbers, and discard all the rest, we may hope to get a fairly accurate picture of the distribution of malaria in those days.
The critically revised results of the inquiry of 1875 are shown in the following map (fig. 3). It is accompanied by another (fig. 4) showing the results of an inquiry undertaken in 1919, during the last extensive malaria epidemie; these results have also been subjected to a critical revision. A comparison .of the two maps shows that, on the whole, the distribution of malaria in the last half century has undergone but little change *. The province of NorthHolland was the most malarious in 1875, as it is now. Still some foei have disappeared, the most important among them being the one in the northeastern corner of South-Holland and the adjoining parts of Utrecht.
Cause of the redudion in malaria incidence
The explanation (if there is any) of the changes which occurred since 1875: the considerable reduction of the incidence of tertian malaria; the almost complete disappearance of quartan; the shift of the
* James (Proc. Roy. Soc. Mei. Vol. 22, 1929, Sect. Epid. & State Med., p. 73) arrived at similar conclusions with regard to malaria in England.



Fig. 3. Dissemination of frankly intermittent tertian fevers through the Netherlands in 1875. The black dots indicate places where such fevers were recorded in great quantities. From unpublished reports.
annual malaria peak from October to June, and the disappearance of some of the former foei of infection, — will have to be postponed till we have more fully explained the present malaria conditions in this country.



Fig 4. Dissemination of malaria through the Netherlands in 1919. The black dots indicate places where much malaria was recorded (from the report of the Central Sanitary Council 1920; corrected by subsequent findings).
There is one factor which goes a long way in explaining the reduction of malaria. It is so obvious that it struck many of the contributors to the inquiry of 1875, at a time when nothing was known of the parasitic nature of the disease, viz. the quinine-



factor. Over and over again one reads the remark: intermittent fevers were very numerous here, but since quinine has become cheaper, and the common people are better able to pay for it, their numbers have been greatly reduced.
We do not belong to those who believe it is possible to eradicate malaria by means of drugs, and in chapter nine we shall explain why we believe this cannot be done. Nevertheless we are ready to accept the above explanation of the reduction of the malaria incidence, and for the following reasons.
Cinchona bark, as a drug to treat intermittent fevers, was known in this country for the whole of the eighteenth century, and quinine salts were extensively used from 1830 onward. But we are not exaggerating in saying that the real malaria patients derived but little benefit from this drug, for the simple reason that they rarely got it. So long as complications were threatening the patient's life bark was extensively used, but as soon as the fever assumed the pure intermittent type the drug was withheld. If it was a pure intermittent from the very start, the disease was permitted to run its natural course till it stopped after six or seven, often not till after sixteen, attacks. Many physicians considered it dangerous to stop the fever too soon, because continuous or putrid fevers, hydrops, anasarca, and icterus might be the consequences of such an untimely interference. There were even some who believed that intermittents cannot be overcome by bark, so long as they are not accompanied by bilious or bronchial symptoms. The more complicated the fever, the less we are able to identify it



as malaria in the modern sense of the word, the more cinchona bark or quinine was given *.
Moreover, cinchona bark, and later on quinine, was very expensive, and so it was logical to reserve it for cases of severe illness and to withhold it from patients who were likely to do well without the specific, be it after weeks of illness. The same financial consideration caused the drug to be withheld from the poor and to be reserved for the rich. Pringle t tells us the common soldiers rarely got it.
As a consequence, numerous patients continued to carry malaria parasites in their blood for weeks at a time and, as the annual incidence of the disease was at its height in late summer and early autumn, they did so at a time when anopheles were most likely to become infected (p. 128-135)- If we reflect how many infected anopheles we sometimes capture in this country (p. 134, 143). we realize what a glorious time we would have had collecting mosquitoes foi our classes in those days, when patients suffering from dysentery, typhus, enteric, scarlatina, phthisis, smallpox even, were dosed with cinchona bark, but when it was the merest chance if a simple malaria
patiënt'got any.
When, in the course of the nineteenth century, diseases like enteric, typhus, and dysentery were clearly defined and it was realized that quinine was not the proper remedy for them 5, the group of
* Nederl. Tijdschr. v. Geneesk. Vol. 70, 1926, -nd half, p. 1109-1111. t Observations on the diseases of the army. 3d I'-d. London 1761, Millar, Wilson, Durham & Payne, pp. xxvii + 435.
S Quinine was used in the treatment of enteric as late as 1871. It was reported to reduce the case-fatality from 24 pet. to 6 pet. (Nederl. Tijdschr. v. Geneesk. Vol. 15, 1871, ist half, p. M8)-



unclassed fevers came to contain an ever increasing admixture of real malaria. So quinine was more and more used for its true purpose and, thereby, became more and more effective.
A second factor, helpful in bringing about the same result, was the decrease of quinine prices. In 1822 the price was as high as 402 Netherland guilders per pound. Then it dropped to 182 in 1878, to 118 in 1880, to 14 in 1902, and to 17 in 1936 *. How much it was valued by the common people may be gathered from a statement, by one of the correspondents of the inquiry in 1875, that quinine was used as cash. That could happen no longer in these days; it has become too cheap and, what is more important still, the less well-to-do receive it altogether free of charge if they are members of a sickclub.
The development of the institution of the sickclubs (p. 7, 8) providing medical assistance free of charge to the members and their dependants, which started in the towns but gradually spread to the country-side, was another and even more important factor which ensured proper treatment for malaria patients. It gave them not only the drug but also the right man to administer it: the country doctor, and all this free of charge so long as they paid their contribution.
Finally, Laveran's discovery allowed of accurately identifying the patients to whom quinine ought to be administered. Unfortunately, this fourth factor which allowed quinine to come to its own was the
* Sulfate of quinine, Pharmac. Brit. 1932, costs 2 sh. 2 d. per ounce.



least effective of all. It took whole decades to convince the local practitioners that blood examination, in the diagnosis of malaria, gives them something of more importance than the satisfaction actually to see the red and blue parasites they know of from the pictures in their textbooks. Even to this day a doctor who assures one that he diagnoses malaria by the typical course of the fever (which, on closer scrutiny, he rarely has taken the trouble to tracé) is still far from being a rarity.
All these influences, tending to direct quinine to its proper address: the malaria patiënt, and to him only instead of to anybody but the malaria patiënt, could not fail to have an enormous success. Instead of the patiënt remaining a source of anopheline infection for many weeks, his parasites were quickly removed from the peripheral circulation. As we said bef ore, this must have had a considerable effect on anopheline infection. We may even assume that these influences were sufficiënt to eradicate malaria altogether in regions where anopheline density was too low to keep the disease going on the scanty food the healthy parasite-carriers (see chapter
eight) afforded it.
This victory over malaria was a fairly easy one. It was like the facile success gained by cleaning a tramp covered all over with lice; a bath and fresh clothes is all he needs to look presentable. But intrinsically he remains as lousy as he was before. So it is with the Netherlands: an accurate diagnosis of other fevers, cheap quinine, and sick-clubs were all that was needful to render it presentable, to remove its eighteenth century reputation of being,



next to Italy, the most fever-ridden country of Europe *. Modern science has contributed nothing to obtain this result.
But, potentially, the country reraains as prone to breed fevers as ever, as would soon become apparent if economie distress were to remove the veil which is hiding its malariousness.
* H. F. Thijssen, Geschiedkundige beschouwing der ziekten in Nederland. Amsterdam 1824, J. v. d. Heij & Zn., pp. xxiv + 426.



CHAPTER III Present-day malaria
Our knowledge of the present incidence and distribution of malaria in the Netherlands is very incomplete. Malaria is not a notifiable disease, so we have to rely on incidental inquiries giving a more or less accurate picture of malaria conditions in a single year, like those of 1875 in the old days and of 1919 in modern times. They require a good deal of careful sifting, since the diagnosis often leaves much to be desired. Local inquiries continued for several years, like that of Amsterdam in the fifties and sixties of last century, are of more value, because they are undertaken by the initiative of the local practitioners and not by government orders. Unfortunately, when the physicians interested in the subject retire, the investigations are often allowed to drop by their successors. In our days similar local inquiries have been taken up again, this time with the aid of modern apparatus. The city of Amsterdam is an example we already mentioned when discussing the reduction of malaria incidence in the second half of the nineteenth century (p. 18). But there are several others going on in rural areas of North-Holland and one in Friesland. The most important is the inquiry Dr. P. C. Korteweg began in Wormerveer (North-Holland) as early as 1880, which has been continued up to this day by his successors. We shall return to it presently.
Finally, since 1927, the sanitary inspectors of the



"Commission to stimulate the control of malaria by the population of North-Holland" (p. 8) have been collecting data regarding the incidence of malaria in that province, with the help of the local practitioners whom they regularly visit. Although these data are not as accurate as those of the local inquiries, because only a fraction of them has been checked by blood examination, they nevertheless furnish very useful information.
Geographical distribution
An inquiry regarding the geographical distribution of malaria in the whole of the Netherlands was undertaken by the Central Sanitary Council in 1919, the first year of the post-war malaria epidemie. The results of this inquiry have been embodied in the map on page 25 (fig. 4), which we already compared with the distribution of malaria in 1875. This map shows the province of North-Holland as the great centre of malaria. The centre in Friesland is the next one of importance. Five other foei are of little moment. They may flare up in the next epidemie upheaval but, as it is, local practitioners do not take an interest in malaria there, for which we cannot blame them as it is of too little consequence to worry about. But without their collaboration we can do nothing and so these five foei will not occupy us any more.
As regards the method of acquiring reliable information on the geographical distribution of malaria by establishing the spleen-rate in various villages, the Central Sanitary Council * had pronounced this
* Centrale Gezondheidsraad, Malaria Commissie, Rapport over 1919, p. 8-14.



method to be useless in the Netherlands, since the observed spleen-rates are too low (seven per cent or less ) to be of any significance. From 1919 onward it was a generally accepted rule that spleen-rates were not to be taken here. It was not till 1935 that this fixed rule came to be ignored. In October of that year we found a spleen-rate of twenty-five per cent among schoolchildren, of the age of six and older, living in the village of Uitgeest. Most of the enlarged spleens, we should add, were palpable on deep inspiration only *. Since then, Dr. de Rook has successfully extended our work, primarily for the purpose of giving a lead in the search for foei of anopheline infection. He found spleen-rates ranging from nineteen to twenty-seven per cent in some littleexplored villages of North-Holland near the new polder. Among the immigrant schoolchildren of the newly reclaimed area, he found, moreover, a spleen-rate of four per cent in children from non-malarious provinces, and of nine per cent in children from NorthHolland and Friesland. So we may take it that the "normal" spleen-rate in this country does not exceed four per cent, a figure which hardly differs from that found in other countries t. Eventually we may now
* Schüflner (Communie. Civ. Med. Serv. Netherl. Indies, year 1919, no- 9, P- 7) warns against overlooking cases of slight splenic enlargement; in this he agrees with the views the late Dr. S. T. Darling was known to hold. It was the neglect of this warning which led the aforementioned commission into their erroneous conclusion. Spleen-rates in this country, moreover, should only be taken at the end of the epidemie season, in October or November. Dr. de Rook found a spleen-rate of 26 pet. among a group of schoolchildren in November. At the end of the non-epidemie season, in March of the following year, the spleen-rate among the same children had fallen to 10 pet.
t Boyd (An introduction to malariology, Cambridge, Massachusetts, Harvard Univ. Press. 1930, p. 152, 153) quotes "normal" spleenrates ranging from one to three per cent.
Malaria. a



hope for a map, indicating the spleen-rate in every village of North-Holland, to materialize as a result of these efforts.
For the present, however, we shall have to rely on the data represented on the map (fig. 4, p. 25). They sufïice to bring out the most salient feature of the distribution of malaria in this country: the sudden transition from a solid block of malariousness in North-Holland to the, practically complete, freedom from the disease in South-Holland and the adjoining low-lands of Utrecht and Guelders. This transition is in no way accounted for by any change in the landscape. In both provinces low-lying meadows prevail, intersected by narrow ditches and broad canals. Sudden transitions, from highly malarious areas to such where the disease is unknown, occur all over the world. But they are usually accompanied by some marked change in the landscape: plains and hills, swamps and dry land, stagnant pools and running water. In this country, however, it requires a practised eye to teil whether the meadows one is looking at lie near Amsterdam, Rotterdam, or the Hague. Andyet, that makes all the difference with regard to malaria.
The freedom from malaria, enjoyed by South-Holland and its neighbouring low-lands, reminds us of the fact that this area corresponds to the "resistant peat-land of the post-glacial period (p. 4), characterized by the fluviatile deposits of the arms of the Rhine passing through it. These deposits contain large stores of fresh water. Fresh water, that is what makes the difference between North- and South-Holland; North-Holland is brackish, South-Holland is fresh. What that stands for we shall explain in chapter five. For the present



it may suffice to summarize the situation by saying: North-Holland, the land largely reclaimed from the sea, is malarious; South-Holland and the adjoining parts of Utrecht and Guelders, the ancient resistant peat-land, are healthy.
Annual variation of malaria incidence
The most complete records we possess on this matter are those of Wormerveer. A comparison of the various data at our disposal leads to the conclusion, that malaria conditions in the village of Wormerveer are fairly representative of the whole of NorthHolland and of the malarious portion of Friesland, although there are many villages much more malarious than this one.
The Wormerveer records were started, as we said, when Korteweg began to practise there in the year 1880. Up to 1902 they are a mere estimate of the malaria incidence. They are based on the quantity of quinine dispensed per head of the population, a figure which is not even known for all years. So these early records do not prove anything beyond the fact that malaria incidence was high in 1880 and that it decreased in subsequent years. They would not even prove that much, if another practitioner had made the older observations. But they were carried out by one and the same physician, from 1880 till 1918, and so he was able to check the reliability of his older observations by his modern ones. In 1902 the two practitioners, Korteweg and van Asperen, agreed to make the diagnosis malaria dependent upon the finding of plasmodia. So, for that one year, we possess figures for the whole village of Wormerveer. All the



others refer to Korteweg's practice only. It remained at a fairly constant figure of round about 3,000 persons entrusted to his care. Lampe, who succeeded to Korteweg's practice in 1919, continued his predecessor's records and method of diagnosis. In 1929 the second practitioner in that village, A.P.N. de
Fie S. Annual variation of malaria incidence. Morbidity due to malaria in Wormerveer according to the data collected by the physicians Korteweg, Lampe, de Groot, and Ris.
Groot, followed suit; so did the third, J. Ris, in 1932. Since that year the malaria incidence of the whole
village is known.
The preceding diagram (fig. 5) shows the number per thousand of persons suffering from malaria each year. The figures before 1902 represent an estimate based on the quantity of quinine dispensed.
This diagram brings öut the fact that malaria has been present all the time; but its incidence has varied



greatly. We can distinguish three major epidemie exacerbations of the endemic condition, occurring in 1880, 1902, and 1922. Moreover, there are four minor exacerbations, in 1905-07, 1912, 1929-31, and 1933-35. Their apparent absence in the period between the first and second major exacerbation is due, no doubt, to the imperfect records of those days.
All villages in North-Holland and Friesland *, where reliable records have been kept, show the same periodicity of malaria in the present century. The major exacerbations of 1900-1902 t and 1919-1922 § have affected most of them. The minor exacerbations, observed in Wormerveer, can all be traced in the other villages, although they sometimes resemble a major rise of malaria incidence. This synchronism in the annual variation of the sick-rate is not without exceptions: In all the villages the maximum of the post-war epidemie occurred in 1922, except for Nieuwendam where it did so in 1920. It stands to reason, that the conspicuous but temporary decline of malaria incidence, following on each major exacerbation, is an event fundamentally differing from the permanent reduction of malaria which occurred in the second half of the nineteenth century (p. 17).
We are ignorant as to the cause of the periodically recurring exacerbations. The last two major exacerbations were preceded by early autumns with an
* Viz. the city of Amsterdam, the villages of Zaandijk, Zaandam, Nieuwendam, and Uitgeest, all near Wormerveer; the mental hospitals at Medemblik (North-Holland) and Franeker (Friesland).
For the sites of these localities compare the map on page 2.
t H. J. M. Schoo, Malaria in Noord-Holland, Haarlem, F. Bohn, 1905, p. 199-208.
§ Report by the Central Sanitary Council on malaria in the Netherlands in 1919.



average temperature of 55 or over; that aiso appnes to the minor exacerbations of 1930 and 1934. We know, moreover, that the mild autumns which preceded them were noted for their unusually large numbers of anopheles (p. 98). Hence, one might feel inclined to regard the exacerbations as the effect of malaria transmission intensified by the mild weather of the preceding autumns. That, however, would be a rash conclusion, since the autumns preceding the exacerbations of 1905 and 1912 were in no way remarkable for their mildness, and since not every mild autumn is followed by an exacerbation of malaria. Of course there remains immunity, which undoubtedly influences the annual periodicity of malaria and which might counteract the anopheline factor. But we cannot measure it, as we can the other, and so it does not take us any nearer understanding the cause of the annual variation of malaria incidence.
* * *
A point of interest brought out by the graph on page 36 is the following: The malaria epidemie, which harassed a great part of Europe by the end of the war, has often been ascribed to infected soldiers returning to their homes and carrying the disease to a population weakened by famine and influenza. Although the Netherlands has borne its share of the general misery, there have been no transports of germs by demobilized soldiers. So, as regards that country, there would seem to be no reason to connect the malaria epidemie of 1920-22 with the preceding war. There are, moreover, the epidemics of 1880 and 1902 to warn us that such an event does not need the



impulse of a world-war or any other great catastrophe. In fact, the epidemie of 1900-1902 was greater than those of 1880 and 1920-22, still it was neither accompanied nor preceded by any great political or economie event. That is the reason why we have always feit some diffieulty in accepting the evidence adduced to explain the malaria pandemic of 1919, and following years, as the direct outcome of the world-war.
We also wish to emphasize the difficulties these periodic rises and falls of malaria cause to anyone who wishes to ascertain the effect of antimalaria measures. As a rule such measures are not taken at the very start of the epidemie rise. It lasts some time before the public and, afterwards, the government become alive to the seriousness of the situation. The epidemie may easily have reached its acme by the time sums of money are voted to combat the disease. After that, all is plain sailing, for malaria is declining. Any measure one has decided to adopt, even Bleuler's * "oudenotherapy" (doing nothing at all), will succeed in bringing about the desired effect. The method recommended to avoid such a mistake, the regular observation of villages not subjected to any measures where malaria ought not to decline by the time it is going down in the protected area, is no absolute safeguard against errors. If we had undertaken antimalarial measures in Nieuwendam in 1920, we would have observed the malaria incidence to drop from 489 cases in 1920 to 192 in 1922. In our "controlvillage" of Wormerveer we would have seen a simultaneous rise from 229 cases in 1920 to 326 in 1922.
* Das autistisch-undisziplinierte Denken in der Medizin. Berlin 1927, J. Springer.



So we would have arrived at the inevitable, but entirely erroneous, conclusion that our measures had been the cause of the decline of malaria in Nieuwendam. Observations in other villages, nearby or farther off, would not have saved us from committing this mistake. For it happened that malaria in all of them was rising synchronously with that in Wormerveer. Malaria in Nieuwendam was the only one declining, rather before its time.
Seasonal variation of malaria incidence
So far for the annual periodicity of malaria in the course of 57 years. But there also exists a monthly, or seasonal, periodicity of malaria, to which reference has already been made (p. 20) when describing the monthly periodicity of malaria in the city of Amsterdam in old and modern times. Here foliows the monthly incidence of malaria in the village of Wormerveer for the years 1902 and 1922. The figure under each month represents the number of persons falling ill with malaria in that month for the first time in that year.
Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
1902 2 6 13 55 in 133 "4 61 27 10 6 2 1922 4 2 7 45 80 75 70 16 11 87 1 1920 4 5 16 31 60 94 178 62 25 9 4 1
The last series represents a major exacerbation in Nieuwendam, which we have added to show that the acme may fall either in June, May, or July (printed in italics).
Faber * has given an interesting example of the
» Nederl. Tijdschr. v. Geneesk. Vol. 66, 1922, ist half, p. 1175.



monthly distribution of malaria reverting to the archaic type, with a peak in May and October (p. 19). He observed this in the village of Sloten in 1921, the year preceding the major exacerbation of 1922. The monthly distribution of malaria (without relapses) was as follows:
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
0 4 16 18 41 30 14 19 21 34 6 o
Here we meet with the true "autumnal" and "vernal" fevers we find described by old writers. We may imagine an army occupying Sloten in July, laughing at the idea that fevers could ever harm them, and then, finding themselves suddenly attacked in August, as happened to the British army in Walcheren. But we cannot imagine any high degree of mortality arising from such a situation, and so Faber's observation cannot alter our opinion of the true nature of the Walcheren epidemie of 1809 (p. 13).
It is rare to meet with such a marked rise of malaria in autumn. Still, it is by no means uncommon to meet with a year of normal monthly distribution of malaria, with a peak in May, June, or July, but with an additional smaller one in September. Korteweg * uses this partial shifting of malaria to the last quarter of the year as a means of forecasting an exacerbation of malaria in the next year. He holds that such an event signifies an intensified autumnal malaria transmission (p. 129). A certain percentage of human beings infected in autumn are bound to fall ill after a short incubation, although the majority are not going to
* Ztschr. ƒ. Hyg. u. Inf. Krankh. Vol. 110, 1929, p. 724-731.



suffer from malaria until seven to nine months later. The more persons infected during the autumn, the higher the number of malaria patients during that season, the higher the morbidity to be expected next summer.
That is a much simpler explanation of the autumnal peak of malaria than the one we suggested (p. 20) to account for the autumnal epidemics in old times. Korteweg's hypothesis assumes that a heavy autumnal anopheline infection is the cause of it. Our hypothesis requires the presence of a peculiar strainof Plasmodium vivax which causes fever within a few weeks of its inoculation by a mosquito into the human host. Of course, we know that such strains exist, e.g. the socalled "Madagascar strain" of James (p. 227). Nevertheless, it is difficult to imagine their sudden appearance in a village and their equally sudden disappearance the next year. If Faber's peak in the autumn of 1921 had been caused by a peculiar strain of PI. vivax, that strain must have disappeared in 1922, since the autumnal rise in malaria was not repeated in 1922. Neither was it repeated in similar cases which Honig * described in Nieuwendam and Korteweg t in Wormerveer.
On the other hand, Korteweg's hypothesis cannot explain old-time conditions. If the autumnal peak in former years was so high because of a particularly intensive autumnal transmission, the next year's summer peak ought to have been still higher. And that was not the case. Year af ter year the autumnal peak
* Studie over de malaria te Nieuwendam en omgeving. Amsterdam 1922, J. H. de Bussy, 79 pp.
■f Geneeskund. Bladen. Vol. 22, no. 1, 1920, p. 8.



was as high or higher than the vernal one. So we conclude that autumnal malaria, as it sometimes occurs in our days, is different from autumnal malaria which was the rule in old times. The two cannot be explained in the same way. Until we find a better explanation, we shall have to stick to the hypothesis of an entirely different strain of benign tertian parasites existing in the past.
Is malaria morbidity an accurate gauge of the incidence of malaria infection?
Malaria in years of epidemie exacerbation affects from twelve to seventeen per cent of the population. In non-epidemie years this figure may fall as low as a fraction of one per cent. These figures signify that twelve to seventeen per cent of the population have been so ill with malaria that they had to call in their physician's help. In this country, owing to the system of the sick-clubs (p. 7), it will rarely happen that a person who is really ill will neglect to ask for medical aid. So we may be sure that these figures represent the true malaria sick-rate, provided the local practitioner's diagnosis is accurate, as it is in the selected villages mentioned in the preceding section.
The following experience made in the village of Uitgeest shows, however, that the sick-rate due to malaria cannot be identified with the malaria incidence, if "malaria incidence" connotes the rate at which malaria infection is spreading among a population.
In that village, which had a malaria sick-rate of twelve per cent in 1936, we kept thirty-one families with 264 members under close observation. Owing to



the focal character of malaria (p. 45) the malaria sick-rate among this group was much higher than the average for the whole village, viz. twenty-nine per cent. But the point we wish to make here is that 67 persons, out of 186 who had not been ill with malaria (not, at least, to the extent of requiring the physician's help), were carrying malaria parasites in their blood. Now the question arises: are we to count these parasitecarriers among the malaria patients, i.e. among those who are suffering from malaria in one way or another, or are they to be considered as "healthy carriers"? The first alternative means that the malaria incidence is nearly doubled, the second that we are considering as healthy a group of persons who are an important source of anopheline infection (p. 165). The existence of so many "healthy" carriers renders it extremely doubtful whether the actual sick-rate due to malaria is a reliable gauge of the importance of malaria as a public-health problem in this country.
There is a second reason for this doubt, viz. the focal character of malaria, i.e. its disposition to occur in clusters.
The map on page 25 suggests that malaria is diffusely dispersed through the whole of the province of North-Holland. Nothing is less true. Malaria is as focal in character here as it is in the United States, where this peculiarity of the disease is so much stressed. As an illustration of this statement a map (fig. 6) is printed here, of the village of Wormerveer, showing the distribution of malaria for the last seven years. In this village, with 9,000 inhabitants, malaria occurs in clusters, leaving wide gaps between where most people know of malaria by hearsay only and never



Fig. 6. Malaria occurring in foei. Map of Wormerveer on which are marked the bloeks of inhabited houses and the site of the houses with i, 2, 3-4, 5 and more malaria patients in the years 1931-1936.
experience it themselves. Every street, every small group of houses, offers its own malaria problem which can be dealt with separately without reference



to neighbouring streets, sometimes even to neighbouring houses, as we shall explain in chapter nine.
In order to give a trustworthy picture of what malaria really means to the part of the population actually afïected by it, the incidence of the disease should be computed with reference to the number of inhabitants in these clusters and not with reference to the total population. If we do that (as in the example quoted on page 44) we find much higher figures for the local incidence.
This remark likewise applies to larger towns, where malaria is much more prevalent on the outskirts than in the centre. In the town of Alkmaar, for instance, A.J. Korteweg * found malaria in 1920 dispersed through the town, in such a way as to cause a morbidity of seven per cent in the centre and eighteen per cent on the outskirts.
The third reason why sick-rates are misleading, no matter how carefully they have been collected, is to be sought in the morbidity often being higher in children than in adults. In highly malarious regions we know that the adults acquire an immunity which renders them less liable to suffer from malaria than children. But one would not expect to meet with this difference between adults and children in the presence of so little malaria as is seen in this country. Nevertheless, the difference is observed in villages like Wormerveer, Uitgeest, and Nieuwendam wheremalaria has existed for a long time. Infants (under one year), as a rule, suffer little from malaria, but the sick-rate in children of one to fifteen years is more than twice
* Nederl. Tijdschr. v. Geneesk. Vol. 65, 1921, ist half, p. 2065.



as high as that in persons of sixteen or older, as exemplified by the accompanying diagram (fig. 7) of the malaria morbidity in Wormerveer, over the years 1930-1936, for each age-group separately *.
The focal distribution of malaria we mentioned before, i.e. the existence of limited areas within a
Fig. 7. Higher sick-rate in children than in adults. Morbidity due to malaria in the village of Wormerveer, from 1930 till 1936, computed for each age-group separately. Age 1 means a child in the first year of its life, age 2 a child in the second year of its life, commonly said to be 1 year old, &c.
village where the malaria morbidity is constantly higher than elsewhere, offers a much better opportunity of acquiring some degree of immunity than would have been the case if malaria had been evenly disseminated through the whole village. So the higher malaria morbidity in children, as compared to adults,
* Festschrift für Nocht. 1937, p. 620-624.



does not require any special explanation, but can be accounted for in the customary way, i.e. by the immunity acquired by the adults.
However, it is not with the object of explaining the higher sick-rate among the children that we have raised this point, but to justify our opinion that the morbidity figures are an inaccurate gauge of the real incidence of malaria in this country.
Parasites and fevers
Practically all malaria in the Netherlands is caused by Plasmodium vivax.
Subtertian malaria is not rare in patients coming from the tropics, but it is never seen to spread. There are many who believe that it was present in this country in former years. Whatever truth there may be in this hypothesis (and on p. 16 we have given the reasons for our scepticism in this matter) there can be little doubt that subtertian malaria could not get a foothold here under the prevailing conditions which limit anopheline transmission to theautumn(p. 140). Plasmodium falciparum requires a high temperature, not only for the development of the oöcysts, but also for keeping the sporozoites alive. Plasmodium vivax, at least the Dutch strain of this species, is not so susceptible to cold.
Quartan malaria undoubtedly is a native to the soil, but it is extremely rare and, moreover, very erratic in its occurrence. Leaving aside all cases of alleged quartan not confirmed by blood examination, we know of eight cases which van Emden * described
* Herinneringsbundel voor Prof. S. S. Rosenstein. Leiden 1902, Ed. IJdo, p. 109-122.



in 1902, one in Leyden and seven in a village east of that town, in the north of South-Holland. Curiously enough, he met with no tertian in that village and only with one case in Leyden, although he saw many cases from other parts of the country. The next record comes from Rotterdam, in 1915, where van der Heijden * gave a detailed description of a case of quartan in a man who, forty-two years before, had been suffering from a fever with exactly the same symptoms, in another village in the above mentioned quarter of South-Holland. During the post-war epidemie the town of Alkmaar in North-Holland, among hundreds of tertian cases, had five cases of quartan in 1920 and two in 1921. They were all of them unconnected and occurred in persons who had never left the country. One more case occurred in a patiënt living in a village in the south of North-Holland, in 1925, but who had moved there from the north of that province.
Finally we come to an example which is the most puzzling of all, namely to the quartan fever which, in 1936, attacked the chairman of the malaria commission of the Sanitary Council, the late professor Aldershoff. He had never lived in any malarious part of the country, but he had visited the Balkans some sixteen years before. However, we are not going to blame another country for the malaria which occurs here. We always feel hurt when we hear the Germans saying they have no malaria except on the Dutch border. So we will not pay others with the like coin and we shall charge Professor Aldershoff's quartan to our own account.
* Nederl. Tijdschr. v. Geneesk. Vol. 59, 1915, 2nd half, p. 1680-1682.
Malaria. A



Quartan can remain latent for a very long time.That may account for the fact that eleven of the eighteen cases, quoted above, occurred in places where malaria was either unknown or rare. The Alkmaar cases, on the other hand, occurring at the height of the postwar epidemie, suggest that they are truly home-bred and that the opportunities of malaria transmission were so good as to offer even quartan a chance *.
* * *
Benign tertian malaria forms, as we said, the bulk of our intermittent fevers. lts causative agent is the ordinary Plasmodium vivax which presents, however, one peculiarity worth mentioning. This peculiarity is the tendency to remain latent for seven to nine months after the parasites have entered the human body by the bite of anopheles. Isolated from cases in North-Holland and Friesland, and continued by alternating passages through patients, suffering from general paralysis of the insane, and mosquitoes, our PI. vivax has been proved to breed true to it through many generations. In mental patients the Dutch strain remains latent in one quarter to one half of the cases. Under natural conditions, i.e. in healthy persons, on whom no more than one or two infective bit es have been inflicted, the tendency to latent infections is even more marked (p. 153).
In this connection we may recall the hypothesis (p. 20) which served to explain the fact that malaria in the past reached its annual maximum in autumn, whereas it does so no longer in our days.
* For experiments on transmission of quartan see p. 245.



It was the difference bet ween the Dutch and the Madagascar strains of Plasmodium vivax which suggested to us an explanation of this change in the seasonal incidence of malaria, by the assumption that in those days a strain of Plasmodium vivax had been prevalent which differed from our present one (and resembled the Madagascar strain) in being little inclined to lapse into long latency on entering the human body.
* * *
There remains for discussion one feature of tertian malaria which is of great importance in this country, because of the bearing it has on the colonization in the new Zuydersea polders. We refer to the so-called initial remittent fever in benign tertian malaria which occurs in those patients only who have never before suffered from that form * of malaria.
Van den Bergh t was the first to draw attention to the existence of the initial remittent in this country, but Korteweg § gave the first detailed description based on numerous observations of cases of induced
* Two healthy European volunteers in Deli (Sumatra), who had never had malaria before, suffered from experimentally induced subtertian in November and, afterwards, from (likewise experimental) benign tertian in December. The benign tertian attack was characterized by the presence of numerous parasites and by the fever being immediately amenable to quinine treatment. The first volunteer took quinine without delay and had one access of fever only; the other, who did not take quinine until the end of the fever-free day, had two accesses, on December i8th and 2oth (Communie. Civ. Med. Serv. Netherl. Indies, Vol. 8, 1919, no. 9, p. 53-71, diagrams 1-4). From this experiment it might be inferred that an attack of subtertian protects the subject from having an initial remittent in a subsequent first attack of benign tertian. But it does not prove this inference to be correct, since we are not sure that the initial remittent is a common feature of benign tertian malaria in all parts of the world. t Nederl. Tijdschr. v. Geneesk. Vol. 65, 1921, 2nd half, p. 1488. § Ibid. Vol. 68, 1924, ist half, p. 1622-1638.



malaria, in patients suffering from general paralysis of the insane, and also in his Wormerveer practice. Korteweg describes the initial fever as a remittent lasting for three to six days and then changing into _ the typical intermittent fever. It is, moreover, characterized by the scarcity of parasites found in the thick film. Since this first description we found, * moreover, that the initial remittent does not respond to the same drugs which promptly cure it once it has become intermittent.
We became thoroughly convinced of the practical importance of the "initial remittent" in benign tertian in the course of two experiments in which twenty-two volunteers were subjected to the bites of infected anopheles. There was a striking difference between the fourteen volunteers who had never suffered from malaria before and the eight others who had. Those who had never had malaria in the course of their existence had to stay in bed for three to five days. They took quinine, or quinoplasmine, from the first or second day of their illness onward, but they had to continue with it for two or three days before the fever abated. Moreover, parasites were extremely difficult to detect so long as the initial fever lasted. So it is very easy to overlook them and it is not surprising to see patients of that description occasionally finding their way to the hospitals with the provisional diagnosis of enteric or appendicitis and, as a consequence, often remaining without proper treatment for a considerable time. In the eight volunteers who had already suffered from malaria,
* Proc. Roy. Acad. Science Amsterdam. Vol. 34, I931» P- 1216-1220.



parasites were so numerous as to be easily detected as soon as a slight feeling of malaise warned them that an access of fever was impending. Those who took quinine at that moment had their expected attack cut short. Others, in whom the exigencies of the experiment required the ascertaining of the type of fever, came down with a typical intermittent, readily amenable to quinine. The former had really been applying Külz's method of prophylaxis as modified by Schüffner *, and we can now understand why this method, useful in many cases, sometimes is of no avail.
There is another observation which likewise finds an explanation in this experience. It is a very old one, relating to soldiers quartered in the fever-ridden regions of Zealand and to immigrants settled in newly reclaimed polders. The indigenous population
* Külz (Arch.f. Sch. u. Trop. Hyg., Vol. 17, 1913. P- 834"835) advised the persons to be protected to watch themselves carefully and to take small doses of quinine (fifteen grains taken in two days) as soon as they experienced the prodromal symptoms of subtertian. Schüffner modified the procedure by adding to it the examination of a thick film. Quinine is taken in therapeutic doses for five days, but only if parasites are found. In that case the temperature, which rose to 99.5°-100°, is stopped from further rising and the impending access of fever is effectively cut short. Külz says it is useless to recommend this method to laymen. They are unable to recognize the fever until they are in the thick of it, especially if it is the first malaria they ever had. We do not believe this failure is altogether the layman's fault. We have observed two cases of experimental subtertian in Europeans (p. 51, footnote) who had never suffered from malaria before, whose date and hour of infection was known, who took their temperature daily ever since, who took quinine from the moment their temperature rose to ioo° (parasites were found after a long search), and who, nevertheless, developed a full-sized access, which lasted for the next three days. A year and a half later one of the subjects developed another subtertian, but that one readily responded to Külz's prophylaxis. So our conclusion is that there likewise exists an initial fever in subtertian, but, owing to the type of fever, it can be identified only by its not responding to Külz's prophylaxis.



had frankly intermittent fevers only, whereas in strangers, who feil ill for the first time, the disease took a more severe course and could not always be overcome by cinchona bark or quinine.
It is, indeed, easy to imagine now what will happen if malaria attacks a population the majority of whom have never had the disease before. The effect will be far more alarming than that which one observes in an epidemie of equal, or even greater, extent in malarious villages of North-Holland, where most adults have had the disease at least once in a lifetime. That is what the directorate of the new Zuydersea polder is afraid will happen in the near future, because a population have settled there four fifths of whom come from regions where malaria is unknown. They have not had an opportunity of acquiring that first stage of immunity which protects the subjects from the initial remittent. The other fifth of the immigrants have come from malarious areas in North-Holland and Friesland, and no one knows how many "healthy" carriers exist among them. So the potential danger of malaria transmission exists, and locally acquired cases are there to show that it is not imaginary. For the present they are few and far between, but the warning which they give is worth heeding, since a malaria epidemie, half the size of that which struck the village of Uitgeest in 1936, would be amply sufficiënt to render immigration to the Zuydersea polders thoroughly unpopular.



