Paper: Recent knowledge about malaria vectors in West Africa and their control

The previous number of these Transactions, Vol. 40, No. 4, was published on M a r c h 8th. 1947.]

TRANSACTIONS OF THE

ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE VOL. 4 0 .

No.

5.

.~IAY, 1 9 4 7 .

ORDINARY MEETING of the Society held at Manson House, 26, Portland Place, Lofidon, W., on Thursday, 16th January, 1947, at 8 p.m. THE PRESIDENT, C.

~.

W E N Y O N , C . M . G , C.B.E., M.B., B.S.~ B.SC.~ F.R.S.,

in the Chair.

PAPER. RECENT KNOWLEDGE ABOUT MALARIA VECTORS IN WEST AFRICA AND T H E I R CONTROL. BY R. C. M U I R t t E A D

INTRODUCTION

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THOMSON,

. . . . . . . . . . . . . . . . . .

THE IDENTITY OF A n o p h e l e s g a m b i a e AND t .

melas

MALARIA TRANSMISSION BY melas AND g a m b i a e BREEDING PLACES OF 2t. melas CONTROL OF 1 . melas

...

IN MALARIA CONTROL

. . . . . . . . . . . . . . . . . . gambiae

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ANOPHELINE REDUCTION AND MALARIA CONTROL REFERENCE~

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BREEDINC PLACES AND CONTROL OF t . DDT

D.se.

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INTRODUCTION.

In this paper I should like to give you a brief outline of some of the work that has been going on in West Africa for the last few years in connection with malaria transmission and control. The rapid advances that have been made in the last 5 or 6 years are partly due to the important, and at times vital, part West Africa played in the supply line to the Middle East and North Africa during the war, and partly due to the increasing funds made available by the Colonial Development and Welfare Act of 1940 for anti-malaria work and trained personnel. In this work many different people have played a part, both military and civil, British and American, and it m a y all be regarded as a combined operation~ in the widest sense. Much of this work has been published, but a great deal ha~ not yet appeared in print, and it is probable that much of that will remain tied up in the original typescript or cyclostyled report form and never become available for reference. This multiplicity of reports and opinions makes it rather difficult for the onlooker to get a clear idea of what has been happening, and is happening just now; and although what I offer today is just one more report, one more opinio_fi, I hope that it may help to clarify the mosquito aspect of the problem at least. Without describing in detail the geography of West Africa, there is one feature which is apt to be overlooked, and one which has a bearing on antimalaria policy. While it is often convenient to think of British West Africa as a single homogeneous unit, in actual fact the four colonies, Gambia, Sierra Leone, Gold Coast, and Nigeria, each with its separate Colonial Government, still remain very distinct, widely separated by non-British territory, and peopled by races whose interests have little in common. Communication between the four colonies is almost entirely by air and sea, and the distances to be covered are still large. Added to this is the fact that while the topography of the four colonies has a great deal in common, the differences, even in the coastal belt, are still wide enough to make generalizations dangerous. The rocky laterite " Colony " of Sierra Leone with its remarkable rainfall obviously presents features very different from Gold Coast with its bare undulating coast of reddish loam, and from Lagos with its porous sandy soil, vast lagoons and coastal swamps. As a result of all these factors malaria control policy in the last few years has pursued a more or less independent course in each colony, and while a great deal has probably been lost by this lack of interchange of ideas and methods, each colony does present its own peculiar problems which can only be tackled successfully in the light of local knowledge. Since 1942 I have~ spent nearly 2 years in Sierra Leone, mostly in Freetown estuary, and 18 months in Lagos. I have also paid a short visit to Accra, Gold Coast, and Bathurst, Gambia. I can only speak with any degree of confidence about coastal districts but, however important the hinterland may be,

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as far as anti-malaria work is concerned it is the coast, on which the capitals of the four colonies are situated, that will be our main concern for some time to come. IDENTITY OF Anopheles gambiae AND A. melas. It has been known for many years that the main vector of malaria in West Africa is A. gam biae, with A. funestus of secondary importance, and that these two mosquitoes together are mainly responsible for malaria transmission throughout tropical Africa. In West Africa it has also been known for some time that in coastal districts a dark form of A. gambiae also occurred. In its most distinct t~orm this dark or irielanic variety was called A. gambiae vat. melas and differed from typical gambiae in having an additional dark band on the palps. Up till about 1940 this was regarded as little more than a "melanic coastal variety," associated with salt water breeding places. Dr. BARriER (1931) in his Nigerian survey had reported finding infections among these dark forms, but nothing more was known about its exact identity or its role in malaria transmission (EVANS, 1931, 1938). Early in the war Professor BLACKLOCKand Dr. CARMICHAELWILSON (1941), in their investigation of malaria among merchant seamen, found odd melas in Freetown itself, and large numbers of them on the Bullom shore, across the estuary from Freetown. In these Bullom villages they found a high rate of infection among the " vat. melas," and considered it must play an important part in malaria transmission in the estuary. About this time, too, house catches in various parts of Freetown estuary were started by the Field Hygiene Section, and later continued by the Malaria Field Laboratory, in connection with airfield and seaplane base construction, and other military installations. They showed regular catches of this "vat. melas" from most parts of the estuary except Freetown proper, and its neighbouring village or suburb of Kissy. The next step was when Major RmBANDS (1944) found that of mosquitoes bred out from eggs laid by distinct " b a n d e d " melas; only about one-third of the females were banded, the remaining two-thirds being identical to typical gambiae. From this it appeared that "var. melas" must include not only those dark forms, with distinct four-banded palps, but also some mosquitoes which had hitherto been regarded as typical gambia& RIBBANDS also, following up BARBER'S original observations, found that larvae of gambiae and "vat. melas " showed distinct differences in their reaction to sudden changes in salinity; and a simple test was devised in which larvae were exposed to chan~es of increasing salinity in such a way that most of the pure gambiae larvae died, and most of the melas survived ('RIBBANOS, 1944). This physiological difference was followed by a morphological one, the pecten of melas and ~ambiae larvae showing consistent differences in relation of the teeth ('RIBBANDS,1944).

