The transmission of malaria in Borneo

537 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE. Vol. 40. No. 5. May, 1947.

COMMUNICATIONS. T H E TRANSMISSION OF M A L A R I A IN BORNEO. ]BY

JOHN MeARTHUR, M.R.C.S., L.R.C.P.,* Malaria Research Officer, North Borneo. INTRODUCTORY. Malaria appears to be hyperendemic throughout the greater part of Borneo. Although some parts, especially on the coast, seem to be quite healthy, there are many places where the spleen rate is consistently 100 per cent.; and' malaria appears to be almost universal throughout the junglecovered interior. Before 1939 no full time research had ever been carried out on the vectors ,of malaria in Borneo nor, consequently, upon rational means for its control. The following pages are an account of work carried out for 3 years under the North Borneo Government, just before the Japanese occupation. Borneo is the third largest island ir~ the world. It is exactly astride the Equator, and is a mountainous, well watered, jungle-covered .and fertile land, with a good climate, and, apart from malaria and those diseases which are a result of the general low standard of living, it is not an unhealthy land.. T H E VECTORS OF MALARIA.

When this work was begun, little was known regarding the vectors of malaria in Borneo. It had long been believed and recorded that Anopheles maculatus, the most dangerous mosquito in the interior of Malaya, was also the most dangerous mosquito in British Borneo. For the existence of this belief there was a good deal of evidence, and upon it anti-malarial measures had frequently been based. SHIRCORE (1937), for e x a m p l e , wrote that " A. maculatus, a notorious carrier in Malaya, is widely, and typically, distributed throughout the areas visited, and must be regarded as the chief vector," and there was a good deal of other published evidence of the virulence with which this mosquito was credited. * Publication of this article--written in 1941--was interrupted by the occupation of Borneo by the Japanese, and my internment by them.

I have pleasure in acknowledging the co-operation of the British North Borneo Government and the helpful interest of Sir MALCOLMWATSON,then Director of the Ross Institute, in this work, a full account of which will be published shortly.

538

THE TRANSMISSION OF MALARIA IN BORNEO

In this belief, therefore, the research was begun under the impression that malaria work there would prove to mean simply the application of those measures which had proved so successful in Malaya, but by some more economical means suited to the far less wealthy land of Borneo. THE

EVIDENCE OF JESSELTON.

Jesselton is a small port on the West Coast of North Borneo. In this town, for 5 months in the early part of 1939, research was carried out on local malaria and mosquitoes. This work at the time appeared fruitless, as it was not possible to follow it to its conclusion, but it later proved to have a quite important place in the picture of Borneo malaria. An examination of some 1,200 children from all parts of the town, to determine the intensity and distribution of malaria, found only 2"5 per cent. to have enlarged spleens; and since these few could all be traced as having recently come from highly malarious areas, Jesselton was accepted as a healthy locality, with no malarial transmission. Areas only a few miles away, however, were found to be highly malarious, with spleen rates of 80 per cent. and more. Next, a larval examination was carried out in the town. This yielded very few anopheles, and seemed to be in keeping with the absence of malaria. After this, however, the larval survey was continued into the hills behind the town, an area which had never been examined before. Here, surprisingly, well within striking distance of the town, were found about fifteen streams, all breeding anopheles, among which A. maculatus occurred in considerable numbers. Thi~ led to much speculation as to why A. maculatus should be present in close proximity to a town with no malaria transmission. Three possible explanations presented themselves: 1. That the breeding m a y have been abnormal, since the survey was carried out following a drought. 2. That the hills and jungle may have constituted a barrier to the flight of the mosquito. 3. That A. rnaculatus might be of little importance as a vector. Ir was not possible at the time to investigate these possibilities, ~ts it was necessary to proceed up country. Later, however, it was possible to fit the peculiar findings of Jesselton into the still more peculiar picture of malaria in the interior. "I'HE EVIDENCE OF TAIVlBUNAN.

Tambunan Valley is an elevated plain in the interior of North Borneo, lying at an altitude of some 2,000 feet. It is about 25 miles from the sea, but separated therefrom by a 6,000-foot mountain range. The plain, some l0 miles long, varying from a quarter to 2 miles broad, is well-watered by rivers, mountain streams and irrigation channels, and surrounded by high

JOHN McARTHUR

539

jungle-covered hills, which rise abruptly from its margin and climb to summits of 5,000 and 6,000 feet. Paddy cultivation, primitive and irregular, practically covers the plain, and herds of buffalo graze over the remaining land. The climate here is equable; temperature is moderate, and rainfall not too great, with an ill-defined wet and dry season. It is an isolated region, and inaccessible to wheeled traffic, but relatively populous, inhabited by some 6,000 people. The population is fairly static, and there is very little export and import, although some movement takes place from the plain into the neighbouring areas, and to supply estate-labour near the coast. The houses, primitive and built of bamboo, are collected into scattered villages, and distributed over the plain and into the hill ravines. Amongst the people of Tambunan, malaria was said to be intense, alcoholism rife, and standards of living low. This area, therefore, with its malaria and its static population, presented itself as suitable for malaria' research. Here, the first step was to confirm the presence of malaria. T h i s was not difficult. Every report from dispensary dressers, and by any visiting medical man, referred to the intense malaria in Tambunan. The survey by Dr. SHIRCORE showed many villages with a spleen rate of 100 per cent., and a rapid and random examination of a few villages for spleens and parasites, soon confirmed the same thing. In this highly malarious area, therefore, work was begun. EXTENSIVE PRELIMINARY LARVAL SURVEY. A preliminary examination of possible breeding places of mosquitoes, revealed the presence of an enormous number and variety of anopheles breeding places distributed widely over the whole plain. Pools, streams, swamps, seepages, paddy fields, irrigation channels, and buffalo wallows--all presented a choice of breeding places, while even the hoof prints of cattle, and the widespread hollow bamboo fences, bred mosquitoes in numbers. The entire ])lain, in fact, seemed to be one vast anopheles breeding place. There appeared to be, therefore, every reason why the area should be subject to very intense malaria, and the problem seemed to be rather how to control it, than how to explain it. This. however, was not altogether the case, as it was very soon discovered that it was by no means easy to explain malaria transmission here. INTENSIVE ROUTINE MONTHLY SURVEYS.