CHAPTER IV
The races of Anopheles maculipennis Korteweg's hypothesis
Now that we have dealt with malaria in man, it would be logical to complete our account by describing malaria in anopheles, especially as we repeatedly have had to touch upon the subject of the autumnal infection of these mosquitoes. Still, we cannot embark upon this matter without first explaining what these mosquitoes are like and how they behave. That is what will occupy us in the present chapter and the next.
The first thing which strikes us here is that anopheles are found all over the country and malaria is not. That, by itself, is not remarkable. It is a mere repetition of the classical observations of Stephens and Christophers in Bengal *, and of James at Ennur t; that no correlation exists between the geographical distribution of malaria and of anopheles. But i n their cases this difficulty was overcome, once it was realized that the species of anopheles are not made for the entomologists to label and to pin into their collecting boxes, but that they are the hygienists' concern as well. Some of them were shown to transmit malaria in nature and others were proved to be
* Rep. Malaria Comm. Roy. Soc. 6th. Ser., 1902, p. 3-10. t Quoted in Ibid. 7th Ser., 1902, p. 22.



harmiess. The reason, why Stephens and Christophers found anopheles numerous in the healthy plains and scarce in the malaria-ridden foot-hills, was simply that the plains bred huge numbers of the perfectly harmiess Anopheles rossi, whereas the foot-hills bred the dangerous Anopheles listoni, in much smaller quantities, but amply sufficiënt to render these regions thoroughly malarious.
In the Netherlands, however, conditions are different. As in Bengal, anopheles are present all over the country, no matter whether there is malaria or not. But, unlike Bengal, they all belong to the same species, Anopheles maculipennis.
Anyone, accustomed to conditions in the Netherlands' Indies and finding the home-country in the grip of a malaria epidemie, would be utterly at a loss what to do. In the Far-East matters look much more hopeful. Out there, anopheles are just as numerous, but they belong to many species, some of them dangerous and numerous others harmiess. Each one of them has its own peculiar habits, and so we can piek out and destroy the dangerous species, while leaving the harmiess ones unscathed; in otherwords, we can practise species-sanitation *. Without speciessanitation one feels helpless. But how can one practise species-sanitation in a country where there exists one species of anopheles only? True: there are three,
* We have been credited, occasionally, with the invention of speciessanitation, because we coined this term or, at least, its Dutch prototype (species-assaineering, see: Indische Mercuur, January gth, 1920). So we wish to make it quite clear that it was Sir Malcolm Watson who invented the method of dealing with one species of anopheles to the exclusion of all others. All the credit we can claim is of having seen and stated, more clearly perhaps than others, the great promise for the future this method holds.



but A. bifurcatus and A. plumbeus do not count.
That was the problem we had to face. lts solution lies wholly to the credit of our old friend Dr. P. C. Korteweg, who showed us the way out of the difficulty.
In the East, dissecting mosquitoes and taking spleen- and blood-census are the first things to do in a place one knows little of. The last were denied us here, because spleen-rates at that time were taboo, and the idea of taking a blood-census seemed preposterous in a country where so much quinine was swallowed daily. So there remained the dissection of mosquitoes as the only subject of investigation. Of course there was no sense in that. In the Far-East, mosquito-dissections indicate which one of the many anopheline species haunting a place is the local vector. With one species present only, as in the Netherlands, dissections serve no useful purpose. Nevertheless, we started on that road to nowhere.
By the end of August we feit thoroughly disheartened. The malaria season was coming to an end and we had hardly found a single infected mosquito, although we had been searching in the most likely places to find them, viz. the malaria houses. It was Korteweg who saved us from throwing up the whole thing in despair, by drawing our attention to a hypothesis he had published eighteen years before *. Here it is, in his own words:
"The presence of a fair number of patients suffering from primary attacks of malaria in April and May 1902, at a time when anopheles are very scarce, strongly suggested to me the possibility of these
* Herinneringsbundel voor Prof. S. S. Rosenstein. Leiden 1902, Ed. IJdo, foot-note on p. 272.



patients having acquired their infection in the autumn of 1901, an infection which had been awaiting the next spring to bring about malarious symptoms. The correctness of this assumption would be proven if a human being, bitten by an infective mosquito, did not develop malarious symptoms until the end of an interval of considerable duration. Unless some therapeutical effect, benificent to the subject, can be hoped for from such an operation — in cancer for instance — an experiment of that description seems hardly justified".
In 1920 Korteweg was as far from the proof of his hypothesis as he was in 1902. Each year a fresh batch of primary spring malaria made him believe he was right, after all; but each year it occurred to him that Erich Martini's * hypothesis, of the warmth of heated rooms in April and May creating the requisite conditions for the amphigony of the malaria parasite, afforded an alternative explanation which had, moreover, the merit of being less fantastic.
To Korteweg it was of the utmost importance to have dissections of anopheles continued during the period anopheline infection ought to occur if he was right in his hypothesis, i.e. in autumn. Accordingly, he urged us to carry on with them and he overcame our objection that it was no use searching for infected mosquitoes at the end of a malaria season.
Fortunately, we adopted Korteweg's suggestion and the result was that we found infected mosquitoes in great numbers, from September onward. We continued like this for two years and always found the same:
* Ztschr. ƒ. Hyg. u. Inf. Krankh. Vol. 41, 1902, p. 147-152.



plenty of infected anopheles in autumn, scarcely any during the rest of the year *. In this way Korteweg's hypothesis received a solid basis; it postulated human beings acquiring their infection in autumn, and now Korteweg could point to the anopheles we actually found infected in autumn. What he needed them for was to explain primary cases in spring. He did not feel that primary cases occurring in June were in need of any special explanation. Anopheles were believed to be sufficiently numerous in June to account for all malaria from that month onward. That there were no infected anopheles in summer did not worry him for the moment. How that difficulty was solved remains for another chapter to explain (p. 96, 151).
* * *
The thousands of dissections we carried out served another purpose. They showed us that the production of mature ova in anopheles comes to an end by the beginning of September. Curiously enough, this pause in the sexual activity of anopheles (it is no more than a pause, since it will recommence in spring) did not greatly impair their appetite; they continued to feed voraciously. A marked falling off of their appetite did not occur till the beginning of November, so as to leave two months, at least, during which anopheles are sexually inactive but taking blood at a rate not much inferior to that observed in summer.
An appetite unimpaired by the cessation of sexual activity is not peculiar to Dutch Anopheles maculipennis. It has been observed in many countries.
* Buil. Soc. Path. Exot. Vol. 15, 1922, p. 116-119.



Grassi * called this behaviour semi-hibernation, to distinguish it from hibernation proper, accompanied by an almost complete cessation of reproduction, nutrition, and motion. In semi-hibernation the first only ceases, for nutrition necessarily entails motion and, moreover, keeping near the animals the mosquitoes feed on by occupying animal or human habitations. In the environment of Amsterdam we found sexually inactive anopheles resting in the bedrooms (usually upstairs) and, in much greater quantities, in animal habitations, notably in horsestables and pigsties, much less in cattlesheds, and least of all in uninhabited outhouses.
The name "semi-hibernation" has the disadvantage of overemphasizing the circumstance that the process occurs in winter. It is continued during the winter, but it starts in August and even as early as the end of July, if one is watching for the first signs of its approach. These first signs do not consist of a marked falling off in the rate of females carrying mature ova. That rate is still high by the end of July. But among the non-pregnant females one notices, by that time, a small number carrying a fully developed adipose body. They are the first ones to enter upon their term of sexual inactivity and they are doing so when summer is in full bloom. On the other hand, reproduction recommences in early spring, and the rate of females carrying mature ova is as high as it ever will be in summer, at a time temperature is still as low as in October or even November.
All this afforded additional evidence in favour of
* Ann. d'Igiena. Vol. 32, 1923, p. 438.



Korteweg's hypothesis. The anopheles found infected in autumn would have been of no use to him if they had been properly hibernating, for how cóuld they transmit malaria without feeding on human beings? And as they no longer carried mature eggs, they remained at home and always at hand to take the infection or to give it. So Korteweg, whose chief interest was in his hypothesis, feit satisfied. But we, with our interest centered on species-sanitation, did not. In fact we would not have got any nearer speciessanitation if van Thiel * had not been carrying out investigations in another part of the country, where little malaria existed, viz. around the South-Holland town of Leyden.
Discovery of the shortwinged and longwinged races
The conditions van Thiel observed around Leyden were exactly the opposite of those we had noted around Amsterdam. The Leyden anopheles were found mainly wintering in outhouses; they were scarce in animal habitations. Although no data were collected regarding the number of engorged and pregnant females in the succeeding seasons, he nevertheless observed that anopheles are not influenced by the presence of food in the choice of their winterquarters. This struck us as so important that we gave as our opinion t that, contrary to our observations around Amsterdam, but in agreement with Wesenberg-Lund's § in Denmark, feeding of anopheles
* Anopheles en malaria in Leiden en naaste omgeving. Leiden 1922,
Ed. IJdo, 97 pp.
f Tijdschr. v. Vergel. Geneesk. Vol. 7, 1922, English summary on p. 303.
§ Acad. Roy. Danemark. Sect. Science, Série vii, 1921, no. 1.



around Leyden is pre vent ed during the winter halfyear by their habit of sheltering in outhouses. We pointed out that this habit cannot fail to render Anopheles maculipennis less successful as a malaria vector.
This was a real step onward. We knew already that malaria transmission is practically limited to autumn and winter. And now, thanks to van Thiel, we had become aware of the existence of a region where anopheles cannot possibly transmit malaria at that time of the year; and in that region malaria actually was scarce in years when the disease had assumed epidemie proportions in North-Holland and elsewhere. Consequently, Anopheles maculipennis is present all over the Netherlands, but it behaves differently in various parts of the country; and this different behaviour influences malaria transmission favourably or adversely. But, can that knowledge lead to species-sanitation? Are these anopheles, behaving differently, to be considered as species? That remained to be proven.
In 1924 van der Hoeven * had propounded the hypothesis that the adults of Anopheles maculipennis bred in salty water are so enfeebled as to become susceptible to malaria infection, and so to act as vectors of the disease. Those, on the other hand, which are bred in fresh water are stronger, less susceptible to malaria infection, and so do not act as vectors. This hypothesis was based upon his laboratory observation that adult anopheles bred in salty water are smaller than those bred in fresh water, and upon
* Uitbreken en verdwijnen van malaria. Rotterdam 1024. Niieh &
* Uitbreken en verdwijnen van malaria. Rotterdam 1924, Nijgh & van Ditmar, 26 pp.



the very old theory that a mixture of sea-water and fresh water breeds swamp fevers.
Van Thiel took up van der Hoeven's suggestion of salty water harming anopheles and rendering their adults smaller, weaker, and more susceptible to malaria infection. He began to compare the size of his Leyden anopheles with those of the malarious regions of Amsterdam and Bolsward *. He found the Leyden ones of larger size, with longer wings, but with a smaller number of maxillary teeth t, than the Amsterdam and Bolsward specimens. Now, the water in the ditches around Leyden is fairly fresh, and that around Amsterdam and Bolsward highly brackish. So he feit inclined, provisionally, to accept van der Hoeven's view that the small size of the Amsterdam and Bolsward maculipennis was the effect of environmental conditions, the salinity of the breedingplaces; the small anopheles were better malaria vectors than the large ones, because they were less resistant to the invasion of the germs.
A survey which covered the southern portion of North-Holland and the adjacent parts of SouthHolland and Utrecht, a region which is traversed by the boundary marking off the malarious area to the north and the healthy one to the south, enabled us § to confirm van Thiel's findings. During autumn and
* P. H. van Thiel, Arch. f. Sch. u. Tropenhyg. Vol. 30, Beih. 1, 1926, p. 67-76.
f With a view to Roubaud's hypothesis (Buil. Soc. Path. Exot. Vol. 14,
1921, p. 577) on the relation existing between the number of maxillary teeth and the ability of A. maculipennis to act as a malaria vector.
§ De variatie bij Anopheles maculipennis in verband met het anophelisme zonder malaria. Amsterdam, 1926, Universiteitsboekhandel, 83 pp.; Proc. Roy. Acad. Science Amsterdam. Dutch Ed. Vol. 35, 1926, p. 1167-1174.



winter (the only time of the year when one is sure to be dealing with a homogeneous material belonging to one and the same generation) the female longwinged paucidentate Anopheles maculipennis (with an average wing of 5.5 millimeters and 16-17 teeth to the maxilla) were markedly more numerous in the southern malaria-free region than in the northern malarious one. The female shortwinged multidentate anopheles (with an average wing of 5.1 millimeters and 17-18 teeth to the maxilla) prevailed in the northern malarious area. Moreover, we found the salinity, on the whole, much higher in the malarious area than in the healthy one. Experiments confirmed van Thiel's view that the length of the wing is greatly influenced by environmental conditions, although we found the salinity by itself of little influence. Food is the most important factor influencing the size of the adults. If the food is adequate, the adults reared are of the size one finds in nature, no matter whether they are bred in salty or fresh water. But if the food is not, one always rears undersized adults, in fresh water as well as in salty water (p. 223) *.
* * *
So far we had been able to confirm van Thiel's results. But we could not follow him to the whole length of his conclusions. With regard to the alleged greater susceptibility to malaria infection of the shortwings, we found that it did not exist. Both longand shortwings could be infected, and to the same
* Riv. di Malar. Vol. n, 1932, p. 138-139; Vol. 13, Sez. 1, 1934, P- 239.



proportion *. But the most important disagreement was this: Although their size is greatly influenced by environmental conditions, we nevertheless found that we canriot breed longwings out of shortwings or vice versa. Starting from eggs laid by longwings, the next generation is longwinged again, longer winged at any rate than a generation bred, under the same conditions, from eggs deposited by shortwings.
The difference was most marked in the niales bred, under exactly similar conditions, from ova deposited by longwinged or shortwinged females. This is shown by the following list referring to an annual average of 338 males. It clearly brings out two facts:
(1) the wings of both long- and shortwings have much increased in length in 1932 as a consequence of an improved technique of feeding (influence of the environment);
(2) the difference between the two races remains the same through all these years (influence of heredity).
Daughter generation bred from longwings and shortwings under exactly similar conditions in each group of experiments
comprising both races Year Males
Average wing in millimeters Longwing Shortwing
1926 4.1 3.7
1927 4-3 3-9
1928 4.6 4.2
1929 4-3 3-9 Ï932 5-3 4-9
So we concluded that longwings and shortwings pass their characters on to the next generation, that
* Proc. Roy. Acad. Science Amsterdam. Dutch Ed. Vol. 35, 1926, p. 1172; Riv. di Malar. Vol. lï, 1932, p. 150.
Malaria. 5



they breed true to type; in short, that they are races possessing heritable characters, and not mere modifications induced by environmental conditions. Van Thiel came to the same conclusion *. He even went to the length of giving the shortwings a Latin name: Anopheles maculipennis var. atroparvus.
The third fact of importance, emerging from our investigations, was the different behaviour of the two races during the winter half-year. Both longwings and shortwings were most voracious in June: about four fifths of the females were found engorged in animal habitations. Later on, the rate of engorged females did not decrease much in the shortwings until November. In the longwings, on the contrary, engorged females disappeared in September. Consequently, the longwings go in for true hibernation and so they cannot transmit malaria in autumn. The shortwings are the ones that semi-hibernate, producing no mature ova but continuing to feed, i.e. continuing in a position enabling them to transmit malaria.
Here was a find of some real importance. These longwings were now proved to be the same as those of which, in 1922, we had said that their habits cannot fail to render Anopheles maculipennis less successful as a malaria vector. And now, A. maculipennis displaying that habit was proved to belong to a separate race with heritable characters. Two separate races, one able to transmit malaria in autumn, the other unable to do so because it does not feed during that season: species-sanitation no longer appeared so far out of our reach as it did before.
* * *
* Buil. Soc. Path. Exot. Vol. 20, 1927, P- 551. &87> 797-



Our next object was to find out * whether these races could be made to account for the difference between the geographical distribution of malaria and of anopheles. This inquiry made us acquainted with the phenomenon we have called segregation, occurring at the time the longwings are hibernating and the shortwings semi-hibernating. This term signifies that the shortwings take shelter in the stables kept warm by the presence of the animals on which they continue to feed, whereas the longwings retire to the cool storerooms above the stables, or to outhouses, away from any source of food or warmth. There is one place only where both races meet, viz. in the attics of human habitations; these are cool, so they suit the taste of the longwings; and they contain food for the shortwings, since these attics are used as bedrooms.
Segregation explained the difference between our winter-findings around Amsterdam and van Thiel's around Leyden (p. 61). In Amsterdam, where shortwings predominate, it was no more than was to be expected that the stables should be full of anopheles and the outhouses empty. In Leyden the contrary obtains, since longwings are in the majority there. But anopheles find their way to human habitations in both places.
Moreover, segregation afforded us the opportunity, we were looking for, to ascertain where the two races occur in winter and in what numbers. The number of anopheles in the stables, and in the stores above them, permitted a fairly accurate estimate of the composition of the anopheline popu-
* Proc. Roy. Acad. Science Amsterdam. Vol. 31, 1928, p. 531-539-



Fig. 8. Geographical distribution ot tne two races, map 01 ine gieiuci part of the Netherlands indicating the site of stables where shortwings were in the majority (black dots) or where longwings were (white dots). The shortwing area, moreover, is darkly shaded and the longwing area lightly.
lation in every single village. In each village the anopheles were counted separately in the stables and the stores upstairs. The average wing-measure was taken, to make sure that the former group was shortwinged and the latter longwinged. In this way we



Fig. 9. Regions with fresh and salty water. Distribution through the Netherlands of water of a salinity of 0.16 pet. of chloride of sodium or more (black dots) and of water of a lower salinity (white dots). The area of high salinity is shaded. The data serving to draft this map have been compiled from various published and unpublished
records.
ascertained, for each village, whether the majority of the anopheline population was long- or shortwinged, and on the basis of these investigations we drew a map of the geographical distribution of



the two races through the greater part of the Nether-
lands * (fig. 8).
This map clearly shows that the shortwings are by no means limited to the coastal provinces. They likewise occur farther inland, in areas where the water is fresh (fig. 9) and where malaria does not exist (fig. 4, p. 25). So it would seem that' we had only shifted the difiiculty. At first it was the fact of Anopheles maculipennis being present everywhere and malaria limited to the coastal provinces (except South-Holland); now it consisted in the shortwings occurring — well, not quite everywhere (in SouthHolland they were in the minority, so that was one point gained) but, at least, in numerous other parts of the country, notably along the eastern border where no malaria existed. However, this difficulty could be removed by taking into account the numbers of anopheles found in the stables, as shown in fig. 10. Anopheles were numerous only in the coastal provinces, everywhere else they were comparativeiy rare.
Here an objection might be raised: Why worry over races of Anopheles maculipennis when it is obvious that the dispersal of malaria through the country is simply determined by the density of the anopheline population? The answer is: the dispersal of malaria through the country is not determined by the density of anopheles as a whole, but by that of the shortwinged Anopheles maculipennis. And
* See also: S. Tillema. Een analyse van Anopheles maculipennis. Bloemendaal, 1931, 56 pp. As far as the provinces of North-Holland, South-Holland, Utrecht, and parts of Guelders are concerned, the data of fig. 8 and 9 have been confirmed, or corrected, by the results of the investigations shown in fig. 14 and 15 (p. 120).



Fie xo Map of the greater part of the Netherlands indicating the anopheline density as estimated by the number of these mosquitoes found in stables. Black: 400 anopheles per stable or over; darkly shaded: 100-400 anopheles per stable; lightly shaded; 1-100 anopheles
per stable.
that density, as we shall prove when discussing the habitat of the larvae, is determined by the presence of brackish water (p. 118-126). Wherever brackish water is prevalent, shortwings are numerous; wherever it is absent, shortwings, if present at all,



are scarce. If the longwings were the vectors of malaria in this country, there would be no malaria in North-Holland, since longwings are scarce in that province, although the total density of anopheles there is higher than anywhere else. On the other hand, the disease would be prevalent in SouthHolland and the neighbouring parts of Utrecht and Guelders, because it is there that longwings are most numerous.
Other charaders of long- and shortwings
It was not enough to have proved that the longwings and shortwings breed true to type through one daughter generation, because that "type" was ill-defined, based as it was on a few morphological characters which had statistical value only, and on the behaviour during autumn and winter. Moreover, the breeding true to type needed to be proved through more than one daughter generation and ought to start from clearly identifiable batches' of ova, before it could be proclaimed really decisive.
Consequently, the first thing to do was to search for other distinguishing characters beside the ones already known, viz. the size (length of the wing), the number of maxillary teeth, and the habits at the time of hibernation or semi-hibernation.
This investigation revealed the existence of certain morphological differences between the two races with regard to the structure of the egg-floats (p. 75), the number and shape of the external harpaginal spines on the male hypopygium (p. 78), and the pilotaxy of the larvae (p. 114). They will receive due consideration in their proper place. Here we shall mention



only two biological differences between the long- and shortwings. At the time they were detected they were of particular interest, because they constituted qualitative, and not merely quantitative, differences between the two races. So they tended considerably to increase our confidence in the reality of the existence of these races.
These biological differences show themselves by the mating habits in confinement and by the way a bloodmeal is digested by sexually inactive females.
No longwinged females are ever inseminated by longwinged males when kept in confinement, no matter what size the cages are. Shortwinged females, on the other hand, readily mate with shortwinged males, even in fairly small cages *. We have not yet met with a single exception to this rule in the nine years we have known of its existence. The curious part about it is, however, that the longwings are immediately cured of their sexual frigidity when they find themselves encaged with the opposite sex of the shortwings. About thirty per cent of the longwinged females, facing this emergency, allow themselves to be inseminated by shortwinged males. The longwinged males derive such a stimulus from the presence of shortwinged females in their cage that they inseminate ten per cent of their number.
The way blood is digested t by hibernating longwings, which have been compelled to feed
* Riv. di Malar. Vol. 9, 1930, p. 104-106.
■f Ibid. Vol. 11, 1932, p. 144-150.
§ "Compelled" means feeding them by putting them in small jars of three by four inches, covered with muslin, which are applied to the human skin. In a cage of two and a half cubic feet they refuse to take blood when a human arm is introduced into it.



in August-November, is entirely different from the same process in shortwings. The blood in the stomach separates into a yellowish, semi-transparent, supernatant liquid portion, and an opaque sediment of red blood-corpuscles which show little agglutination. The corpuscular mass is covered by, or includes, a fibrin-like coagulum. This condition appears half an hour after feeding and continues for a day at least. It may last as long as eight or ten days in about ten per cent of the longwinged females maltreated in this way. The saliva shows little agglutinating effect on human and animal red bloodcorpuscles and it delays coagulation for no more than half an hour to two hours. Digestion at y6°-yg° is very slow; it often takes five to six days, sometimes even ten, for completion. Longwings fed in this way show a heavy mortality. From March onward digestion becomes normal.
In shortwings, treated in the same way, the supernatant fluid is almost completely resorbed within half an hour after feeding. The stomach is then filled throughout with the red corpuscular mass which is very sticky, owing to the strong agglutination of the red blood-cells. The saliva strongly agglutinates human and animal red blood-corpuscles and delays coagulation for six to twenty-four hours or more. Digestion at y6°-yg° is completed within one or two days and there is little mortality.
We may add that the state of abnormal digestion in the longwings greatly reduces their chance of becoming infected with malaria parasites, if they are forced to ingest them during the period this abnormal mode of digestion occurs. Fed on human



gametocyte-carriers, ninety-four per cent of the shortwings, showing normal digestion, had their stomachs infected. Longwings, on the other hand, fed on the same carriers as the shortwings, but showing abnormal digestion, acquired a stomach infection of no more than thirty-one per cent. Not only the rate of infection, but also its intensity is much reduced by the influence of abnormal digestion. The average number of oöcysts per mosquito was seventy in the shortwings and eleven in the longwings.
* * *
The other thing required was, as we said, to prove that our races breed true through more than one generation in experiments starting from clearly identifiable batches of ova. To attain this end, we tried to identify the ova of the two races. The ribs supporting the egg-floats are more numerous, on an average, in longwings (22-23) than in shortwings (17-18) *. Moreover, the intercostal film spreading between the ribs of the floats in longwing eggs is reinforced by rod-like thickenings, forming reticular designs or placed one above the other like the rungs of a ladder. In the shortwings the intercostal film is smooth t as a rule, although a certain number show a reticular design which, however, is much less conspicuous than in the longwings. It was not till after Hackett, Martini, and Missiroli § had made known the pattern of the upper surface of the exochorion of the ova in Italian and German varie-
* Centr. Bit. f. Bakter. Org. Vol. 109, 1928, p. 269.
•j- Riv. di Malar. Vol. 11, 1932. P- r4J§ Amer. Jrl. Hyg. Vol. 16, 19.32, p. 137-162.



ties of A.maculipennis, and after we had confirmed their findings in the Netherlands *, that we could feel completely sure of the accuracy of the identification of every single batch of eggs, if not of every individual egg, deposited by one female.
The exochorion in all eggs of Anopheles maculipennis is transparent. It looks black, however, because of the underlying black chorion. On its upper surf ace are implanted slender whitish rods, shaped like tiny pillars, which we have called columellae. The upper sides of the capitals of these pillars appear as white, star-shaped or polygonal, discs. Interspersed between the columellae stand numerous smaller globular excrescences, which we have called papülae. Where the columellae are long and close together the upper surface of the egg has the appearance of a white mosaic ("white patches"); where they are short and far apart the upper surface looks black, with white pin-points scattered over it ("dark patches").
In the longwings the columellae, even in the white patches, have narrow capitals, but this is compensated to some extent by their standing close together. The tips of the eggs are black, and two more dark areas exist at both ends of the floats, stretching over the whole breadth of the upper surface of the egg, called by Hackett, Martini, and Missiroli the transverse bars. Between these four black areas the upper surface is gray with rather indistinctly outlined dark patches.
In the shortwings the white patches may be more brilliant than in the longwings, but they are often
* Proc. Roy. Acad. Science Amsterdam. Vol. 35, 1932, p. 335-339; Vol. 37, 1934, p. 578-579.



blurred. There are numerous dark patches irregularly dispersed. Sometimes they are so little marked as to render the upper surface of the egg almost wholly white. Often they appear as strongly marked oblique bars. It may happen that two of them, running at almost right angles to the long axis of the egg, are placed in such a way as to simulate the transverse bars of longwing ova. As a consequence, it may be difficult, sometimes, to identify a single egg. But it is always possible to identify a batch of eggs laid by a single female and, consequently, to identify every individual pregnant female by the ova it deposits.
On the strength of the characters of the ova they had discovered, Hackett, Martini, and Missiroli identified the longwings of this country with an Italian race of A. maculipennis Falleroni had called A. messeae. They applied the name of A. atroparvus, which van Thiel had given to our shortwings, to other Italian anopheles they had identified with that Dutch race. For reasons to be explained later on (p. 90) we shall continue to call our races by the non-scientific and non-committal names we have used up till now.
Pedigree cultures prove that the long- and shortwings are taxonomie units without deciding whether they are proper objects for species-sanitation
The upper aspect of the ova of A. maculipennis has been described at some length, because it was to be the starting point for investigations to decide whether the races of this species are, or are not, independent taxonomie units. The first object was to ascertain whether the characters of these races



breed true through a number of generations, and not, as we had shown already, through one daughter generation only. In other words, our object was to raise -pedigree cultures, a very convenient botanical expression for cultivating a putative taxonomie unit (i.e. species, subspecies, or variety) through several generations, in order to make sure that it is a proper taxonomie unit. The late Professor Hugo de Vries might have had our special problem in mind when he wrote *: "The real units are the elementary species; their limits often apparently overlap and can only in rare cases be determined on the sole ground of field-observations. Pedigree culture is the method required, and any form which remains constant and distinct from its allies in the garden is to be considered as an elementary species."
The first series t was carried on through four successive shortwing generations, hatching on April 2ist, June 3d, July 2oth, and September ioth. In all of them the characters of the larvae and the male hypopygium bred true to type. The number of branches of the antepalmate hairs on the fourth and fifth abdominal segment of the larvae varied in the four daughter generations from four to six; so it kept well outside the range of eight to nine branches observed in longwings. The external harpaginal spine was never blunt, except in 0.3 per cent of the specimens in the first daughter generation. In longwings a blunt harpaginal spine occurs in thirty-seven per cent of the specimens.
* Hugo de Vries. Species and varielies. Chicago 1905, the Open Court Publ. Cy., p. 12.
f Riv. di Malar. Vol. 9, 193°. P- io2-



The second series * of shortwing cultures was based on the pattern of the upper surf ace of the ova only. In four successive filial generations, hatching on April uth, May ioth, June ioth, and July 3ist, this pattern remained exactly the same as that described on p. 76 as characterizing the ova of the shortwings. In both series the adults of the successive generations continued to mate in confinement, so they likewise bred true to that character t.
Our investigations suffered a serious check by the pedigree cultures in longwings proving an absolute failure, sirxce these gnats consistently refused to mate in confinement through a succession of greatly varied experiments §. All we could accomplish was to breed a filial generation from ova which, by their exochorion (which is a purely maternal product), testified of the longwing nature of the maternal generation. Then the females of the filial generation were inseminated by shortwinged males to cause them to oviposit hybrid eggs. The exochorion of these eggs, being again purely maternal and not (like its content) hybrid, vouched for the longwing nature of the filial generation. That, of course, took us no further than we were before. It only proved that longwings breed true to type through one daughter generation, the only difference being that we had confirmed this conclusion with regard to another character than we had used in our earlier experiments.
* Riv. di Malar. Vol. 13, Sez. 1, 1934. P- ,239- .
t A similar pedigree culture of A. maculipennis atroparvus (cor~
responding to our shortwings) has been carried out by Rivera and Hill for six filial generations (Medicin. Pais. Calid. Vol. 8, 1935. p. 313-319).
§ Riv. di Malar. Vol. 9, 193°. P- 104-106.



Consequently, we arrived at the conclusion that the shortwings, Anopheles maculipennis var. atroparvus van Thiel of the entomologists *, are to be regarded as a taxonomie unit, differing by heritable characters from their closest allies in this country, the longwings, Anopheles maculipennis var. messeae Falleroni of the same entomologists. The claim of the longwings to the title of taxonomie unit could not be confirmed by pedigree cultures, owing to their inability to mate in confinement. Nevertheless, since their closest allies, the shortwings, are a taxonomie unit, a simple exercise in subtraction shows that the longwings must likewise be one.
* * *
The term taxonomie unit may mean species as well as variety. The entomologists have decided that they are varieties. What does that mean? Turrill t says: Species must be morphologically distinguishable and determinable, but so must varieties. Species, however, must be isolated from one another, otherwise they inbreed to form one polymorphic multivarietal species.
If anopheles are to be controlled successfully by species-sanitation, they should belong to a taxonomie unit which remains constant in all circumstances; i.e. to a unit which does not only breed true to type, but which is prevented, by some mechanism, from mating with allied forms. It is useless to apply species-sanitation to the control of an inconstant taxonomie unit, since it will show its inconstancy in its breeding habits and in its ability to act as a
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 3, 1934, p. 657. ■f Nature. Vol. 137, 1935- P- 563-566.



malaria vector. So we have gained nothing by having our races recognized as varieties. Varieties, as Turrill understands the word, can never be proper objects for species-sanitation. They may emerge, once in a while, out of Turrill's "multivarietal species", but they are not reliable, since they are destined to lose their individuality by mating with other members of that multivarietal species.
But are the long- and shortwings varieties, according to Turrill's definition? Do they inbreed to form one polymorphic multivarietal species? On the face of it they are, most certainly. The species of which they form a part, Anopheles maculipennis, is undoubtedly polymorphic, very much so in fact. And as to their inbreeding, nothing is more likely to happen, since they are so extremely alike in appearance, without being geographically isolated. Both races occur all over the country, although in variable numerical proportions. They have similar habits during the time of their sexual activity. They enter human habitations, and there they take man's blood with equal eagerness *. They enter animal habitations in much larger numbers, and there take little but animal blood *. So the two races are not isolated from each other by the longwings being "zoophilous" and the shortwings "anthropophilous", both are zoophilous. As to the mating in confinement of the shortwings and the inability of the longwings to do so: the way the longwings have of mating with the shortwings ought to caution us not to count on the different sexual behaviour
* Proc. Roy. Acad. Science Amsterdam. Vol. 32, 1929, p. 772-779. Malaria. 6



of the two races as a means of bringing about their sexual isolation.
So the morphological likeness to each other of the long- and shortwings is prima facie evidence of their being able to hybridize, and there are no facts to prove their isolation on geographical or ecological grounds. Nevertheless, that consideration does not settle the case, for, as Turrill remarks, the most complete isolation is, undoubtedly, inability to cross. This inability cannot, usually, be judged a priori on the basis of degree or kind of morphological difference. That the species cannot be made to cross, Turrill continues, may cause the geneticist to lose interest in a genus, but such genetical isolation may have considerable taxonomie importance.
Here is a kindred spirit speaking. The interest Turrill feels is ours, although the cause of this interest is different. To him it means an answer to the question of how the taxonomie units should be subordinated on a sound basis of facts. To us it simply means deciding whether the shortwings are a proper object of control by species-sanitation.
Crossmating experiments furnish the evidence pedigree cultures failed to provide
These considerations led on to our crossmating experiments which proved that the shortwings are not a variety, but a species sexually isolated from the other Dutch race of A.maculipennis, the longwings.
Attempts at crossing longwings with shortwings proved entirely negative. In five experiments, under-



taken in 1929-1932 *, 236 longwinged females were crossed with shortwinged males, 37 per cent were found inseminated t. One batch of ova was deposited, but none hatched. In five others, 187 shortwinged females were crossed with longwinged males, 7 per cent were inseminated. Two batches of 67 and 128 ova were deposited; both hatched, one producing 34 larvae, the other 57. All larvae died within a few days.
These experiments were repeated in 1933 §. In eleven experiments, 428 female longwings were crossed with male shortwings, 32 per cent were found inseminated. They deposited 73 batches of ova, 15 of which hatched producing 32 larvae. They all died within 24 hours. In nine experiments, 493 female shortwings were crossed with male longwings, 4 per cent were found inseminated. They deposited 17 batches of ova, 2 of which hatched producing 80 larvae. All but one died on the first tofourth day after hatching. That one lived for sixteen days.
In 1934 similar experiments were undertaken with Swedish anopheles II which, by the pattern on the upper surface of their ova, could be identifxed with the Dutch long- and shortwings. We shall call them atroparvus (corresponding to Dutch shortwings) and messeae (corresponding to Dutch longwings), because we wish to use the names longand shortwings for anopheles found in the Netherlands only. In five experiments, 319 female Malmö or Stockholm messeae were crossed with male
* Riv. di Malar. Vol. 9, 1930, p. 105 and Vol. 11, 1932, p. 143-144.
f Inseminated means that their spermathecae were found to contain
spermatozoa.
§ Riv. di Malar. Vol. 13, Sez. 1, 1934, P- 240-244.
II Proc. Roy. Acad. Science Amsterdam. Vol. 38, 1935, p. 554"555-



Malmö atroparvus, 31 per cent were inseminated. They deposited 10 batches of ova; one of them, with 155 ova, hatched 4 larvae which died in one day. In three experiments, 175 female Malmö atroparvus were crossed with male Stockholm and Malmö messeae, 1 per cent were inseminated. They deposited 5 batches of ova; none of them hatched.
Finally, as a consequence of Diemer's * publishing a few successful crosses between long- and shortwings, which van Thiel had undertaken at a time the morphology of the egg was still insufficiënt ly known, we repeated our experiments with longand shortwings in 1936 and 1937 t. In eight experiments, 314 female longwings were crossed with male shortwings, 48 per cent were inseminated, the largest percentage we ever found in crossmating. They deposited 116 batches of ova, 49 hatched producing 386 larvae which all died on the first to fourth day after hatching. In eight other experiments, 407 female shortwings were crossed with male longwings, 10 per cent were inseminated. This percentage is again the highest ever met with in reciprocal crosses. They deposited 49 batches of ova, 20 of which hatched producing 1,455 larvae. Out of these, 46 reached the second instar in 9-14 days and 5 of them the third instar after 19-21 days, but they did not grow any more. As to the length of life of the larvae: 143 lived up to the tenth day, 13 up to the twentieth, and the last died on the thirty-sixth day.
* Over de biotypen van Anopheles maculipennis. Amsterdam 1935, W. ten Have, p. 102-115.
t Buil. Soc. Path. Exot. Vol. 30, 1937. P- 699"7°3-



As all these experiments show completely negative results, they rccjiiire a careful and critical scrutiny in view of the considerable practical importance we must attach to them. If these results are true, if longwings and shortwings cannot produce a viable progeny, species-sanitation in this country merits careful consideration. But if they are wrong, if, as others maintain *, the two races can intermingle, species-sanitation can be dismissed without further thought.
Why is it that our results have been so consistently negative? We believe the reason is that we have always been extremely careful to keep our material pure. At the beginning of each experiment we selected the ova, which were to produce the males and females to be used for crossmating, by relying on whole batches, never on single ova, so as to exclude any doubtful case. Afterwards, when the females were inseminated by males of the other race, we always re-identified these females by the exochorion of the ova they deposited, since this exochorion belongs to the female which laid the egg and not to the hybrid embryo therein contained.
Our results have proved negative because all hybrid larvae died. We have called them non-viable, thereby indicating that there was something unusual in their dying, but is there? So many larvae may be expected to die in the artificial environment of our laboratories. There are species, like Anopheles leucosphyrus, which it is almost impossible to rear in confinement, even from the full-grown larval
* E. Roubaud, Buil. Soc. Path. Exot. Vol. 30, 1937, P- 7°3-



stage onward. To this objection we answer that, since we worked out the proper method of rearing larvae in this country (p. 223), we never experienced any high mortality. As an example we may quote the fate of homozygous and hybrid larvae in a parallel experiment:
hybrid larvae: normal larvae:
i st day: oviposition oviposition
3 d day: larvae hatched
4 th day: 95 larvae hatched
5 th-i5th day: death of 73 larvae i5th day: 14 larvae moult to
become 2nd instars
I7th-2ist day: death of 14 larvae a few larvae die,
the others pupate 2oth-24th day: adults hatched
23d-25th day: 5 larvae moult to
become 3d instars 25th-3Óth day: death of 8 larvae
A final objection we can think of is this. If these experiments were repeated of ten enough, would they not finally yield positive results? It is quite likely they would. In our former experiments, one larva out of 171 moulted once and lived to the sixteenth day; in our last, 13 moulted once out of 1,455 and 5 moulted twice. Proceeding at the same rate, 11,640 young larvae would be required to produce the fourth instar, and 93,120 to produce pupae and adults. But, unless they pass their larval life in a way entirely different from what we observed in our hybrid larvae, we are not afraid of these adult hybrids contaminating the purity of our races. For the few third instar hybrid larvae we reared would never have lived in natural surroundings. That they continued so long was due to our unre-



mitting care. Any adult, produced by such larvae, would have but little chance successfully to compete with its congeners, and (supposing it was not sterile)
to mate in nature.
So we conclude that there exists an insuperable barrier of sterility which precludes the possibility of the long- and shortwings' intermingling.
* * *
This is not to say, however, that we cannot rear adult hybrid anopheles; for we can, when crossing the shortwings with the third Dutch maculipennis race: typicus, discovered by Hackett, Martini, and Missiroli in Germany, Italy, and elsewhere.
That is a race we only found in some of the eastern provinces of the Netherlands (Overijssel, Guelders, and Limburg). We detected it 6 times out of 589.
Typicus on the whole is intermediate between short- and longwings, except for its male hypopygium. It is impossible to teil it apart from one or the other, except by its ova which are white with two black transverse bars. They are entirely different from the ova of the long- and shortwings, and so we wondered what a hybrid exochorion derived from typicus would be like. High hopes were raised when females of that race crossed with shortwinged males laid batches of ova which all hatched, producing a thriving brood of larvae which, in due time, became adults. But these hopes were shattered, since all of 84 hybrid males were completely sterile and out of 175 hybrid females ten only carried follicles in their ovaries. The back-cross of these hybrid females with male shortwings proved a complete



failure. The same results were obtained by crossing male shortwings with Swedish typicus: all of the 42 hybridmales and 48 out of 50 females were completely sterile * (Plate I).
Although typicus merits little attention as a malaria vector in this country, these experiments are of great interest, because they show that the two races, shortwings and typicus, are likewise sexually isolated by a barrier of mutual sterility. Moreover, this sterility presents itself in a way much more fascinating and, at first sight, more convincing, than it does in crossing longwings with shortwings.
* * *
The barrier of interspecific sterility, isolating our shortwings from the mediterranean races of Anopheles maculipennis, is less complete t. Crossing male shortwings with Hackett's race melanoon §, we reared hybrid males most of which were sterile, but half of the hybrid females were fertile. We also crossed male shortwings with Falleroni's race labranchiae *. All females, and about one male in seventy, proved fertile in the ensuing hybrid generation. In the reciprocal cross all the females
* Riv. di Malar. Vol. 13, Sez. 1, 1934, p. 252-255; Proc. Roy. Acad.
Science Amsterdam. Vol. 38, 1935, p. 555-556. Similar results have been obtained by Corradetti (Riv. di Malar. Vol. 13, Sez. 1, 1934, p. 708-720; Riv. di Parassit. Vol. I, 1937, P- 329-34°)-
f Riv. di Malar. Vol. 13, Sez. 1, 1934, P- 244"252 and 255-259; Proc. Roy Acad. Science Amsterdam. Vol. 38, 1935, p. 557.
§ In 1933 that name did not exist. We called the race "Italian messeae", to signify that we found it different from true messeae. There exists some uncertainty, at present, whether this race Deiongs to Hackett's varieties melanoon or subalpinus, since it is not quite certain that these two are really different.