The next step apart from preliminary map-making and organization, was to determine what species of anopheles were actually present, with, if possible, some assessment of the relative importance of each. This was done by taking an experimental area of about a square mile, which included paddy fields and irrigation channels, streams, swamps, pools, seepages, buffalo wallows, water-

540

THE TRANSMISSION OF MALARIA IN BORNEO

filled bamboos, and water-logged hoof prints. These were situated in native villages, in open country, and in jungle, so that every possible variety of breeding place might be examined. The area was carefully surveyed for the breeding of anopheles larvae, and records were kept of the type of breeding place preferred by each species. This survey ,was repeated every month for 24 months, keeping careful records of the breeding habits, in the belief that, when the vector species were incriminated, it might be possible to have full information regarding their habits, and an effective strategy might be available for a campaign against them. It was found that most of the species of anopheles present were very fastidious in their choice of breeding places. Certain species selected conditions which were shunned by others, preferring-to die rather than to be compelled to breed in conditions other than their preference. It was found that twelve species of anopheles were breeding, four of these in abundance, and the remaining eight more rarely, in the following numbers: and proportions out of a total catch of over 44,000:-SPECIES OF ANOPHELES

A. A. A. A. A. A. A. A. A. A. A.

barbirostris V . d . W u l p . . . . . . kochi D 6 n . . ... . . . . . maculatus T h e o . . . . . . . philippinensis L u d l . . . . . . . karwari ( J a m e s ) barbumbrosus Strickl. & " C h o u d l leucosphyrus D 6 n . . . . . . . tessellatusTheo . . . . aitkeni J a m e s . . . . . . "'" aitkeni bengalensis P u r i iii aitkenipalmatus ( R d w t . ) ...

TOTAL

P e r cent.

25,107 = 57 7,561 = 17 6,147 = 14 4,804 = 11 267 = 1 79 = 0 74 = 0 ~') F i g u r e s lost d u r i n g .~ | J a p a n e s e o c c u p a t i o n , ~ - b u t all less t h a n 0.2 i[ p e r cent.

A. aitkeni (var. nov.) ...... ?J The problem was to determine which of these were vectors, and which were harmless mosquitoes. Suspected Species.--It was assumed that the first species which sl~ould be suspected as vectors should be those which were found to be the most widely breeding, and those which had been proved to be dangerous elsewhere. For this reason, consideration was given chiefly to the first four of this list, mosquitoes which were not only found in numbers in Tambunan, but which had all been found to be vectors of malaria in other places. The most common anopheles in Tambunan--A. barbirostris--is a vector both to the east and to the west of Borneo, in Celebes and in parts of Malaya. Being found in such large numbers in Tambunan, breeding in pools and paddy fields over the entire plain, it had to be under great suspicion. A. kochi, found breeding generally in dirty water on the plain, is a vector in parts of Sumatra, and required to be considered. The paddy-field breeder, A. philippinensis, is a vector of malaria in parts of India, and, breeding in enormous numbers toward the end of the irrigation ~ason in Tambunan, was under considerable suspicion.

JOHN McARTHUR

541

A. maculatus, although not in great numbers, was found breeding typically in hill seepages and streams in cleared ravines; and on account of its evil reputation in other parts of Borneo, and its dangerous nature in Malaya, was naturally under the greatest, suspicion. Each of the remaining eight anopheles numbered less than 1 per cent., and none was under any suspicion in Tambunan, nor, as far as could be ascertained then, in any other part of the world, and could not therefore be very, seriously suspected. INTENSITY AND DISTRIBUTION OF TAMBUNAN MALARIA.

A study was next made of the intensity and distribution of malaria in Tambunan. A survey was carried out in twenty-five villages and in a representative area starting in the centre of the plain among the rice fields, working to the jungle-covered hills at the periphery, and up into the inhabited ravines. This includedspleen and parasite examinations in some 1,200 children. As a result, a peculiar and interesting situaton was found. It was found that villages in the middle of the plain, in the rice fields, and therefore in the situation of the most intense anopheles breeding, were relatively healthy, with spleen rates of only about 20 per cent., but that malaria steadily increased on approaching the surrounding hills, until in the hills and in hill ravines it was 100 per cent., although there was far less anopheles breeding there. There appeared to be no appreciable seasonal variation in malaria throughout the year. This was taken as evidence that A. rnaculatus, being a hill-breeding mosquito, was the vector of malaria. It had to be admittedv however, that it pointed equally to the other hill breeding species. A. karwari, A. leucosphyrus, and A. aitkeni in its four varieties, as possible vectors. These were all found only in very small numbers in Tambunan, and were not under great suspicion, as they were believed to be harmless in Malaya. It was also evidence that A. barbirostris, A. kochi, and A. philippinensis, being plair~ breeders, were of little significance, as they were all found in greatest numbers where malaria was slight. Thi~ was an excellent illustration of the fact that water is by no means necessarily dangerous in malaria; that, in fact, the vast areas of water covering the plain, t h e paddy fields and irrigation channels, pools, streams, seepages, buffalo wallows, wells and swamps, were at least relatively harmless; whereas the much smaller areas of water in the hills, the seepages, springs, and rapidly flowing streams, generally too swift for breeding, were for some reason more dangerous. Thus, it was accepted as a rule, that the healthiest villages were those farthest from the hills, and the most malarious were those in the hills.

Exceptions.--To this apparent rule there was one peculiar exception, for which no explanation could for long be found. To the north-west of Tam-

542

THE TRANSMISSION OF MALARIA IN BORNEO

bunan Plain there was a ravine, typical except that it was more open than most. In this ravine there were two villages---Sunsurun and Tantalob--and in accordance with the rule it was estimated that they would both be highly malarious, probably with a spleen rate of 100 per cent. An examination found that Tantalob had, in fact, a spleen rate of I00 per cent., but an examination of Sunsurun, less than a mile away and within sight of it, yielded; surprisingly, a spleen rate of only 25 per cent. Several attempts were made to explain this. It was assumed that probably there were fewer mosquitoes in Sunsurtm: but it was soon clear that there were actually more mosquitoes breeding there than in the malarious villages. It was then assumed that probably there were fewer A. rnaculatus than in other ravines; but a survey showed that there was, in fact, more A. rnaculatus breeding, and that recent clearing had encouraged this rather than otherwise. A search of the records showed that, I0 years before, this village had had a high spleen rate of 85 per cent. What had caused the recent fall in malaria, although with a rise in A. maculatus, it was difficult to see. Further surveys in the vicinity of Sunsurun only deepened the mystery, for the village was found to be in close proximity to cleared ravines of a type which in Malaya which would be considered very dangerous ; and the ravines were in fact breeding A. maculatus in considerable numbers, to the very doors of the houses. This anomaly was a constant challenge to conservative thinking, but ultimately it proved the key to the problem of malaria in Tambunan and, in fact, throughout at least a large part of Borneo. A D U L T MOSQUITOES.

Although valuable information may sometimes be obtained in this way from the evidence of larval breeding, this is only indirect evidence, and cannot be accepted as proof. The only absolute proof that a species is responsible for transmitting malaria is by the finding, on dissection, of a proportion of adult mosquitoes infected with malaria parasites. Adult mosquitoes were, therefore, sought for dissection, and it was proposed to dissect at least 100, and if possible 500, of every species in the locality. Here, too, a puzzling situation presented itself, because searches for mosquitoes failed to find any, and the natives declared that there were none. House Searches.--In Africa, India, Malaya, and in fact in most countries in the tropics, numbers of anopheles can generally be captured settling on the walls inside houses during the evening. In Tambunan, however, no anopheles were ever seen either during the day or in the course of the evening. Careful searches in every possible type of house, in native huts, bungalows, cow-sheds, hen-houses, and elsewhere, in every kind of situation, by day and night, aided by the disturbance of the furniture and spraying with "Flit," on literally hundreds of occasions, yielded not a single mosquito either resting or attempting to feed; and the natives even in the most malarious villages were insistent

JOHN McARTHUR TAMBUNAN

PLAIN

SPLEEN

543

RATES,

1939-40.

Showing distribution of villages, healthy on the plain and malarious in the hills ; and the one important exception to the rule : Sunsurun. |.

i

SUNSURUN

O JUNGLE COVERED HILLS

O ~

0 PLA

IN

~

O

JUNGLE COVERED HILLS

i

SPLEEN RATES 0- 2~ % "

25 - 50

O (}

SPLEEN RATES 50 - 75 % "

O

7 5 - 100 C

544

THE

TRANSMISSION

OF

MALARIA

IN

BORNEO

that they were not bitten by mosquitoes there. Dr. SI-IIRCOREfound the same thing during his survey in 1936. During .several months' assiduous and skilful search, he saw only one anopheles in a native hut. The picture presented itself, therefore, of an intensely malarious area, with widespread breeding of anopheles, but with apparently no adult mosquitoes to account for the transmission of the disease. Consequently, it was difficult to explain how malaria transmission was being maintained, and it was impossible, to carry out any dissections.

Human Bait Trapping.--In other parts of the world, a " h u m a n bait t r a p " may, be a valuable means of capturing mosquitoes. This consists essentially of an inner net to protecv the operator, who acts as bait, and a larger outer net, provided with an opening which can be rapidly closed, for capturing anopheles which are attempting to feed. It is often used in Malaya and elsewhere, and to obtain dissection material in Tambunan, therefore, such a trap was made, and operated irt a variety of situations; in healthy villages and in the most highly malarious; in empty houses and in houses full of people; in houses with cases of malaria and where children had died of the disease; in the middle of villages, and in solitary huts and open verandahs. It was operated every hour, all through the night, from sunset to sunrise, on hundreds of nights, during every season, and in every variety of situation. The results were astonishing. In contrast to other countries, practically no anopheles were ever seen. On most nights nothing was taken whatever during the entire night. On an average, only one anopheles was taken during every 15 hours of w o r k . On an average, only one A. rnaculatus was taken every 3 weeks, after more than 200 hours of work. In one group of highly malarious villages, for example, with spleen rates of 100 per cent., all night trapping, every night~ for sixteen nights, resulted only in the capture of one harmless anopheles. Such a paucity of material made dissections an impossibility. It was calculated that to obtain the desired 500 dissections of A. rnaculatus, it would require an attendant to operate the trap hourly, all through the night, from dusk to dawn, 7 days a week for 25 years. The trap was carefully checked with recording alarm clock, running commentary by the operator, periodical visits, and occasional personal operation, and there was no doubt about the reliability of the results. Comparison~ with the operation of a similar trap in Malaya showed that no error lay in the technique. The most common species taken, though that only rarely, was the poolbreeding A. kochi, generally of little importance, but a vector in some countries; next was A. leucosphyrus, which in Malaya is welcomed rather than otherwise since it is not only considered harmless, but its presence indicates the absence of A. rnaculatus. Present also, and during the paddy season by far the most common, was A. philippinensis.

JOHN McARTHUH

545

Thus the results of human bait trapping did not yield any convincing evidence, and an analysis of the figures simply deepened the mystery. More anopheles were taken, as a rule, in healthy villages than in unhealthy, although still in astonishingly small numbers. More A. maculatus were taken in healthy villages than in the most malarious. There was no indication of a seasonal increase over the course of 2 years' trapping, and no information of any value seemed to accrue from the different situations of the trap. A review of the first year's work left one with the impression of an area of intense malaria, in parts with spleen rates of 100 per cent., and widespread anopheles breeding, but without the adult anopheles to account for it. Suspicion still rested on A. maculatus, although it was difficult to understand how its apparent scarcity could maintain malaria at such an intensity. Suspicion also rested, as a result of human bait trapping, on A, kochi and A. philippinensis, although their distribution was against this. To be logical, suspicion had also to rest u p o n A. leucosphyrus, although this, taken in such minute numbers--only one every three or four nights, found breeding also in very minute numbers, and believed to be harmless elsewhere, could not seriously be considered to be carrying malaria and maintaining it at 100 per cent. Dissections, however, continued to be carried out on every anopheles taken, and always with negative results.