Plate I
Normal testis of the "typicus" race.
Testis of hvbrid typicus X shortwing.
Normal ovarium of the "typicus" race.
Ovarium of hybricl typicus x shortwing.







were again fertile; but so, in this case, were half of the males. Consequently, if we had melanoon or labranchiae here, we would not be so sure of the purity and constancy of our races. Fortunately, the mediterranean basin is a long way off. Geographical isolation, in these cases, fills the gap sexual isolation leaves open.
On the nomenclature of the races of Anopheles maculipennis
It has always been the bane of Latin species names that they carry with them the suggestion that all living beings bearing that name behave the same. A man in Java says that Anopheles ludlowi breeds in salty water only; another in Sumatra tells him he is wrong: A. ludlowi breeds in fresh water. Java says ludlowi carries malaria, the Philippines say it does not. One man in Sumatra says ludlowi breeds in fresh-water fishponds only; another maintains it breeds in swamps. All this strife arises from the idea that the anopheles, around which it centres, behave alike because they have the same name.
And that, we are much afraid, is going to happen in the species (variety of the entomologists) called atroparvus. That name suggests that anopheles, all over Europe, are the same so long as they are entitled to bear that name. The effect is already being feit: In Holland, they say, atroparvus is supposed to be a salt-water breeder, but the Dutch are wrong, it breeds in fresh water. In Holland, they say, atroparvus is supposed to be a potent malaria vector, but the Dutch are wrong, it is not. And the proof of it is that malaria in Holland "is



so mild as to be more of a curiosity than a problem".
The fact is, we believe, that atroparvus is one of Turrill's polymorphic multivarietal species whose varieties, by constant crossmating, have no chance to become independent, unless they are geographically isolated. As a consequence, in the various localities atroparvus has succeeded in occupying, here one, there another of its varieties will come to the front, as determined by local ecological conditions. They will all be called atroparvus, but they will all be different. None of them are homozygous, and so it is next to impossible to breed them in pure lines. Nevertheless, the occasional occurrence of a whole brood of atroparvus with mainly white eggs, or with branched hairs (instead of palmate hairs) on the second abdominal segment of the larvae, has convinced us of the heterogeneous make-up of this species. The same applies to messeae. We know a variety of this species characterized by a proximal branched antennal hair of the larvae, so long and so much branched as to render the larvae indistinguishable from those of Anopheles hyrcanus. But we cannot rear them separately because the longwings do not mate in confinement.
For these reasons we have sedulously avoided using the narnes messeae and atroparvus. We have kept to "longwings" and "shortwings" for no other reason than to signify that these two Dutch forms are to be kept separate, although they undoubtedly belong to the multivarietal groups of messeae and atroparvus.
Finally there remains one point to settle. We have assigned to our long- and shortwings the rank



of species. But the entomologists have decided that Anopheles maculipennis atroparvus and Anopheles maculipennis messeae are so much alike that they are no better than varieties. The argument that the races are sexually isolated does not impress them. If they are to take notice of details like that, they will never know where they are. How many species or varieties of anopheles are known to be sexually isolated? None but the maculipennis varieties. The basis of systematics ought not to be changed for the sake of a few varieties which happen to have been studied by methods vastly different from those accepted in entomology.
Divested of its outer shell of irrelevanties, the question of whether our races are varieties or species is easily answered by applying once again to Hugo de Vries' genius. Here are his own words *, like the others we quoted, expressly adapted to our needs.
"Species is a word, which always has had a doublé meaning. One of them is the systematic species, which is the unit of our system. Today the vast majority of the old systematic species are known to consist of minor units. These minor entities are called varieties in systematic works. However, there are many objections to this usage. The subdivisions of species are by no means all of the same nature, and the systematic varieties include units the real value of which is widely different in different cases. Some of these varieties are in reality as good as species and have been elevated to this rank. This conception of the elementary species would at once get rid of all
* Species avd varieties, p. 10-12.



difficulties, were it not for one practical obstacle. The number of the species in all genera would be doubled and tripled, and the distinction of the native species of any given country would lose most of its charm and interest. In order to meet this difficulty we must recognize two sorts of species. The systematic species are the practical units of the systematists and florists, and all friends of wild nature should do their utmost to preserve them as Linnaeus has proposed them. These units, however, are not really existing entities; they have as little claim to be regarded as such as the genera and families have. The real units are the elementary species".
Our long- and shortwings are elementary species. Let that suffice us, and do not let us hurt the feelings of "systematists and all friends of wild nature" by claiming for our races a systematic position they are really entitled to. "In this world" John Buchan says "you often can get success, if you do not want victory". Well, we do not want it, and so we are quite content to let their rank be that which was given to them at a conference in 1934.



CHAPTER V
Behaviour of the shortwinged anopheles
maculipennis Seasonal variation of anopheline incidence
As has been said before, Korteweg invented his hypothesis of the spring-cases of primary malaria originating from infections acquired in the preceding autumn, in order to explain why people get malaria at a time when anopheles are rare, viz. in April and May. This observation of the scarcity of anopheles in April and May was remarkably exact, as we know now; but it was only part of the truth, since anopheles are hardly more numerous in June and the first half of July.
The following diagram (fig. n) shows the average daily catch, in each succeeding week, of anopheles (practically all shortwings) occupying an open shed on the outskirts of the town of Medemblik. The diagram refers to one of the seven years during which these observations were continued *. We have noticed t that the percentage of male anopheles in such shelters is always very high (fifty per cent or over), as high as among anopheles hatched from
* Unpublished records mainly collected by j. a. Nijkamp. a year has been selected when antilarval measures had been discontinued, as thev are apt to disturb the natural course of events.
"t" Proc. Roy. Acad. Science Amsterdam. Vol. 32, 1929, p. 669-678.



pupae in the laboratory, and much higher than among anopheles caught in stables. Moreover, the quantity of anopheles in such shelters is not in the least influenced by daily catches, their number is renewed every day. In stables, on the other hand, daily catches keep the number of anopheles low, because there is no such uninterrupted entrance
Fig. li. Seasonal variation of the numbers of anopheles. Weekly fïgures of the average number of male and female anopheles caught daily in an open unprotected shed on the outskirts of Medemblik in 1932.
and exit of mosquitoes *. Hence, we believe that daily catches in open unprotected shelters afford a more exact picture of the daily output of the neighbouring breedingplaces than those in stables. Nevertheless, catches in stables and human habitations yield, on the whole, similar results. There is this difference, however, that anopheles disappear
* 36 pet. of stained anopheles remain in the stables for two days, 3 pet. for five days, i pet. for as long as 10 days. See: Proc. Roy. Acad. Science Amsterdam. Vol. 32, 1929, p. 675.



from the open unprotected shelters by the end of September, whereas they remain in human habitations and stables, and even become more numerous. This is shown in the accompanying diagram (fig.12) of the average half-monthly number of shortwinged females per human habitation in the village of Uitgeest *.
Both diagrams clearly bring out the fact that the anopheline season proper, in this country, does
Fig. 12. Seasonal variation of the numbers of anopheles. Average number of female anopheles per house found in human habitations in the succeeding half-monthly periods of 1935 in the village of Uitgeest.
not really commence till the second half of July or August. In the open shelters it lasts for less than three months, in human and animal habitations it continues till the beginning of March. At that time it is brought to an end by the females developing mature ova and leaving their winterquarters, in search of breedingplaces.
The same variation of the seasonal incidence is seen in the breedingplaces, where the number of larvae is low in April, May, and June, to rise in July and still more in August, to drop again to the level
* Quart. Buil. Hlth. Set. Lg. 0. Nt. Vol. 5, 1936, p. 310.



of May in September, and finally to become zero in October or November. *
As a consequence, it is not only true that the malaria season commences bef ore the anopheline season, but the malaria season is actually on the wane by the time the anopheline season has properly started. So, the primary cases of malaria occurring in June and the first half of July are as much in need of an explanation as those in April and May. If Korteweg's hypothesis is to account for them, it must be extended by assuming that all patients having their primary malaria in these months (two thirds to three quarters of the total annual number) have been infected during the preceding autumn.
* * *
As has been pointed out, a comparison of fig. n and 12 shows that anopheles disappear from open unprotected shelters by the end of the summer and, simultaneously, become more numerous in human (or animal) habitations. These two events coincide with the cessation of sexual activity. Sexually inactive females need no longer visit distant breedingplaces; they are free to follow other instincts, which cause the shortwings to leave open sheds and suchlike shelters, and to accumulate near animal or human blood reservoirs. Not only to accumulate there — they did so earlier in summer — but to stay on there, not for three days or ten days, but for as long as their sexual inactivity lasts.
This staying on we have called autumnal fixation 1.
* Malaria in the Kingdom of the Netherlands. Hlth. Org. Lg. o. Nt., no. C. H. 196, Geneva 1924, p. 61.
"t Ann. Inst. Pasteur. Vol. 43, 1929, p. 1374-



Autumnal fixation has a logical, but nevertheless unexpected, effect. When one catches all anopheles in a house or a stable in summer, the building, nine days later, will be as full of anopheles as it was before *. But as soon as sexual inactivity has become well established, a locality which has been artificially depleted of its anopheles remains empty, or almost so, till the next spring. This is a highly important effect of autumnal fixation. It means that if one continues the killing of anopheles in a single house till the end of October, one need not worry how many anopheles remain in neighbouring houses or stables; that particular house will harbour no more of them till the next spring.
Annual variation of anopheline incidence
Observations continued for several years, in stables in which anopheles were counted every month, have made it clear that the incidence of anopheles varies from year to year. Moreover, a certain parallelism exists in the annual variation observed in stables situated in different parts of the country. So we may conclude that these variations reflect general upward and downward trends in the anopheline population.
The following diagram (fig. 13) shows the average number of anopheles counted in a stable at Amsterdam on the i6th of the months of September and October, i.e. at a time they are most numerous. The average temperature during the second half of September and the first half of October is also indicated.
* Proc. Roy. Acad. Science Amsterdam. Vol. 32, 1929, p. 669-678. Malaria. 7



We have mentioned already (p. 38) that major or minor malaria exacerbations are often, though by 110 means invariably, seén to follow in the wake of years notable for the mildness of their early autumn. Here we see the peaks in the curve, marking the
Fig. 13. Comparison of the incidence of anopheles during the season of their greatest profusion, with the average temperature during that time, in the years 1928-1937. Continuous line: average number of anopheles in a stable at Amsterdam on September iöth and October i6th. Dotted line: average daily temperature at 8 a.m. during the 2nd half of September and the ist half of October.
autumnal anopheline incidence, coinciding with a rise to 550 or over of the average temperature in early autumn. We wish to add that no such close relationship is to be observed between the average temperature in July and August (the height of the breeding season) and the numbers of anopheles found during the subsequent autumn.



Seasonal variation in the reproduction and nutrition
of anopheles
In summer reproduction and nutrition run parallel in both races, but in the shortwings reproduction ceases, and nutrition continues, on a date somewhere between the end of July and the beginning of September. In the longwings no such difference exists between reproduction and nutrition; when the one is active the other is so too, and when the one ceases the other follows suit.
Feeding habits, fertility, and condition of the adipose body in longand shortwings staying together in human habitations in late summer, autumn, and winter
Shortwings Longwings
Month No. Pet. of females No. Pet. of females en- preg- with en- preg- with gorged nant transient gorged nant permafat nent fat
August 2,768 45% 29% 16% 120 2% 10% 87%
September 3,145 31% 2% 18% 598 o o 99%
October 1,596 35% o 35% 617 o o 95%
November 1,031 11% o 37% 668 o o 79%
December 881 10% o 15% 602 o o 59%
January 325 10% o 18% 128 o o 48%
February 384 6% o 4% 144 o o 8%
This habit the shortwings have in late summer and autumn, of disconnecting the functions of reproduction and nutrition, we have called * gonotrophic dissociation, as distinct from the longwings' habit of keeping the two together which we have termed gonotrophic concordance.
The preceding list shows gonotrophic dissociation and gonotrophic concordance, in shortwings and longwings sheltering together in the same attic bedrooms of human habitations. It indicates, for each month of late summer and autumn, the percentage of engorged
* Ann. Inst. Pasteur. Vol. 43, 1929, p. 1377-1379.
.



females (with blood in their stomach) and of pregnant females (carrying mature ova).
The list clearly shows that the races behave similarly with regard to the time of cessation of their reproductive activity. But their feedinghabits are different. The appetite of the longwings is non-existent from September onward. It is always present in the shortwings, although they are decidedly off their feed from November onward. As a consequence, gonotrophic dissociation is most marked in the months of September and October.
♦ ♦ *
As we have said before (p. 60), one of the earliest signs of anopheles losing their sexual activity is their growing fat. In summer shortwings respond to a meal of blood by maturing their ova. From the end of July onward an ever increasing number do so by growing fat. But they soon grow lean again, until a fresh meal of blood causes the adipose body to regain its size for a time. So the fat they store is "transient fat". In this way periods of fatness and leanness alternate through the autumn and winter, in agreement with the ingestion of blood. As a result, a comparatively small fraction of shortwings are found to be fat on any given day (see the list on p. 99) and, moreover, the shortwings cannot stand fasting in autumn and winter. If food is withheld, by keeping them caged in a cool storeroom from October till March, five per cent of them only survive until the end of that period *. In spring the
* Riv. di Malar. Vol. 13, Sez. 1, 1934, p. 406.



renewed production of mature ova puts a stop to the storing of fat.
Longwings also grow fat when their sexual activity comes to an end, but the fat they store is "permanent fat". They develop an enormous adipose body, much bigger than the shortwings do, and in nature it takes them months to get rid of their store of fat. As a consequence, the percentage of fat mosquitoes, to be found on any given day, is much higher than in the shortwings (see the list on p. 99). Moreover, longwings stand fasting much better in autumn and winter. When kept under the same conditions as the shortwings, from October till March, nearly sixty per cent of them are alive at the end of that period.
Stabular deviation
Stabular deviation is a term which indicates that there are many more shortwinged anopheles in animal habitations than in human. From this fact it is inferred that they are more attracted by stables than by houses. It is expected that a group of female shortwings, returning (after oviposition) to a neighbouring village in search of food, splits into two groups, a large one going for the stables and a small one for the houses.
The fact, mentioned above, is beyond all doubt. This is shown by the following figures of the average number of anopheles caught in houses and stables from July till September:
average number of anopheles proportion of
(mostly shortwings) these averages per per inhabited house stable Medemblik 1933 22 9,592 1 to 436 Amsterdam i92o-'23 15 3,794 1 to 253
Stabular deviation is a term which indicates that there are many more shortwinged anopheles in animal habitations than in human. From this fact it is inferred that they are more attracted by stables than by houses. It is expected that a group of female shortwings, returning (after oviposition) to a neighbouring village in search of food, splits into two groups, a large one going for the stables and a small one for the houses.
The fact, mentioned above, is beyond all doubt. This is shown by the following figures of the average number of anopheles caught in houses and stables from July till September:



As to the conclusion derived from this fact, that animal habitations are more attractive to anopheles than human habitations, we do not actually observe anopheles, returning from the breedingplaces, passing by a house without alighting, and going straight for a stable. Still, nobody will hesitate to call one theatre more attractive than another if he sees the one full of visitors and the other empty, even if he has not seen the public entering the one and passing by the other. And so it is in this case, unless there is some other obvious reason for the stables to be preferred to the houses, as would be the case if they were within easier reach of the breedingplaces, which they are not, as a rule. So we believe we may call the stables more attractive to anopheles than the houses.
But why use the term "deviation"? We do so because we actually see a portion of the anopheles which have passed some time in a human habitation, and have left it, missing their way to a house and getting lost in some stable.
In the months of July and August we examined a number of houses with rabbitcages attached to them. Inside these houses we found young oöcysts in twelve of the anopheles dissected. Inside the rabbitcages no such infection was detected. As to mature (salivary) anopheline infection, conditions were different: eight anopheles carrying sporozoites were found in the houses and six were detected in the rabbitcages. In September twenty-four intestinal infections and fifteen salivary infections were found in the houses, whereas neither the one nor the other could be detected in anopheles caught in the



rabbitcages *. Consequently, about two fifths of the anopheles carrying sporozoites, but none of those carrying young oöcysts, are turned aside from their way to human habitations by the attraction of the rabbitcages in July and August. No such deviation is to be observed in September.
Why are the carriers of more advanced stages of infection susceptible to the influence of the rabbitcages in summer, and why are they no longer influenced in the same way in September? There is nothing mysterious in that. In summer a number of anopheles become infected with malaria; obviously this happens in human habitations only. They remain long enough in the house to grow oöcysts of a size sufficiënt to be detected on dissection. They leave the house, at the latest, by the time their eggs reach maturity. It is during their flights to and from the breedingplaces, when their infection reaches the final stages, that they become susceptible to stabular attraction. In September there are no such long flights, since no excursions to breedingplaces are undertaken by anopheles which, at that time, have already entered upon their term of sexual inactivity. Consequently, there are no signs of stabular deviation to be detected in that month.
This example justifies the word "deviation", since it applies to anopheles which conspicuously fail in taking the parasites, they are carrying, to their proper destination, by being turned aside on their way to human habitations. At the same time it indicates the time-limit beyond which stabular deviation ceases to
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 5, 1936, p. 317-319.



act. That limit is the end of sexual activity in anopheles; once that is finished, stabular attraction loses its object: the anopheles flying long distances.
The kind of stabled animals plays a prominent part in determining the strength of stabular deviation. We observed this during the first years of human settlement in the new polder reclaimed from the Zuydersea (fig. i on p. 2) *. At that time there were no other stabled animals than rabbits and poultry. Although there existed no anopheline breedingplaces in the new polder t, the average number of short wings per human habitation, from July till September, was twenty-one. That number is as high as in areas teeming with breedingplaces. But the total number of anopheles was low, owing to the fact that each group of chickenpens and rabbitcages, belonging to one house, sheltered on an average no more than 123 anopheles. That is hardly the sixfold of their number in human habitations. As a comparison we refer to the figures in the list on p. 101 showing stable-mosquitoes outnumbering house-mosquitoes by the three- and fourhundredfold.
In the village which provided us with the example to justify this term stabular deviation was no stronger than in the new polder. This was to be expected, since there existed little other animal habitations than rabbitcages. In other villages, however, animals especially attractive to anopheles, like horses and pigs, are numerous; each stable counts, on an average, as many anopheles as in
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 3, 1934, P- 441-460. f Breedingplaces occurred on the mainland, at a distance of three miles or more.



the examples cited on p. 101. There we may expect the chance of an infected anopheles to find its way back to a human habitation to be infinitesimally small in early summer.
Longwings behave much the same as shortwings with regard to stabular deviation. We need not repeat, however, that they pass their time in entirely different localities when autumnal fixation commences (except for human habitations, which the two races share). During the time of their sexual activity stables are not so attractive to them as to shortwings. The average number of longwings found in stables is slightly less than the hundredfold of their numbers in houses *.
Both long- and shortwings found in stables in summer contain little else than animal blood *. In human habitations, the proportion of mosquitoes having ingested human blood in summer varies considerably. In shortwings it was 92 per cent in a village, like Uitgeest, where stabular deviation is of little importance t. Around Medemblik, with a strong stabular deviation, it was 16 per cent in summer; but in the new polder, where stabular deviation was of as little consequence as in Uitgeest, it was 31 per cent during the same period §. A comparison of long- and shortwings, found in houses in various areas *, shows that the proportion of mosquitoes having ingested human blood, during the time of their sexual activity, is somewhat higher in the shortwings (84 per cent) than in the longwings
* Proc. Roy. Acad. Science Amsterdam. Vol. 32, 1929, p. 772-779. t Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 5, 1936, p. 312.
§ Nederl. Tijdschr. v. Geneesk. Vol. 78, 1934, P- 3439-344°•



(63 per cent). But this difference is not significant when compared with the considerable variations shortwings from various regions show in this respect.
Range of flight
Malaria in this country is focal in character (p. 44), even to the extent of affecting a portion only of a village, leaving the remainder almost untouched. This has led to the belief that anopheles cover only short distances by flight, and that antilarval measures limited to the immediate neighbourhood of inhabited centres would be quite effective. Although there can be no doubt as to the focal character of malaria, this conclusion is unwarranted, because it makes no distinction between the distances covered by shortwinged anopheles during the time of their sexual activity (long flights) and those taken by anopheles which have settled down in houses by the time reproduction ceased (short flights). No malaria is conveyed by the long flights, because anopheles are not infected at the time they take them, but it is by the short flights.
We have studied the long flight of the shortwinged anopheles in this country in two ways. One is the classical method, invented by le Prince and Orenstein *, of releasing stained mosquitoes and trying to identify them, by the dye they are carrying, among anopheles captured at various distances from the place where they were liberated. The other was specially suited to local conditions which the new polder offered during the first two years after its
* Mosquito control in Panama. New York and London 1916, G. P. Putnam Sns., p. 94-114.



reclamation, at a time when no breedingplaces existed there.
Experiments with stained anopheles were carried out in the neighbourhood of Medemblik *, a little town on a peninsula, surrounded by the sea on all sides, except to the south and south-west. They were primarily undertaken to prove that the anopheles, still to be found in that town (notwithstanding it was protected by antilarval measures covering an area with a radius of nearly two miles), could be accounted for by an invasion from uncontrolled regions.
The result of two experiments, undertaken in June and September, was that 38 stained females were recaptured, out of 2,830 released, at a distance varying from 1 to i4/5 miles from the starting point. Southerly to south-westerly winds passing over the uncontrolled breedingplaces were found to favour dispersion, winds passing over the sea counteracted it. Favourable winds carried anopheles much farther than their need of food required, right up to the centre of the area protected by antilarval measures, where they were stopped by a townlike arrangement of the buildings.
If no such obstacles stand in the way, a favourable wind can carry anopheles much farther, as we observed t in the new polder. At that time it offered a monotonous sight of flat dreariness, with only one village between the spot where our mosquitoes were released and the coast towards which the wind was to carry them. That spot was situated on the
* Amer. Jrl. Hyg. Vol. 10, 1929, p. 419-434.
t Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 3, 1934, p. 453"454-



western border of this polder where it joins the mainland of North-Holland. There we released 3,000 stained anopheles, including 700 males, on June i8th. A slight wind was blowing in the direction of the east coast of the new polder, where we had built a small goatshed on the shore of the impounded Zuydersea. On June i9th-2ist we recaptured one stained male in a stable 5V5 miles to the east of the spot where it had been released, and two stained anopheles (one male and one female) in the goatshed 83/4 miles to the east of that spot. No stained mosquitoes were found in stables to the south-east of that same spot. Neither did we recapture them in the village which the mosquitoes had to pass on their flight to the coast, at a distance of three miles from the place of release. This confirms our earlier experience, that the wind carries mosquitoes much farther than their need of food requires them to go.
Incidentally, this experiment proves that even the males may travel long distances by flight.
These are impressive figures which might well discourage any attempt at control. Still, they teil us no more than that stained anopheles may travel these long distances. Now, in all things pertaining to the practice of sanitation we cannot afford to be interested in things that may happen. If we do, we will find ourselves doomed to complete inaction. So we must focus our attention on the things which commonly happen, to the exclusion of all extraordinary events.
Applying this rule, we shall have to face this question: It has been proved that anopheles can



fly several miles. But do they do so every day? And
by the way — do ordinary anopheles behave
that way, and not stained ones only? The poor insects have been terribly maltreated by spraying them with a one per cent watery solution of methylene blue, till they lay all sprawling at the bottom of their cage, the survivors being released thereafter. Is it known how they feel in their coat of paint? Can one be sure that their long flights are not a kind of running amuck? We confess we cannot answer these questions; we agree that these experiments savour too much of the laboratory, and too little of the field, to be taken at their face value. Nevertheless they are; people invariably get inordinately impressed by experiments of this kind. They lose all sense of solid criticism in the face of them. Well, they may be excused in their amiable attitude. These experiments are not easy to carry out, and so the onlookers probably say: "la critique est aisée, 1'art est difficile". We, however, avail ourselves of the privilege of having practised the art, which entitles us to criticize experiments of this description and to ask for other proof that anopheles fly such long distances.
We are in possession of that proof *, through no experimental skill though, but by an exceptional opportunity. That opportunity offered in the shape of the conditions existing in the new polder, in the year following on that in which so much water had been pumped out of it that it had been pronounced to be "dry" (August 1930).
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 3, 1934. P- 44I"46°-



In 1931 no anopheline breedingplaces were found. There was plenty of water, not only in the big canals (which had been cut by dredging when the land was still submerged, and so were emerging ready-made with the land) and in the ditches which were being cut: the whole polder was a quagmire. The statement that breedingplaces were absent may therefore appear hardly credible. Nevertheless, it was perfectly justified by an often repeated, but always negative, search within an area with a radius of over three miles. This result was no more than was to be expected. The salinity of the water was far too high (1.32 per cent of salt) for our shortwing larvae to breed in, since they will not stand more than 1.15 per cent.
Within that area we built two pigsties, each holding two pigs, one at nearly two miles', the other at a little over three miles' distance from the nearest breedingplaces. In these stables we caught and counted all anopheles every day, from June ist till September i5th 1931, and we found fresh ones almost daily. In other words, almost every day fresh anopheles were entering the new polder and travelling the distance of two and three miles to reach these pigsties, since no other animal or plant life existed in that barren new land capable of providing anopheles with subsistence or shelter of any kind. The average daily catch was small: thirteen anopheles, including two males, in the stable at two miles', and seven anopheles, including one male, in the one at three miles' distance from the nearest breedingplaces.
So we have the proof in hand that shortwinged



anopheles, the sole representatives of the maculipennis-tribe in this area, can not only travel distances of two and three miles, but that a certain number of them continually do so under entirely natural conditions.
The long flights are undertaken by sexually active anopheles. The short flights, to which we shall now turn our attention, are peculiar to sexually inactive anopheles which are breaking through the bonds of autumnal fixation. They are doing so in a very fainthearted way, no doubt, but still that is what their behaviour amounts to.
We have studied the short flights in marked mosquitoes. Not, however, in those marked with a dye, but in those marked by the malaria parasites they carry. The cage from which the mosquitoes were released was substituted by the house where infected anopheles occur in large numbers (p. 157). Within a radius of 100 yards around such a focus of infection houses exist which harbour neither malaria patients nor healthy parasite-carriers. Nevertheless, infected anopheles occur in these houses. The parasites they carry indicate whence they came, and which distance they travelled on their short flights *.
As we said before, the short flights determine the dispersal of malaria, the long flights have little to do with this. Mosquitoes taking long flights do not, as a rule, carry malaria. If they do, they will most probably take it to the wrong place, the stables. Nevertheless, the long flight is of great importance. The long flight carries anopheles to inhabited
* Jrl. 0. Hyg. Vol. 38, 1938, p. 62-74.



centres, from far-away breedingplaces, and stocks human habitations with mosquitoes. These insects, when the proper time has arrived, are going to stay there, entrapped by autumnal fixation. In the presence of a parasite-carrier they will acquire malaria infection, transmit it to the inmates of that house, or spread it to neighbouring houses by short flights. As a consequence, the apparent contradiction, of the long distances anopheles travel by flight and the crawling progress of malaria, is no contradiction at all, but wholly explained by the habits of the insect vector.
Brtedinghabits
Once the principal insect vector of malaria is known in a district, the applicability or non-applicability of species-sanitation depends on the answer to the question: Where does that species of anopheles breed? If its larvae are catholic in their taste, all collections of water will have to be treated, and then there is no longer any advantage in having to deal with one species only. But if the larvae of the malaria vector show special breedinghabits which keep them within particular kinds of water (seepage areas, hill streams, fishponds, &c.), leaving the others unoccupied, then species-sanitation has the chance of showing what it can do.
The proof that our shortwings are the principal malaria vectors in the Netherlands will be stated in the next chapter, but we may take it for granted that they are, and so the point to settle here is: Where do the shortwings breed?
How do we find out the breedingplaces of an



anopheline species? We do so by the method of the "inventory of the breedingplaces" *. In a great and varied number of breedingplaces larvae are caught, identified, counted, and listed, for each species and breedingplace separately. Next, these variousbreedingplaces are grouped into classes, according to certain characteristics they have in common. Finally the larval inventory of each class of breedingplaces is made out by compiling the lists of all breedingplaces belonging to that class. In this way one cannot fail to find out the class of breedingplaces favoured by the local malaria vector, on condition that one is able to identify its larvae.
Here we might ask whether it is necessary to identify the larvae of the shortwings. The ova might do as well, and better, since there is no difficulty in identifying them. But the ova are useless for our purpose, as they teil us only in which classes of breedingplaces the long- and shortwings oviposit. What we want to know is in which kinds of water the eggs will hatch and the larvae will grow to maturity. In salty breedingplaces we can rely on the eggs to give us an answer, for there the number of larvae and the number of eggs of each race perfectly agree (p. 124). But in breedingplaces of fairly low salinity it may happen that shortwings are in the majority as ova, but that the longwings far outnumber the other race when both have become fourth instar larvae. Going by the eggs we would
* As an example we may quote the inventory of the breedingplaces in the district of Great-Mandailing, Sumatra, which proved that A. ludlowi sundaicus (fresh-water form) breeds almost exclusively in the fishponds (Communie. Civ. Med. Serv. Netherl. Indies. Vol. 8, 1919. no. 3, p. 65-88).
Malaria. 8
.



pronounce such a collection of water to be a shortwing breedingplace, whereas, judging by fourth instar larvae, it turns out to be a longwing one. For practical purposes there can be no doubt which of the two results must take precedence: obviously it is the second one. That is the one which tells us what kind of adults this breedingplace is going to produce. Here is an instance of four ditches showing a shortwing egg fauna turning into a longwing larval one:
average number of eggs found number of 4th instar
saimity on certain days in June larvae caught 2I-3
m NaCl and J"ly Weeks later
Shortwing Longwing Shortwing Longwing
0.16 174 71 2 107
0.07 147 42 3 38
0.04 xio 72 9 80
0.02 105 164 10 130
So we have to rely on the larvae, and to do so we must be able to identify them individually. Well, we are able to do so with reasonable accuracy, i.e. we are wrong in less than five per cent * of the identifications of fourth instar larvae reared from longwing and shortwing ova. We identify them by the presence of a pair of hairs, either branched or palmate, on the second abdominal segment, and by the number of branches of the antepalmate hairs on the fourth and fifth abdominal segment. A larva is pronounced a longwing if it has at least one
* 128 4th instar larvae out of 3,651 reared from shortwing batches of ova were indentified as longwings, an error of 3.5 per cent; 143 4th instar larvae out of 3,620 reared from longwing batches of ova were identified as shortwings, an error of 3.9 per cent. We are not aware that larval characters in other anopheline species have ever been so rigorously tested.



branched hair on the second abdominal segment. If it has two palmate hairs in that position it is still entered as a longwing, if the number of branches the antepalmate hairs on the fourth and fifth abdominal segment possess between them is nine or more on both sides; but if it is eight or less, even on one side only, and if there are two palmate hairs on the second abdominal segment, the larva is regarded as a shortwing. These distinguishing characters do not allow of identifying the larvae of our third maculipennis race: typicus. They will pass as shortwings. Considering, however, that typicus has not been found in our coastal provinces, and very rarely in our eastern ones (p. 87), this difficulty is of no practical consequence.
* * *
So we are in a position to make an inventory of the breedingplaces. All we have to do is to classify them. But, here another difficulty arises in the way to species-sanitation: There are no classes of breedingplaces in the malarious provinces. There exists one class only: the narrow ditch. Swamps there are, but they hardly breed any larvae; in fact, they are the most disappointing breeding areas conceivable in this country. The name "swampfever" would never have been invented here. "Ditchfever" would have been more appropriate; for the surest way, in this country, to start anopheles breeding in a swamp is to drain it by cutting ditches through it *. Of lakes there remain quite a number, although all the larger ones have been pumped dry. But lakes are not
* Versl. en Meded. betr. Volksgez. Year 1927, p. 867-868; 1929, p. 563.



breedingplaces of any consequence, neither in the reed-covered margins, nor in the open centres. Here again it is land-reclamation which creates breedingplaces. The same applies to land reclaimed from the sea. Obviously, no breeding of anopheles occurred in the Zuydersea before it was impounded. But in the reclaimed part of it breeding is going on at a great rate at present, although it took some years before it commenced, owing to the high salinity of the water in the drainage ditches during the first three years *. In former years (and even in our days) the tidal marshes, which gradually form by silting outside the protecting embankments along the coast of Groningen, Friesland, and Zealand, were held in great awe because of the miasma supposed to emanate from them. Whatever this miasma may have been, it is certain that these marshes do not produce a single anopheles t in our days.
So the drainage ditch is the only breedingplace: the narrow ditch, that is, not exceeding three yards in width (the broader canals are of little importance in this respect). Their breeding capacity wholly depends upon the kind of vegetation growing in them. A "vertical vegetation" (to use Sella's § expression), like reeds and rushes, is quite unfavourable. It must be "horizontal", floating just below the water's surface, in order to promote breeding, like many algae do and also narrow-leaved phanerogamous plants. But horizontal vegetation floating
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 3, 1934, p. 441-459. f Versl. en Meded. betr. Volksgez. Year 1926, p. 346-347. § Int. Jrl. Publ. Hlth. Vol. 1, 1920, p. 316.



at, or on, the water's surface, either as broad leaves, like water-lilies, or as small units holding close together, like duckweed, are distinctly unfavourable. Absence of vegetation, as a rule, means absence of larvae.
As a consequence, we can split the one class of breedingplaces, we possess in our country, into a number of classes according to the kind of vegetation. But there exists another possibility of subdividing the ditches which, in the light of previous findings, appears more logical. That is a subdivision according to the salinity of the water.
As we pointed out bef ore (p. 63), van Thiel was the first to demonstrate the association of shortwings with brackish water and of longwings with fresh water. Later on we found (p. 70) the association not quite so close as it appeared at first sight, shortwings being found in areas where all the surface water is fresh. Nevertheless, it was close enough to make us contemplate the possibility of species-sanitation in the shape of controlling the brackish breedingplaces to the exclusion of the fresh ones, a procedure for which this country seemed to offer particularly favourable opportunities. It was not sufficiënt, however, to have proved that the geographical distribution of adult shortwings roughly coincides with that of brackish water (fig. 8, 9, p. 68). It was necessary to prove that shortwings mainly breed in brackish water and longwings in fresh.
Experiments in the laboratory did not support the assumption that longwings grow better in fresh water and shortwings in salty water. They both



did equally well in fairly brackish (0.49 per cent of salt) and in fresh water *. In the laboratory shortwings do not oviposit more readily in brackish water than in fresh; if anything, the contrary is true. As to the longwings, they are more attracted by brackish water for oviposition than the shortwings. Unless the salinity rises to a height approaching the limit compatible with the breeding of shortwings in nature (0.99-1.15 per cent of salt), there is little difference between the percentage of shortwing ova and longwing ova hatching. Nor is it possible to detect any greater tolerance in shortwing larvae than in longwing larvae to sudden rises in the salinity of the water. The only experimental evidence we could obtain that shortwing larvae are more at home in brackish water than longwing larvae was the following. An equal number of larvae of the two races were reared together in fresh and brackish (0.74 per cent of salt) water. In fresh water the two races grew into an equal number of mature larvae and adults, but in brackish water the longwings feil far short of the other race t.
* * *
Two sets of field-observations, carried out in order to establish the inventory of breedingplaces of varying salinity, have had an entirely different tale to teil. One of them, the "three provinces" survey, refers to more than 900 breedingplaces, dispersed through the whole of the provinces of North-Holland, South-Holland, and Utrecht, including the adjoining
* Proc. Roy. Acad. Science Amsterdam. Vol. 30, 1927, p. 61-68.
t Riv. di Malar. Vol. 11, 1932, p. 150-155.



parts of Guelders, in which over 24,000 larvae were identified *. The result was that the "fresh" breedingplaces (0-0.08 per cent of salt) were found to contain a large majority of longwing larvae, the "salty" breedingplaces (0.25 per cent of salt or more) an even larger majority of shortwing larvae, andthe"brackish" breedingplaces (0.08-0.25 Per cent of salt) were intermediate between the two t:
_ ,. . No. ol larvae per 10 dips
Breedmgplace Shortwing Longwing
fresh, o—0.08 per cent of NaCl 10 35
brackish, 0.08—0.25 per cent of NaCl 58 26
salty, over 0.25 per cent of NaCl 125 8
The two accompanying maps show the same results. One (fig. 14) marks the breedingplaces with a salinity above, or below, 0.16 per cent of salt; the other (fig. 15) whether shortwing or longwing larvae (if at all present) occur in a majority in these breedingplaces. The principal thing these maps bring out is the difference between North- and South-Holland, in respect of the salinity of the water in the breedingplaces and of the composition of the larval fauna. Without regard to details, the difference may be stated as follows: North-Holland is salty, its anopheles are shortwings; South-Holland is fresh, its anopheles are longwings. Taking into account the distribution of malaria, as shown in fig. 4 (p. 25),
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 5, 1936, p. 280-294. Also: G. van der Torren, De geographisehe verspreiding van Anopheles maculipennis atroparvus en messeae. Amsterdam 1935, P.H. Vermeulen, 81 pp.
t To put it more simply: Below a salinity of 0.16 pet. there are 18 shortwing larvae and 39 longwing larvae per 10 dips, a proportion of 1 to 2; at a salinity of 0.16 and over, there are 117 shortwing larvae and 10 longwing larvae per 10 dips, a proportion of 12 to 1.