Adult Resting-Places.--It was clear, that, if mosquitoes were breeding widely, they must exist as adults somewhere, and therefore they must feed and rest. Since they did not appear to rest in the houses, nor in such cattle sheds and hen-houses as existed, it was assumed, therefore, that they must be resting in the iungle or elsewhere outside. Consequently, to obtain further specimens for dissection, and to discover where the mosquitoes rested, search was made out of doors; and it was found that, by beating bushes in jungle, and along the banks of streams, resting mosquitoes "might be started into flight, and if followed carefully they might be captured settling in the grass. This provided considerable dissection material, until it was found that mosquitoes captured in this way were nearly all A. kochi or A. philippinensis; and when 500 of each of these had been dissected with no positive results the search was abandoned, as not worth further time spent on it. Animal Bait Trapping.--The question then arose of where the anopheles, breeding in such enormous numbers, were feeding. There were large numbers of buffaloes on Tambunan Plain, more than one for every inhabitant, in addition to cows, ponies, pigs, and other animals, and it was suspected that these might be providing the blood meals for the adult mosquitoes. Searches were therefore, carried out, by tethering animals and searching these at night with a torch, for feeding anopheles: This also led at first to disappointment, as only one or two anopheles were

546

THE

TRANSMISSION

OF

MALARIA

IN

BORNEO

found during several evenings' search. In the conviction, however, that the mosquitoes found' breeding must also be feeding, it was determined to watcb the animals all night long, constantly, from sundown to sunrise: and this led to the discovery that about dusk, enormous numbers o.f anopheles appeared, to feed on ponies and buffaloes. They fed avidly for quite a short time after sundown, and in an hour or so had practically all disappeared. This explained where most of the mosquitoes fed, and why the previous animal observations, made at night, had failed. It also provided much more material for dissection, but it did not explain in any way the transmission of malaria from man to man. Dissections.--By this time, therefore, Anopheles kochi and A. philippinensis, by providing adequate negative dissections, were exonerated from suspicion as vectors. Dissections of other species, however, also continued to be negative, and when 1,500 mosquitoes, obtained with great effort over the course of nearly 2 years, bad all been dissected without positive result, hope began to yield to despair. All the conventional methods, and many' unconventional ones, had failed to discover the vector mosquito, or even to suggest the solution to the problem. House searching and human bait trapping had proved useless. Animal bait trapping and jungle searching, although at first promising, yielded no positive result, and the problem of the identity and habits of the vector mosquito remained. One was compelled to consider the possibility of Tambunan malaria being carried in some unusual way, for example by a jungle-feeding anopheles, or possibly not by 'an anopheles at all. COMPARISON OF HUMAN AND ANIMAL BAIT r~RAPPING.

At this point, the results of past work were considered, not only by themselves, but in relation to each other; and it was found that, whereas the results of human bait trapping and of animal bait trapping did not, by themselves, yield any information, a comparison of the two yielded information which was of the highest value, and which led ultimately to the solution of this curious problem, and to the incrimination of the vector. A study of 1,300 anopheles captured while they were in search of a blood meal, indicated the following preferences of each species for man and animals : - HUMAN ANIMAL PER CENT. PER CENT.

A. A. A. A. A. A.

kochi ... philippinensis barbirostris... maculatus ... karwari ... leucosphyrus

Total

...

3 6 10 15 17 88

97 94 90 85 83 12

5

95

JOHN M c A R T H U R

547

It was found, firstly, that practically all the species of anopheles present showed a much greater preference for animal blood than for human blood, and could not, therefore, be of any importance in regard to human malaria. It was found, with some surprise, that A. macuIatus, hitherto regarded as under greatest Suspicion, had an interest in human blood in only about 15 per cent. of cases, the remaining 85 per cent, seeking their blood meal on animals. Further, the equally remarkable fact was discovered that of all the anopheles present, only one had a preference for h u m a n blood, and that this was A. leucosphyrus, hitherto disregarded, and considered to be the most harmless of all insects in Malaya. This mosquito showed a preference for human blood in no less than 88 per cent. of dissections. The few precipitin tests carried out completely confirmed this finding, all A. leucosphyrus captured being engorged with human blood, and all other species, including A. rnaculatus, with animal blood, although unfortunately it was not possible to obtain sufficient numbers to make conclusive series. Thus, the impression arose that the previously unregarded A. Ieucosphyrus might prove to be the vector, and that the suspected A. maculatus, together with all the other mosquitoes breeding in the locality, might be harmless. THE EVmENCE REGARDINGA. leucosphyrus. With this information, the whole work was reviewed. It was noted that A. leucosphyrus is a purely jungle mosquito, and that if it were a vector, and A. maculatus harmless, it would explain a number of anomalies. It would explain why Jesselton was healthy. It would explain why the centre of Tambunan Plain was healthy. It would explain why the hills of Tambunan were malarious, and it would explain the anomaly of the village of Sunsurun, cleared of jungle and healthy in spite, of the increased breeding of

A. maculatus. Against the acceptance of A. leucosphyrus as a probable vector, however, was its innocent reputation in Malaya, and, above all, its apparently great rarity in the locality. In the human bait trap, only one was taken, on an average, about every three whole nights of operation, and none captured in this wayj had ever been found infected. Moreover, of 44,000 larvae taken in the locality, only seventy-three were A. leucosphyrus. During most months of the year it was never found breeding at all, and it had never been found consistently in a permanent breeding place. With the information gained, however, a thorough search was made of the literature, and here the evidence was rather encouraging, It was found, firstly, that although generally regarded as harmless, A. leucosphyrus

548

THE

TRANSMISSION

OF

MALARIA

IN

BORNEO

had been recorded as under suspicion, although on indirect evidence, and together with several other species, as a vector of malaria in at least four places in Borneo. Finally, it was found that one A. leucosphyrus had actually been found infected on the east coast of Dutch Borneo after a discouraging search for mosquitoes, by GOELARSO (1934); and that in an island off the south-east of Dutch Borneo no less than seven of the same species had been found infected by STOKER (1934). Thus, the evidence for A. leucosph~rus as a vector became stronger although its reputation, and especially its apparently extreme rarity, still made it difficult to regard it very seriously.

THE INCRIMINATIONOF THE VECTOR. With this information a drive was made to capture more A. leucos#hyrus for dissection. H u m a n bait trapping was pursued vigorously in spite of its poor returns. House examinations were renewed, traps were experimented with, and possible natural resting-places of anopheles were sought for. Searches were made at nights on the backs of ponies, buffaloes, pigs, and small boys. Payment was offered to the natives for any anopheles captured, and a prize was offered for the first infected specimen. It was this that finally rewarded the search with success. The domestic staff, having been encouraged to use anti-malarial precautions for themselves and their families, began bringing in anopheles taken in defective mosquito nets, in spite of the previous protests that t h e r e w e r e no mosquitoes. Sometimes they would bring in ten or fifteen in a morning, and these all proved to be A. leucosphyrus, thus strongly supporting suspicion. It was from this source, in April, 1941, after more than 2 3~ears' search for mosquitoes, and after more than 1,500 negative dissections, that the first infected specimen--an A. leucosphyrus--was found, proving this species to be a vector, Following this, the search was continued, and out of nearly 800 di,sections on A. leucosphvrus obtained in this way, twen.ty-five specimens were found infected, while all other species, including A. maculatus, remained consistently negative. Positive dissections were carried out not only in Tambunan, but in two other localities in North Borneo, thus showing that it was not limited to Tambunan Plain. Thus, A. leucosphyrus, which in Malaya is considered to be one of the most harmless of insects, was proved to be the vector, and almost certainly the only vector, of malaria in T a m b u n a n ; while A. maculatus, for long regarded as the chief vector in Borneo, was found to have little, if any, relation ro the disease in the locality.

549

JOHN M c A R T H U R

Total

and

on

1940 T a m b u n a n ,, ,, ,, ,,

Purutan Apin Apin ,, ,, ,, ,,

Jesselton ,, ,, ,,

Men'ggatal ,, ,,

1941

following

Dissections.--~'The

interior

Tami~unan ~J

,, ,, ,, ,, ,,

Apin"Apin (June) Purutan J~

Apin Apin (October) ,,

the

west

A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. .4. A. A. 21. A. A. A. A. A. A. A. A. A. A.

leucosphyrus karwari ... maculatus... barbirostris leucosphyrus maculatus ... leucosphyn*s barbirostris kochi ... maculatus... philippinensis litoralis ... leucosphyrus philippinensis maculatus... barbirostris philippinensis kochi ... barbirostris litoralis ... leucosphyrus barbirostris kochi ... philippinensis maculatus ... karwari ... tessellatus ... barbumbrosus leucosphyrus maculatus ... leucosphyrus maculatus ... leucosphyrus barbirostris philippinensis

are summarized

...

Borneo

were

carried

during

POSITIVE POSITIVE GUT. GLAND. . . . . . . .

POSITIVE TOTAL. . .

41

.

.

.

.

. . .

. . .

. . .

. . .

6 1

. .

. .

. .

. .

34

.

.

.

.

6 6 5 2 30 2 1 1 1

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

58

.

.

.

.

. .

. .

-. .

639

-. .

7

13

483. 290

. .

. .

. .

. .

274 138 70 12

. . . .

. . . .

. . . .

. . . .

3

.

.

.

.

.

11

.

0

1

.

13

2 .

--

3

.

15

1 .

1

3 1

. .

.

.

. .

. .

2 . .

2,715

as follows:-761 567 541 521 177 102 31 12 3

... 2 , 7 1 5

25 positive = 3.3 per cent. all n e g a t i v e ,, ,, ,, ,, ,, ,, ,,

out

in

the

1940-41 :-

32 28 26

8 2 1

A. leucosphyrus A. philippinemis A. kochi ... A. barbirostris... A. maculatus ... A. karrzari ... A. litoralis ... A. tessellatus ... A. barbumbrosus

Total

dissections

of North

NUMBER • EXAMINED. A. kochi ... 237 A. philippinensis 231

Total These

coast

PER CENT. POSITIVE

--

=-

19

3-0

2

18.2

1

7"7

3

20"0

550

THE TRANSMISSION OF MALARIA IN BORNEO

Explanation of Anomalies.--The discovery of A. leucosphyrus as the only vector in Tambunan explained all the difficulties that had arisen with the assumption that A. maculatus was the vector. A. leucosph.yrus is a purely jungle breeder, demanding dense shade, in contrast t o ' A . maculatus which demands sunlight; and that A. leucosphyrus Was the vector in Tambunan, explained the peculiar distribution of malaria, which was least in the paddy fields, and greatest in, the hills. It showed that the hills were malarious, not because they were hills, but because they were jungle-covered. It explained why Sunsurun, highly malarious 10 years previously when under dense jungle, was now healthy when jungle had been cleared, although A. maculatus has been encouraged, and it explained why Jesselton was healthy in spite of the presence of heavy A. maculatus breeding, in contrast to its jungle-covered environs which were highly malarious. THE

A P P A R E N T SCARCITY OF

A. leucosphyrus.

With the discovery of A. leucosphyrus as the vector of malaria in Tambunan, another problem presented itself. A. leucosphyrus appeared to be a very rare mosquito, both in the larval and adult stages ; so rare, in fact, as to be easily missed in an ordinary survey. It was a mosquito which had never been found resting, either by day or night. Unlike other mosquitoes, it was rarely found feeding on animals. It was in the human bait trap only once every 36 hours of work. 1 have never seen a single specimen of A. leucosphyrus at liberty, flying, feeding or resting ; and the natives were equally emphatic that they were not visited b y this or any other mosquito. The apparent absence of all anopheles from the houses was explained very largely by the feeding of most anopheles on cattle; but this did not explain the absence of A. leucosphyrus which did not often so feed. Again, in extensive larval surveys, only one A. leucosphyrus was found in every 5,000 larvae taken, and during most months of the year it was never foun4 at all. It is no disparagement of the work of Dr. SHIRCORE or of other previous observers, to say that in skilled and assiduous searches they never even recorded A. leucosphyrus as present in the locality. It appeared to be an area of intense malaria, but in which the only vector seemed to be present in such insignificant numbers as to be incapable of maintaining malaria transmission at all. It seemed clear that for some peculiar reasort this mosquito was being missed, both in the larval and adult stages, and searches were therefore directed to the solution of this problem. The Elusiveness of A. leucosphyrus Larvae.--First, search was made for A. leucosphyrus breeding. Since it was found to be a purely jungle breeder, although searches had been virtually negative for nearly 2 years, renewed surveys were made in the area previously selected for routine larval