Fig. 14. Dissemination of fresh and salty water. Map of the provinces of North-Holland, South-Holland, Utrecht, and the adjoining parts of Guelders, indicating the site of actual, or potential, breedingplaces containing water of a salinity of 0.16 per cent of chloride of sodium or over (black dots), or of a lower salinity (white dots), in 1933, 1934. and 1935-
we may add: North-Holland is malarious, SouthHolland is not.
The fresh breedingplaces are the most prolific



Fig. 15. Dissemination of the larvae of the two races. Map of the provinces of North-Holland, South-Holland, Utrecht, and the adjoining parts of Guelders, indicating the site of actual breedingplaces harbouring a majority of shortwing larvae (black dots), or of longwing larvae (white dots), in 1933, 1934, an(ï 1935.
in producing longwings and the salty ones in producing shortwings. The total larval density, however, is much higher in the latter type of breedingplaces. Disregarding the existence of the two races, we



might be tempted to conclude that North-Holland is malarious simply because salty breedingplaces produce vastly larger numbers of Anopheles maculipennis than fresh ones do. But the fallacy of this reasoning would become at once apparent if the longwings were the malaria vectors in this country, and not the shortwings. For, then, the malarious province would be South-Holland where the total larval density in the breedingplaces is much below that in North-Holland. We have made that remark bef ore (p. 72), but we repeat it here because we can now explain why shortwinged adults in fresh-water areas are comparatively rare: It is because they have to breed in fresh water. It is only in salty water that their larvae are sufficiently numerous to uphold the high density of adult shortwings which is indispensable to keep malaria going in this country (p. 172).
It may be objected that there is nothing surprising in the fact that longwings mainly breed in fresh water and shortwings in salty water. These kinds of breedingplaces occupy widely separate areas. How could longwings breed in salty water, when they mainly occur in South-Holland where the breedingplaces are predominantly fresh? And how are the shortwings to breed in fresh water, when their own particular province is salty? That means reversing the argument: shortwings live in NorthHolland, longwings in South-Holland, for some unknown reason. And because North-Holland is salty and South-Holland fresh, the shortwings, if they are to breed at all, have to be satisfied with salty water and the longwings with fresh.



We are ready to meet this objection with our other set of field-observations, the "one polder" survey. It was continued for several years in a very limited area, a small portion of a polder, fifteen to eighteen feet below sea-level, in the north-western portion of the province of Utrecht. There the salt- and freshwater regions meet, and so fresh and salty ditches lie side by side. Here nature carries out the same experiment as we attempted, when we offered longand shortwinged pregnant females the choice of ovipositing in fresh or salty water in the laboratory (p. 118). Both longwings and shortwings are present in this polder and they are offered the same opportunity to oviposit in fresh, brackish, and salty breedingplaces within an area of little over half a square mile. But they do not avail themselves of this opportunity, as may be gathered from the following table. It shows the result of one year's investigation only (1937) and refers to 46 ditches in which 14,169 larvae and 20,975 ova were identified:
„ , No. of larvae per 10 dips
Breedingplace Shortwing Longwmg
"fresh", o—0.08 per cent of NaCl 13 16
"brackish", 0.08—0.25 per cent of NaCl 18 14
"salty", over 0.25 per cent of NaCl 159 2
The results of the "one polder" survey, compared with those of the "three pro vinces" survey quoted on p. 119, show the effect of breedingplaces of all gradings of salinity being brought together within a narrow compass. The shortwings continue to hold their undisputed sway over the salty breedingplaces, but in those of lower salinity the two races become more or less intermixed.



Whatever difference there may exist between the two sets of observations, one outstanding feature they have in common: salty water is the only medium that breeds shortwings in profusion. In spite of all the larvae of that race found in fresh water, the shortwings are salt-water breeders. In salty water their larval density reaches the twelvefold of that in fresh water, in both sets of observations. The longwings can hardly lay such a well-substantiated claim to the title of fresh-water breeders. Their larval density in fresh water is no more than the eightfold of that in salty water in the "one polder" survey. It is even less, namely the fourfold of the larval density in salty water, in the "three pro vinces" survey.
Why do the salty breedingplaces produce shortwing larvae in such quantities? Do the shortwing larvae die in fresh water or do the shortwinged adults oviposit more in salty water than anywhere else? The "one polder" survey gives an answer to this question, since we identified and counted, not only the fourth instar larvae, but the eggs as well. This is shown in the following table:
Breedinrolace No" of eggS Per 10 diPs
tsreeamgpiace Shortwing Longwing
"fresh" 19 18
"brackish" 38 18
"salty" 242 5
Consequently, the large majority of the shortwings in the salty breedingplaces finds a ready explanation in their preferential habits of oviposition: 99 per cent of the larvae in salty water are shortwing and so are 98 per cent of the ova. Shortwing larvae



and shortwing ova are both twelve to thirteen times as numerous in salty water as in fresh.The longwings, on the contrary, show once more that they do not possess the same discrimination as the shortwings. Their larvae are eight times as numerous in fresh water as in salty water, hut their ova four times only. Going by the eggs there are too few longwing larvae in salty water; half of them must have died before reaching maturity, or must have failed to hatch.
The same discrepancy has been signalized already with regard to shortwing ova and larvae in some fresh and brackish breedingplaces. In the table on p. 114 we have quoted four cases in which we identified 536 shortwing ova and, two to three weeks later, 23 shortwing larvae. In the coexisting longwing population there were 349 ova and, later on, 355 larvae. Here, a large proportion of the shortwing larvae must have failed to reach the fourth stage. Whether they fail to do so because conditions in breedingplaces of low salinity are not quite congenial, or because coexisting longwing larvae interfere with the breeding of shortwings in ditches of that kind, we do not know.
It sometimes looks like Sir Malcolm Watson's dictum * coming true, of one species of anopheles being bidden to clear out and another one to come in. There is some reason to put the question whether shortwings will disappear (or dwindle to a harmiess scarcity) by rendering the water in North-Holland fresh, or whether this hoped-for result is only to be obtained by the help of longwings settling in
* Malay Mail. June 21, 1910.



great numbers in the fresh-water breedingplaces *.
However that may be, one thing is certain: shortwings are salt-water breeders, although their larvae and ova may be found in perfectly fresh water. But it is in salty water only, i.e. in water with 0.25 per cent of salt or over, that they occur in great numbers. Whether it is the salt itself which brings about this effect, or other unknown factors associated with it, we do not know. The influence of salinity is sufficiently evident to conclude that, whatever these other factors may be, salinity is a reliable gauge by which to measure them.
* Van Thiel's experiments (Fesischr. f. Nocht. 1937, P- 625-630), showing that in the laboratory neither shortwings nor longwings succeed in crowding out their rivals, are certainly not in favour of this hypothesis. But then, they are laboratory findings, and we know that they do not always agree with field-observations.



CHAPTER VI
Anopheline malaria
In the preceding chapter we have shown that our shortwings are a proper object for species-sanitation, on condition that they are vectors of malaria. The proof that they comply with this condition can be given in two lines:
Shortwings * dissected 44,167, infected 2,467; 5.58 per cent infected Longwings * dissected 2,880, infected 1; 0.04 per cent infected
All these mosquitoes were caught in human habitations. In animal habitations we dissected 2,640 shortwings, 7 of which were found infected, and 8 uninfected longwings. So the grand total amounts to 49,695 dissections t.
Hence, the rate of infection in the shortwings is 139 times as high as in the longwings. This, we believe, is sufficiënt proof that the shortwings are the principal, and practically the only, vectors of malaria in this country. Adding to this that no more than 18 out of 2,464 infected shortwings were found
* During the time of their sexual inactivity, when long- and shortwinged adults are sharing the attic bedrooms of human habitations, we cannot identify them by the eggs. Fortunately, it is possible, at that time, to identify the females by the shape of their salivary glands (see the upper figures on the plate facing p. 130. For further details see: Proc. Roy. Acad. Science Amsterdam. Vol. 38, 1935, p. 452-454)-
t Not including the 9,549 anopheles we dissected in 1920-1922, since we did not differentiate between long- and shortwings at that time (Malaria in the Kingdom of the Netherlands. Hlth. Org. Lg. o. Nt., no. C. H. 196, Geneva 1924, p. 48 and 71).



between July ist and August i5th, during the period of their sexual activity, we have the further proof that the occurrence of malaria infection in anopheles is closely associated with their sexual inactivity which commences in the month of August.
These are the principal facts. Here follow the details which substantiate them and which render them useful for practical purposes.
Periodicity of anopheline malaria
The following diagram (fig. 16) depicts the periodicity of anopheline malaria in human habitations. Although it refers to one village and one year only, Uitgeest in 1935 *, it not only confirms our earlier investigations, continued for two years in the malarious areas on the outskirts of Amsterdam, but it is itself confirmed by investigations in other villages. So we may take it as representative of the conditions in this country, especially so as the number of mosquitoes dissected is sufficiënt (24,898) to inspire confidence. It shows that sporozoite-carrying anopheles are to be found in human habitations from the second half of August of one year till the end of April of the next. They are absent in May, June, and July. The columns pointing downwards show the number of inhabitants of Uitgeest who had malaria in 1935. Each person is recorded only once, in the half-monthly period when he had his first attack in that year.
In chapter four (p. 59) we have said that the finding of mosquitoes infected in autumn lent a
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 5, 1936, p. 296-314.



Fig. 16. Seasonal variation of the sporozoite-rate compared with the seasonal variation of the number of malaria patients. Columns pointing upwards: Half-monthly incidence of anopheline infection (sporozoites only) as observed in 1935, mainly in Uitgeest. Black columns: percentage of anopheles carrying sporozoites the large majority of which are normal. White columns with a black vertical bar: percentage of anopheles carrying sporozoites the large majority of which are degenerated. Columns pointing downwards: Half-monthly number of persons having suffered from malaria in 1935 in the village of Uitgeest. Each person is counted once only, the first time he had malaria that year.
Malaria. 9



strong support to Korteweg's hypothesis, that persons falling ill with malaria in April and May had acquired their infection in the preceding autumn. But this diagram suggests that the cases occurring in April and the first half of May need no hypothesis to explain them. There are sporozoite-carriers in the second half of March and in April to account for such cases. It is on the explanation of this finding that our recent investigations have brought more light. They confirm our earlier observations on the existence of anopheles infected from January till April. But, they show at the same time that the infectivity of these anopheles (as distinct from their infection) may be regarded as non-existent, because the sporozoites they carry are degenerated (Plate II).
Long ago Roubaud * discovered degenerated sporozoites in infected mosquitoes he had kept the winter over. Since that time, several other workers have noticed them. They are easily recognized in fresh preparations in saline, since they are smaller than normal sporozoites, and excessively curved in the shape of a "c" or an "s", quite different from normal sporozoites which are slightly curved only. In fixed preparations degenerated sporozoites stain rather faintly, their chromatine is sometimes broken up into a chain of granules. But, on the whole, the difference between normal and degenerated sporozoites is much better marked in fresh preparations than in stained ones. The difference is brought out best of all if one tries to infect patients suffering from general paralysis of the insane. In the practice
* C. R. Acad. Sciences Paris. Vol. 166, 1918, p. 264-266.



Plate II
Salivary gland of semihibernating shortwing.
Salivary gland of hibernating longwing.
Normal sporozoites.
Degenerated sporozoites.







of therapeutical malaria-infection of these patients (p. 227) we know that degenerated sporozoites are no longer of any use as infecting agents *.
To us degenerated sporozoites are of particular importance, for the following reason:
In fig. 16 anopheles carrying sporozoites which are all, or in majority, of normal shape (black columns) have been kept separate from those carrying sporozoites which are all, or in majority, degenerated (columns with a longitudinal black bar). The latter make their appearance in the first half of November. In the following half-monthly periods they rapidly increase in numbers. In the second part of December less than half of the sporozoite-carriers have a majority of normal sporozoites in their glands. By the first half of January this proportion has dwindled to one fifteenth; from then onward no more normal sporozoites are to be found. The gap in human malaria, from November till March, seemed to be bridged over by anopheline malaria lasting from September till March. One might actually have doubted the correctness of Mitzmain's t saying that man, and not mosquito, is the winter-carrier of malaria organisms. But now, by taking note of the shape of the sporozoites, we perceive that anopheline infection would be a very rickety bridge for human malaria to cross the gap between one year's season and the next. If it had no better conveyance, there would be no malaria in this country (p. 162).
* Amer. Jrl. Hyg. Vol. 24, 1936, p. 7; also: Boyd and StratmanThomas, Ibid. Vol. 19, 1934, P- 539-54°. t Public Health Buil. Washington, no. 84, 1916, 32 pp.



Consequently, the whole of anopheline infection in the first four months of the year is nothing but a dead thing, the mummy of an infection which was living in the preceding autumn, embalmed within the salivary secretion of the mosquito. The dead sporozoites in March and April can never be the origin of spring-malaria, and so they cannot invalidate Korteweg's hypothesis. The nearest living anopheline infection to account for these spring cases is still to be looked for as far back as December.
* * *
So far for the theory. But there also is a practical side to the existence of degenerated sporozoites. When our investigations of 1920-1922 revealed the fact that anopheline infection is limited to autumn and winter, practical application of this knowledge was immediately contemplated. It was proposed to destroy anopheles in human habitations in autumn and winter, with the express purpose of killing the infected mosquitoes. Nothing came of it, however, since the killing would have had to be continued till March. At present, we know that we may stop by the end of December.
We may stop even earlier than that.
Fig. 16 shows none but sporozoite-carrying anopheles, and so the diagram does not bring out the fact that oöcysts likewise degenerate in the course of the autumn. Normal young and medium-sized oöcysts, examined in saline, have a smooth and shiny appearance. Degenerated oöcysts have lost that; their content is granular, often with tiny yellowish dropiets. In the medium-sized oöcysts the



pigment is no longer arranged in strings, and vacuoles appear in the protoplasm. In full-grown oöcysts these vacuoles become the principal sign of degeneration.
Anopheles carrying none but degenerated oöcysts make their appearance in the second half of October, half a month before the sporozoites begin to degenerate. At that time degeneration of oöcysts occurs in one sixth of the anopheles showing intestinal infection, a month later in more than three fifths of them; in the first half of December nearly all carry degenerated oöcysts. Consequently, no anopheles acquiring its infection as late as December will carry sporozoites. Provided one succeeds in killing all anopheles carrying malarial infection by the end of November, one need not worry what will happen after that time. No fresh infection has a chance to establish itself under the existing conditions of housing, in the unheated attic bedrooms.
We can further reduce the length of the period of anopheles destruction by taking account of the size of the normal-looking oöcysts during the autumn. The small and medium-sized oöcysts signify a recent infection. Mosquitoes carrying them are two to three times as numerous as mosquitoes carrying fullgrown and mature oöcysts, in the second half of August and in September. In the first half of October anopheles carrying full-grown oöcysts are in the majority. In the second half of that month those carrying young oöcysts regain the upper hand; but thereafter their numbers rapidly go down.
Hence, fresh infections of anopheles occur in August and September and again in the second



half of October, but rarely later. This is confirmed by the fact that the rate of anopheline infection in human habitations continues to rise till the second half of October, but remains stationary, or declines, after that time. Here is an example of the change in the rate of anopheline infection observed in one single house:
August i6th; 10 anopheles infected out of 118: 8 pet. September 2oth; 79 „ „ „ „ 307: 26 „
October i7th; 84 „ „ „ „ 219: 38
November 27th; 75 „ „ „ „ 232: 32 „
December i7th; 38 .. „ „ „ 101: 38 „
December 3oth; 20 „ „ „ „ 77: 26 „
So we conclude that fresh infections continue till the end of October, and the ripening of the, then existing, oöcysts till the end of November. Of course, this is not a hard and fast rule; one may find a mature oöcyst at the end of December. Nevertheless, we feel satisfied that it is not necessary to continue the killing of anopheles inside a human habitation after October 3oth, provided all anopheles present in that house on that date have been destroyed. As there are, practically, no infected anopheles carrying sporozoites before August i5th, it comes to this that the killing of anopheles in houses, for the purpose of destroying infected mosquitoes, can be limited to two months and a half.
One question remains: How long does it take an anopheline infection to mature during that period or, to put the question in a practical way, how often need the killing of anopheles be repeated? The proportion of mosquitoes carrying full-grown oöcysts shows periodic rises and falls. So does the proportion of mosquitoes carrying young oöcysts. The rises



of the latter precede those of the former by half a month until the middle of October, and by twice that time in the second half of that month. Consequently, it takes the oöcysts half a month to mature, except in the second half of October, when it takes them doublé that time. So the killing of anopheles can be limited to twice a month. In how far this conclusion tallies with practical experiences will be discussed in chapter nine (p. 213, 214).
The cause of the periodicity of anopheline malaria
Anopheline infection with a maximum in autumn is not a feature peculiar to the Netherlands only. Long before our first communication on the subject *, Martiranot had found that the rate of anopheline infection in southern Italy rose from zero in April to nine per cent in October and eighteen per cent in December, the months of June, July, and August showing intermediate figures. Sella's figures § are even more like our own. In human habitations, near Rome, he found an anopheline infection of four to five per cent in October, November, and December, and of one to three per cent in January, February, and March. In August mosquitoes are infected for a half per cent only, but in September the rate rises again to three per cent. So it would appear that conditions regarding anopheline malaria were much the same as they are here.
There are three explanations to account for the high rate of anopheline infection in autumn and
* Nederl. Tijdschr. v. Geneesk. Vol. 65, 1921, 2nd half, p. 1486-1488. t Atti Soc. Stud. Malaria. Vol. 2, 1901, p. 262-265.
§ Int. Jrl. Pübl. Hlth. Vol. 1, 1920, p. 341.



its low rate in summer. Two of them, set forth by Martirano, maintain that there really are more anopheles infected in summer than in autumn. It is the rate of infection onlywhich is lower in summer, because the mosquito population is "diluted" by theadmixture of newly hatched adults; and it is the detection of infected mosquitoes which is rendered more difficult in summer, because the adults take long flights. As a third explanation, Grassi * accepts the absence of infected anopheles in summer as a fact; he accounts for it by their high mortality during that season.
The second and third theory have continued to this day, but the first has fallen into oblivion. Hence, we shall commence with the neglected one by asking the following question: If we correct the summerrate of anopheline infection by eliminating all records of dissections of freshly hatched mosquitoes, do we then arrivé at a summer-rate which equals or surpasses the autumn-rate of anopheline infection? The answer to this question is that the elimination of all mosquitoes which were not carrying mature, or nearly mature, ova raises the rate of infection in summer from 0.4 to 0.5 per cent. That is no more than a small fraction of the autumnal rate of infection which is 9.2 per cent t.
* Rendic. Re. Ac. Naz. Lincei. Sez. 5, Vol. 20, 1920, p. 307-313, 339-344. Schüffner and Hylkema (Communie. Civ. Med. Serv. Netherl. Indies. Vol. 10, 1921, p. 48-91), working under entirely different conditions in Sumatra, likewise stressed the influence of an early death of anopheles on the malaria infection of these mosquitoes.
t Out of 2,702 anopheles dissected from May ist till August I5th, ten were infected, or 0.4 pet.; 1,846 certainly were not newly hatched adults, since they were bearing ripe or nearly ripe ova. Éxcluding all other adults as possibly young ones, the rate of infection rises to 0.5 pet. Out of 16,498 anopheles dissected from August i6th till December 3ist, 1,519 were infected, or 9.2 pet.



Martirano's second explanation of the comparative scarcity of summer infection in anopheles is still held in connection with Roubaud's theory of zoophilism and animal protection *, to which we have referred by the term stabular deviation (p. 101). On page 102 we have quoted an observation of infected anopheles losing their way and straying into animal habitations, an event which is only observed during the period of sexual activity of anopheles. Hence, the question arises: If the summerrate of anopheline infection is corrected by including all infected mosquitoes found in animal habitations, does this correction raise the summer-rate to, or above, the autumn-rate of anopheline infection in human habitations? Uitgeest is a village eminently suited to elucidate this point, since there are little other than rabbitcages to represent animal habitations. They harbour an anopheline population which is not too numerous in summer to be examined exhaustively. By adding the infected anopheles found in animal habitations during the summer to the females, carrying mature ova, caught in houses, we can raise the summer-rate of infection to 0.6 per cent t. That is one and a half times as much as the uncorrected summerrate; still, it is no more than one fifteenth of the autumnal-rate of anopheline infection.
We realize that a stronger stabular deviation, than that which we found in Uitgeest, will raise the number of infected anopheles straying into
* E. Roubaud, C. R. Acad. Sciences Paris. Vol. 186, 1928, p. 329-331. t Twelve infected anopheles found from May ist till August i5th (ten in houses, two in rabbitcages) to 1,848 females (1,846 carrying mature ova in houses and the two infected ones from the rabbitcages).



animal habitations during the time of their sexual activity. What is of still greater importance: it will reduce the number of sexually inactive anopheles settling down in human habitations, to become infected later on *.
But the point we wish to make here is another one. In Uitgeest neither stabular deviation, nor the interference of freshly hatched adults, nor the two factors combined, can account for more than a small fraction of the difference existing between the rate of infection in summer and in autumn. This difference, we may add, is no less conspicuous than around Amsterdam where stabular deviation is most marked.
* * *
Grassi's theory, explaining the scarcity of summer infections by the infected anopheles dying when the oöcysts are still quite young, has been put on a firm experimental basis by James t. Of batches of anopheles infected in April and May, James says, less than ten per cent live until the batch becomes
* The rate of oöcyst infection in the rural areas to the south of Amsterdam, 5,1 pet. in the autumn of 1921 preceding the great outbreak of malaria in those parts, was but little inferior to the rate of oöcyst infection in Uitgeest, 5.9 pet. in the autumn of 1935. But stabular deviation in the Amsterdam rural area was far greater than in Uitgeest, as many of the homesteads had pigsties or horsestables on the premises. Accordingly, the number of anopheles per house, in the last quarter of the year, was 31 around Amsterdam, whereas it was 53 in Uitgeest. Consequently, the average number of infected mosquitoes per ten houses was 31 in Uitgeest, or almost twice as much as around Amsterdam, where it was 16 (Nederl. Tijdschr. v. Geneesk. Vol. 68, 2nd half, 1924, p. 750-763, 1113-1125). f S. P. James, W. D. Nicol and P. G. Shute, Medicin. Pais. Calid. Vol. 1, 1928, p. 161-164; Transact. Far East. Assoc. Trop. Med. 7th Congress, Vol. 2, 1929, p. 712-717. S. P. James, Transact. Roy. Soc. Trop. Med. Vol. 24, 1931, p. 491; Proc. Roy. Soc. Med. Vol. 22, 1929, Sect. Epid. & State Med., p. 71-85.



infective, but of batches prepared between the end of August and the middle of November, at least fifty per cent of the mosquitoes with which the batch was begun will be available for use in infecting patients. Going by James' observations, the year can be divided into two seasons; one is favourable to malaria transmission: August to November, the other is not: December to July. The latter includes the whole period of sexual activity. This sexual activity, endangering life by the processes of ovulation and parturition, seriously impedes malarial transmission by mosquitoes in the laboratory. It may be expected to have a still more deleterious effect in the field, since anopheles are exposed there to the dangers consequent upon the long flights they are forced to take for the sake of oviposition. We have encountered these long flights already as entailing the loss of infected anopheles by their straying into animal habitations. Here we meet them again, imperiling the existence of anopheline infection by causing the early death of the prospective sporozoite-carrier.
We have been able to confirm James' experimental findings *: Four weeks after the infecting meal, two weeks after the glands became infected, the rate of survival of shortwings caught in nature t, and infected in the laboratory, is nearly sixty per cent from September till December, but no more than one quarter of that rate from April till August. From January till March the rate of survival is half of that observed during the favourable season.
* Proc. Roy. Acad. Science Amsterdam. Vol. 38, 1935, p. 335-343f Laboratory-bred anopheles show a much lower mortality(p. 222).



Consequently, we may take it that from three quarters to four fifths of the mosquitoes infected during the time of their sexual activity die before, or shortly after, they have become infective.
When sexual activity comes to an end all this changes. Mosquitoes becoming infected by that time are no longer removed by early death. Freshly infected ones are added to their number and likewise remain alive. In this way the stock of infected anopheles keeps accumulating, as in the example quoted on page 134. This accumulation primarily depends upon the long life of the mosquitoes. There is no doubt, however, that their autumnal fixation is an additional help in bringing it about.The absence of newly hatched mosquitoes contributes to make it more marked.
Whatever explanation of the cause of the periodicity of anopheline infection may be considered the right one (and we believe longevity is the most important), they have this in common, that they assume that sexual activity of anopheles adversely influences their showing infection. That assumption, at least, is perfectly correct as may be seen in the following diagram (fig. 17).
The percentage of pregnant females suddenly drops from the high level it had sustained since April to fifteen per cent in the second half of August, and to practically zero in subsequent months. This decline coincides with an equally sudden rise of the percentage of sporozoite-carriers, from zero (or next to it) in all breeding months to one per cent in the second half of August, and to four and six per cent in the three subsequent months. The second half of August is particularly appropriate



to show the intimate relation existing between the biological condition of the females and their rate of infection. In houses where infected anopheles were detected within that period of time we found:
/ Z, ,3 H 5 6 7 8 9 10 11 11
month 1 { 
Fig. 17. Sporozoite-rate inversely related to the rate of pregnant females. Uninterrupted line: percentage of females carrying normal sporozoites. Broken line: idem, carrying mature, or nearly mature, ova. Dotted line: average half-monthly temperature at 8 a.m.
229 females in a state of ascertained sexual activity, i.e. with ripe or nearly ripe eggs; three were infected or 1.3 per cent;
590 females in a state of ascertained sexual inactivity, i.e. with a well-developed adipose body; 39 were infected or 6.6 per cent;
1,655 females whose biological condition could not be determined, since they were neither fat nor pregnant; 15 of them were infected or 0.9 per cent.



This comparison, of the rate of infection of sexually active and inactive females, clearly brings out the fact that the latter are the principal initiators of autumnal infection. They are the ones which, being infected, do not waste the treasure they are carrying by straying into the stables. This last point is brought out by the fact that the few infected anopheles we caught in animal habitations, during the same season, were all bearing mature eggs.
Hence, we feel justified in concluding that the periodicity of malarial infection in shortwinged anopheles is determined by, and inversely related to, the periodicity of their reproductive functions. Sexual activity declines in late summer, hence the incidence of anopheline malaria rises in that season. Reproductive functions are most active in spring and early summer, hence anopheline malaria is rare or absent at that time of the year.
Irregular distribution of anopheline malaria
When we discussed the sick-rate due to malaria in a village, we said that such figures are misleading, because it may happen that most of the malaria occurs in some parts of the village, leaving the rest almost untouched (fig. 6, p. 45). The same remark applies to anopheline malaria. If we say that the autumnal rate of anopheline infection is nine per cent, in a group of 201 houses with 1,131 inhabitants, this does not mean that we may expect to find a mosquito infected in every batch of eleven dissected. It means that we find the infected mosquitoes dispersed through these houses in the following manner:



1,265 anopheles in 133 houses with 591 inhabitants, none infected.
5,300 anopheles in 55 houses with 412 inhabitants, 3.8 per cent infected.
8,407 anopheles in 13 houses with 128 inhabitants, 13.9 per cent infected.
The first group of houses harboured on an average 9 anopheles per house during the autumn, the second group 96, the third 645. Of course, there is nothing astonishing in finding more infected anopheles among many mosquitoes than among few. But here the rate of infection increases with increasing numbers; that is quite another matter and not at all to be expected. In fact, we had expected the contrary, on the strength of Buil and King's observation * that the more anopheles there are in a house, the smaller is the proportion of them containing human blood.
There is another difference between the three groups of houses: the first harbours an average of four inhabitants per house, the second of seven and the third of ten. That is a result which was to be expected; the more inhabitants, the greater the chance of one of them having malaria and infecting mosquitoes. In reality conditions are not so simple, but that remains to be discussed in another chapter (p. 167).
* * *
It is reasonable to assume that the number of anopheles attracted to a house is determined by the density of its human population. That would explain the existence of certain houses where one is always sure to find numerous anopheles, although
* Amer. Jrl. Hyg. Vol. 3, 1923, p. 497-513.



they are no nearer the breedingplaces of the larvae than others. Here is an instance of such a house, examined through four consecutive years, all that time occupied by the same family. This example, moreover, shows how such a house may become a permanent hotbed of anopheline infection. In this house we detected in the months of:
December 1934 *83 anopheles, 48 infected, 45 sporozoite-carriers Jan.-Feb. 1935 483 „ 68 „ 64 Sept.-Dec. 1935 414 „ 26 „ 10 Aug.-Dec. 1936 191 „ o „ o „
Aug.-Sept. 1937 132 „ 4 „ o
Still, the number of anopheles found in a house does not all depend on the human population, but likewise on the house itself. The following is an example of the influence of the house, independent of the number of people living in it, on the number of anopheles it shelters and the extent of the anopheline infection it is fostering.
In a family of eight members four had malaria in 1935; moreover, there were two parasite-carriers * among them. In 1936 they had two malaria patients and four parasite-carriers. In 1935 52 anopheles were found infected in their house, among 172 captured on three visits; in 1936 none was found infected among 8 anopheles captured on four visits. The family had remained the same, the parasite reservoir had remained the same, but anopheline infection had become extinct for lack of anopheles. Here is a case which contradicts the conclusion we arrived at, that the mosquitoes each year tend to return in large numbers to the same house. But it is no con-
* The importance of "healthy" parasite-carriers as a source of infection for anopheles will be discussed later on (p. 165).



tradiction, since in this case the mosquitoes were prevented from following their inclination. The fact is that the family had moved to another house on August ist 1936, before autumnal fixation of anopheles commenced. The new house and the old one were in the same street, 82 yards apart. They were of the same type, but the attic of the old house possessed secluded dark shelters, next to the bedrooms, difïicult of access and therefore never cleaned out. All of the 172 anopheles were captured there in 1935- The attic of the new house, on the contrary, had no such dark shelters, it was well lighted allthrough. Hence, anopheles taking up their abode in that attic had little chance to stay on unnoticed and, consequently, unmolested.
The direct influence of the internal construction of the house, independent of the number of its inhabitants, is brought out quite clearly in this example. At the same time it shows that the change conditioned by the different construction of the new house would have had no effect without the intervention of the inhabitants. They destroyed the mosquitoes in the new house, because its construction rendered this process easy of execution. They failed to do so in the old house, because they did not see the anopheles. If they had been utterly indifferent to the presence of anopheles in their house, the new one, probably, would have harboured as many as the old. If they had been thoroughly alive to the danger of autumnal anopheles, we would not have captured 172 in the old house in 1935. So, the cleanly or careless habits of the family living in the house determine, after all, how many anopheles shall
Malaria. Tri



take up their abode there by the time sexual activity of the mosquitoes ceases. Since these habits largely depend on the management of the household, there is some truth in the saying that one has only to have a look at the house-wife to predict the number of anopheles in her house.
All this would make no sense if the anopheles transmitting malaria were in the habit of entering the house one night and leaving it the next. Then we would experience conditions like those Barber and Rice * described in Greece, where infected anopheles in stables are nearly as numerous as in houses, and where malaria transmission occurs in summer only. In Greece the most careful housewife cannot prevent mosquitoes entering her house unless by screening. Nor can she in this country. But here she gets the chance to prevent anopheles from permanently taking up their abode in her house, and to kill them at the time it really pays to take that trouble.
* * *
To us there always is something ludicrous in the existence of these opposites: the huge masses of infected anopheles we find in North-Holland and the comparatively slight effect they produce. It is like using artillery to kill a moth. It is an anomalous situation that it is far easier to get naturally infected anopheles in North-Holland than in the highly malarious areas along the north coast of Java. We believe we ought to look at it like this: In most
* Amer. Jrl. Hyg. Vol. 22, 1935, p. 512-538.



malarious countries transmission is, probably, carried on by sexually active anopheles; often by those which stay no longer in human habitations than is necessary to take their meal of blood. Their shelters need not be human habitations. They may hide in crevices in rocks, or under cover of vegetation, where they are difficult to find. As a consequence, one may be sure that one gets no more than a fraction of the infected anopheles that actually are about. Moreover, little is known of the relation as to time existing between human and anopheline malaria. It is taken for granted that one should look for infected anopheles when human malaria is rife. But in this country we know exactly where to look for infected mosquitoes, and when to look for them; and so we practically get them all. If conditions would allow of doing the same in highly malarious countries, one would probably find many more infected anopheles than one does now. Even in these circumstances, however, we should not expect to find as many as in North-Holland. In a country which is thoroughly fit to be malarious, by climatic and other conditions, the transmission of the disease can be run on lines of rigid economy. The process requires a minimum of anopheles and anopheline infection, since there is no waste of either. But nature has to be lavish to the extreme in providing anopheles, and she has to accept heavy losses of sporozoites by degeneration, if it takes her fancy to keep malaria going in a country, like our own, which is not really suited to that purpose.



CHAPTER VII
Transmission of the malaria parasite from mosquito to man
Evidence that anopheles transmit malaria parasites to man in autumn but that man does not have his attack of fever until the next year
So far (chapter three and six) we have been occupied with statical conditions. In this chapter, and the next, we shall discuss the dynamics of malaria in this country. The first problem, requiring our attention, is when and how malarial infection passes from infected anopheles to man.
The question "when" has been answered already: it is at a time anopheles carrying normal sporozoites are numerous, from the moment they lose their sexual activity until the end of December.
How do they accomplish this feat? Obviously by biting man. But do they take man's blood in that season? We have more than once said they do, and here follows a diagram to prove it. It shows the percentage of female shortwings in human habitations with human blood in their stomachs, or with normal sporozoites in their salivary glands, for each half month of the year, from March onward (fig. 18).
Accordingly, the shortwinged females continue to feed on human beings from the second half of August till the end of October, at a rate not much



below that observed in July and the first half of August. Considering that September and October are the months showing the highest rate of anopheline infection with undegenerated sporozoites, it may be safely assumed that sporozoite-carrying anopheles do feed on man and infect him as a consequence. That assumption need not be taken on trust. We
Fig. 18. Sporozoite-rate is high at a time when females are still voracious. Uninterrupted line: percentage of females carrying normal sporozoites. Broken line: idem, with blood in their stomachs, identified as human by the precipitine test.
have often found sporozoites in the salivary glands and freshly ingested human blood in the stomach of one and the same anopheles.
Such observations are equivalent to our actually witnessing malaria infection passing from mosquito to man in autumn.
* * *
One part of Korteweg's hypothesis, the one referring to the autumnal origin of malaria infection,



has been proven up to the hilt. The proof of the second part, which is that man infected with malaria falls ill after a long incubation, consists in the following evidence. First, in order of time, come the observations of German workers * who made it clear that soldiers, returning from the malaria infested eastern front, occasionally showed a period of incubation lasting for many months. James t is the next, whose epidemiological observations in south-eastern England brought out the same facts in an even more striking fashion. He also inaugurated the experimental elucidation of the problem, by showing § that twelve patients, suffering from general paralysis of the insane, did not fall ill with malaria until six to ten months after being bitten by infected mosquitoes.
In collaboration with Schüffner and Korteweg, we have repeated this experiment with eight healthy volunteers, who were living under absolutely normal conditions11. Five of them were bitten by one infected shortwing, two others by two, and one by twelve. One volunteer was infected on September 4th; he developed malaria on April 23d of the following year. Six were infected between October 3oth and November 7th; they had their malaria
* Quoted by: E. Martini, Berechnungen und Beobachtungen zur Epidemiologie und Bekampfung der Malaria. Hamburg 1921, W. Gente, p. 10; P. Mühlens, Handbuch d. Pathog. Protoz. Leipzig 1931, A. Barth, p. 1474, 1600.
"f Malaria at home and abroad. London 1920, J. Bale Sns. & Danielsson, p. 110.
§ Communication to the malaria commission of the League of Nations at its session of October 1927. See also: Transact. Roy. Soc. Trop. Med. Vol. 24, 1931, p. 498, 499.
II Buil. Soc. Path. Exot. Vol. 22, 1929, p. 642-645; Proc. Roy. Acad. Science Amsterdam. Vol. 35, 1932, p. 912. For personal reasons the case of our 8th volunteer has never been published before.



between June 2Óth and August 2nd of the following year. The eighth was infected on November 2gth and feil ill on August 2Óth of the following year. The period of incubation lasted for an average of eight months and fifteen days. James found it lasting for an average of eight months and twenty-seven days. So we may take it that the "long incubation" in benign tertian malaria lasts for somewhere between eight and nine months.
These experiments show that malaria infection acquired in autumn remains latent, not only until the next spring, but likewise until the next summer. A case of spring malaria is shown to be caused by an infection acquired early in September; cases of summer malaria are seen to follow infections acquired in October or later. October is the month of the highest incidence of anopheles carrying normal sporozoites, eight or nine months more carry us into June or July which happen to be the months of the highest malaria incidence. All this fits perfectly.
And yet we do not feel satisfied. We would, if this were simply a matter of finding out interesting details to help us to understand the epidemiology of malaria in this country. But that is not our object. We wish to control malaria. Nature has packed all infected anopheles within a few houses and a few months, and in doing so she invites us to destroy them. But, before acting upon this suggestion, it behoves us to halt for a moment, to ask whether these mosquitoes found infected in autumn are the cause of all, or most, of next year's malaria.
It seems foolish to ask such a question. Is there any sound reason to doubt the infectivity of anopheles
.



taking human blood and carrying healthy-looking sporozoites during the same season? As far as we are aware, a similar question has never been asked. It has, in relation to anopheles carrying oöcysts only, or degenerated sporozoites, or normal looking sporozoites five months old *, or sporozoites suspected of being of avian origin. None of these cases, however, apply to ours; yet, there is reason for doubt, as we shall see in the next two sections.
Is autumnal malaria transmission responsible for most of the malaria occurring during a whole year?
We shall call autumnal human malaria the number of malaria patients falling ill, for thefirst time in a given year, from the first of September onward. Autumnal human malaria may be fairly numerous in some years (p. 41), but its incidence never exceeds one third of the total annual incidence. We may assume that most of it is caused in the ordinary way by the bite of mosquitoes infected at that time. The only departure from what is customary is that this bite is followed by an attack of malaria after a short incubation, instead of a long one.
On the strength of the experiments detailed above we assume that autumnal anopheline infection is responsible for all malaria occurring during a whole year, and not for malaria in spring or in autumn only. This assumption implies that two thirds, at least, of all infections man acquires during the autumn must remain latent till the next year.
* Edm. Sergent, Ann. Inst. Pasteur. Vol. 32, 1918, p. 382.