JOHN MEARTHUR

551

surveys, but with greater thoroughness, and confining attention to the jungle. The previous breeding places were again examined, searches were made in rock pools, tree holes, water-filled leaves, rapidly flowing streams and other unlikely place~. Finally, these having all proved negative, the jungle itself was felled, and it was this that finally exposed the breeding places. Under dense jungle shade, so dense that access was often difficult or impossible without clearing vegetation, in little seepages right up at the sources of hill streams, breeding places were discovered which for 2 years had been overlooked. It was found that breeding had been missed for two reasons. Firstly, it had been missed because the work had been too conservative. Larval surveys had been made on conventional lines found to be successful in other countries. Searches had been made particularly for A. maculatus, and had been too unenterprising to include the denser jungle as important. Secondly, A. leucosphyrus was found to be a very shy larva. It was discovered that it not only readily hid in the sedil~aent at the bottom of the dipper, but that it remained submerged for a very long rime, much longer than was the case with other species. It was often timed to remain submerged for more than 5 minutes, and it seemed clear that water had often been rejected from a dipper whicl~ might, in fact, have contained A. leucosphyrus. T h u s , the rule was made that A. leucosphyrus could not be claimed as absent from an area unless jungle had been cleared right to the sources of streams, and unless the dipper had been observed for at least 5 minutes after dipping. When this was realized, it was discovered that, far from being a rare mosquito, it was in fact quite a common one. Breeding was found in no less than 150 permanent breeding places in this area which had previously been regarded as negative during twenty-four careful routine monthly surveys. Although quite widespread, however, once the type of breeding place was understood it was found to be easily recognizable, as this insect was found breeding almost exclusively in clear spring water in tiny seepages at the sources of streams or along a hillfoot, and always under complete shade. It was concluded, therefore, that the great number and variety of other breeding places in the locality, streams, swamps, paddy fields and irrigation channels, pools, puddles, and the innumerable other collections of water, were harmless, although breeding anopheles freely. The Elusiveness of A. leucosphyrus Adults.--It remained to be discovered where the adult A. leucosphyrus fed and rested, and why it was apparently so rare. It appeared to be completely absent from the houses on examination by conventional methods, and in the opinion of the natives, and almost completely so in experience with the human bait trap. That it was feeding

552

THE TRANSMISSION OF MALARIA IN BORNEO

in the houses, however, was proved by the fact that it was taken in fair numbers, engorged and trapped in defective mosquito nets in the early morning. How it was feeding was only discovered by the tedious method of sitting up all night and actually watching for individual mosquitoes; and it is to the credit of the staff that they did, in fact, undertake this successfully. For four nights nothing whatever was seen during 12 hours' observation; but on the fifth night it was seen that several mosquitoes--all A. leucosphyrus-entered the house long after midnight, fed on their sleeping victims, then after a short rest fled quickly away to their undiscovered hiding-places, presumably in the jungle, long before dawn. W h y this mosquito came out on some nights and not on others was never discovered; but this late feeding, in the early hours of morning, long after people were asleep, explained why it was never seen, and why it was apparently absent from areas where it was responsible for intense malaria. Further, it was seen that, when searching for a blood meal, if A. leucosphyrus entered a human bait trap, unlike other anopheles it did not wait long enough to get caught. This explained its relative rarity in the trap. It was observed to be present and feeding on sleeping natives, in houses where the human bait trap was being operated unsuccessfully. Thus this dangerous mosquito, although apparently rare and almost absent, was in fact quite common, and was being missed because of its elusive breeding under dense and often impenetrable jungle, and of its feeding in the early hours when everyone was asleep, and not resting on the walls for any length of time. The Importance of A. leucosphyrus in Borneo.---Tambunan district represents a relatively small part of Borneo. and the findings of one locality cannot be applied without question throughout the whole of the island, so that further work was necessary, to define its importance elsewhere. Positive dissections were carried out in two other places in the interior of North Borneo, Purutan and Apin Apin, finding natural infection indices of 8 per cent. and 20 per cent. respectively, so that this mosquito is a vector elsewhere than in the immediate vicinity of Tambunan Plain. On the coast of North Borneo, as in the interior, the same vector was sflggested by finding that malaria was intense in jungle-covered areas, and that it was slight or absent in cleared areas. Here, also, A. leucosphyrus was the only mosquito found with a preference for human blood, while A. maculatus had a preference for animal blood. On the east coast the same thing was found. Although not actually incriminated, the observations of local medical men suggested the importance of A. leucosphyrus and the danger of jungle (CARLILE, 1941, etc.). As a parallel to these observations a study of the literature yielded a number of important facts, which it is planned to publish in full later. It

JOHN McARTHUR

553

was found that, in spite of its elusive habits, A. leucosphyrus had been recorded in more localities in Borneo than any other mosquito, being referred to by more than a score of observers, and found in every locality so far examined. It appeared to-be distributed throughout the whole of Borneo, wherever there was jungle shade. Again, it was found to have been suspected as a vector of malaria on epidemiological grounds in at least five places in Borneo, and actually incriminated, as already stated, in one place in Dutch Borneo, and in an island off the south-east of Dutch Borneo, as well as" in the three places examined during the work described in North Borneo. This species appears to be distributed throughout the jungle which is almost universal in Borneo, and has been either suspected or incriminated in the north, east, south, west, and in th~ very centre of the island. Further, an analysis of the available figures shows that A. leucosphyrus has exhibited the highest index of natural infection of any anopheles so far studied by dissection in Borneo, and to'have been consistently infected wherever it has been so studied. Thus, in spite of its elusiveness, and of its innocent reputation in Malaya and elsewhere, A. leucosphyrus has proved to be the most widely distributed, the most frequently suspected, the most frequently incriminated, and the most highly infected mosquito so far studied in Borneo, and therefore to be probably the most dangerous insect throughout the island, In contrast to this, 212 dissections of A. rnaculatus--all that could be obtained throughout Borneo--have proved to be negative, and this, together with the indirect evidence of its preference for animal blood, its absence from many malarious areas, and the absence of malaria from many areas where it breeds freely, supports the belief that this mosquito, for long regarded as the chief vector of malaria in Borneo, is actually harmless throughout the island. The Importance of Varieties.---During this work superficial observations suggested the existence of at least two types of A. leucosphyrus, differing in their habits, and, therefore, probably in their relation to malaria: and it would appear of importance to make a further study of this. THE IMPORTANCETO OTHER COUNTRIES OF THESE FINDINGS.