None of our volunteers had malaria after a short incubation. But the experience with patients suffering from general paralysis, infected by mosquitoes with Dutch tertian, has another tale to teil (p. 228 footnote). No less than half of the patients bitten by three to five mosquitoes had malaria after a short incubation. It is true that the conditions of infection in these patients differ widely from those which obtain in nature. But let us leave that consideration aside and accept the evidence of therapeutic malaria at its face-value. Then, it follows that autumnal human malaria must represent one half of all malaria caused by the bite of anopheles infected in autumn. As the incidence of autumnal human malaria represents one fourth to one third of the total annual incidence, this means that anopheles infected in autumn are the cause of no more than one half or, at best, two thirds of the annual malaria incidence. In other words, by killing all anopheles infected in autumn one may look for a suppression of one half to two thirds of the fresh infections otherwise to be expected in human beings.
* * *
So far for the experiments and the conclusions they lead up to. Now we are to see what field-observations can teach us.
If anopheles infected in autumn are at all responsible for the next year's malaria, this ought to show by the cases of malaria being much more numerous in houses where infected anopheles occurred in the preceding autumn than in houses where they were not detected. In this expectation we are, however,



disappointed *. The incidence of malaria in the houses with infected anopheles is not quite the doublé of that in the houses where no such mosquitoes were found in the preceding year. This conclusion, however, underrates the effect of the presence in human habitations of anopheles infected during the autumn, for the following two reasons:
(1) Malaria is prevented from materializing in a given house, in a year following on an autumn during which infected anopheles were present in that house, if the human occupants have acquired a certain measure of immunity to the disease. This condition increases the number of houses without malaria fever but with infected anopheles.
(2) Infected anopheles escape, in small numbers, from houses harbouring them in great profusion ("foei of anopheline infection"), and enter neighbouring houses. So there exist undetected shelters of small groups of infected anopheles in the neighbourhood of foei of anopheline infection. This condition raises the number of houses with malaria but, apparently, without infected mosquitoes.
The first condition, the house harbouring infected mosquitoes during the autumn and yet failing to produce malaria during the next year because of the immunity of the inhabitants, is illustrated by the following two cases.
A certain family, in the village of Uitgeest, had much malaria in 1934 but none in 1935. In 1935 and 1936 there were, however, five "healthy" parasite-carriers. As numerous infected mosquitoes
* Jrl. of Hyg. Vol. 38, 1938, p. 69.



had been found in the autumn of 1934, the house was listed in 1935 as an example of anopheline infection without malaria. Since an even larger number of infected mosquitoes reappeared in the autumn of 1935, the house would have been put on the same list in 1936 but for the birth of a child on March i3th 1935, who, consequently, was exposed to the autumnal anopheline infection of that year. The infant promptly responded to this exposure by having malaria on June 25th 1936.
The second case resembles the first, inasmuch it was a non-immune child who saved the situation. It differs, however, from the first, since it took that child an uncommonly long time to have its attack of malaria.
A family, in the village of Wormerveer, who had suffered much from malaria previous to June 1932, came to occupy a house on October i5th 1934. The house had been vacated by a second highly malarious family on October ist. A few days before the second family moved out of the house, and during the weeks it was left unoccupied, we had detected quite a number of infected anopheles. We had captured not nearly all of them, as we perceived on revisiting the house (now occupied by the new family) in the beginning of November. One of the new occupants, a girl of fourteen months who had never had malaria before, was evidently infected by them, since she was found to carry parasites in the course of November. She promised to pro vide us with a most beautiful natural repetition of our experiments on long incubation *, but she disappointed us: the
* There is this diöerence, however, that we never found parasites in our volunteers during the long incubation.



child had no malaria in 1935. We are quite sure about this, because we engaged the family-physician's interest in the case, and nothing of the slightest moment could happen without his knowing it. In 1935 and 1936 the girl continued as a parasitecarrier and infected anopheles were found in her house during the autumn of these years. Finally, in June 1937, s^e an attack of malaria, two years too late. All that time the house had been listed as an example of the presence of infected anopheles not being followed by human malaria.
* * *
The conditions described above suggested that it makes little difference whether a house has harboured infected anopheles during the preceding autumn or not. This, however, was an unwarranted conclusion, since malarial infection had been transmitted to man, but it had failed to cause a fever. The second condition, which leads us into the same erroneous belief, does so by our failing to discover all the details of the dispersal of infected anopheles. Since this point requires rather full explanation, we shall deal with it in a separate section.
Movements of anopheles infected in autumn
We have pointed out already that there exists a comparatively small number of houses in a malarious village where infected anopheles are to be found in great profusion. Among the seventy-six houses in the village of Uitgeest where we detected anopheline infection, during the autumn of 1935, thirteen only harboured twenty-two infected ano-



pheles or more; another thirteen harboured from six to fifteen, and the rest from one to five only. The houses belonging to the first group have been called the foei of anopheline infection.
On the whole these infected anopheles move about very little, no more than is necessary to take their fill of blood once in a while. How sluggish they are may be gathered from findings in one house where, early in September, anopheles were collected separately, in two cupboard-beds * downstairs and in the attic bedrooms. In the cupboard-beds 145 anopheles were captured with 29 infected ones (20 per cent). In the attic we caught 270 with 10 infected ones (4 per cent). So, here are two groups of anopheles in a one-family house, each group sufficiently numerous to justify the rate of infection being expressed as a percentage. They have kept so well separate that the rate of infection of the downstairs-group is the fivefold of that of the upstairs-group.
One ought not, however, to acquire an exaggerated idea of the lack of energy of mosquitoes which find themselves in the grip of autumnal fixation. There are infected mosquitoes breaking this blockade, as we found out on some occasions, one of which is related here.
In a hamlet near Uitgeest, consisting of a row of seventy-five houses, there existed two foei of ano-
* A cupboard-bed is a recessed bed closed by doublé doors. The recess contains sleepingroom for two persons or more, in addition to a shelf on which an infant's cot can find accomodation. If the cupboard-bed is in a room downstairs, it rarely contains numerous anopheles, the attic being the shelter they prefer. So the above instance, in this respect, is rather an exception.



pheline infection. Twenty-four houses were situated at a distance not exceeding 109 yards from the first focus; eleven of them were harbouring one to three infected mosquitoes, to a total of sixteen. Seventeen houses were situated at a distance not exceeding 72 yards from the other focus; six of them were harbouring from one to three infected mosquitoes, to a total of eleven. The intervening space of 127 yards, between the areas of influence of the two foei, was occupied by thirty-four houses; one of them harboured a single infected mosquito *.
Hence, a definite agglomeration of infected mosquitoes existed around each focus of anopheline infection, extending no farther than 109 yards from the focus. Although the two foei were no farther apart than 311 yards, they still left a space of 127 yards between them almost empty of anopheline infection. This distribution strongly suggests that the infected anopheles had actually escaped from these foei. The size of the cluster of infected anopheles around each focus marks the distance which they travelled.
In agreement with these, and similar, observations we found, in the village of Uitgeest, that 44 of the 50 houses harbouring one to five infected mosquitoes, during the autumn of 1935, were situated within a radius of 109 yards around the foei of anopheline infection. Six only were found some distance outside this range.
Accordingly, any case of malaria occurring within a radius of 109 yards around a focus of malaria
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 35, 1936, p. 301.



infection, which existed during the preceding autumn, is strongly suspect of having been caused by the bite of a mosquito which escaped from that focus. Applying this principle to the case in hand, namely to the question of how much malaria is due to the preceding autumn's anopheline infection, we find the following conditions * in Uitgeest: The areas which are likely to be invaded by stray sporozoite-carriers are the site of 179 (or 78 per cent) of the total of 228 malaria houses; 342 (or 84 per cent) of the total of 409 malaria patients are living within these areas. By killing infected anopheles in autumn we may hope to bring about a reduction of malarial incidence to about one fifth of the original fïgure.
* * *
We ought to add, however, that the measure mentioned above will never succeed in preventing all malaria transmission, because of the presence of sexually active anopheles carrying sporozoites in summer. They are rare, no doubt, and they are lured away from man by stabular deviation, yet they are there all the same. Their existence is recognized in cases like the following t; A child living in a non-malarious village in the province of Guelders was staying for some nights, in the beginning of August 1934, with a family in the village of Uitgeest. In April 1935 this child had an attack of malaria, in its native village, coinciding with an attack in one of the children of its Uitgeest
* Jrl. of Hyg. Vol. 38, 1938, p. 70-74.
f See also: P.H. van Thiel, Nederl. Tijdschr. v. Geneesk. Vol. 78, 1934. P- 997-1007-



host. Sexually inactive (fat) shortwings begin to appear as early as the end of July, and the children may have been bitten by one of them. But that is unlikely, considering that most of the mosquitoes found infected before August i6th were carrying mature ova.
Malaria in infants (already mentioned on p. 46) furnishes evidence to the same effect. As a rule it is rare in this country. lts incidence amounts to no more than one eighth to one twentieth of that in children over one year of age. The almost exclusively autumnal transmission of malaria and the long incubation make it easy to understand this condition *. We have collected data regarding malaria in seventeen children under one year: Eleven had their attack after they had lived through a complete season of malaria transmission; five feil ill with malaria in the midst of their first season of transmission, consequently their attack followed after a short incubation; one had malaria before it had ever experienced a season of autumnal transmission.
One case in seventeen does not impress us with the importance of summer infections. We must admit that they exist, but they should not influence our activity in the field of malaria control by speciessanitation.
Transport of malaria by anopheles
We have discussed the movements of infected anopheles, with the object of showing that their
* Festschrift Nocht. 1937, P- 620-624.



dispersal during the autumn closely corresponds with the dissemination of human malaria in the next year. In this section we wish to describe a case which clearly brings out all stages of the transport of malaria by mosquitoes, from a focus of anopheline infection to another house.
Family A, two parents and five children from a non-malarious area, settled in Uitgeest on July ióth 1935. On October i8th none of the four schoolchildren had enlarged spleens, none carried parasites. Their house was twenty-seven yards away from house B, a focus sheltering numerous infected anopheles on September 3oth. On November i2th anopheline infection had completely disappeared in house B. On December i2th house A was found to shelter three infected anopheles carrying none but degenerated sporozoites. They could not have arrived from any other place but house B, since there was no other house near producing infected mosquitoes. Moreover, they must have invaded house A some time before November i2th, because on that date the stock of infected anopheles in house B was exhausted. At that time their sporozoites were probably normal, because they are normal in most anopheles in the beginning of November (fig. 16, p. 129).
The father in family A was the first to suffer by the presence of these infected mosquitoes: he had malaria on December igth, after a comparatively short incubation. The other members of the family must have been infected at or about the same time. They feil ill in the course of 1936, after the usual long incubation. Three became healthy parasite-carriers.
Malaria. r T



No anopheline infection was detected on repeated visits to this house in July and August 1936. But on September uth we found eight mosquitoes carrying mature oöcysts. If it had not been for the process of spraying, to which it was repeatedly subjected, the house might have developed into a regular focus of anopheline infection.
This family history shows both what the anopheles do and what they leave to man. Anopheles take the malaria parasites to a house during the autumn, and hand them over to the human inmates. But then their part is finished, and the stock of sporozoites which still remains in their salivary glands is seen to degenerate. It is now the róle of the human carriers to take sole charge of the parasites for the whole of the winter, spring, and summer, until the next autumn. Then the same human carriers return some of the parasites to anopheles; not to the original vectors — they have been dead months ago — but to a brood some four or five generations younger. We mention this point in order not to spoil the beautiful clearness of this case, although it really belongs to another section. Incidentally, we may notie e that this is a repetition by nature of our experiments on long incubation. Here it is carried out with three infected anopheles and seven human beings, one of whom falls ill after a rather short incubation.
The short flights (as we called them on p. 111 to distinguish them from the long flights of sexually active anopheles) of the infected mosquitoes, together with the existence of definite foei of anopheline infection, explain the focal character of malaria



in the villages of this country, mentioned on p. 44. If the foei of anopheline infection are limited to some parts of the village, the dispersal of malaria is patchy, as represented on fig. 6 (p. 45). But if they are present in many parts, malaria appears to be evenly distributed through the village, although it really is as focal as in the first case.
A special case of the transport of malaria by anopheles, which is by no means uncommon, is that of the infected anopheles remaining stationary but of a human family moving into the house in which the mosquitoes are sheltering. The case mentioned on p. 155 is an example of this kind.
Since infected anopheles occasionally occur among the sexually active females of the earlier half of summer, there always exists the possibility of their carrying sporozoites a long way, even to the extent of alighting in a neighbouring village. Considering the scarcity of these summer infections, and the extreme likelihood of the infected vagrant alighting in a stable, and not in a house, of the new village to which it found its way, we feel reluctant to attribute any practical importance to this theoretical possibility. We do not need it as an explanation of malaria travelling long distances, and so we had better leave it altogether out of consideration, especially as it could not in any way modify our practical activity. Moreover, we know that the idea of infected anopheles travelling all over the country is extremely dangerous, once the general public gets hold of it. It is so convenient to lay the responsibility of one's own malaria to one's neighbour's door and to do nothing oneself. And once one admits



the notion of infected anopheles travelling long distances (by flight or by conveyances of any kind), anybody, no matter how far away, may be inculpated. Our principle has always been "Malaria is a local disease, to be dealt with by local efforts" *. Like any other principle, we are prepared to relinquish it at a moment's notice, if forced to do so by irrefutable scientific evidence; but until that evidence is available, we shall continue to act on it.
* Jrl. Med. Ass. Sth. Africa. Vol. 5, 1931, p. 418.



CHAPTER VIII
Transmission of the malaria parasite from
man to mosquito Róle of the healthy carrier
Since anopheline infection can never be of any practical importance previous to the month of July, because of the scarcity of the mosquitoes before that time, we distinguish between spring patients falling ill with malaria before July ist, and summer patients falling ill from July ist till December 3ist. Summer patients, suffering from a primary attack of malaria or from a relapse, are the only ones likely to infect anopheles. On the whole, these patients are treated without delay, if they are members of a sick-club. So the mosquitoes must be fairly quick about it, if they are to become infected by taking these patients' blood.
Beside the malaria patients there exist healthy carriers. We do not know that they are really healthy and, for our present purpose, we do not care either. All that matters is that they do not feel sufficiently ill to call in their physician, although they can do so as often as they like, being members of the sick-club. Their number may be quite considerable on occasion. Among a group of 264 persons *,
* Proc. Roy. Acad. Science Amsterdam. Vol. 40, 1937, p. 368-374.



78 of whom had had malaria during the year, 103 healthy parasite-carriers were detected. Out of these, 67 had no malaria during the whole year, the others had, although they were quite well at the time their blood was examined.
The number of parasites these persons carry varies greatly, both with regard to the individual examined and to the time the same individual is tested. Hence, the following figures serve no other purpose than to convey an idea of how many parasites healthy carriers may be harbouring in their blood on a certain day, without being ill. Out of 103 parasitecarriers:
1 had 50 parasites per xoo leucocytes including male gametocytes 7 „ 12-25 parasites per 100 leucocytes, with male gametocytes in 3 4 >• 6-9 „ „ 100 „ „ „ „ „ 1
i° .. 1- 5 » 100 „ „ „ „ „ 3
22 >. 1- 5 >. „ 1,000 „ „ „ „ „ 5
59 .. 1- 5 » .. 6,000 „ „ „ „ „ 1
Healthy parasite-carriers are slightly more numerous in children than in adults. In the example quoted here there were 24 carriers among 80 adults and 79 among 184 children, a proportion of three in ten among adults and of four in ten among children.
These healthy carriers are of considerable importance, because nobody is aware of their existence; so they receive no treatment and anopheles taking their blood become infected.
In the laboratory we could infect our shortwinged Anopheles maculipennis by allowing them one feed only, on patients suffering from general paralysis, undergoing a malaria cure, who had entered upon their last, non-febrile, stage of malaria infection. They were carrying one to fifteen parasites (Plas-



modium vivax, Madagascar strain) per 100 leucocytes. The male gametocytes never numbered more than one in 3,000 leucocytes, of ten they were so scanty as to escape detection. Still the rate of infection in anopheles was 100 in 130, or 77 per cent. Even carriers with no more than one to six parasites in 6,000 leucocytes, no male gametocytes being found (although they must have been present of course), succeeded in infecting 4 anopheles in 123 after one meal of blood.
Observations on anopheline infection in nature bear out the result of these laboratory findings, as exemplified by the following cases.
In a family who had suffered from malaria in 1934, no more illness occurred in 1935. But three children were healthy carriers, of one to six parasites per 100 leucocytes, in the spring and autumn of that year. In the autumn of 1934 thirty-five anopheles were found infected. None were detected in the summer of 1935, but in the beginning of September they reappeared to the number of thirtynine. Thirty more were found in November and fifty-five in December.
In this instance no malaria had occurred in the family during the whole year of observation. To our repeated questioning as to the state of health of the parasite-carrying children we always got the answer that they never came home except for their meals and at bed-time, so there could not be anything amiss with them. We call particular attention to this answer, because it is in perfect agreement with our observation that there is more fever detected among adult carriers than among children who belong



to that group. This may mean that a child who is a carrier is less liable to have an attack of fever than an adult. But we believe the true explanation to be this, that a slight fever is more likely to escape attention in a child than in an adult, especially if so little attention is paid to the children as in the example mentioned above. And that example, we should add, is by no means an exception. In poor families with numerous young children we may even go so far as to say that it is the rule.
The case remains practically the same as that quoted above, if malaria occurs in the family during the year of observation, provided it is does so bef ore July ist. That was the position in a family who had had two cases of malaria on May 5th and 6th 1935, but not later in the year. They developed a heavy anopheline infection in the autumn of that year: 335 mosquitoes infected as a result of two of the children having become healthy carriers.
The following family history is similar to the preceding one, except for the two spring patients falling ill on April 28th and June 6th, instead of in May, 1935. In that year there were two healthy carriers among the five children. In 1934 four out of seven inmates had been suffering from malaria; in the autumn of that year 116 anopheles were found infected. No infected anopheles were found in April, May, and June, but four of them reappeared on September uth, 3 months after the last case of malaria had occurred in that house. Later on in the autumn their number rose to twenty-two. This rise shows that anopheline infection bore no relationship to the case of malaria in early June.



Finally we quote an instance in which anopheline infection might, at first sight, be explained by a case of malaria on October i6th. But a closer scrutiny reveals the fact that anopheline infection has not followed that one case of malaria, but has preceded it by twenty days. In a family, four of whom had malaria in 1934 and one in 1935 (on October i6th), there were two healthy carriers among the children. Over a hundred anopheles were found infected during the autumn of 1934. No infected mosquitoes were detected in February, May, July, and August 1935. They reappeared, to the number of ten, on September 2óth. Next came the one and only case of malaria on October i6th, and five days later twenty-seven infected anopheles were found, seven with sporozoites. Even this rise in the anopheline infection cannot be accounted for by the case of malaria in October; it occurred much too soon for that.
These instances may suffice as a proof of the great importance of the healthy human carrier as a source of anopheline infection. The foliowing shows that they are actually more important than the patients suffering from an acute attack of malaria, although the latter harbour many more parasites in their blood than the healthy carriers.
In the village of Uitgeest 92 houses were continuously kept under observation during the year 1935. Ih some of them healthy carriers were living, in others summer patients, still other houses harboured both. According to the presence or absence of healthy carriers these 92 houses fall into two groups (see fig. 19, p. 171).
A first group of 43 houses with healthy carriers



among their inmates, 83 in all, or about two per house.
A second group of 49 houses where healthy carriers were absent.
The first group, of 43 houses with healthy carriers, contained 23 houses in which summer patients were living to a total of 48, or about two per house in one half of these houses.
The second group, of 49 houses without healthy carriers, contained 33 houses in which summer patients were living to a total of 75, or about two per house in two thirds of these houses.
So the number of summer patients per house is the same in both groups, but the number of houses harbouring them is greater in the second group. Supposing the summer patients to be the only source of anopheline infection, we may expect to find infected anopheles in equal numbers in both groups of houses, perhaps a few more in the second group, as it comprises a greater proportion of houses with two summer patients. If, on the other hand, the healthy carriers are the only human beings responsible for anopheline infection, we ought to find infected anopheles in the first group exclusively.
Neither of the two suppositions was confirmed by the actual findings of infected mosquitoes. But the second one, incriminating the healthy carriers, almost hit the mark. During late summer and autumn of 1935, we captured a total of 1,329 infected anopheles in these 92 houses; 1,279 (or 96 per cent of the total) were captured in the 43 houses with parasite-carriers, half of which were harbouring an average of two summer patients; 50 (or 4 per cent of the total) were captured in the 49 houses without



Fig. 19. Infected anopheles in houses with healthy carriers (right section) and without healthy carriers (left section), but with summer patients in both. White or shaded columns: number of houses; the shaded portions indicate the number of houses with an average of two summer patients. Black columns: number of infected mosquitoes caught in these houses.
parasite-carriers, two thirds of which were harbouring an average of two summer patients.
All this may seem difficult to believe: Surely, a person with numerous parasites in his blood, with quite a number of gametocytes among them,



is better able to infect anopheles than one with few parasites and fewer gametocytes? Of course he is, so long as man does not interfere by giving drugs. But he does interfere, and so effectively at that, that the malaria patients simply have no chance to infect anopheles. So there remain the healthy carriers as the only source of anopheline infection, and the above figures are there to show that they are quite successful in infecting numerous mosquitoes.
The human habitation as a centre of malaria infection
It is the healthy carrier who, by maintaining a source of infection for anopheles, causes the house he lives in to become a focus of anopheline infection. There can be no doubt that the spraying of such a centre of infection with suitable insecticides, for the purpose of killing infected anopheles, is a true measure of malaria control *. But this holds good only under special conditions, depending upon the biological characters of the shortwinged Anopheles maculipennis. Healthy carriers have been shown to work wonders, no doubt, in the way of infecting anopheles. But, after all, they are a very poor material for malaria to carry on with. Malaria could never succeed in this, unless there were numerous
* Chagas (Ztschr. f. Hyg. u. Inf. Krankh. Vol. 40, 1908, p. 321-334) initiated this method in Brazil, by fumigating houses with sulfurous vapours once a week. Later on, le Prince and Orenstein (Mosquito control in Panama, 1916, p. 207-217) tried mosquito catching and found it very successful. So did Schüfïner in Sumatra (Communie. Civ. Med. Serv. Netherl. Indies, year 1919, no. 3, p. 65-88). Finally, Park Ross reported on highly satisfactory results of "hut-spraying", twice a week, among the natives of Zululand {Buil. trimestr. Org. Hyg. Soc. d. Nat. Vol. 5, 1936, p. 124-146). Our own experience in this field will be detailed in the last section of chapter nine.



anopheles ready to fill the houses by the end of summer. That is a condition our shortwings, bred in salty water, meet in an admirable way. In late summer the breedingplaces may truly be said to rise to the situation. At that time they produce the huge numbers of adults to stock the human habitations with a profusion of sexually inactive adults, even in the face of stabular deviation which claims still greater numbers.
Moreover, the healthy carriers could never have taken over their part without the help of the peculiar habit, the shortwings develop in late summer, which we have called gonotrophic dissociation: the habit of continuing to take blood without the consequent ripening of the eggs (p. 99). This habit causes anopheles to stay on, and to feed, in the shelters they select at the time their sexual activity is coming to an end, from the end of July or the middle of August onward. If these shelters are human dwellingplaces inhabited by healthy carriers, anopheles, by their continuous presence, are in a position to feed repeatedly on the human carrier. Repeated bloodmeals taken on such a carrier are an essential condition for anopheles to become heavily infected, since the number of parasites in the blood of the healthy carrier is subject to considerable variations. Anopheles sharing a house with a healthy carrier, i.e. being present in that house every night, cannot fail, sooner or later, to feed on the carrier at a moment his parasites are on the rise. The following is an example of a rise in the number of Plasmodium vivax in the blood of a patiënt (suffering from general paralysis and treated with malaria) during the last,



afebrile, stage of his infection. It shows the effect of this rise on the number of anopheles infected after taking his blood.
This "healthy" carrier had two plasmodia in 2,000 leucocytes, apparently without male gametocytes. He infected three anopheles out of 81 taking his blood on that day. A week later he had 29 plasmodia in 200 leucocytes, still apparently without male gametocytes, and he infected 19 anopheles out of 30 which took his blood on that day. Three days later the number of plasmodia had risen to 1x2 in 200 leucocytes (with one male gametocyte among them). The batch of anopheles fed on him that day became so heavily infected that it was used to infect patients suffering from general paralysis. As a consequence, the exact rate of infection could not be established, but it approximated 75 per cent. Parasites remained numerous for three more days, then they rapidly disappeared. Except for one day, when his temperature rose to 99.50, a rise which would not have induced him to take medical advice if he had been a free agent, the carrier's temperature remained quite normal all the time.
Only the anopheles which live continuously in the house of a healthy carrier of this kind are in a favourable position to become infected from him, for they are the only ones which, by their habit of feeding frequently on him, are likely to feed during the short period when his parasites have risen in numbers. If our shortwings behaved like theFarEastern Anopheles maculatus, which never stays in a house but only enters it to feed and to acquire (or to transmit) a malarial infection in the act, spraying



of houses with the object of killing infected anopheles would be useless. This difference in behaviour between some malaria vectors may be put in other words by distinguishing between house-born malaria, transmitted by anopheles which do not only visit the house but stay there permanently after becoming infected, and field-born malaria, transmitted by anopheles which never stay in a house for any length of time.
In house-born malaria spraying is useful, but in field-born malaria it is not. Anopheles causing house-born malaria can infect themselves on a healthy carrier because they share the house with him and so they can repeat indefinitely their bloodmeals, one of which is sure to be an infecting meal. Anopheles causing field-born malaria require a more potent parasite reservoir than the average healthy carrier, since they never really share a house with him. As a consequence, they rarely have the opportunity of biting him more than once, and so these anopheles cannot meet the principal requirement to become infected by a healthy carrier, viz. often repeated feedings.
Healthy carriers may play an important part in malaria transmission. But not necessarily or inevitably; and whether they do so or not depends on the biological characters of the particular species of anopheles which acts as the local malaria vector.
* * *
We have said (p. 172) that the healthy carrier causes the house he lives in to become a focus of anopheline infection. Of course this is only partly



true, since no parasite-carrier can convert a house into a focus of anopheline infection, unless the house has been stocked with a great profusion of shortwinged anopheles at the time their sexual activity ceased. And whether that will happen does not depend upon the healthy carrier, but upon the capacity of the country to breed and to feed shortwings, upon the construction of the house, and on the number and habits of the people living in it. In fact, his remaining a parasite-carrier at all largely depends upon the last two factors. It depends on the faulty construction of the house, on its being overcrowded, and on the uncleanliness of its inmates, since these factors promote the reinfection of the healthy carrier by allowing anopheles to take up their permanent autumn-quarters in that house. It depends on the overworked condition, and the ensuing carelessness, of the parents in large, and consequently poor, families, since these factors are the cause of the little attention paid to the state of the carrier's health and to minor ailments which, otherwise, might have led to his being detected as a source of infection. It is the combination of all these conditions which renders a house a focus of anopheline infection and, as a consequence, a focus of human malaria. The term "Housing and Malaria" * is not the
* A problem raised for the first time, we believe, by Chagas (see footnote on p. 172). It did not really come to the fore, however, until James (Malaria at home and abroad, 1920, p. 85-88, 93) put it most clearly and forcibly to the public attention. Since then James' views have been criticized by Hackett and Missiroli (Transact. Roy. Soc. Trop. Med. Vol. 26, 1932, p. 65-72), defended by Claytonj Lane (Publ. Soc. Nat. III, Hygiene, 1931, III, 6), and exhaustively discussed in an admirably detached spirit by Christophers and Missiroli (Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 2< J933. P- 379-516)-



name of a problem in this country, but of an existing condition. A condition, however, depending on factors which do not exist everywhere. There are some who believe they are nowhere to be found but in northwestern Europe. They may be right in that belief. Even if they are wrong, it remains true, nevertheless, that a house can never claim the status of a focus of malaria unless in the presence of an insect vector whose habits resemble those of our shortwings.
Transport of malaria by man
Anopheles, infected during the autumn, can be relied upon to transport malaria over short distances within the precincts of one village (p. 161). Ofcourse, a human carrier, coming to occupy a house in another part of the village, may cause the same effect, if he succeeds in infecting mosquitoes in his new quarter. But we principally look to him for the explanation of the transport of malaria over long distances. In the following case he was actually up to our expectations.
In a hamlet of 64 inhabitants living in sixteen houses, the seventeenth house came to be occupied by the ten members of family K, on November 8th 1933- Before, they had lived in another village, where nine of them had had malaria. In their new abode four had malaria and three were parasite carriers since November 1934.
In house K we found 107 infected anopheles in the autumn of 1934, and 44 in the autumn of 1935. In the autumn of 1934 we captured all the anopheles we could find in every house of this hamlet. We detected one infected anopheles in house S, and
Malaria. I2



another in house WR, both 60 yards distant from K. Three more were detected in house G, next door to K. As no malaria had occurred in these houses for at least five years, we believe we are justified in concluding that their infected mosquitoes came from house K.
The subsequent history of these houses was as follows:
There never was any malaria in house S.
Two inmates of house G had malaria in 1935, three others did not fall ill till 1936. Probably all of them were infected in 1934, as we never found a single infected mosquito on repeated visits to house G in August, September, and October 1935.
In house WR malaria was still further postponed, as it did not make its appearance, in one patiënt, till June 1936.
Malaria occurred in three houses where no infected anopheles had been detected in the autumn of 1934 and 1935. Three inmates of house JR, next door to house WR, had malaria in June 1935; three others had malaria in house Sm, situated between house WR and K, in July 1935 and October 1936; one case occurred in July 1936 in house M, 44 yards distant from K.
Consequently the immigrants had caused malaria in six and, probably, in another se ven of the fortyeight neighbours living at a distance of seventyone yards or less from house K. It is worth noticing that no malaria occurred in 1934 (except in the immigrants), the first year after the family K's arrival. The explanation is that they settled in the hamlet on November 8th 1933, too late in the season



to start anopheline infections, as anopheles rarely become infected after October 3ist (p. 133).
Anopheles remained numerous in family K's house in the autumn of 1936, but none were found infected. Neither was this the case in 1937. Anopheline infection had not succeeded in establishing itself permanently in any of the other houses of the hamlet, not even in house G next door toK. Hence, thewhole of the malaria in the hamlet hinged on the production of anopheline infection by family K. When that failed, malaria had to disappear. It actually disappeared in 1937,the year following upon the cessation of anopheline infection in house K. Infected anopheles disappeared notwithstanding the fact that three children continued to carry parasites in November 1936. These healthy carriers had evidently lost their power to infect anopheles.
We have seen * more cases like this, of parasitecarriers, after one or two years of success, quite unexpectedly failing to infect anopheles. This phenomenon is probably to be explained by a local population of parasites losing their vigour from always having to circulate among the same group of healthy human carriers living in the same house, with never a change, except for a short interlude of insect transmission once a year. This assumption is supported by the experience in a house, a focus of anopheline malaria in 1935, where mosquito infection never revived in 1936, although there were three parasite-carriers and four malaria patients. But in another house, to which the malaria parasites
* Jrl. of Hyg. Vol. 38, 1938, p. 62-74.



from the first house had been transplanted in 1935 and where some of the patients had newly developed into healthy carriers, anopheline infection flourished in 1936. It probably was the change to fresh healthy carriers entailed by this transplantation, which made all the difference between the new house and the old one.



CHAPTER IX Malaria control
The chief difficulty with malaria in this country is that it rarely kills. So it is not a major health problem calling on all available resources to deal with it; but neither is it a mere curiosity that can be simply dismissed on the plea that every malaria patiënt is adequately treated. The case (p. 155) of a child, remaining a carrier for years before it was treated, gives a fairly accurate picture of what is happening daily in our malarious areas. It shows that malaria in the Netherlands is of more consequence than meets the eye. But it does not allow of assessing the damages in terms of money.
The facts we dispose of to meet these difficulties are the habits of the insect vectors, the shortwings. They are: (1) Their preference for salty water which causes their extreme abundance in the salty coastal provinces. (2) Their gathering in great numbers in human habitations by the time sexual inactivity commences. (3) Their habit of gonotrophic dissociation (p. 99) during that time, which causes them to feed on the healthy carriers in the houses called foei of anopheline infection and so to grow a surprisingly rich harvest of sporozoites. (4) Their habit of taking short flights (p. m) during that time, which enables them to carry some small portion of this harvest to neighbouring houses.



Various attempts at malaria control have been, or are being, made in this country. Some of them simply ignore the shortwings as a special tribe; they only know of anopheles. Others aim at control by speciessanitation, the term taken in its wider sense of any anti-malaria measure finding its rationale in the existence of the shortwings as the sole and only vectors of malaria in this country. We should add, however, that those who attempt species-sanitation are not all aware of the fact; but it is speciessanitation all the same.
Malaria control by means of drugs without regard to the species of the insect vector
In chapter two (p. 26) we arrived at the conclusion that the incidence of malaria has greatly decreased in the course of the nineteenth century, and that there are many reasons for explaining this by a more efficient and more general treatment with quinine salts.
But the incidence of malaria has not continued its decrease. It has descended to a lower level, but there it has stopped. Further improvements have not been achieved. Why is that so? Do not all who complain of malaria receive adequate treatment whether they can pay for it or not? They do, most certainly, yet there are fresh malaria cases every year.
We well remember an exposé on malaria in the province of North-Holland discussed at a meeting of the malaria commission of the League of Nations. The graphs of the annual incidence of the disease, with high peaks in some years (p. 36), caused one



of the members to express as his opinion: "ces médecins n'ont pas fait leur devoir". He refused to believe that malaria could continue, as it did in North-Holland, unless the physicians neglected to treat their patients as they ought to be treated.
It all depends on what is to be understood by "the duty" of the general practitioner. We hold that he neglects nothing that can be reasonably expected of him, if he is always ready for anybody who seeks his aid, and if he does everything within his power to ensure that his directions are properly followed. We may expect him to visit his malaria patients repeatedly during the first week after their attack of malaria, to see to it that they take quinine. We are not sure that we can expect this vigilance to continue during the next week, and we certainly cannot during the third. A treatment continued for many weeks is a fiction. The general practitioner has no means of knowing whether the drug is taken or not, unless he can spend all his time on running after his patients. But then he is no longer a physician but a specialist, and his patients will clamour for another doctor to supply their needs. The requirement to be always ready for all who seek his aid assuredly means more than simply to provide any person who believes he is suffering from a relapse with a fresh box of pills. The relapses require as much attention as the primary attacks. But we cannot expect the physician to be about importuning his former malaria patients with injunctions to watch themselves, and to come to him as soon as they feel in the slightest degree



unwell. A well-meaning physician will refuse to do so, on the plea that he will either ruin his own practice or his patients' nerves.
The villagers claim the right to feel reasonably ill before they call in the doctor, even if they can consult him free of charge. "I'll let you know when I am ill" is a saying as common among panel patients as among paying patients. Any system of malaria control which neglects this home-truth is bound to be a failure in this country.
Quinine in the Netherlands has failed to reduce the incidence of malaria beyond a certain limit, because people do not go to the doctor unless they feel they need his advice, and because there are many who do not feel this need and who, nevertheless, continue to carry plasmodia in their peripheral circulation. As we observed before (p. 165), it is quite irrelevant to the problem in hand whether these carriers are really healthy or only apparently so. What matters is that they feel in good health, and so they are up and about carrying plasmodia without taking quinine.
Still, these healthy carriers would not be of any practical importance if they did not infect anopheles. But we know they do, experimentally as well as in the field. In chapter eight we have quoted several instances of this happening. They make it amply clear that the local physician could not possibly have been aware of the existence of healthy carriers in a family where no malaria occurred during the year, unless he was in the habit of repeatedly examining the blood of all the inmates of the house. This supposition could only arise in the brain of one who



has not the slightest notion of the claims a rural practice lays on the physician.
Nobody, we trust, will charge him with neglect of his duty when he carefully treated two patients in May, saw to it that they continued taking quinine for a fortnight after their recovery, but then left them alone, trusting they would call him in again if anything were amiss. But nobody came to him and when, at the end of the year, he wanted to examine two healthy carriers, we had found in the same family, to discover any hidden blemish in their constitution, he encountered the greatest difficulty in persuading them, and their parents, to submit to the examination.
Preventing the sick from being a source of anopheline infection, that is what systematic quinine treatment has done in this country. But the healthy carriers could not be prevented from taking the patients' place; and they have managed so effectively that, at present, malaria transmission wholly depends upon them.
* * *
The reappearance of vivax parasites in the prospective parasite-carrier is nothing but a relapse which, however, is left untreated because it is not accompanied by fever. It is a parasite relapse, not a fever relapse. If we could prevent these parasite relapses, or if we could render them noticeable by turning them into fever relapses, our problem would be solved. We have no means of carrying out the latter object. But the prevention of relapses does not seem so impossible as it did thirteen years



ago. Then we had quinine only; at present plasmoquine and atebrin have been added to our arsenal. True, these synthetic drugs are reported to prevent fever relapses, and what we aim at is to prevent parasite relapses. But Sinton * has shown that the combination of plasmoquine and quinine reduces the number of parasite relapses as much as that of fever relapses, and it is reasonable to assume that the same holds for atebrin.
With the help of the three physicians in Wormerveer, de Groot, Lampe, and Ris, we have investigated the influence of the new synthetic drugs on the rate of anopheline infection in the daily routine of rural practice.
Our collaborators were ready to vouch for their directions being followed to the letter, if the treatment was a simple one (one kind of drug only), and if it did not last for more than a fortnight. But they were sure that an extension beyond that limit, or a complication of the treatment by prescribing first one drug and then another, would vitiate the results. So we had to abandon the three weeks' treatment with quinoplasmine, and to content ourselves with the fortnightly treatment. As to atebrin, the seven days' treatment was chosen, and the more complicated ones (one or two courses of atebrin followed by an equal number of shorter courses of plasmoquine) had to be discarded. A simultaneous administration of the two drugs was not to be thought of, in view of the risks attending such a course.
* Ind. Jrl. Med. Res. Vol. 17, 1930, p. 793.



Treatment with quinoplasmine for a fortnight (adult daily dose fourteen grains of quinine and half a grain of plasmoquine) has been the only treatment of malaria in Wormerveer since 1934, except for 1936 in which year every malaria patiënt was treated with atebrin for seven days (adult daily dose four and a half grains).
* * *
The treatment with atebrin had to be abandoned after a full year's trial, although this drug proved itself an adequate remedy, because it acted too slowly in reducing the fever. The patients are accustomed to have their fever stopped at once; an attack after the course of treatment has commenced is a rarity (except, of course, in a case of Korteweg's initial remittent). Not so, however, in atebrin treatment; one, two, and even three, attacks are far from uncommon. Patients will not stand this, and so the Wormerveer physicians were forced to abandon atebrin.
If the relapse-rate, compared with the relapserate after quinine treatment, had been greatly reduced, this might have been regarded as a sufficiënt compensation. But the relapse-rate was only slightly less than after quinine. In 1936 eighty-five persons were treated with atebrin; thirty relapsed in the course of that year. In the same year the two physicians Beeker and Brugman, in the neighbouring village of Uitgeest, treated 452 patients with sulfate of hydroquinine (adult dose fourteen grains) for eight days. Out of these, 168 relapsed in the course of that year.



Hence, the relapse-rate over one year was thirtyfive per cent in Wormerveer after a seven days' treatment with atebrin, and thirty-seven per cent in Uitgeest after an eight days' treatment with sulfate of hydroquinine. The difference is so slight as to be practically of no account.
As to the effect atebrin treatment has in preventing anopheline infection, the following observation may be quoted as an example.
In a family of four adults all the members had malaria in 1936. They all received atebrin treatment and none of them relapsed in the course of that year. Two of them became healthy carriers in the autumn; one was found to harbour twenty-two plasmodia (one male gametocyte among them) in 500 leucocytes, the other three plasmodia in 6,000 leucocytes. The family-physician went to the length of visiting them more than once, but he could do nothing as they scouted the idea of being ill and of having to take medicine. During the autumn of that year we captured 113 shortwinged maculipennis in their house, twenty-nine of which were found infected, twenty-five with sporozoites in their salivary glands.
* * *
Quinoplasmine treatment of a fortnight's duration appeared more likely to reduce anopheline infection, for it considerably reduced the relapse-rate in the village of Wormerveer.
Out of 382 persons, treated with quinoplasmine for two weeks, forty-eight relapsed during the year they were treated and thirty-nine during the next
.



year: a relapse-rate over the first year of twelve per cent and over two years of twenty-two per cent. Dr. Piebenga obtained similar results with a fortnightly quinoplasmine treatment of patients who acquired a natural malaria infection in the mental hospital of Franeker (province of Friesland). Out of 119 patients, thirty (twenty-five per cent) relapsed during the year of the treatment or during the next *.
Quinoplasmine treatment at Wormerveer has always been continued for two weeks since 1934, it has reduced the relapse-rate, and so we may ask what is the effect of this reduction. In other words: Where are more infected anopheles to be found, in a village where quinine is the only drug or in one where quinoplasmine is used?
To answer this question we quote the foliowing rates of anopheline infection, in October, in the villages of Wormerveer (quinoplasmine) and Uitgeest (quinine):
1934 r935
Wormerveer 151 out of 1,277 = 12 pet. 60 out of 695 = 9 pet. Uitgeest 53 out of 372 = 14 pet. 288 out of 2,941 =10 pet.
There exists a slight difference in favour of Wormerveer, but it is so insignificant, and the rate of anopheline infection is so high in that village, that we cannot attach any value to it.
With regard to the prevention of anopheline infection we are no better off with the new drugs
* This is the final outcome of observations continued for four years. The result of the first year (P.J. Piebenga, Nederl. Tijdschr. v. Geneesk. Vol. 76, 1932, ist half. p. 1564-1578) was much better, but it did not last.



than we were with the old ones. We certainly value the reduction in the rate of fever relapses, obtained with the aid of quinoplasmine, for the sake of the individual patients. But it is evident that parasite relapses continue in numbers sufficiënt to maintain an anopheline infection which is hardly inferior to that found among a human population treated with quinine salts only.
* * *
The three weeks' treatment with quinoplasmine, in general practice, is of too long duration to be sure of its being regularly taken by all patients. But in the mental hospital of Franeker Dr. Piebenga inaugurated the three weeks' treatment with quinoplasmine in 1934 and his successor, Dr. van Andel, continued it till the end of 1937. Out of 115 patients, treated in this way, two relapsed during the year they received their treatment and eight during the next year, a total of ten per cent in two years. This result is better than any of the preceding ones. Unfortunately the trial came to an untimely end as a consequence of gastric complaints occurring rather frequently during the third week of the treatment. Hence, they were, not unreasonably, ascribed to that extra week of treatment. As a consequence, it is generally feit that it would be unsafe to recommend the three weeks' treatment in general practice.
Malaria control by means of drugs with due regard to the species of the insect vector
The conditions permitting malaria to hold its own in this country, with no better support than



the healthy carriers, have been discussed in chapter eight. Malaria which is able to carry on under these conditions cannot be influenced by drugs, unless all carriers swallow them during the whole time malaria infection can pass from man to mosquito, i.e. from the middle of August until the end of October.
That is an experiment which Dr. P.C. Korteweg * tried once, in 1902, in the village of Wormerveer. At any rate he came near it, although on the one hand he did not do quite enough, since he neglected the healthy carriers in the supposition that they did not exist, and on the other he did too much, by watching cases of malaria in spring and early summer as carefully as in late summer and early autumn. His object was to eradicate malaria by treating all the sick with a daily adult dose of fifteen grains of sulfate of quinine for eight to ten days. After this, he prevented his patients from relapsing by making them take fifteen grains of sulfate of quinine every eighth and ninth day. He continued in this manner until the malaria season was well over, i.e. till the first of November. The direct result was frankly discouraging, although relapses were comparatively few in number during the preventive treatment. After it had been stopped, however, no fewer than 85 out of his 541 former patients came down with fever during the months of November and December, at a time when, without man's interfering, relapses are extremely rare. But in the light of more recent knowledge, Korteweg t
* Deutsch. Mediz. Wochenschr. 1903, no. 46 and 47.
t Versl. en Meded. betr. Volksgez. Year 1923, p. 200.



realized many years later that, from the point of view of preventing the spread of malaria, he had hit upon an extremely useful expedient. He had done nothing less than pilot his patients through the dangerous season when shortwinged anopheles can be readily infected, in such a way as to prevent the patients from infecting the mosquitoes.Theirrelapsing in November and December was of no consequence in this respect, because, as we know now, fresh anopheline infections rarely occur in these months (P- 133)- Consequently, the after-effect of his efforts, which did not show till the next year and so counted for nothing in those early days, was by no means a failure. In 1902 541 persons had suffered from malaria, in 1903 111 only. It is less than what can be obtained, at present, by means of house-spraying (p. 216). This is due, no doubt, to the fact that Korteweg's method was based on the assumption that anopheles are solely infected by taking the blood of people who are actually ill with malaria. Of course, many healthy carriers had been malaria patients during the same year, and these Korteweg kept under treatment. But many others (more than three fifths of them in the case mentioned on p. 166) had not been ill with fever in the course of the year. These Korteweg did not treat, and so he was bound to neglect families, like the one mentioned on p. 167, who infected large numbers of mosquitoes.
Nevertheless, it is a reduction justifying the strenuous efforts which were put in motion to obtain it. Unfortunately, these efforts are useless without the whole-hearted coöperation of all people concerned. Korteweg had succeeded in securing it for that one



year. Neither he, nor any other local practitioner, ever repeated the experiment.
The method might be simplified by limiting the actual "piloting" to the period during which anopheles acquire fresh infections, i.e. from the second half of August till the end of October. On the other hand, the group of subjects to be piloted ought to include the healthy carriers. This requirement might be met in practice by adding to the malaria patients of the current year those of the preceding year, without having to go to the length of actually finding out the healthy carriers by blood examination.
Malaria control by antilarval measures without regard to the species of the insect vector
With the financial aid of the International Health Division of the Rockefeller Foundation rather extensive attempts * have been made to protect a small town in North-Holland, Medemblik by name (fig. x on p. 2), from the inroads of anopheles bred in the ditches draining the polders around the town. This antilarval campaign was continued through the years 1926-1931 t.
The town of Medemblik was situated on a triangular peninsula. Two sides of it bordered on the Zuydersea, the third connected it with the mainland. Three quarters of the area of a circle, with a radius of nearly two miles around the town, were covered by sea; one quarter only was land. Hence, this was
* The greater part of the researches referred to in this book have been carried out with this same financial aid.
f Nederl. Tijdschr. v. Geneesk. Vol. 78, 1934, ist half, p. 345-352. H. de Rook, Parijsch groen als anopheleslarven doodend middel. Amsterdam, 1927, Universiteitsboekhandel, 103 pp.
Malaria. r,



a particularly appropriate site for carrying out antilarval operations, since it required no more than one quarter of the work which would have been necessary to protect a town surrounded by land on all sides. The area to be protected contained 906 ditches with a surface of nearly 58 acres. They had to be treated once every fortnight, from May till the end of September, by three gangs of three men doing their work in a boat.
The breedingplaces were dusted with parisgreen or oiled with spindle oil. The parisgreen was applied in the customary quantities * mixed with a substance called "tarras", a finely powdered volcanic rock used for making hydraulic cement. Like Italian road-dust it has the advantage, over other diluents, of being just a shade heavier than parisgreen. Hence, the diluent sinks almost immediately while the parisgreen keeps floating t.
Since dusting has the disadvantage of having to be repeated if a shower follows within twentyfour hours af ter the operation, we used oil wherever possible. In many ditches, however, the water is set in motion whenever the pumps, regulating the waterlevel in the polder, are working. In these circumstances we had to rely on parisgreen, because the non-toxic oil we used acted too slowly in killing the larvae, and so the oilfilm was carried away by the current bef ore it had done its duty. We had to avoid smelling oils, because the dairy-farmers objected to their use. They were afraid the milk would taste of oil if the cattle drank the water of
* A. Missiroli, Riv. d. Malar. Vol. 6, 1927, p. 501-572.
•f Riv. d. Malar. Vol. 8, 1929, p. 34-37.



oiled ditches. So we used spindle oil, a kind of impure liquid vaseline. Giving up smelling oils means giving up toxic oils. Hence, we had to rely on the suffocating power of the oil only, i.e. on the blocking of the tracheae. That process kills the larvae in three to four days. On the other hand, spindle oil does not evaporate, and so it continues to keep the surface of the water free from larvae for a much longer time than kerosene *. Spindle oil is sprayed in the usual way, in quantities of eight to eleven cub.cm. to the square yard. lts price is two and a half times as high as kerosene. So it is in no way to be recommended, unless antilarval activity is hampered by public opinion in the way it was here.
The daily work of the gangs was checked by ascertaining the presence of an oilfilm in the sprayed ditches and of particles of parisgreen in the dusted ditches. These particles were collected with a dipper shaped like a small sieve, the bottom covered by a piece of black blotting-paper through which the water percolated, leaving the parisgreen on the paper, where it could be seen under the microscope. Of course these tests, to make sure that the gangs of oilers and dusters had done their duty, had to be supplemented by dipping for larvae, in order to ascertain that the larvicides had done theirs.
* * *
The result of these operations was measured by comparing the anopheline density in the centre of the area treated by antilarval measures with that
* Buil. Soc. Path. Exot. Vol. 21, 1925, p. 109-112.



observed outside this area. The anopheline density was estimated by the number of adults caught in pigsties situated on the outskirts of the town of Medemblik and at a distance of two miles from this town. Both groups of pigsties were situated in the open field, near breedingplaces of exactly similar description. From 1927 till 1931 the anopheline density outside the controlled area was from four to six times as great as in the centre of the area *.
Could we not have done better than this? We believe not. While we were trying our best to keep anopheles out of Medemblik, nature was trying hers to keep anopheles out of the new Zuydersea polder. We were preventing breeding with oil and parisgreen, nature was doing the same by maintaining an excessive salinity in the ditches during the first years after the new polder had been pumped dry (p. 1x0). We might make mistakes by overlooking breedingplaces, nature made no such mistakes. And yet, her results were no better than ours. In the area of artificial larval control the anopheline density was one fifth (to be exact: one in four and a half) of that beyond the limit of this area. In the area of natural larval control, the new Zuydersea polder, the anopheline density, as estimated by the number of adults caught in pigsties at a distance of two miles or more from the nearest breedingplaces on the mainland, was likewise one fifth* (to be exact: one in four and seven tenth) of the anopheline density beyond the limit of that area. So we conclude that our antilarval measures
* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 3, 1934, p. 449.



had effected as much as could be reasonably expected in the circumstances: the "circumstances" being the long flights of anopheles (p. 107) which wrecked nature's scheme as well as our own.
In any case, the antilarval measures have had a measurable effect on adult anopheles. The effect they had on malaria was estimated by the incidence of the disease among the three hundred inmates of the mental hospital at Medemblik.
After the major exacerbation of 1922 malaria incidence in that hospital remained at the comparatively high level of round about ten per cent. In 1927 it suddenly dropped to a much lower level and has stayed there ever since. It would have been reasonable to charge this lasting improvement to the credit of the antilarval operations, since it commenced in the year following on the first campaign. Unfortunately, this improvement imperturbably continued for six years after the operations had come to a close. So we are not quite sure whether we have achieved anything in the way of malaria control.
Of one thing we are sure, however: antilarval operations of this description, indiscriminately aiming at the destruction of all anopheles, are much too expensive for this country. Deducting all expenses which were not absolutely necessary, we spent four thousand guilders annually to protect from malaria a town of an equal number of inhabitants. That is too much for a disease which rarely kills, and so antilarval measures of this kind, whatever their merits may be, have to be ruled out as a means of malaria control in this country.



Species-sanitation in the control of the larvae
Species-sanitation has never, consciously, been tried to control the larvae of the shortwings in this country. Nevertheless, measures are being planned, or actually put into execution, which are as good as speciessanitation, although they were never intended to serve that purpose.
These measures are the partial reclamation of the Zuydersea. The Zuydersea was a land-locked bay. lts water was not quite so salty as that of the North Sea, because an arm of the Rhine, the river Yssel, discharged into it. Still the salinity reached a half to one per cent.
The reclamation of the Zuydersea aims at two entirely different objects. One is the creation of a fresh-water lake. It has been achieved by the building of a barrage, over twenty-five miles in length, across the mouth of the bay. This barrage has impounded the Zuydersea. Since the river Yssel still discharges into it, the water of this bay has gradually lost its salinity. At present the water of the Yssel-lake (as the impounded Zuydersea has been rechristened) contains no more than 0.03 per cent of salt.
The other object is land-reclamation. It is being achieved by building a dam which encircles a portion of the lake on three sides, the coast of the mainland constituting the fourth. When the dam is finished the isolated part of the lake is pumped dry. In this way a portion of the bottom of the former Zuydersea is converted into a polder. There are to be four of these polders, but as yet only one



has actually been completed. It is the smallest, with an area of about fifty thousand acres, referred to in this book (and on fig. 1) as the "new polder".
The presence of salty water all over North-Holland is causing heavy losses to agriculture, horticulture, dairy-farming, and industry, especially in times of drought. Hence, the presence of a fresh-water lake in the midst of the country is regarded of as much importance as land-reclamation *, since it offers unprecedented possibilities for the freshening of the North-Holland canals and ditches. If these possibilities could be realized, they would, incidentally, influence for the good malaria conditions in North-Holland. Freshening the waters of that province, provided it was carried on sufficiently far (namely to a salinity of less than 0.16 per cent), would prove to be equal to species-sanitation. It would effect a reduction in the number of shortwing larvae to less than one sixth of that in salty water. A reduction of the salinity to 0.08 per cent would bring the larvae down to one twelfth of their number in salty water (p. 124). At the same time, it would not harm the longwings, it would actually stimulate them to increased multiplication. In short, the anopheline population of North-Holland would come to be like that in South-Holland.
The freshening of the water in North-Holland would be no less an instance of species-sanitation because it was undertaken with the object of helping agriculture and not of controlling malaria. The classical instance of species-sanitation, the control
* Report of the Commission Lovink. The Hague 1924, Landsdrukkerij, p. 33 and 48; J. P. Mazure, De Ingenieur. Vol. 48, Sect. A., p. 42-44.



■
of Anopheles umbrosus in Malaya, was the outcome of a purely agricultural activity: feiling and draining the jungle for the purpose of growing rubber *
* * *
We ought not, however, to conceal the fact that this effect, the existence of a fresh Yssel-lake is expected to bring about, has not yet materialized. There are those who believe it never will. The fact is that there are more reasons than one for NorthHolland to be salty. The salty water in the former Zuydersea undoubtedly is one reason, but it is not the only one. Mrs. Wibaut t has pointed to the sea-water entering the locks at the mouth of the two canals, which join the port of Amsterdam to the North Sea, as an important cause of maintaining the high salinity of the North-Holland ditches. Moreover, the deeper strata of the subsoil in that province store salty water in large quantities. It is kept down by the supernatant fresh water. But if this is removed, by pumping a lake dry, the underlying salty water rises to the surface and the resulting polder becomes a salty polder, breeding shortwings §. In fact, it is not too much to say that the malariousness of North-Holland, which is such a marked feature of that province, is largely due to extensive reclamation of lak es and pools (p. 6, 116).
The importance, but also the magnitude, of the
* Sir Malcolm Watson. The prevention of malaria in the Federated Malay States. London. 1921, John Murray, p. 46-76.
"f Nederl. Tijdschr. v. Geneesk. Vol. 79, 1935, p. 722-727.
§ W. A. J. M. v. Waterschoot v.d. Gracht. De recente bouw van de Nederlandsche delta. Amsterdam 1915, Stadsdrukkerij, p. 56; J. G. Bijl. Nw. Verhand. Bataafsch Gen. 2e Rks. Vol. 10, 1930, 60 pp.



task of counteracting this deleterious influence of land-reclamations with the help of the huge reservoir of fresh water, which has now actually come into being, is well realized in our days *. At present, however, there still exist pessimists t who believe that continued land-reclamation in the Yssel-lake will finally defeat its own purpose, by the great quantities of salty water they expect these new polders to produce. We can only hope that these birds of ill omen will be proved in the wrong, like those have been who predicted that the Yssel-lake would never become fresh. But, even if they are, the ditches in North-Holland will never contain fresh water without greater incentives to sustained efforts than the presence of malaria in that province will ever be able to provide. Fortunately, there are the more powerful economie incentives which, in this case, aim at the same object as malaria control. They may achieve what the fear of malaria by itself cannot bring about.
Control of adult anopheles without regard to the species of the insect vector
In 1920-1923 an effort was made, in this country, to arrivé at a wholesale destruction of anopheles by killing the hibernating adults in stables.
The work was carried out in late autumn and winter, a period during which the whole anopheline population consists of females only, hibernating in stables, outhouses, and human habitations. Outhouses were never thought of as places in which anopheles take
* Water, bodem en lucht. Vol. 27, 1937, P- 17~57-
t J. M. K. Pennink, De Ingenieur. Vol. 48, 1933, Sect. A., p. 2-5.



shelter. As to human habitations, anopheles are so numerous in stables that it was believed to make little difference whether houses were included in the work or not. Moreover, the method employed in the killing of anopheles could not be applied in houses, since it consisted in the spraying of a three per cent solution of lysol, which spoils the furniture. Around Amsterdam anopheles were caught with a vacuum-cleaner. That is a more elegant method; still it cannot be used in houses, because the hidingplaces of anopheles are too difficult to reach with that apparatus. So houses had to be omitted, since no method existed to free them from anopheles.
This campaign proved a failure. Even around Amsterdam, where it has been conducted with the greatest energy and care, it was impossible to detect any lasting decrease in the anopheline population *. The immediate results were of course quite satisfactory. A stable which had been sprayed in November remained empty for the rest of the winter. The supposed effect on malaria was satisfactory too, its incidence diminished year by year. The same, however, had been observed af ter the major exacerbation in 1901, without any spraying. Moreover, the decline of malaria after 1922 occurred in all villages, whether they had been well sprayed, badly sprayed, or not sprayed at all.
Species-sanitation in the control of adult anopheles
In chapter seven (p. 151) we summarized the focal occurrence of anopheline infection in North-Holland
* Nederl. Tijdschr. v. Geneesk. Vol. 68, 1924, 2nd half, p. 1113-1125 and 3120-3124.



villages, by saying that nature had crowded all infected anopheles within a very small compass; small with regard to time — the autumn; and to space — the foei of anopheline infection. In doing so, she is actually inviting us to kill them there at one stroke; the opportunity appears so favourable that it is difficult to hold one's hand. Doubts whether anopheles infected in autumn actually cause all, or the greater part, of next year's malaria, and not only the cases arising before the first of June, did, however, cause us to hold our hand. An analysis of the facts led to the conclusion that four-fifths of the malaria, occurring in one year, could be prevented by killing infected anopheles in the preceding autumn. This was the most optimistic estimate, based on the observation that a large majority of fresh cases of malaria occur within an area of a hundred yards around the preceding autumn's foei of anopheline infection (p. 159).
We realized, however, that such an estimate served no other purpose than to encourage us to recommend the method of house-spraying in autumn to the responsible authorities, the "Commission to stimulate malaria control by the population in North-Holland". To them, a satisfactory result would mean the possession of a reliable method of malaria control, to be used while awaiting the reduction of the shortwings consequent upon the hoped-for freshening of the North-Holland ditches. To us, any result, whether satisfactory or not, would be valuable, since it would give us an answer to the question of how much malaria is caused by anopheles infected in autumn.



Killing anopheles in human habitations by spraying was rendered practicable by the use of concentrated extracts of pyrethrum. They are prepared by treating powdered pyrethrum with petroleum aether, in a Soxhlet apparatus, at a temperature of ii 3°. The concentrated extract, which has the consistence of treacle, is used by dissolving half an ounce of it in two pints of white kerosene. This solution can be kept for a long time, provided it is not exposed to light *.
A solution of pyrethrum extract in kerosene can be sprayed in a house without causing any damage to the furniture. Moreover, it not only renders the mosquitoes insensible for a time, it kills them. In a series of experiments we have carefully noted the killing power of home-made pyrethrum extracts and of a number of proprietary products. The homemade extracts, and the better class of proprietary products, all have the same effect in killing the mosquitoes, if the solution of the extract is sprayed in quantities of five cub. cm. to thirty-five cub. feet. "Killing the mosquitoes" means that not one anopheles, out of a hundred collected after each operation, revives after twenty-four hours.
An advantage, the spraying of pyrethrum insecticides possesses over the older method of fumigation, is that the spray acts as a cloud of minute dropiets, and not as a gaz. This cloud does not escape through cracks and crevices, and so it is not necessary hermetically to close them. It is quite sufficiënt to close doors and windows. Neither is it necessary to keep them closed for a long time.
* Nederl. Tijdschr. v. Geneesk. Vol. 78, 1934, P- 2327-2338.



A handpump is the apparatus usually recommended to spray the insecticide. In small rooms it may do very well, but in large ones the person working it feels tired and stops long before the necessary quantity of insecticide has been used. For spraying we used an apparatus built on the same principle as the ordinary handsprayer, but the reservoir was larger, and the liquid was ejected by the force of air at a constant pressure of ten to twelve pounds to the square inch. Such an apparatus ejects a cloud of dropiets over six feet long. In a slightly modified form it is used, at present, for spraying those houses which the inhabitants are unable to deal with themselves.
* * *
Many years ago the discovery that anopheles are only to be found infected in human habitations in autumn and winter * had suggested the idea to have malaria controlled by the inhabitants themselves, each family being responsible for the destruction of anopheles in their own home. Accordingly, the "Commission to stimulate the control of malaria by the population in North-Holland" was called into being. Their task, besides urging the people to seek medical advice and conscientiously to follow it in every case of fever, was to propagate house-spraying. Unfortunately, most people thought it absurd not to spray the house when mosquitoes were troublesome, and to commence with it by the time no more mosquitoes (i.e. no more Culex
* Nederl. Tijdschr. v. Geneesk. Vol. 65, 1921, 2nd half, p. 1486-1488; Vol. 68, 1924, 2nd half, p. 750-763.



pipiens) were about. In this way the habit arose to do the spraying in summer, and to leave it at the moment the proper season for it had arrived.
Another difficulty, the plan had not allowed for, was the internal construction of the attic of the houses which require spraying. Often it is a large room under a high slanting roof, partitioned off into small bedrooms which have their own ceiling, made of a kind of paper not strong enough to stand on. These cubicles do not fill the whole attic. On both sides and above them some space is left, a convenient shelter for anopheles but very difficult of access. These are the kind of houses which become foei of anopheline infection, when occupied by a large family with parasite-carriers among them. We cannot blame the occupants if they give up in despair any attempt at dealing with such an impossible situation.
These are the kind of people who really need help. Once the North-Holland Commission had realized the situation, they were not slow at changing their tactics. The principle of "malaria control by the population" remained of course, but it was not applied too rigidly. When local conditions demanded it, the spraying, in its technically improved form (p. 205), was to be carried out by the sanitary inspectors of the Commission, especially in the foei of anopheline infection. It was not expected that the killing of infected mosquitoes in their foei would always prove of striking benefit to the family living in such a house. In fact, that family may have been hardly aware that they needed any help. It may never have occurred to them to use the hand-



sprayer, since they had suffered from no malaria and the sanitary inspector had not visited them for a year. He had not received any notification to do so, since the local practitioner directs the sanitary inspectors to houses with malaria patients only. That circumstance, we may add, was another reason why the spraying carried out by the population was not always a success. It was limited to the houses where a malaria patiënt was living. And we know (p. 167) that not all of these houses become foei of anopheline malaria; neither do the latter always harbour a malaria patiënt (p. 170).
It was hoped, however, that the extinction of these foei by efficient spraying would prove beneficial to the neighbouring families. If that expectation came true it would be time to charge the costs to the municipality. For the moment this was not to be thought of. The method was still on its trial.
* * *
As to the costs, a preliminary experiment in Wormerveer, carried out by Dr. de Graaf in 1935 *, showed that one operation, in a house with an average family of five, costs 1.48 guilders, labour included. The spraying would have to be done once in August, and twice in September and October. This plan was based on the assumption that the inhabitants of the house are the main source of anopheline infection. If all anopheles are killed on August i5th, fresh ones will enter it very soon
* Versl. en Meded. betr. Volksgez. Year 1936, p. 334.



afterwards. They will become infected, but it will take them about half a month (p. 135) to have their salivary glands invaded by sporozoites. By that time a repeated spraying will kill them on September ist. The anopheline invasion continues, but the resulting anopheline infection is again checked by spraying on September i5th. Since few anopheles enter the house after that date, subsequent operations may be altogether omitted in many houses. Judging by the first signs of sexual inactivity (p. 60) spraying ought to commence on the first of August. In view of the scarcity of anopheline infection in the first half of that month, it was deemed justifiable to postpone the operation till the middle of August. To continue the spraying after November ist was judged superfluous, since infections acquired after that date are not likely to reach maturity (p. 133).
Accordingly, spraying a hundred foei of anopheline infection three to five times, in a village of eight hundred houses and four thousand inhabitants, would cost 622 guilders, or fifteen cents per inhabitant. That is a sum which would be repaid by the reduced consumption of quinine. So the economie basis of the plan was sound, provided the number of houses to be sprayed did not exceed the estimate.
That estimate was based on our findings at Uitgeest in the autumn of 1935, where seventy-six houses were harbouring infected anopheles among a total of 771. They were not all foei of anopheline infection, but our estimate had better keep on the safe side. In Uitgeest the houses harbouring infected mosquitoes had been detected by catching and



dissecting great numbers of anopheles. That, however, is a procedure which cannot be applied to all villages. It requires a skilled staff, and so it would increase the oosts of the spraying to an extent which would upset the whole plan.
Accordingly, we have tried to find other criteria by which the houses likely to harbour infected mosquitoes could be identified without having to dissect anopheles. With regard to a group of two hundred and one houses, in sixty-eight of which we found 1,376 infected anopheles, we have asked ourselves how many of these infected mosquitoes would have survived if we had sprayed the houses in which had been found: (1) summer patients (p. 165); (2) cases of malaria irrespective of the season; (3) splenomegaly in schoolchildren; (4) healthy parasite-carriers; (5) four children or more under sixteen years of age; (6) fifty female anopheles or more captured on one day *. The answer to this question is the following.
If we had sprayed houses with:
1. summer patients 579 infected anopheles would have survived
2. spring or summer
patients 171 „ 
3. enlarged spleens 105 „
4. healthy carriers 95 „
5. tour children under sixteen 82 „
• 5o anopheles 45 „
The fourth criterion, healthy carriers, can be ruled out since the Commission in North-Holland has no staff to detect them. The fifth criterion is easily established, but the number of children under
♦ Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 5, 1936, p. 336 Malaria.



sixteen (four in the village of Uitgeest) will have to be modified according to local conditions. In Wormerveer it had to be put down at three, in Marken at two. The sixth criterion, the number of anopheles found on one day, is also dependent on local conditions; in Wormerveer it would have to be twenty instead of fifty. As to schoolchildren with splenic enlargement, they require some special skill in detecting (p. 33) and so they hardly come within the scope of a survey as envisaged here.
In order to arrivé at a practicable plan, it was decided primarily to rely on the number of young children without applying this criterion too rigidly. For instance, a family with numerous young children who had never suffered from malaria, or who were living in a house attracting but little anopheles, would be excluded. On the other hand, houses known to attract numerous anopheles would be sprayed in any case. As to the occurrence of malaria, including the cases of the preceding year, that criterion would be decisive in villages where the incidence of the disease was not so high as to upset the financial basis of the plan.
* * *
In late summer and autumn of 1936 the villages of Uitgeest and Marken were sprayed.
In Marken, a village on a small island in the Yssel-lake, operations were directed according to the presence of malaria. This involved the spraying of over a hundred houses. The quantity of the insecticide employed was three quarters of a pint for each operation in one house. That is much more than is required



for the killing of anopheles, especially as the spraying was limited to those parts of the house where anopheles were sheltering: the attics and the cupboardbeds downstairs. Still, this waste was countenanced, because it was observed that abundant spraying not only kills anopheles, but continues to act as a repellent for some two weeks afterwards (p. 212). The spraying was carried out three times: at the end of August, in the middle of September, and in the first week of October. It was not repeated after that, since no more anopheles could be found on subsequent inspections.
The operations carried out in Marken are an example of what can be done along these lines, without a special survey, by a sanitary inspector of the North-Holland Commission under the direction of the local physician, Dr. J. H. Wagenaar.
In Uitgeest, with 771 houses, no more than fiftynine were sprayed once or repeatedly. Most of them were houses with four or more young children, but some were included because of the number of anopheles found in them.The average number of anopheles caught in each of these fifty-nine houses was a hundred and thirteen. In 331 untreated houses, examined for this purpose, it was five. Some houses with numerous young children were excluded because too few anopheles were present. That was a mistake. When it was too late to remedy the neglect we found that one of them had developed anopheline infection.
The first round of spraying, on August 24th, did not prevent the houses from being full of anopheles again shortly before the second round on September



i4th. Still the number of anopheles had been reduced to half of what it was shortly before the first round, whereas it had increased to the seven or eightfold in unsprayed houses. This shows that the spraying has the additional effect of a repellent, provided the insecticide is lavishly applied (nearly a pint for each operation in one house). After the second round the number of anopheles remained very low for a fortnight at least, whereas it continued to increase in the unsprayed houses until the end of October. In houses remaining unsprayed at the third round, on October 5th, a slight increase in the number of mosquitoes was observed in the second half of October, shortly before the fourth round on November 4th. That is the effect of the short flights undertaken by sexually inactive anopheles by which parasites are carried from the foei of anopheline infection to neighbouring houses (p. 157). In November, after the fourth round, there was no more evidence of anopheles entering the houses.
The mistakes, we had made, had this advantage at least, that they enabled us to estimate the effect of the spraying on anopheline infection. Out of twenty-two houses in which anopheline infection was detected, eight had been sprayed from the end of August onward. In the remaining fourteen anopheline infection came as a surprise, and so they were not sprayed until the beginning of October. In the eight houses which were sprayed for the first time in August two per cent of the shortwings were found infected; one infected mosquito in eleven was carrying sporozoites. In the fourteen houses which were not sprayed until October eight per



cent of the shortwings were found infected; one infected mosquito in every two was carrying sporozoites. The difference would have been even more striking if the spraying had been commenced on August isth, as was originally arranged, instead of on August 24th, a delay which entailed the appearance of two of the three sporozoite-carriers in the houses sprayed at that time. The third appeared as a consequence of the interval between the first and the second round of spraying, which ought to have been a fortnight, being prolonged to three weeks. As to the fourteen houses left unsprayed till October, most of them harboured four or more young children. If we had rigidly kept to that criterion, the results would, probably, have been better. But we were led astray by the small number of mosquitoes present in these houses.
The costs of spraying the houses in Marken and Uitgeest amounted to a total of 875.73 guilders to protect a population of 5,000 inhabitants. Consequently, the expenses per capita, fifteen cents and a half, do not greatly exceed the estimate (p. 208).
* * *
The question of how long it takes anopheline infections to mature has been answered (p. 135) to the effect that it takes them about half a month from August until the beginning of October, and a whole month by the end of October. It is a matter of some practical importance to fix that period of maturation as accurately as possible, since the length of the interval to be allowed between two rounds of spraying depends on it. The operation



of spraying may itself be used to decide this point. The immediate effect of such an operation is to deplete a house of all the mosquitoes it sheltered. Anopheles found in it on a subsequent date must have come from elsewhere and, if they happen to be infected, they must either have brought the infection from some other house or they must have acquired it in the sprayed house after the operation. In the houses, referred to lower down, infected anopheles could not have been imported, because no foei of anopheline infection existed in the neighbourhood. Hence, anopheles found infected in these houses after the spraying must have ingested the parasites on the spot, the earliest possible date of infection being the night after the operation.
In three houses all mosquitoes were destroyed by spraying on August 2Sth, so the earliest possible date of infection was August 2Óth. On September nth, sixteen days later, we found in these houses twelve infected anopheles, four with mature oöcysts, one carrying sporozoites.
In another house all mosquitoes were destroyed by spraying on October 6th, so the earliest possible date of infection was October 7th. On October 27th, twenty-one days later, we captured one anopheles carrying sporozoites.
In the first three houses the period of sixteen days appears fairly accurately to represent the time required to mature a salivary infection, since both mature oöcysts and sporozoites were detected. The period of twenty-one days in the fourth house may be more than was actually required to mature the



sporozoites in October. In any case these data fully
confirm our previous estimate.
* * *
The effect of the spraying in late summer and autumn of 1936 became apparent in 1937.
At the village of Marken 148 persons had been ill with malaria in 1936; 129 had not had malaria in 1935 (primary cases), 19 had already suffered from the disease in that year (relapses). In 1937 10 persons suffered from malaria, 9 primary cases and 1 relapse.
At Uitgeest 414 persons had malaria in 1936, 343 primary cases and 71 relapses. In 1937 there were 56 malaria patients, 31 primary cases and 25 relapses.
Hence, the primary cases of malaria at Marken in 1937 have been reduced to se ven per cent of their number in 1936, and at Uitgeest to nine per cent.
In view of the mistakes we made at Uitgeest it is not astonishing that the results in that village are nor quite so good as at Marken. The effect of these mistakes shows even better when comparing the malaria which occurred in the practice of the two physicians of that village, which we shall indicate as "the first" and "the second" practice. For reasons it would take too long to explain, the first practice was somewhat better served by spraying than the second. Most of the mistakes were made in the latter.
Taking the two practices separately, the reduction the number of primary cases of malaria underwent in 1937, as compared with 1936, works out at seven per cent (from 193 to 14) in the first practice, and at eleven per cent (from 150 to 17) in the second.
I



In the outlying parts of Uitgeest, which have not been sprayed at all, the primary cases have fallen to twenty-eight per cent of their number in 1936 (from 32 to 9).
In the following list the above data are compared with similar ones collected in some neighbouring villages, where no spraying was carried out:
Uitgeest central, 2nd practice primary cases reduced to 11 pet.
Uitgeest central, ist practice „ „ „ „ 7 „
Marken „ M n n 7
Uitgeest, outlying parts „ „ „ „ 28 „
Akersloot „ „ „ „ y0 n
Wormerveer „ „ „ „ 83 „
Taking the village of Akersloot as a Standard of spontaneous reduction of malaria, the primary cases in Uitgeest and Marken have been reduced to ten per cent as a consequence of the spraying.
The mental hospital at Franeker illustrates the saying that malaria is a local disease to be dealt with by local measures (p. 164). There are two hospitals, one in the town, the other some distance in the country. Both have separate wards for men and women. The male ward in the country hospital was repeatedly sprayed in the autumn of 1936, none of the other wards were. The number of malaria patients was as follows:
Hospital' Ward- Number of mental Number ill with mal-
^ ' ' patients in: aria in:
1936 1937 1936 1937
Country male 119 119 42 8
„ female 128 129 5 13
Town male 156 153 9 12
„ female 178 183 2 2
So the malaria in the male ward in the country hospital has been much reduced; in the female ward it has increased.
Taking the village of Akersloot as a Standard of spontaneous reduction of malaria, the primary cases in Uitgeest and Marken have been reduced to ten per cent as a consequence of the spraying.
The mental hospital at Franeker illustrates the saying that malaria is a local disease to be dealt with by local measures (p. 164). There are two hospitals, one in the town, the other some distance in the country. Both have separate wards for men and women. The male ward in the country hospital was repeatedly sprayed in the autumn of 1936, none of the other wards were. The number of malaria patients was as follows:



Such a localized reduction of the incidence of malaria would be incomprehensible without the knowledge of the habits of infected shortwings during the period of their sexual inactivity. The case (p. 157) of a one-family house, with a rate of anopheline infection of twenty per cent downstairs and of four per cent upstairs, renders it possible to understand the conditions obtaining in a much larger building, where the intercourse between the different parts is necessarily more limited than in a small one.
* * *
The above results suggest that spraying in the autumn greatly reduces malaria in the next year, even if full allowance is made for spontaneous reduction in neighbouring villages. They support the view that autumnal anopheline infection is the principal cause of next year's malaria. The spraying of houses is a kind of species-sanitation, directed against the adults of one single species: the shortwings. If the longwings had been the vectors of malaria in this country, transmission would have been limited to the summer, since longwings cannot act as vectors in the autumn. Consequently, autumnal spraying would have been perfectly useless in that case.
We do not forget the example of Nieuwendam (p. 139) where malaria decreased in 1921, whereas the incidence of the disease continued to rise in the neighbouring villages. Nevertheless, we venture to regard the decline of malaria at Uitgeest and Marken as a result of the spraying, because it was observed in two different villages, and because it was localized,



as shown by the difference between the two practices at Uitgeest and the two wards of the mental hospital outside Franeker.
From a practical point of view the results of house-spraying are encouraging. But we regard these operations as an experiment. In Uitgeest it has been possible to limit the spraying to fifty-nine houses out of 771 because of the perfect knowledge of local conditions, the outcome of a survey continued for two years previous to the operations. In Marken no such survey had been carried out, but the small size of the village allowed of the spraying of about one quarter of the houses. Further experiments will have to decide what spraying can do in large villages, without the help of the data provided by a thorough and long continued survey.
Even if the results should be favourable, we regard spraying as no more than a step on the road to malaria control. Spraying is like anti-malaria drugs. It reaches deeper than drugs, no doubt, since it prevents human infection instead of concealing it. But, like drugs, it does no more than hide the intrinsic malariousness of the country, without even touching its source: large numbers of shortwinged Anopheles maculipennis. As a consequence, our ultimate hope remains with the freshening of the water in North-Holland. It is only because we expect to have still to wait for it a long time that we have devised the means which, we hope, will help to bridge over the gap between the present day and that future, when malaria will be banished from this country where it so long held its undisputed sway.