The fact that A. leucosphyrus was the only vector of malaria in Tambunan, and that this was such an elusive insect, made it difficult to find any answer to the question of malaria transmission there. What would have been the result of the work had another vector, for example, A. rnninirnus, been present in the locality, is quite certain. There is not a shadow of doubt that A. rninirnus would have been incriminated, and A. leucosphyrus would have been missed. To what extent, therefore, is this occurring elsewhere ? To what extent is A. leucosph.yrus carrying malaria where A. rninimus, or A. rnaculatus, or

554

THE TRANSMISSION OF MALARIA IN BORNEO

some other mosquito, is bearing all the blame, and where control are consequently inadequate or inappropriate? A search of the literature shows not only that A. leucosphyrus incriminated in other places, and its infection generally regarded thing of a curiosity, but that wherever it has been dissected in

measures has been as gome-

adequate numbers, it has always been found infected. A. leucosphyrus is a widespread mosquito, being distributed almost right

across Asia. It is often so difficult to find that it is reasonable to suppose it to be more widespread than the records show. To what extent it is of importance throughout the East, and to what extent its importance has been overlooked, remains to be determined by further work. Ceylon~ for example, claims that it is a harmless mosquito; but the evidence for thi~ rests upon a record of only four negative dissections over the course of many years. Malaya claims it to be a harmless mosquito; but only one dissection, by HODCKIN (1934) appears to have been recorded. A. leucosphyrus is considered to. be a harmless insect in India, but no dissections can be traced, with the exception of a series byCLARK and CHOVDnVRY (1941) in Assam, where it was proved t o be positive. It is suggested that if dissections are more widely carried out--not always an easy matter on account of the difficulty of obtaining material even where it abounds--it may prove to be a dangerous vector in areas where it has never been suspected; and it may be found that the occasional incriminations of the insect throughout Asia are not such curiosities as has been supposed. More dissections are necessary throughout its range, and this m a y demand a new technique for the capture of such an elusive insect. MALARIA CONTROL.

The aim of malaria research is practical malaria control, and these discoveries are of-no value unless applied to an effective campaign against the disease. With the incrimination of A. leucosphyrus as a vector in Tambunan, therefore, it remained to be discovered what methods of control were most likely to be effective there, and to do this two things were required. It needed, firstly, to be proved whether A. leucosphyrus control actually resulted in malaria control; and, secondly, it needed to be shown what were the most effective and economical methods of permanently accomplishing this. Malarial Control by A. leucosphyrus Control.---Firstly, in order to prove whether A. leucosphyrus control results in malaria control, a locality was selected with a spleen rate of I00 per cent. and where the spleen rate had been maintained at this figure for at least 5 years. It lay in a heavily jungled valley associated with about fifteen streams and over 150 A. leucosphyrus breeding places. This represented probably the most difficult area to control in the district, and was selected partly for this reason--in order to

JOHN McARTHUR

555

make a convincing experiment--and partly because of the consistently high spleen rate, so that any reduction might not be attributed to chance. The breeding places were carefully defined and mapped, and then dusted weekly, by hand, with paris green, avoiding the breeding places of other spec!es. It was hoped that this, by temporarily abolishing the breeding. would be reflected in a reduction of malaria in the villages. For some weeks it was fairly effective, in that breeding was diminished, although not abolished. After some time, however, heavy rains drenched the jungle, washing away the paris green, and it was soon clear that breeding was no longer controlled. Thus, this experiment failed. It could not be expected to influence the degree of malaria in such a short time, and it proved that dusting with paris green is likely to be unsuitable as a means of malaria control where A. leucosphyrus is a vector. Practical A. leucosphyrus Control.--Concurrently with this experiment in malaria control, parallel experiments were also being carried out to determine the most effective economical methods of A. leucosphyrus control. Throughout the greater part of Borneo, especially in the intensely malarious areas, anti-malaria measures on conventional lines are out of the question for economic reasons. Oiling, dusting with paris green, drainage, and drug therapy, for example, are quite impracticable in vast isolated areas inhabited only by a few thousand primitive people living in the utmost poverty, even if these measures are effective. The only hope for malaria control is such a land lies in the discovery of some simple method of controlling the one dangerous species of mosquito; a method which is cheap, permanent and practicable under existing conditions, and which if possible will go hand in hand with some agricultural or o t h e r activity of the people. The most hopeful way of accomplishing this appeared to lie in the discovery of some means of naturalistic control. Naturalistic Control of A. leucosphyrus.--It was noted that A. leucosphyrus in Tambunan always bred in clear spring water, with an acid reaction, pH. 60 to 6'4, in the presence of dead leaves, in seepages, under dense jungle shade, and was never found in any other situation. 'This suggested that breeding might be controlled by changing any of these conditions; by admitting sunlight, by abolishing the dead leaves, by changing the reaction of the water or by polluting it; and it seemed obvious that all these might be accomplished by the simple clearance of jungle for a short distance around the breeding place, and cultivating it 0r admitting cattle; and an experiment was undertaken to investigate this. Three apparently identical seepages were selected,' each breeding A. leucosphyrus consistently. The first of these was cleared, and the vegetation was left lying on the ground; the second was cleared and then burned: and the third was left alone as a control.