CHAPTER X
Malaria induced in patients suffering from
general paralysis of the insane in collaboration with Dr. Ch. W. F. Winckel
In the preceding chapters we have occasionally made use of the results of observations on malaria induced in patients suffering from general paralysis of the insane. The behaviour of the strain of Plasmodium vivax, called James' Madagascar strain, suggested to us an explanation of the autumnal rise of malaria as recorded in the past (p. 42). Induced malaria taught us that degenerated sporozoites are no good as infecting agents (p. 131). By infecting mosquitoes on mental patients immune to fever, but still carrying parasites, we were able to confirm our field-observations on the infectivity of healthy carriers (p. 166). Korteweg derived a good deal of information on his initial remittent from the study of malaria induced in mental patients (p. 51). As far as this country goes, our knowledge on the long incubation derived no benefit from malaria induced for therapeutic purposes. All experiments on this subject were performed by infecting healthy volunteers (p. 152). Nevertheless, we are sufficiently indebted to the study of malaria in mental patients, for the increase of our epidemiological knowledge,



to justify our giving an account of the source of this information.
The Amsterdam organization to supply mental hospitals with malaria parasites
In 1921 the director of the mental hospital at the University of Amsterdam, Professor K. H. Bouman, commenced treating patients suffering from general paralysis of the insane, by means of the method Wagner von Jauregg had introduced into mental practice some four years earlier. To advise him on all matters relating to malaria he turned to Dr. P. C. Korteweg *, honorary member of the staff of the Zoological Department at the Institute of Tropical Hygiene of the Royal Colonial Institute, Amsterdam. In this way a cooperation was established between the two institutes which has proved profitable to both. Dr. Ch. W. F. Wïnckel t taking over Dr. Korteweg's functions has continued it to this day.
The incidence of syphilis in this country is not a very high one. As a consequence, none but fairly large cities, like Amsterdam, are in a position to collect patients suffering from general paralysis, in numbers (50 to 60 every year) sufficiënt to keep the strains of parasites going uninterruptedly. Other mental hospitals in this country are in a less fortunate position and so they have to apply to Amsterdam to supply their need of plasmodia, in the shape of blood containing parasites or of infected anopheles.
* P. C. Korteweg, Nederl. Tijdschr. v. Geneesk. Vol. 77, 1933, P- 4547-457°.
t Psychiatr. en Neurol. Blad. 1935, no. 3.



We ought to explain that we have almost wholly to rely on patients suffering from general paralysis of the insane, since they form the large majority in whom malaria is induced to bring about a therapeutic effect. A preventive treatment of patients ill with syphilis, whose cerebrospinal liquor shows a positive reaction of Wassermann, is not yet regularly practised in this country.
So the mental hospital of the University of Amsterdam, and the "Valerius" mental hospital (Professor van der Horst) in that city, have to supply the whole country with malaria parasites. This centralization has had the advantage of rendering it possible to attach a mosquito-station to the Department of Zoology at the Institute of Tropical Hygiene. The working of this station will be described in the next section.
The close collaboration with the above institute has had the additional effect that the data, collected by the observation of patients undergoing a malaria cure, were utilized to increase the knowledge regarding malaria in this country. The full benefit to be derived from this collaboration becomes apparent, when comparing conditions existing in the Amsterdam mental hospitals with those in other similar institutions. In Amsterdam induced malaria is an object of close study, elsewhere it merely is an instrument for certain therapeutic ends.
The technique of rearing and infecting anopheles in the laboratory
Amsterdam is very favourably situated for obtaining adult anopheles. At any time we choose



we can catch them in stables and pigsties in the neighbourhood of that city. It is only in late spring and early summer (p. 95) that they are scarce. Even then, it is not so much their scarcity as the high mortality, they are subject to, which has forced us to discard the "wild" anopheles of that season and to employ laboratory-bred mosquitoes in their stead. From the end of August until March, however, wild anopheles give complete satisfaction. In the first place, they belong to the proper race, the short wings. The longwings, as we have explained before (p. 74), are difficult to infect during the time of their sexual inactivity, which occurs within the period mentioned above. Sexually inactive shortwings can be readily infected up to a hundred per cent after one single meal, so long as it is a good gametocyte-carrier who pro vides it. In the second place, they are available in great profusion during these months. Finaily the sexually inactive shortwings offer the great advantage of being longlived. The various processes they are subjected to, in order to have them infected, do them no serious harm. Their term of life, however, rapidly decreases after December (p. 139). In May no more than eight per cent of the mosquitoes live till the end of the fourth week after the infecting meal. Fortunately, laboratory-bred anopheles do better during that unfavourable period. In May thirty per cent of infected laboratory-bred mosquitoes survive the period of four weeks after the infecting meal. So we have come regularly to use them, instead of wild mosquitoes, not only in May but for the whole period of sexual activity, from April till August.



■
We have hit upon a method of breeding our mosquitoes which requires little apparatus but a good deal of care. So we do not recommend it, unless the person presiding over it possesses what we might call "nursing spirit". This allows him to watch and nurse his broods without spending too much time on it.
After a batch of eggs has hatched in a small dish (diameter two inches and a half), the young larvae are given a modicum of food only. If they are seen to feed satisfactorily, they may be allowed a second helping. On the second or third day they are transferred to a larger dish, five inches in diameter, and the quantity of food is increased. If they thrive, half of the larvae are transferred to another dish of the same size. During the next days the process of distributing the larvae over two dishes is repeated. So we find ourselves with quite a number of dishes on our hands (each one containing ten to twenty larvae) by the time they are well up into the third stage. If vigorous individuals are required, of a size equalling that which we meet with in nature, special precautions are necessary. In that case we isolate the larvae which have just entered upon their third stage, by using a small dish for every single larva. The water in the dishes should not be changed. It requires filling up, however, from time to time.
The quantity of food is an important point. We are in the habit of scattering fresh food on the water's surface twice a day, morning and evening. We succeed in measuring out the exact quantity required to keep the larvae feeding nearly all the time, till they get their next allowance. To give more than



that is worse than to leave them without food for a few hours a day.
The food of the larvae consists of two parts of dried Daphniae, two parts of unicellular algae and one part of piscidin (No ooo). The Daphniae have to be powdered and this can only be done when they are quite dry. In summer, in unheated rooms with a high relative humidity, drying in the incubator is necessary. The algae are scraped off the bark of trees or, better still, off wooden fences, during dry weather. They should be lightly powdered, without using a mortar, so as to avoid crushing the individual cells. The algae, we believe, have little nutritive value. Nevertheless, they are an indispensable constituent of the diet. After passing through the gut of the larvae, they are ejected, for the greater part undigested. Then they sink to the bottom of the dish and there they help in maintaining a favourable biological equilibrium in the water, by counteracting the putrefaction of the sunken remnant of the other foodstuffs.
This food-mixture has a remarkable way of spreading over the surface of the water as a very thin film. This film does not sink to the bottom. If the food, when scattered on the water's surface, does not spread, this means that the larvae have not cleaned the surface. That is a sign that they are either ill or near moulting. In both cases it is advisable to keep them fasting for some time. "Having cleaned the water's surface", as healthy larvae do, does not mean that they have eaten the last morsel of food. In fact, there may be plenty of food left on the surface, and yet a small amount of fresh



food will spread over it, be it a little slower than over an empty surface. But if the larvae are off their feed, the surface may be quite clean to the eye, yet fresh food refuses to spread.
As a rule, the growth of the larvae proceeds satisfactorily until the end of the fourth stage. Then a high mortality may supervene. We know neither its cause nor how to avoid it. The last days preceding pupation seem to constitute a critical period in the larval life. As a rule, however, most of the larvae manage to live through it, if well nursed. Our breeding method allows of collecting the pupae without difficulty, there being no vegetation of any kind for the pupae to hide in. The pupae, collected in a dish, are put in a cage for the adults to hatch.
* * *
The freshly hatched adults (and likewise mosquitoes caught in nature) are kept on a diet of sugarwater, until they are required to take the blood of a suitable gametocyte-carrier. The cages containing adults ready for infection are stored in a cool room with a high relative humidity. Anopheles kept there for some weeks, on a diet of sugar-water, are by no means unsuitable to serve as malaria vectors. Nevertheless, we prefer freshly caught or freshly hatched females. To infect them, they are removed from their cage by means of a test-tube and transferred to a glass jar, with a diameter of two inches and a half, covered by muslin gauze. Each jar may contain 40 females, without risk of their being damaged. The muslin cover is applied to some part of the gametocyte-carrier's Malaria.



skin, usually his back. After the females have taken their fill, the jars, containing them, are packed in a bag in which a hot-water bottle keeps the temperature above 65°. In this way they are returned to the laboratory. There, the anopheles are put into a cage. The mosquitoes which have imbibed an infecting meal are transferred to a second cage, which is put at a temperature of 8o° and a relative humidity of 90 per cent. All the time infected anopheles stay there they are kept on a diet of sugarwater. We never found that this diet unfavourably influences the amphigony of the malaria parasite. In these circumstances, salivary sporozoites make their appearance at the end of nine days. When the first sporozoites have been detected in the salivary glands, the cage containing the infected females is allowed to remain in the tropical chamber for a couple of days longer. After that, it is stored in a cool room till the mosquitoes are required to infect a fresh patiënt. The greater the demand for them, the less is the time they will have to pass in this room; for it is necessary to return them to warmer surroundings after each meal, in order to allow them quickly to digest the blood they imbibed.
In these conditions, numerous mosquitoes may survive for three or four months, notably during the favourable season from September until December. Unfortunately, the sporozoites are not so longlived. Long before their hosts have died, or have got rid of their salivary infection, the sporozoites are beginning to show the symptoms of degeneration described on p. 130. Usually this happens already after one month, and mosquitoes carrying them



are no longer to be relied upon to infect patients *. So it is useless to infect great numbers of anopheles at the same time or at short intervals. On the other hand, it is advisable to infect fresh batches of mosquitoes every two or three weeks.
Induced benign tertian malaria
The two strains of Plasmodium vivax employed to induce malaria for therapeutic purposes. — We shall have to refer to these strains more than once and so we begin by saying what they are. One is called the home-strain, indigenous in North-Holland and Friesland. No difference can be detected between parasites from these provinces. The other is James' Madagascar strain. They differ in regard of the period of incubation, the tendency to lapse into latency, the type of fever they cause, their susceptibility to neosalvarsan, and by an incomplete reciprocal immunity. These differences will be discussed later on. There remain two which, having no further consequence, will not be included in that discussion. They refer to the number of merozoites and to the virulence.
Merogony in the Madagascar strain of P. vivax produces, on an average, between seventeen and eighteen merozoites. In our home-strain the average number of merozoites lies between twelve and thirteen t. Korteweg § calls attention to the fact that the fever caused by the Madagascar strain is not only more intense by its quotidian type, but also
* Amer. Jrl. Hyg. Vol. 24, 1936, p. 7.
t Arch. f. Sch. u. Tropenhyg. Vol. 39. 1935, p. 342-345.
§ Nederl. Tijdschr. v. Geneesk. Vol. 77, 1933, p. 4547-4570.



by the temperature running higher than in the fever caused by our home-strain.
Long latency in benign tertian malaria. — When malaria therapy of general paralysis of the insane was first commenced in Amsterdam, the homestrain of Plasmodium vivax was the only one used. It yielded satisfactory results so long as malaria was transmitted by blood-inoculation only. This state of general contentment did not, however, last from the moment mosquitoes came in use to transmit malaria. It soon became painfully clear that this method was not reliable. Thirty-eight per cent * of the patients bitten by infected mosquitoes did not develop malaria within the normal period of incubation. They had to be reinfected, either by the bite of more mosquitoes or by blood-inoculation. These reinfections precluded the possibility of ascertaining whether the failures were due to the patiënt not having been infected at all, or to the infection being successful but having lapsed into prolonged latency (p. 150). On the strength of protracted incubation occurring in healthy volunteers infected with the home-strain (p. 151), it was generally assumed that these failures were due to the effect of this protracted incubation. This view was also justified by James' statement of 1927, that patients who failed to get malaria, but who could be kept under observation for a much longer time than was possible at Amsterdam, finished by developing a fever.
At first, it was taken for granted that infections
« Viz. half of the patients bitten by 3-5 mosquitoes, one quarter of those bitten by 6-20 mosquitoes and, quite unexpectedly, again half of those bitten by more than 20 mosquitoes.



induced by the bite of mosquitoes in autumn or winter were the only ones to lapse into long latency. Infections in spring or summer were supposed to be followed by fever after a normal period of incubation. It soon became apparent, however, that failures to induce malaria with the Dutch strain of P. vivax, transmitted by mosquitoes, occurred at all seasons. Neither was it possible materially to reduce the incidence of failures by increasing the number of mosquitoes biting the patiënt.
In all these cases, of fever failing to appear after the bite of mosquitoes infected with our home-strain of benign tertian, reinoculation was postponed for a time equal to the maximum of the period of normal (short) incubation. When that period had elapsed, some more days were usually allowed to pass, in the hope that a belated fever would still appear. In the end we found that it was no use waiting for more than four weeks or a month. That was the longest period of short incubation we ever encountered. To the alienist this meant postponing for a month the treatment of two fifths of his patients. Of course, he was far from pleased with this and he urged us to give up mosquito transmission altogether. James helped us out of this difficulty by letting us have his Madagascar strain of P. vivax.
We found that strain far superior to our indigenous P. vivax. Transmitted by mosquitoes, it failed to cause a fever in no more than six * per cent of the cases. Some of these failures need not be charged to that parasite's account at all, since they were due to
* Amer. Jrl. Hyg. Vol. 24, 1936, p. 1-18.



other causes. Moreover, in using the Madagascar strain it was no longer necessary to effect the transmission at often repeated sittings, and by the application of numerous mosquitoes. As a rule one sitting proved sufficiënt. This put us in a position accurately to fix the length of the period of incubation. In the Madagascar strain it proved to be twelve days, on an average, whereas it was twenty-one days in our home-strain. This difference is apparent in mosquito-transmitted malaria only. After subcutaneous inoculation the incubation lasts for nine days in both strains.
Initial remittent fever. — We meet with Korteweg's initial remittent (p. 51) in patients infected with the Madagascar strain as well as with the home-strain, provided, of course, that the patiënt never had had an attack of tertian malaria in all his life. If the initial remittent fails to appear at the beginning of the induced fever, we are sure we will never carry it to a completely successful termination because of the immunity of the patiënt, even in the absence of a definite history of malaria. A previous attack of quartan malaria is no obstacle for the initial remittent to appear in benign tertian.
Type of fever. — In patients infected by the bite of mosquitoes the Madagascar strain tends to cause a duplicate fever from the very beginning (fig. 21, p. 236). Malaria caused by the home-strain often starts as a tertian fever; it is not till after some paroxysms have passed that a tendency to turn quotidian becomes apparent * (fig. 20).
* P. C. Korteweg, Jrl. Trop. Med. & Hyg. May 15, 1931.



The type of the fever caused by blood-inoculation does not depend upon the strain of parasites. It depends upon the stage of development the parasites had reached at the moment they were injected. If all these parasites have reached the same stage, the fever in the patiënt, into whose body they have been injected, will run a simple tertian course, for a while at least. But if the injected parasites belong
Fig. 20. Simplel tertian, turning duplicate on December 2nd-4th. The fever is stopped with quinoplasmine on December 4th.
to two widely divergent stages of development, the fever they cause will be of the quotidian type from the start.
Once the fever has become quotidian, it remains true to this type through all subsequent accesses, no matter whether the parasites have been conveyed to the patient's body by the inoculation of blood or by the bite of a mosquito. The only way to make a fever abandon its quotidian course is the administration of suitable drugs (p. 235). Once in a while,



we meet with a spontaneous conversion of a quotidian fever into a tertian, but that happens in cases of incipient immunization only. In such cases one of the fever peaks is seen to disappear, but the second one does not wait long in folio wing suit. The patiënt has made a spontaneous recovery, as a consequence of the immunity he acquired in the course of his attacks.
Effect of drugs. — A fundamental difference exists between malaria caused by the subcutaneous, or intravenous, inoculation of blood containing parasites, and malaria caused by the bite of infective mosquitoes. To simplify the wording of our explanation, we shall call the former inoculated malaria and the latter mosquito-borne malaria.
Inoculated malaria is invariably and definitely cured by a daily dose of 15 grains of sulfate of quinine continued for five days. We never met with a single exception to this rule in a series of over six hundred patients infected and treated in this way. We occasionally hear, from other mental hospitals, of relapses in patients who have been treated with inoculated malaria and whose fever has been suppressed by quinine taken for five days. We are sure such failures are due to the mental state of the patients. Unless they are strictly supervised, they cannot be relied upon to swallow the drugs they are expected to take *. Occasionally one has to resort
* The "quinine-resistant" parasites, which we occasionally find reported, and which we know not to be resistant at all, since they belong to the strain we have been familiar with for years, owe their "resistance" to the same condition which gives rise to the relapses.
In former years, when the home-strain of P. vivax was still



to intramuscular injections to overcome this passive resistance.
In mosquito-borne malaria the case is different. It is likewise amenable to quinine taken in doses as quoted above. But relapses occur in a certain proportion of the cases. They are not so numerous, however, as in spontaneous malaria. Out of 58 patients, suffering from mosquito-borne malaria and subsequently treated with a daily dose of 15 grains of sulfate of quinine for five consecutive days, nine had a relapse within the next eighteen to twentyfour months: a relapse-rate of fifteen per cent. Out of 81 similar patients, treated with quinoplasmine (fourteen grains of quinine and half a grain of plasmoquine daily) for one or two weeks, nine relapsed within the same period: a relapse-rate of eleven per cent.
In general practice about fifty per cent of the malaria patients, treated with quinine for seven days, are reported to relapse during the year they were treated and the next year. Our patients were much better observed than they ever can be in general practice. Nevertheless, their relapse-rate was less than one third of the above. Apart from the absence of reinfection in our patients and the likelihood of their occurrence in general practice, this difference is due, no doubt, to the immunity our patients develop as a consequence of the ten
in use, there was another reason for the occurrence of relapses in inoculated malaria, viz. an infection (by the bite of mosquitoes) which had lapsed into long latency and so had to be followed by blood-inoculation. Obviously, cases of this description cannot be quoted as an instance of inoculated malaria giving rise to relapses. (P. C. Korteweg. Nederl. Tijdschr. v. Geneesk. Vol. 77, 1933, P- 45&2)-



to twelve accesses of high fever they have been subjected to. A patiënt suffering from spontaneous malaria has much less chance of acquiring a solid immunity, since his fever is usually nipped in the bud. This is certain to happen in North-Holland, with its welldeveloped system of sick-clubs (p. 7).
With a view to the insignificant difference between the relapse-rate after a five days' treatment with quinine and after a weekly or fortnightly treatment with quinoplasmine, we have, for the last two years, adopted the rule to treat patients suffering from mosquito-borne malaria with 15 grains of sulfate of quinine daily, continued for seven days.The susceptibility of the Madagascar strain to quinine is in no way different from that of the home-strain.
When patients suffering from general paralysis have been subjected to a sufficiënt number of paroxysms, they are treated with neosalvarsan. According to Professor K. H. Bouman it is in the patiënt' s interest to begin with the administration of neosalvarsan without delay. Since Plasmodium vivax is highly susceptible to this drug, this method of treatment renders it difficult to test the effect of various antimalarial drugs. This is unfortunate, but it cannot be helped, since the patients are treated with malaria to cure them of their general paralysis and for no other purpose.
Nevertheless, certain remarkable facts have been revealed, notably the great difference in susceptibility to neosalvarsan between the home-strain and the Madagascar strain *. This difference may be described as follows:
* P. C. Korteweg, Nederl.Tijdschr. v.Geneesk.Wol. 77,1933,p. 4563-4567.



It olten happens that a patiënt s conaition is seriously affected by the tertian, he is suffering from, taking on a quotidian type. Of course, the physician wishes to slow down the tempestuous course of the fever without actually suppressing it. The simplest course open to him, in a case like this, is to administer an intravenous injection of 150 milligrammes of neosalvarsan. Since half-grown parasites are more susceptible to this small dose than full-grown ones, the effect of this intervention is that one of the two generations of parasites, which combine to keep the fever quotidian, is suppressed, whereas the other carries on. As a consequence, the quotidian fever is converted into a simple tertian, and both the physician and the patiënt feel much better. The same dose of neosalvarsan applied to a simple tertian fever, during the apyretic interval, usually stops the fever altogether, but it will return after a week or so.
All this applies to malaria caused by our homestrain of Plasmodium vivax. Twenty-four hours old parasites of the Madagascar strain are much more susceptible to neosalvarsan: 75 milligrammes, even 50 milligrammes, suffice to convert a quotidian into a tertian (fig. 21) by suppressing one of the two alternating generations. Occasionally, and unintentionally, that dose may suppress them both and stop the fever for a whole week.
Although neosalvarsan, administered in small doses, is quite capable temporarily to interrupt a series of paroxysms, it cannot be relied upon to bring about a permanent cure. Even during the prolonged treatment with neosalvarsan following on the full course of malaria attacks fever may reappear at



any time, so long as no quinine has been given.
Number of parasites in the blood. — It is obvious that Ross' "pyrogenous limit", the number of parasites present at the onset of the fever, must be highly variable. To realize this, one has only to remember the small numbers of parasites found during an initial remittent, and their vastly greater quantities at
Fig. 21 Initial remittent (October 28th-November ist) immediately followed by duplicate tertian, which is converted into simple tertian by a dose of 50 milligrammes of neosalvarsan administered on November 6th. Nine grains of quinine given on November i4th end the fever.
the commencement of the first typical intermittent attack. Even if the initial remittent and the intermittent fevers are considered separately, the variability remains. Van Assendelft * showed that the number of parasites may vary from less than one to nine hundred to the cubic millimetre of blood at the onset of the initial remittent, and from four
* Beihefte Arch f. Sch. u. Tropenhyg. Vol. 35, 1931, p. 5-103.



to twelve thousand in the first typical attack following the remittent. Relapses commence with a still greater number of parasites, which van Assendelft never found under a thousand parasites to the cubic millimetre. In the subsequent course of the infection the number of parasites rarely exceeds twenty-five thousand, although eighty thousand per cubic millimetre have occasionally been counted. In other words, not quite two per cent of the red bloodcells are infected as a maximum, a figure remaining far below that observed in subtertian, in which up to thirty per cent of these cells may be found infected. The initial stage of the infection is characterized by a rapid increase in the number of parasites. During the next stage their number remains stationary. We may take this as a sign that no more than one merozoite, out of the batch resulting from every schizogony, succeeds in reaching the stage of a full-grown parasite. Finally, the parasites gradually decrease in numbers, overwhelmed by the ever growing immunity of the host. At that stage it is usual to put a stop to the series of paroxysms by the administration of quinine, or other drugs. They gain an easy victory over the miserable remnants of a tribe of parasites already subdued by the host's immune bodies.
Occasionally, the ease with which this victory is gained may prove more of a hindrance than a help. So, at least, Korteweg found it in the early days, when the indigenous strain was the only one in use and when no mosquitoes were employed to act as a parasite reservoir. At that time Korteweg was in the habit of utilizing, as parasite reservoirs,



patients who were undergoing the after-treatment with neosalvarsan, since this drug keeps the parasites down without exterminating them. But he had carefully to keep the plasmodia in check by very small doses of quinine, so as to quench any attempt of theirs at undue multiplication. If, as occasionally happened, they managed to circumvent him, Korteweg would, all of a sudden, find his patiënt in a high fever. Then there was nothing for it but to treat him with one full dose of quinine. The desired effect never failed to materialize but, unfortunately, that one dose of quinine, given to a highly immunized patiënt, occasionally had the additional effect of causing the parasites to disappear for good and all *.
Immunity. — In spontaneous malaria we highly value immunity as an asset in man's struggle against that disease. In the practice of inducing malaria for therapeutic purposes it is frankly a nuisance, and we would be better off with less of it. It has caused us difficulties in patients with a history of malaria from the Netherlands' Indies or other tropical countries, and it has given us endless worry when the alienist wanted to repeat the treatment with malaria in a patiënt who had been subjected to it one or two years ago. The fact is that such a repe-
* P. C. Korteweg, Nederl. Tijdschr. v. Geneesk. Vol. 77, 1933, P- 4563-
A similar experience once happened to one of us, a likewise fairly thoroughly immunized individual, who had tried the effect of a fïve days' course of atebrin in his own case of tertian (Madagascar). Since the result was frankly discouraging, he wished to try again, in case of a relapse. The relapse materialized a month later, at a very inconvenient moment, though, for a repeated trial with atebrin. So he took 15 grains of quinine once, thinking thereby to postpone the relapse for a fortnight or so. Now, six years later, he is still waiting for it.



tition usually results in a dismal failure, since the patiënt proves immune to the fever. All our homestrains are the same in this respect. A patiënt who formerly has been infected with a North-Holland strain does not respond with fever to a subsequent infection with a Friesland strain. The Madagascar strain does slightly better in this respect. A patiënt previously infected with one of our home-strains may develop some (never more than four) fever attacks up to 104° when infected with the Madagascar strain. But a really satisfactory course of ten to twelve paroxysms cannot be obtained in this way. Nevertheless, this experience shows that there exists a complete reciprocal immunity to our various indigenous strains, but not to a home-strain and James' Madagascar strain.
In about ten per cent of the patients showing no previous history of malaria immunity makes its appearance before the course of ten or twelve paroxysms is completed. This precocious immunity may even occur in cases which commenced with a typical initial remittent. Such an abortive course of paroxysms is usually followed, after a week or two, by a slight recrudescence. This event, however, is of no avail, since it reinforces the acquired immunity so as to completely prevent the fever from returning ever after.
In the face of these difficulties the quartan strain, we obtained from Vienna in 1933, was a veritable godsend (p. 245). In fifty-seven patients who had become immune to tertian malaria we succeeded in inducing a complete course of quartan paroxysms (fig. 22). In patients immune to quartan malaria



the way to a complete treatment with tertian malaria is likewise open.
Provocation of malarial fever. — In the literature on the subject, the term provocation means an intervention intended to force into the peripheral circulation malaria parasites concealed in the internal organs. Thus, in the pathology of malaria, provocation is an aid to diagnosis.
Fig 22. Quartan in patiënt immune to tertian. No initial remittent. In spite of a course of neosalvarsan, commenced on November 5th, there are 6 further paroxysms, 3 of which are shown here.
In the literature relating to the treatment of general paralysis of the insane the term sometimes has a different connotation. Here it means an intervention to reduce the period of incubation, or to reactivate a fever which has disappeared spontaneously or by the administration of drugs in small quantities. This intervention might better be called stimulation.
In the Amsterdam mental hospitals it has been found impossible to stimulate the fever. This applies to inoculated as well as to mosquito-borne malaria.



In immunized patients, who show neither parasites nor fever, no injection of pyrifer, adrenaline, or any other substance, ever results in the reappearance of the parasites in the peripheral circulation. Neither do we succeed in reactivating the fever in immunized patients whose blood still contains parasites. Finally, we always fail in reducing the length of the period of incubation or of the apyretic interval following a small dose of quinine or neosalvarsan.
Some psychiatrists hold that neosalvarsan acts as a provocative. Considering the destructive action of this drug on tertian parasites, it is hardly necessary to say that it never did in our experience.
Induced malaria as a source of danger to the population. — With regard to this point, it is necessary separately to consider the risks of malaria spreading from patients treated with inoculated malaria and from those treated with mosquito-borne malaria.
There was a time when it was believed that parasites in inoculated malaria lose their capacity of growing gametocytes. Korteweg *, however, has made it amply clear that gametocytes occur in cases of inoculated malaria, in Amsterdam as well as in Vienna. The number of gametocytes is variable no doubt, but no more so than in cases of mosquitoborne malaria. Consequently, patients treated with inoculated malaria may be a source of infection so long as the treatment lasts, but no longer. For, once the patients have taken their final five days' course of quinine they will have no relapses, and so no more danger is to be apprehended.
* Wiener Klin. Wochenschr. 1930, no. 26.
Malaria.
16



In the treatment with mosquito-borne malaria the position is different. Like in inoculated malaria all patients may be a source of infection during their treatment. Afterwards, however, some ten or fifteen per cent are likely to relapse and an unknown number may become healthy parasite-carriers. The danger of their spreading malaria was all the more to be apprehended in this country, since all our patients treated with mosquito-borne tertian malaria are infected with James' Madagascar strain. This strain, as we pointed out already, is not prevented from causing a fever by the immunity conferred by an infection with our home-strain; it is, moreover, more virulent than the latter strain. So it was to be feared that the Madagascar strain, spreading in this country, would cause an epidemie of unusual extent and malignity in some unsuspecting and unprepared North-Holland village. Moreover, such an epidemie was likely to coincide with the season of anopheline malaria, since the Madagascar strain rarely shows a long incubation. This, undoubtedly, would aggravate the situation by facilitating anopheline infection.
Investigations * into the influence of temperature on the amphigony of the Madagascar strain have shown, however, that our fears were unfounded. In view of the fact that the principal season of malaria transmission by anopheles in this country is late summer and early autumn (p. 129), a series of experiments were carried out from the end ot August till the beginning of October. Their object
* Amer. Jrl. Hyg. Vol. 24, 1936, p. 1-18.



was to ascertain whether mosquitoes having ingested gametocytes of the Madagascar strain would become infected at ordinary room temperature. Other mosquitoes infected with the same gametocytes, but kept in the tropical chamber, served as controls. Natural anopheline infection provided us with a clue as to the behaviour of indigenous P. vivax in the mosquito at room temperature.
The results of these experiments were definitely reassuring. Although outdoor temperature had been rather above the average, and room temperature accordingly, the development of the infection proceeded very slowly. Sporozoites took a whole month to appear in the salivary glands. The majority of the oöcysts did not reach maturity. Salivary infection, accordingly, was very poor. It often failed altogether to show in mosquitoes carrying great numbers of oöcysts. But then, these oöcysts were all degenerated.
Experiments, continued in November, proved that oöcysts starting their development in the tropical chamber fail to continue their growth when transferred to a room at 46°-57°. They may revive after being returned to the tropical chamber, but the resulting infection is a poor one, most of the oöcysts being degenerated. And if they remain too long in the cold room there is no revival at all. In houses, where anopheles acquire their infection under natural conditions, cold spells are not followed by tropical temperatures, but by less cold or, at best, mild weather. So we may infer from this that there is small chance for the Madagascar strain of ever obtaining a foothold in this country.



Case-fatality. — The Amsterdam mental hospitals always see to it that every patiënt who is to be subjected to treatment with malaria is thoroughly examined by a specialist, to make sure that he is fit for this treatment. Nevertheless, sixty-two out of eight hundred and seven patients, in whom benign tertian malaria had been induced for therapeutic purposes, died during the period of incubation, during the treatment, or within a week after its cessation. That represents a case-fatality approximating eight per cent. As a rule, the patients died notwithstanding the parasites in their blood proved perfectly amenable to quinine treatment.
This case-fatality is to be accounted for by the circumstance that a series of twelve paroxysms affects much more seriously an individual weakened by general paralysis (usually aggravated by syphilitic vascular lesions, an incipient ataxia, and other morbid symptoms) than an otherwise healthy person.
In some instances autopsy clearly revealed other causes of death, like pneumonia, septic conditions following bed-sore, syphilitic aortitis, or myocarditis. There can be no doubt, however, that malaria is a contributory cause of the death of many patients. There is little use in minutely scrutinizing this deathroll to exonerate the therapy of induced malaria. It is better to face the truth that this method of treatment carries with it grave risks to the patiënt who is to profit by it. At the same time we ought to bear in mind that it is amply justified by the absence of any other means to stop the insidious progress of general paralysis.



Induced quartan malaria
We have shown (p. 239) that it is not possible to induce a complete course of tertian paroxysms in a patiënt who has been subjected to the same treatment at some earlier date. So we decided to try quartan malaria. Professor Pötzl of Viennakindly supplied us with the strain of Plasmodium malariae he was using in his hospital. In September 1933 the first patiënt was infected with this parasite.
Transmission. — This Viennese strain shows one inconvenience, which it shares with all quartan strains used at present in various mental hospitals: it rarely succeeds in growing in anopheles. We found this particularly disappointing, since our first attempt was quite successful. The very first patiënt inoculated with the Vienna strain of quartan showed six male gametocytes in 5ooleucocytes. That is a quantity which in tertian warrants success. Twentyseven per cent of the mosquitoes which had taken this patient's blood showed salivary infection. By their bite they caused quartan malaria in two of us and in two patients suffering from general paralysis, after an incubation of twenty-three to thirty-one days. Since then we have often tried to repeat this experiment, but all further attempts were unsuccessful, except one in which we found a few infected glands *. As a consequence, the quartan strain is maintained by blood-inoculation only.
Period of incubation. — After subcutaneous inoculation the length of this period averages twenty-
* Ann. Trop. Med. & Paras. Vol. 29, 1935, p. 171-175.



six days. To obviate the inconvenience resulting from this long delay it has been found necessary to have recourse to intravenous injections. They reduce the incubation to an average of ten days. As we pointed out already, one of the disadvantages of inoculated malaria hes in the fact that the recipient shows the same type of fever as the donor. In the treatraent with quartan malaria this disadvantage becomes particularly conspicuous after intravenous inoculation. If the donor's fever has become triplicate, the recipient is sure to have a triplicate fever from the start. Wherever possible, we select as a donor a patiënt whose fever is a simple quartan. But, on an emergency, it is not possible always to keep to that rule.
Initial remittent fever. — As far as the type of the fever allows of deciding this point, there is no tracé of an initial remittent in quartan malaria in persons who never had malaria in the course of their life.
Duplicate and triplicate fever. — There must exist a marked divergence in the length of the time the individual quartan parasites require to complete their development. That, at least, would explain why the majority of the patients develop a duplicate or triplicate fever after the first few simple quartan attacks. These daily recurring paroxysms greatly exhaust the patients (fig. 23). Of course, the same obtains in duplicate tertian, but in quartan conditions are worse, since this fever is far less amenable to drugs than tertian. It is easy enough to convert a doublé tertian into a simple one with the help of 50 to 150 milligrammes of neosalvarsan. But neosalvarsan in



those, or even larger, quantities has no effect whatever on quartan fever (fig. 22). Quinine has, but it does not act selectively by killing one generation of parasites and leaving the other.
At one time we believed that methylene blue in quartan malaria could take the place neosalvarsan occupies in tertian malaria. A patiënt suffering from a triplicate quartan fever was given fifteen grains
Fig. 23. Quartan turning triplicate on October 2nd, reduced to simple quartan after (not necessarily by) the administration of 15 grains of methylene blue on October 6th.
of methylene blue by the mouth. The fever promptly returned to the simple quartan type (fig. 23). In other cases, however, the same conversion of a duplicate, or triplicate, into a simple quartan fever was observed to occur without intervention of any kind. So we are afraid our first success in the application of methylene blue was apparent only. We are forced to admit this, since this drug, on further trials, proved highly unreliable in its effect.



t
Effect of drugs. — There is one fundamental difference between tertian and quartan: quartan is not susceptible to neosalvarsan. We have quoted this as a disadvantage in the preceding section. But it is an advantage too, since the patiënt can be treated with neosalvarsan while the treatment with quartan continues uninterruptedly. For this reason the Amsterdam mental hospitals are in the habit of commencing a course of neosalvarsan injections after the second quartan paroxysm. The dose administered increases from 150 milligrammes in the first week, to 300 in the second, and to 450 in the third. It remains stationary at 600 milligrammes during the following six weeks. Fig. 22 shows that the course of the fever is in no way influenced by this intervention.
Quartan fever, moreover, is less susceptible to quinine and atebrin than tertian. Eight grains of quinine stop a tertian fever without delay, but it takes them three or four days to bring about the same effect in quartan fever. As in cases of induced tertian malaria, the treatment is ended by giving the patiënt fifteen grains of sulfate of quinine or four grains and a half of atebrin daily for a week.
Number of parasites in the peripheral circulation. — The number of parasites found at the onset of the fever (pyrogenous limit) ranges from fourteen to five hundred-sixty to the cubic millimetre; so it is not nearly so variable, nor so high, as in tertian malaria at the onset of the first typical paroxysm (p. 236). In the subsequent accesses of fever this number rises to an average of nine thousand, ranging from



sixteen hundred to twenty-five thousand. Figures of eighty thousand parasites to the cubic millimetre, as sometimes observed in tertian fevers, are never met with in quartan. Hence, on an average, no more than 0.2 per cent of the red blood-cells are infected. At the end of the regular course of attacks the parasites have decreased to numbers ranging from two hundred fifty to six thousand to the cubic millimetre.
Complications and case-fatality. — Quartan is occasionally blamed for causing "nephrosis", a renal condition characterized by albuminuria and oedema, in the absence of urea retention and of hypertension. We have never met with it in our hundred and thirteen patients treated with quartan malaria. A few of them developed a slight and transient albuminuria; that is the nearest approach to the above named condition we ever came across.
Eleven of our patients died: a case-fatality of ten per cent, slightly higher than in patients treated with tertian malaria in whom it approximated eight per cent.



EPILOGUE
In the preface we announced this book as a pocket edition on the epidemiology of malaria. To some extent we have made good our word. We have seen the germs entering the human body by finding anopheles carrying both sporozoites and human blood. We have followed anopheles, on their short flights, carrying the infection from a focus of anopheline malaria to a house full of fresh victims. We watched the fate of the plasmodia in these new surroundings, traced their way into a fresh host, identified the long incubation under field conditions, observed the parasites preserved in human hosts between the seasons of anopheline transmission, and finally saw these hosts establishing a new focus of anopheline malaria. We have witnessed the rise and decline of a malaria epidemie, the infection imported by immigrants, continued by their infecting anopheles, brought to an end by their failing to keep anopheline infection going. We have seen the energy of the most conscientious family-physician set at nought by "healthy" carriers, whom we have proved to be the principal source of anopheline malaria in this country. We have admired the exquisitely adjusted adaptation allowing the parasites to carry on, in spite of climatic conditions, stabular deviation, the precarious life of pregnant anopheles, and sick-clubs, threatening



them with extinction. We have realized that they would never have been able to hold their own against these odds without the shortwings, anopheles specially adapted to act as vectors in the most adverse circumstances.
Was it not a stroke of true genius rigorously to separate the periods of reproduction and malaria transmission in anopheles? First the mosquitoes are permitted to devote all their time to reproduction. Then, when this activity has been carried to the highest pitch, it suddenly comes to an end. Huge masses of sexually inactive females are now available for malaria transmission in human habitations, in spite of the stables claiming a vastly larger share. The shortwings, protected from the vicissitudes of outdoor life by being confined in these houses, are now in a position to direct all their energy to ingesting, growing, and distributing malaria parasites, by continuing to feed notwithstanding their sexual inactivity. Finally, their confinement is not too strict to allow their taking short flights to distribute parasites in a wider circle.
Even this plan would not have been successful if anopheles had not found powerful allies in a human population who were in the habit of extending their territory at the cost of the sea. This condition offered the anopheline population the opportunity of adapting themselves to the salty water in the reclaimed land. This adaptation has been carried to the extreme by the shortwings. They find conditions for unlimited reproduction realized nowhere but in salty water, and they are protected from being tainted by the touch of their nearest



relatives, the longwings, by an insuperable barrier of sexual isolation. Malaria in this country exists by virtue of the shortwings; of the shortwings breeding in salty water, that is. If we had none but longwings here, or none but shortwings breeding in fresh water, there might occur an odd case of malaria once in a while, but nothing like what we now experience.
So this country will never get rid of malaria, unless all efforts are directed to the elimination of salty water. Drugs have helped us some way, by eliminating the malaria patients as a source of anopheline infection. But there remain the healthy carriers against whom drugs are of no avail. Housespraying may carry us a step further, since the shortwings select particular houses in which to set the stage on which malaria transmission is to be enacted. But drugs and house-spraying are no more than makeshifts. The permanent cure lies with the freshening of the surface waters of the coastal areas.
In all this we have acted up to the expectation we created. But we have failed to explain the permanent reduction malaria has undergone in the last quarter of the nineteenth century, the shift of the annual peak of malarial incidence from autumn to early summer, the disappearance of malaria from South-Holland and some sites farther inland, and the disappearance of quartan malaria. Of course, we may imagine shortwings, and even longwings, acting as carriers, even of quartan parasites, in areas where their numbers were far below those required for successful malaria transmission in our days, provided stabular deviation was non-operative and malaria patients did not take quinine. We may



imagine all this, but imagining is not explaining; no more than an imaginary variety of Plasmodium vivax, resembling James' Madagascar strain, can be said to explain the autumnal peak of old-time malaria. We shall never be able to grasp the situation of the past, unless our rural population slides back to conditions existing in the late eighteenth and early nineteenth century. We hope that we shall never be offered this opportunity to fill the gap in our knowledge.



INDEX
Adipose body. See under Longwings and Shortwings, other subjects.
Akersloot, village of, 2, 216 Aldershoff, Professor H., quartan malaria, 49 Alkmaar, town of, 2, 46, 49, 50 Amsterdam, 1, 2, 34, 220, 221, 228, 244, 248 anopheles, 61, 63, 67, 97, 98,
101, 128, 138, 202, 221 malaria incidence, middle igth century, 17-19, 21, 40 quartan, 19, 21 tertian, 19, 20 since 1920, 19, 20 Andel, van, J.C., house-spraying, 2x6
quinoplasmine, 190 Andel, van, Dr. M.A., fevers on
ships, 14 Anopheles, bifurcatus, 57 hyrcanus, 90 leucosphyrus, 86 listoni, 56 ludlowi, 89, 113 maculatus, 174
maculipennis, atroparvus of van Thiel (see also Shortwings), 66. 77. 79, 80, 83, 84, 89-91 from Sweden, crossed with Swedish messeae, 83, 84 breedingplaces, salinity, supposed to influence malaria transmission, 62, 63 density in various parts of Netherlands, 70, 71 geographical distribution, not correlated with — of malaria, 55, 56
Italian messeae. See A. maculipennis melanoon. labranchiaeofFalleroni, crossed
with shortwings, 88, 89 malaria infection. Sec Longwings and Shortwings, malaria infection. melanoon of Hackett, crossed with shortwings, 88, 89 messeae of Falleroni (see also Longwings), 77, 80, 84, 90, 91 from Sweden, crossed with Swedish atroparvus, 83, 84 sexual activity, ceases in full summer, 59, 60 commences in [early spring, 60 sexually inactive females, feeding around Amsterdam, 59
fasting around Leyden, 61 size of adults, smaller in salty regions, 62-64, II7 influenced by nutrition of
larvae, 64, 65 little influenced by salinity of breedingplaces, 64 subalpinus of Hackett, 88 type of Hackett, Martini, and
Missiroli, 87, 115 crossed with shortwings, 87, 88
from Sweden, crossed with
shortwings, 88 prevalence in Netherlands, 87, "5
plumbeus, 57 rossii, 56 umbrosus, 200 Anopheline malaria. See Longwings and Shortwings, malaria



infection.
Asperen, van, Dr. F., activity at
Wormerveer, 35 Assendelft, van, Dr. F., number of malaria parasites in peripheral blood, 236 Atebrin. See under Malaria, treatment.
Autumnal fevers. See under Malaria, history. fixation. See under Shortwings, other subjects. human malaria. See under Malaria, epidemiology.
Baak, Dr. J.A., geology of Ne-
therlands, 3 Bakker, Professor G., Groningen epidemie of 1826, 15 Barber, Dr. M.A., malaria in
Greece, 146 Bay-bar, formation of, after last glacial period, 3 Beeker, H. B., activity at Uitgeest, 215, 216. hydroquinine, 187, 188 Bengal, anopheles in, 55, 56 Bergh, van den, D., initial remittent in benign tertian, 51 Bolsward, town of, 2, 63 Bouman, Professor K.H., in-
duced malaria, 220, 234 Boyd, Dr. M.F., degenerated sporozoites, 131 normal spleen-rate, 33 Brugman, J.J.F., activity at Uitgeest, 2x5, 216 hydroquinine, 187, 188 Buil, C.G., preferential feedinghabits in adult anopheles, 143 Bijl, J.G., subsoil salty water, 200
Chagas, Dr. C., fumigation, 172 housing and malaria, 176 Christophers, Sir Rickard,housing and malaria, 176 species of anopheles in relation to malaria, 55, 56
Cinchona bark. See under Malaria, history.
Corradetti, Dr. A., crosses between Italian races of A. maculipennis, 88 Couvée, Dr. A., alleged malaria epidemie, 10
Darling, Dr. S.T., slight splenic
enlargement, 33 Davis, Dr. J.B., Walcheren epidemie of 1809, 11, 13, 14 Dawson, G.P., Walcheren epidemie of 1809, 13 Diemer, Dr. J.H., crosses between
long- and shortwings, 84 Dobreitzer, Dr. J.A., quartan, 21
Emden, van, Dr. J.E.G., quartan, 48, 49 Enkhuizen, town of, 2, 21 Ennur, anopheles, 55
Faber, L.A., autumnal malaria
in modern times, 40-42 Falleroni, Dr. D., A. maculipennis labranchiae, 88 messeae, 77, 80 Flevo, lake, 4, 5 Flushing, town of, 2, 13 Foei of anopheline infection. See under Shortwings, malaria infection .
Franeker, mental hospital at, 2,
37, 189, 216, 218 Friesland, province of, 2-5, 31-33, 37, 50, 116, 189, 227, 239
Galama, Dr. S.J., Groningen
epidemie of 1826, 15 Gonotrophic concordance. See under Longwings, other subjects. dissociation. See under Shortwings, other subjects.
Graaf, de, Dr.W., house-spraying, 207
Gracht, van Waterschoot van



der, subsoil salty wa¬
ter, 200
Grassi, Professor B., mortality in anopheles, 136, 138 semi-hibernation, 60
Groningen, province of, 2-5, 116 town of, epidemie of 1826, 2, 14-16
Groot, de, A.P.N., activity at Wormerveer, 36, 186 atebrin, 187, 188 quinoplasmine, 188, 189
Guelders, province of, 2, 4, 5, 34, 35. 7°. 72, 87, 119-121, 159
Hackett, Dr. L.W., A. maculi-
pennis, melanoon, 88 subalpinus, 88 type (typicus), 87 characters of ova, 75-77, 87 housing and malaria, 176 Hague, the, x, 2, 34 Healthy carriers. See under Malaria, epidemiology.
Heijden, van der, I., quartan, 49 Hibernation. See under Long-
wings, otner subjects.
Hill, Dr. R.B., with Rfvera, J. Hoeven, van der, J., salty water and A. maculipennis, 62 Holland, explanation regarding
namg of, 1 Honig, Dr. P.J.J., autumnal malaria in modern times, 42 malaria at Nieuwendam, 40 Honkoop, Dr. E., quartan, 21 Horst, van der, Professor L., in-
duced malaria, 221 House-spraying. See under Malaria, control.
Housing and malaria. See under
Malaria, epidemiology. Hydroquinine. See under Malaria,
treatment.
Hylkema, Dr. B., with Schüfiner, W.A.P.
Immunity. See under Malaria,
epidemiology and parasites. Incubation, long, in benign tertian. See under Malaria, epidemiology.
Infants, malaria in. See under Malaria, epidemiology.
James, Lt. Col. S.P., housing and malaria, 176 long incubation in benign ter-
tian, 150, 151, 228 Madagascar strain of PI. vivax,
42, 51, 227, 229, 239, 242 malaria in England, 23 mortality of Anopheles, 138, 139 species of Anopheles in relation to malaria, 55 Jauregg, Wagner von. Professor J., induced malaria, 220
King, W.V., with Buil, C.G. Korteweg, Dr. A.J., malaria in town of Alkmaar, 46 Korteweg, Dr. P.C., alleged malaria epidemie, 10 autumnal malaria in modern times, 42 cause of, 41, 42 home-strain and Madagascar strain of PI. vivax, 227, 230, 233. 241 hypothesis to explain primary malaria in spring, 55, 57-59, 61, 93. 96, 130, 132, 149, 150 initial remittent in benign ter-
tian, 51, 52, 187, 219, 230 long incubation in benign ter-
tian, 150 malaria, at Wormerveer, 31, 35,
36
induced, 219, 220 prolonged treatment with quinine, 190-193 susceptibility, to quinine, of PI. vivax in immune individuals, 237. 238 to salvarsan of Madagascar strain, 234



Külz, Professor L., malaria pro- Longwings, other subjects (cont.)
phylaxis, 53 mating in confinement, absence
of, 73. 79, 82 with shortwings, 73
Lampe, H.A., activity at Wor- predominant in fresh-water re-
merveer, 36, 186 gions, 68, 71, 72
atebrin, 187, 188 rare in salty regions, 68, 71, 72
quinoplasmine, 188, 189 stabular deviation, 81, 105
Lancisi, J.M., land-reclamation zoophilous, 8i, 105
an anti-malaria measure, 6 adult females sexually active,
Land-reclamation, Netherlands choice of food, 81, 105, 106
owing its existence to, 5, 251 oviposition, preferential,
risk to public health, 6, 116, 200 apparently absent in labo-
Lane, Col. C., housing and mal- ratory, 118
aria, 176 conspicuous in the field, 124,
Laveran, Dr. A., land-reclamation 125
an anti-malaria measure, 6 adult females sexually inactive,
modernization of diagnosis by adipose body, 99, 101
discovery of, 28 agglutinins in saliva, 74
Leyden, town of, 2, 49, 61-63, 67 anticoagulins in saliva, 74
Limburg, province of, 2, 87 capable of passing winter with-
Long incubation in benign ter- out food, 101
tian. See «jfcferMalaria, epidemio- digestion of blood, 73, 74
logy. fasting in autumn, 61, 66, 99,
Longwings, general subjects, 100
breeding true, through one ge- geographicaldistribution, 68-72
neration, 65, 79 gonotrophic concordance, 99
crossed with shortwings, 83-85 hibernating, in attic bedrooms,
isolated from shortwings, by 67, 99, 105
interspecific sterility, 87 in uninhabited shelters, 67
not by geographical or eco- maxillary teeth, 64
logical barrier, 81, 82 permanent fat, 101
nomenclature, 90, 91 salivary glands, during hiber-
species or variety, 81, 82, 91, 92 nation, 127, 130
strain of, peculiar, 90 segregation, 67
Longwings, malaria infection, wing, 64
in laboratory, in summer, not adult males, hypopygium, 72, 78
inferiorto — in shortwings, 64, wing, 65
65 eggs, columellae, 76
in winter and autumn, inferior fresh water, commonin,124,125
to— in shortwings, 74, 75, 222 intercostal film of floats, 72, 75
in nature, in autumn, greatly number of ribs of floats, 75
inferiorto — in shortwings, 127 papillae, 76
Longwings, other subjects, pattern of upper surface, 76
adults, animal habitations, 61, salty water, rare in, 124, 125
67, 105 transverse bars, 76
geographical distribution, 68, larvae, breedingplaces, 119, 121-
72 124
Malaria. 17



Longwings, other subjects (cont.) Malaria, control (cont.)
fresh water, predominant in, village of Marken, applied in,
119. 123. *99 210, 211
geographical distribution, 1x9- Uitgeest, applied in, 2x1, 212,
122 pyrethrum extract,
pilotaxy, 72, 78, 1x4, 1x5 preparation, 204
preferential breeding habits, solution of, acting,
apparently absent in labo- as an insecticide, 204
ratory, 117, 118 as a repellent, 211, 212
conspicuous in the field, 118- in the shape of dropiets, 204
124 not as a gaz, 204
salty water, rare in, 119, 123 method of spraying, 205
shortage of — in salty water, quantity required, 204, 210,
with regard to number of eggs, 212
125 renders not merely insensible
Loo, van, Walcheren epidemie of but kills, 204
1809, 14 strength of, 204
Lovink, Dr. H.J., importance of use in human habitations,
fresh Yssel-lake, 199 204, 210-212
by destruction of adult anophe-
Malaria, control, les,without species-sanitation,
by destruction of adult ano- lysol, 202
pheles, with species-sanitation, results, 202
J34> *35. 202-218 vacuum cleaner, 202
house-spraying, circumstances by destruction of larvae, with
rendering—useful, 172,175 species-sanitation, by-product
costs of, 207, 208, 213 of measures serving other pur-
criteria guiding choice of poses, 198
houses to besprayed, 209,210 hoped-for effect of, equal to
difficulties encountered in, species-sanitation, 199
205-207 uncertainty of — materializ-
effect on, anopheline infection, ing, 200
162, 212, 213 by destruction of larvae, with-
malaria incidence, 215-218 out species-sanitation,
numbersofanopheles,2ii,2i2 costs of, 197
flights, short, of anopheles, parisgreen, 194, 195
rendered noticeable by, 212 results, 196, 197
interval allowed between two spindle oil, 195
operations, 134, 135, 213 tarras, a diluent of parisgreen,
method of, 132, 204, 205, 210, 194
212 by drugs, with species-sanita-
mistakes, 211, 212 tion, 190-193
palliative, no more than a, 218 by drugs, without species-sani-
proper season for, 132-134, tation (see also Malaria, treat-
173, 206, 210 ment), 182, 187-189
time anopheline infection re- organization of, commission in
quires to mature estimated North-Holland, 8, 32, 205-207,
by, 214 209, 211



Malaria, control (cont.)
commission of Sanitary Coun-
cil, 8, 32, 37, 49 cross-societies, 7 sick-clubs, 7, 28, 29, 43, 165, 184, 234 screening, 146 Malaria, epidemiology, healthy carriers, adults and children, 166-168 anopheles infected, by repeatedly feeding on, 173, 174 by sharing a house with, 173-
175, 181 under field conditions, 167-
169, 188 under laboratory conditions, 166, 167, 219 comparison of anopheline infection due to — and to summer patients, 169-171 continuing their existence in spite of sick-clubs, 7, 165-167, 172, 181, 184, 185, 188, 250 losing their capacity of infecting anopheles, 179,180,250 number of parasites — carry, 166, 167, 174, 188 occasional increase of, without notable morbid symptoms, *73. 174 anopheline infection increased by, 174 prevalence, 44, 54, 144, 154,
165-170, 177 source of anopheline infection, circumstances rendering — a, 173-175
more important than summer patients, 169 state of health of, uncertainty regarding, 44, 165, 167, 174, 176, 181, 184 initial remittent in benign tertian, 51-54, 187, 219, 230 absent, after previous attack of subtertian, 51 in quartan, 246 Malaria.
Malaria, epidemiology (cont.) disabling effect on non-im-
munes, 52, 54 doubt as to its invariable oc-
currence, 51 importance for colonization in
New Polder, 54 little influenced by drugs, 52 present after previous attack
of quartan, 230 scarcity of parasites, 52, 236 subtertian, in, 53 long incubation in benign tertian, experiments, 50,150,151, 219, 228, 229 field observations, 57, 58, 150,
155, 162, 178, 250 incidence, 50, 152, 153, 160, 162 malaria, in spring, explained
by. 58, 151
in summer, explained by, 151, 161
malaria, accumulation of, around foei of preceding year's anopheline infection, in accordance with dispersal of infected anopheles, 157-159, 161, 177, 178, 203 apparent absence of, in houses with anopheline infection in preceding autumn, 153-156, 207
explained by immunity of inhabitants, 154-156 autumnal human, 152 incidence of, 20, 41-43, 152 field-born, 175
focal distribution of, 44-46, 106, 142, 162, 163, 178 explained by short flights of anopheles, 111, 112, 158, 159, 162, 178, 250 geographical distribution, 23, 25. 32, 34. 35, 119. 120 house-born, 175 housingand —, 143-146,172-177 incidence, annual variation, 35-4°
17*



Malaria, epidemiology (cont.) cause of, 37, 38, 98 not world-war, 38, 39 causing difficulties in estimating effect of anti-malaria measures, 39, 40 exacerbations, major, 37, 98 minor, 37, 38, 98 synchronism showing in, 37, 40
higher when calculated according to parasite-carriers thantofeverpatients, 44,166 in centre and periphery of a
town, 46 seasonal variation, 40-43 with vernal and autumnal peak, 41 explanation of, 41-43 infections acquired in summer from sexually active anopheles, 147, 159, 160, 163 incidence of, 160 man acting as only host from December until August, 131, 162, 250 morbidity, adults, 46, 47 children, 46, 47, 48 infants, 46, 47, 160 not the same as malaria incidence, 43 mortality, statistics unreliable, 22.
mosquito and man acting as hosts from August until December, 131, 162 presence of, in houses apparently without anopheline infection in preceding autumn, 154 explained by infected anopheles escaping from neighbouring foei, 157-159, 161,178 reduction of, permanent, cause of, 23-30, 37 temporary (see also Malaria, incidence), 37 relapse, fever —, 185, 186
Malaria, epidemiology (cont.) parasite —, 185, 186 transport by anopheles, on long flights, 163, 164 on short flights, 158, 160-163, 178, 212, 250 transport by man, 177-179 spleen-rates, pronounced useless in North-Holland, 32, 33, 57 proved valuable in North-Hol-
'and, 33, 34 small degrees of enlargement not to be neglected, 33 to be taken in autumn only, 33 without malaria (normal spleenrate), 33 spring patients, 165, 168, 209 summer patients, 165, 169, 170, 171, 209 Malaria, history, cinchona bark.supposed effect, 11
use of, 26, 27 epidemics. See Amsterdam, Groningen, Walcheren, fevers, autumnal, middle igth century, 19, 20 different from autumnal malaria of modern times, 43 season of, coinciding with — anopheline malaria of present time, 20 explained by supposed prevalence of parasites resembling James' Madagascar strain, 20, 21,42, 43,50,51, 219. 253 intermittent, different connotation in the past, 11, 12 simple tertian, geographical distribution in 1875, 22-24, 252
vernal, middle igth century, 19, 20
malaria, changes the meaning of the word — underwent during igth century, 9, 10 subtertian, doubts as to presence in the past, 16, 48



Malaria, history (cont.) misconceptions regarding meaning of apparently familiarterms, 9-12
Malaria, induced for tberapeutic purposes, epidemiological evidence derived from, 42, 50-52, 130, 131, 166, 167, 173, 174, 219 organization at Amsterdam, 220,
221, 228, 244, 248 paralysis, general, of the insane, incidence at Amsterdam, 220 quartan, case-fatality, 249 tertian, case-fatality, 244 provocation, 240, 241 risk of spreading, 241-243 stimulation, 240, 241 Malaria, parasites,
Plasmodium malariae, spontaneous occurrence, early records, 21 present time, 48-50 Vienna strain, incubation, inoculated, 245, 246 mosquito-borne, 245 parasites in peripheral circulation, number of, 248, 249
reciprocal immunity, absent to PI. vivax, 239, 240, 245 susceptibility to methylene blue, doubtful, 247 neosalvarsan, absent, 247, 248
transmission by anopheles, 245
type of fever, 246, 247 Plasmodium vivax home-strain, incubation, inoculated, 230 mosquito-borne, 230 long incubation. See under Malaria, epidemiology. induced infection, gametocytes in, 241
merozoites, number of, 227 reciprocal immunity, absent to PI. malariae, 239, 240, 245
Malaria, parasites (cont.)
complete to other home-
strains, 239 incomplete to Madagascar strain, 239 susceptibility to neosalvarsan, 235
type of fever, 230-232 Plasmodium vivax James' Madagascar strain, amphigony defective at room temperature in early autumn, 242, 243 explains formerautumnalfevers
See under Malaria, history. incubation, inoculated, 230 mosquito-borne, 230 long incubation exceptional, 229
merozoites, number of, 227 parasites in peripheral circula-
tion, number of, 236, 237 relapse-rate, after inoculation, 232, 241 after mosquito transmission, 233, 234, 242 susceptibility to neosalvarsan,
234. 235 type of fever, 230-232 Malaria, treatment, atebrin one week, anopheline infection, 188 duration of fever, 187 relapse-rate, 187, 188 hydroquinine one week, anopheline infection, 189 relapse-rate, 187, 188 plasmoquine. See quinoplasmine. quinine, factors assisting in having — used for its proper purpose, 27-29 influence on malaria incidence,
26, 27, 29, 182 malaria patients, of limited use
to, in former times, 26, 27 poor, not for the, in former
times, 27 price, 27, 28



Malaria, treatment (cont.)
treatment prolonged through whole season of anopheline infection, 190-193 quinoplasmine, three weeks, relapse-rate, 190 two weeks, anopheline infection, 189 relapse-rate, 188, 189, 233 Marken, village of, 2, 210, 211,
213, 2x5, 216, 218 Martini, Professor E., A. maculipennis type (typicus), 87 characters of ova, 75-77, 87 long incubation in malaria, 150 malaria epidemics in the past, 20 Martini, Dr. Erich, malaria in
spring, 58 Martirano, Dr. F., periodicity of anopheline infection, 135-137 Mazure, J.P., importance of fresh
Yssel-lake, 199 Medemblik, town of, 2, 37, 93, 94,
iox, 105, 107, 193, 196, 197 Melville, Col. C.H., Walcheren epidemie of 1809, 12 Meuse, river, 2, 4 Missiroli, Professor A., A. maculipennis type (typicus), 87 characters of ova, 75-77, 87 housing and malaria, 176 parisgreen, 194
two annual peaks of benign tertian, 20 Mitzmain, B., man, not anopheles, reservoir of parasites in winter, 131 Mühlens, Professor P., long incubation in malaria, 150
New polder, in former Zuydersea, 2, 33, 104,107-110,196,199 colonization in, 33, 51, 54 natural larval control in, 196 Netherlands, explanation regarding name of, r origin, 3-5 topography, 2
Nicol, W.D., with James, S.P. Nieuwendam, village of, 2, 37, 39,
40, 42, 46, 217 North-Holland, province of, 1-8, 49, 5°. 63, 72, 108, 118, 193 malaria, 23, 31-35, 37, 44, 62, 146, 147» *82, 183,200,202,227, 239, 242 surface-water brackish or salty, 34. 69, 119-122, 199, 200, 201, 203, 218 Nijhofï, Dr. I., Groningen epidemie of 1826, 15 Nijkamp, J.A., anopheline incidence, 93 long flights, 108 pyrethrum, 204
Oil, spindle. See under Malaria, control.
Oöcysts. See under Shortwings,
malaria infection.
Orenstein, Dr. A.J., with Prince, le, J.
Overijssel, province of, 2, 87
Paralysis, general, of the insane.
See under Malaria, induced. Parisgreen. See under Malaria, control.
Park Ross, Dr. G.A., house-
spraying, 172 Peat-land, resistant, 4, 34 Pennink, J.M.K., land-reclamation in former Zuydersea, 201 Piebenga, P. J., malaria in mental hospital at Franeker, 37 quinoplasmine, 189, 190 Popken, F.A.L., epidemie of
1826, 12 Prince, le, J., destruction of anopheles in houses, 172 range of flight in anopheles, 106 Pringle, Sir John, cinchona bark
in the army, 27 Pyrethrum. See under Malaria, control.



Quinine. See under Malaria, treatment.
Quinoplasmine. See under Malaria, treatment.
Republic of the United Provin-
ces, 1 Rhine, river, 2, 4 Rice, Dr. J.B., with Barber, M.A. Ris, T-, activitv at Wormerveer, 36, 186 atebrin, 187, 188 quinoplasmine, 188, 189 Rivera, Dr. J., pedigree cultures
in A. atroparvus, 79 Rockefeller Foundation, International Health Division of, activity in North-Holland, 193 Rome, 135 Rook, de, Dr. H., long flights of anopheles, 107 non-toxic oils, 195 parisgreen, 193
spleen census in North-Holland, 33
Ross, Sir Ronald, pyrogenous
limit, 236, 248 Rotterdam, 1, 2, 34, 49 Roubaud, Professor E., criticizes cross-mating experiments, 85 degenerated sporozoites, 130 maxillary teeth, 63 zoophilism and animal protection, 137
Scheldt, river, 2, 4 Schoo, Dr. H.J.M., malaria epidemie of 1902, 37 Schüffner, Professor W.A.P., long incubation, 150 malaria control, by killing adult anopheles, 172 prophylaxis, 53 mortality of anopheles, 136 slight splenic enlargement, 33 Segregation. See under Longwings and Shortwings, other subjects. Sergent, Dr. Edm., inactive sporozoites, 152
Shortwings, general subjects, breeding true through one generation, 65 several generations, 78, 79 crossed with labranchiae, 88, 89 longwings, 83-85 melanoon, 88 typicus, 87, 88 discussion of cross-mating experiments, 85-87 isolated, from longwings, by interspecific sterility, 87, 251, 252 not by geographical or ecological barrier, 81, 82 from mediterranean races, by geographical barrier, 89 incompletely, by interspecific sterility, 88, 89 nomenclature, 89-91 pedigree cultures, 78, 79 species or variety, 81, 82, 91, 92 strains of, peculiar, 90 Shortwings, malaria infection, in laboratory, females, caught in nature, suitable for infection in autumn, 139, 222 unsuitable for infection in spring and summer, 139, 222 laboratory-bred, suitable for infection in spring and summer, 139, 222 in autumn and winter, superior to — in longwings, 75, 222 in summer, not superior to — in
longwings, 64, 65 technique, of infecting females, 225, 226 of keeping infected females alive, 226, 227 Shortwings, malaria infection, in nature,
during sexual activity, 127, 128,
I41. I59, 160, 163 during sexual inactivity, accumulationinautumn, 134,140 December, no longer important after, 132



Shortwings, malaria infection, in Shortwings, malaria infection, in
nature (cont.) nature (cont.)
different portions of same rate of — infection, in presence
house, in, 157, 217 of strong and slight stabular
excessive in North-Holland, deviation, 138
27. *46, *8i stage of development indicates,
explained as only means for time anopheline infection re-
malaria to hold its own, 147 quires to mature, 134, 135
wholly dependent upon high period duringwhich anopheles
density of anopheline popu- acquire fresh infections, 133
lation, 70, 122, 172,173,176, periodicity, 128, 129, 162, 167-
181, 218, 251 169
fat females, in, compared with cause of, mortality during
— of sexually active females, sexual activity, 138-140
Ï41 new-born adult females, 136,
females carrying —, sluggish- 140
ness of, 157 stabular deviation, 102, 103,
focal distribution of, 143, 156, 137, 138, 142.
I57> I7^, 179- sporozoites, degeneration, 130,
foei of anopheline infection, 131, 161, 162, 219, 226, 227
!57. 203, 208 females carrying — and human
spreading infected anopheles blood at the same time, 149
to neighbouring houses by infection in January-April not
short flights, iii, 157,158, contradictory to Korteweg's
161, 162, 177, 179, 212, 250 hypothesis, 130, 132
no longer after October, 212 rate of — infection, inversely
greatly superior to — in long- related to rate of pregnant
wings, 127 females, 141, 142
influenced by, construction of maximum of, coinciding with
house, 144, 145, 176, 206 high rate of engorged
customs of inhabitants, 145, females, 148, 149
146, 176 preceding by 8-9 months the
number of inhabitants, 143, maximum of human mal-
176 aria, 151
vitality of parasites in human Shortwings, other subjects,
carriers, 179, 250 adults, animal habitations, 67,
maturation, length of period of, 97, 98, 101, 104, 105
I34. I35, 213-215 annual variation in number,
October, no more fresh — 97, 98
after, 133, 134, 178, 192, 208 in relation to temperature, 98
rate of, rising with number of comparatively rare in fresh-
anopheles per house, 143 water regions, 71, 72, 122
responsible for most malaria, geographical distribution, 68-70
i53"i56, 158, 159 mating in confinement, 73, 81,
Greece, comparison with con- 82
ditions in, 146 profusion of, nowhere but in
oöcysts, degeneration, 132, 133, salty regions, 68, 69, 122, 181,
243 251



Shortwings, other subjects {cont.) season of, not starting before
July, 93. 96 seasonal incidence, in human habitations, 95 in open shelters, 94 stabular deviation, 81, 101-105, 173
zoophilous, 81, 101, 105, 106 adult females sexually active, choice of food, 81, 105, 106 first appearance, 141 long flights, experiments, 107, 108 field observations, 109-111,197 oviposition, preferential, apparently absent in labora-
tory, 118 conspicuous in the field, 124,
125
adult females, sexually inactive, adipose body, 99, 100, 141 agglutinins in saliva, 74 anticoagulins in saliva, 74 autumnal fixation, 96, 97, 111,
112, 157 digestion of blood, 74 feeding in autumn, 66, 99, 100, 181
first appearance, 99, 100, 128,
141, 173, 208 gonotrophic dissociation, 99
173. i8r incapable of passing winter
without food, 100 maxillary teeth, 64 salivary glands during hi-
bernation, 127, 130 segregation, 67 semi-hibernation, animal habitations, 67 attic bedrooms, 60, 67, 99,
206, 211 begins by the end of July, 60. cupboard-beds, 157, 211 short flights, 111, 112, 158, 162,
181, 212, 250 transient fat, 100
Shortwings, other subjects (cont.) wing, 64 adult males, hypopygium, 72, 78 long flights, not inferior to those in females, 108, 110 more numerous in open shelters than in stables, 93, 94 wing, 65 eggs, columellae, 76 fresh water, comparatively rare
in, 124, 125 intercostal film of floats, 72, 75 number of ribs on floats, 72, 75 papillae, 76
pattern of upper surface, 77 salty water, greatest profusion in, 124, 125 larvae, breedingplaces, broad canals, 116 lakes, 5, 6, 115, 116 narrow ditches, 6, 115, 116 swamps, 6, 115 vegetation, 116, 117 fresh water, comparatively rare
in, 119, 123, 199 geographical distribution, 119122
pilotaxy, 72, 78, 114, 115 preferential breedinghabits, apparently absent in labora-
tory, 117, 118 conspicuous in the field, 118124
salty water, greatest profusion in, 6, 119, 123, 181 the first cause of the malariousness of the Netherlands, 70, 71, 122, 173,181, 218, 251 seasonal incidence, 95, 96 shortage of — in fresh and brackish water, with regard to number of eggs, 114,125 suggests crowding out of shortwings by longwings, 125, 126 not confirmed by laboratory observations, 126



Shortwings, other subjects (cont.) technique of rearing in laboratory, 65, 86, 223-225 Shute, P.G., with James, S.P. Sick-clubs. See under Malaria, control.
Sinton.Col. J.A.,plasmoquine,i86 Sloten, village of, 2, 41 South-Holland, province of, 1-5, 22, 63, 70, 72, 118 malaria rare in, 23, 34, 35, 252 surface water mostly fresh, 34, 69, 119-122, 199 Species-assaineering, Dutch prototype of term species-sanitation, 56 Species-sanitation, 56, 61, 62, 66, 77, 80, 81, 85, 115, 117, 182, 198, 199, 202, 217 conditions to be fulfilled for — to be useful, 1x2, 127 finding principal vectors by rate of infection in nature, 57, 127
inventory of breedingplaces, 113, 118 to be based on the larvae only,
"3. "4 Sporozoites. See under Shortwings, malaria infection. Stephens, Professor J.W.W., species of anopheles in relation to malaria, 55, 56 Stratman-Thomas, W.K., with Boyd, M.F.
Tesch, Dr. P., geology of Netherlands, 3 Thiel, van, Dr. P.H., anopheles around Leyden, 61, 62, 67 of larger size than around Amsterdam, 63, 64, 66, 80 explained by absence of salty water, 63, 64, 117 crosses between long- and shortwings, 84 no outcrowding of one race by the other, 126
Thiel, van, Dr. P.H. [cont.) sexually active anopheles in-
fecting man, 159 shortwings called atroparvus, 66 Thuessink, Professor E.J., Groningen epidemie of 1826, 15, 16 Thijssen, Dr. H.F., former malariousness of Netherlands, 30 Tillema, Dr. S., geographical distribution of short- and longwinged adults, 70 Torren, van der, Dr. G., geographical distribution of short- and longwing larvae, 119 Tropical Hygiene, Zoological Department in Amsterdam Institute of, 220, 221 Turrill, W.B., difïerence between species and variety, 80, 81, 90 significance of interspecific sterility, 80, 82 Typicus. See A. maculipennis type.
Uitgeest, village of, 2, 33, 37, 43, 46. 54. 95. ï°5,128,129,137,138, 154, 156-159, 161, 169, 187-189, 208, 210, 211, 213, 215, 216, 218 Utrecht, province of, 2, 4, 5, 23 34. 35. 63, 70, 72, 118, 120, 121, 123
Vernal fevers. See under Malaria, history.
Vries, de, Professor Hugo, elementary species, 78, 91, 92 misuse of the term variety, 91 pedigree cultures, 78
Wagenaar, J.H., activity at Marken, 211 Walcheren, island of, epidemie of 1809, 2, 12-16, 41 doubts as to its malarious nature, 13, 14, 16 Watson, Sir Malcolm, antagonism between anopheline species, 125 control of A. umbrosus, 200 species-sanitation, 56



Wesenberg-Lund, C., Danish A.
maculipennis, 61 Wibaut, Mrs. N.L., salinity of
water in North-Holland, 200 Winckel, Dr. Ch.W.F., induced malaria, 220 quartan, 21 Wormerveer, village of 2, 31, 35-37. 39, 4°. 42, 44"47, 52, 155. 186-189, 191, 207, 210, 216
Yssel, river, 198 Yssel-lake, freshening of, 5, 198, 201
malaria in relation to, 201
Yssel-Lake, (cont.) new name of impounded Zuydersea, 2, 5, 198
Zaandam, town of, 2, 37 Zaandijk, village of, 2, 37 Zealand, province of, 2-5, 22, 116 Zuydersea, dangers attendant upon land-reclamation in, 201 impounding of, consequences, 116, 201 objects, 198 origin of, 5