556

THE T R A N S M I S S I O N OF MALARIA IN BORNEO

Disappointingly, the first seepage continued to breed Without interruption, and was still breeding 3 months later, although some other species had also become established. The second seepage, cleared some time later and burned, immediately ceased to breed A. leucosphyrus and began to breed other species. The third seepage continued to breed without interruption, proving that the results in the controlled seepages were not due, for example, to weather conditions. Thus it was suggested that the most effective method of controlling A. leucosphyrus might be the clearing and burning of jungle for a very short distance around the dangerous breeding places; and with the failure of the paris green as a means of experimental control, it was decided to apply this method of clearing to the attempted control of malaria. Malaria Control by Naturalistic ",4. leucosphyrus Control.---The same locality was selected as before, and 3 months were spent in attempting to clear all the seepages within half a mile of the malarious villages, and in making observations on the most effective ways of undertaking this. It was found that complete clearing and burning, for quite a short distance around dangerous seepages, generally for about 30 feet, resulted in every case in the immediate disappearance of A. leucosphyrus breeding, although this breeding tended to return with the growing up again of jungle. Even better than this, however, it was found that only partial clearing for quite a short radius, with the admission of cattle to graze, resulted also in the immediate control of A. Ieucosphyrus---a control which was accelerated by the pollution of the water by the cattle; and since the cattle grazed down the young shoots, the control was apparently permanent, transforming the previously inaccessible iungle-covered soil into turf. It was shown that these clearings, being moist, sheltered, with good soil, were ideal situations for cultivation, and even--although previously deadly-for housing sites, and that the creation of grassy lanes for grazing and access to the jungle, following the course of streams, was a simple means of control, and a benefit to the natives. Unfortunately this wo'rk was interrupted, when only half completed, by the occupation of the country by the Japanese troops. It was not possible to expect any reduction of the malaria with such widespread breeding still in progress. Thus the experiment failed as a demonstration of actual malaria control. It succeeded, however, in adequately demonstrating the success of the method as a means of immediate, effective, and apparently permanent A. leucosphyrus control.

Other Evidence of Malaria Control by this Method.--Although this experiment failed, owing to its non-completion, to control malaria, there is a

JOHN McARTHUR

557

considerable weight of evidence throughout Borneo that jungle clearance in this island, unlike Malaya and many other countries, results in malaria control. For example, all towns, being cleared of jungle, appear to be healthy in strong contrast to their highly malarious jungle-covered environs which provide the cases of malaria. Cuhivated areas, like Tambunan Plain with its paddy fields, are healthy, although they may be mala.rious if close to jungle. Sunsurun, in the hills of Tambunan, had a spleen rate of 80 per cent. in 1930 when still under jungle, which was reduced to 25 per cent. in 1940, however, with the extension of jungle clearance. On the coast, the northern Chinese Settlement, Jessehon, provided a great many cases of malaria and many deaths when it was first established 20 years previously, but in 1939, with the abolition of jungle and its replacement with rubber, it had no malaria and a negligible spleen rate of 0"5 per cent. It was stated by WEBSTER (1941) that Miri, although previously very malarious, became quite healthy when extensively cleared of jungle about 1924, with only sporadic cases since, until 1940. There are other evidences of the disappearance of malaria with jungle clearance in Borneo, and no evidence has yet been found to t h e contrary. Thus it would appear that in Borneo jungle clearance .results in malaria control.

The Importance o[ ]ungle Clearance.--In most other countries, malaria control is carried out by measures which cost money, and which in themselves contribute nothing further to the prosperity of the land. Whether such measures, for example for the control of A. maculatus, could have been applied to such a primitive rural population as is found in Tambunan, and with the small financial resources available, is however, very doubtful; and before the discovery of A. leucosphvrus as the vector, the possibility of effective control seemed very remote indeed. With the discovery of A. leucosphyrus as the vector in Tambunan, however, control of malaria presents itself not only as a possibility, but as, an activity which is cheap, easy, effective and permanent; able to be carried out by the natives themselves, if directed, in the course of their daily work; and above all, an activity in keeping with the agricultural and economic development of the country. Thus the simple methods described, of localized jungle clearance, should result in benefit not only to the health, but to the wealth, of the people of Borneo. SUMMARY.

A. maculatus has for long been regarded as the chief vector of malaria in Borneo. As a result of the work described, however, this mosquito has been shown to have little and probably no importance as a vector there.

558

THE

TRANSMISSION

OF MALARIA IN

BORNEO

A. leucosphyrus, on the other hand, g e n e r a l l y regarded as a more or less harmless insect, has been shown to be the most dangerous vector in N o r t h Borneo, and, on the existing evidence, probably t h r o u g h o u t the greater part of the island. This is i m p o r t a n t because measures which control the one m a y encourage the other. T h u s measures directed against A. rnaculatus where A. leucosphyrus is the vector, m a y not only be ineffective, but m a y increase the disease they are intended to eliminate. A. leucosphyrus was f o u n d to be a very elusive insect, avoiding discovery and incrimination for nearly 2 years, until demonstrated b y less conventional methods. This mosquito eluded search as a larva because it bred u n d e r dense jungle which often required to be cleared for its exposure, and because it remained submerged for abnormally long. "It eluded search as an adult because it fed in the early hours of m o r n i n g when its victims were asleep, and because it did not rest in the houses. It is considered that had some more easily discovered vector been present, A. leucosphyrus would certainly have been missed. It is suggested that, since the mosquito is so elusive, it m a y be of wider distribution and greater virulence and importance in other countries than has h i t h e r t o been supposed. It is pointed out that, in spite of its innocent reputation in m a n y countries, wherever it has been e x a m i n e d in adequate numbers it has always proved to be a vector. Experimental measures for the control of A. leucosphyrus showed that this mosquito m a y be easily and permanently abolished, and by measures which are in keeping with agricultural and industrial development of the land. It is hoped to publish a full account of this work as a volume later.* REFERENCES. CARLILE, J.P. (1941). Personal communication, 31.7.41. Sandakan. CLARK & CHOUDHURY. (1941). Observations on Anopheles leucosphyrus in the Digboi Area, Upper Assam. J. mal. Inst. Ind., 4, 103-107. GOELARSO. (1934). Larven en Muskietenvengsten in de onderafdeeling Boeloengan (OostBorneo). Geneesk. Tidjschr. Ned.-Ind., 74, 1345-1352. HODGKIN,E.P. (1934). Quoted in GATER,B. A. R., Aids to the Identification of.~nopheline Imagines in Malaya. Singapore, 1935. p. 117. SHIRCORE, J. O. (1937). Report on Native ttealth. Government Publisher: Sandakan, British North Borneo. STOKER, W. J. (1934). .Over de malariagevaarlijkheid can A. leucospkyrus. Geneesk. Tijdschr. Ned.-Ind., 74, 1342-1344. Batavia, 0th October, 1934. WEBSTER, F. L. Personal communication, Miri, 5.10.41. * Work carried out by the British and Americans during the Burma Campaign, and shortly to be published, confirms the belief expressed in this paper, that the mosquito incriminated by unorthodox technique in Borneo, although previously unconsidered, may p r o v e to be of much wider importance than has hitherto been realized.--ED.