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The situation at this stage was that when we found adults with the extra dark band on the palps we knew we were dealing almost certainly with rnelas, but where there was no extra dark band we did not know whether we were dealing with unbanded melas or with typical gambiae. Then in 1942 we discovered that the eggs of the two forms were quite distinct, and that the wellmarked differences were consistent (MumrlZAD THOMSON, 1945). Here at last we had a method of knowing exactly what mosquitoes we were dealing with. When we caught mosquitoes in villages the banded ones were practically all rnelas, while the unbanded ones which might be melas or garnbiae were put aside to lay eggs on which the identity of the mosquito was established. The egg Character remains the only accurate means we have of identifying individual wild-caught females on the coast. ROBERXSON(1943) has found that the range of variation in certain tibial markings is different in the two species, but we still lack any morphological feature which can be used to distinguish all adult gambiae from all adult melas. We now felt that with so many differences to go on we could regard A. gambiae and A. melas as distinct species (RIBBANDS,1944; MUIRHEADTHOMSON, 1945). In the last 18" months in Lagos we have been able to carry the business a stage further. In captivity gambiae and melas which have been bred out in the laboratory show no inclination to mate. u n t i l this happened we could not offer proof that the two species breed perfectly true to type. In Sierra Leone we tried small cages full of gambiae or rnelas, large cages, inside and outside at dusk, and fed the females regularly on blood. But there was still no mating and fertilization, the ovaries did not develop, and no eggs were laid. The same methods were tried out in La~os -usin,~ large numbers of laboratory-bred mosquitoes, but still nothing happened, till finally one night the cage was kept illuminated by a table lamp with an orange coloured shade for several hours at night. Under those conditions both gambiae and rnelas mated readily, and laid eggs after a blood meal. In this way we could finally show that adults bred -from rn:elas eggs in turn laid nothing but rnelas e ~ s , and ffambiae also remained true to type. One of these laboratory-bred and m a t e d garnbiae females lived 86 days in a small 6 by 6 by 6 inch cage; in that time it took thirty blood meals and laid twenty-eight batches of eggs, all of them distinct gambiae type. In order to bring our observations into line with those of the European A. ~maculipennis complex, We persuaded melas males tO fertilize ~arnbiae females, and ,ffarnbiae males to fertilize rnelas females, illuminating the cages as described above. The eggs laid as a result of these cross matings showed no transitional forms, but were determined by the identity of the female. These eggs produced healthy larvae Which ~ave rise to a hybrid generation of healthy adults: Attempts to get these hybrids to mate with each other, or with pure garnbiae and rnelas, were unsuccessful, and examination showed that while the

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females were apparently normal sexually, the males were in most cases sterile, with underdeveloped or atrophied testes. The sterility of hybrids is a severe test of specific status, and considered in conjunction with all the morphological and physiological differences just described forms a pretty firm basis for regarding gambiae and melas as two entirely distinct species. MALARIA TRANSMISSION" BY

melas AND gambiae.

However satisfactory it may be to have established the exact identity of

melas and gambiae in West Africa, at this stage one naturally asks whether it is anything more than a nice piece of pure systematics, and whether it has any bearing on the problem of malaria control. On this point I can assure you that on many parts of the coast the efficiency of control is, and will be to an increasing extent, determined by the full appreciation of the relative abundance and relative infectivity of the two species in the area. In Freetown estuary we are fortunate in that about one-third of the melas have distinctly banded palps, and we can form a rough idea of its incidence in different places by catching mosquitoes in houses, and multiplying the number of four-banded females by three to give an estimate of the total melas in the catch. This is a rough and ready method which can only reasonably be applied to large samples. In small catches it may lead us astray. But by this means we could tell, for example, that while melas was abundant in most of Freetown estuary, Freetown itself and the neighbouring village of Kissy were almost pure gambiae areas. Irt and around Lagos, on the other hand, only about 5 per cent. of the melas have this distinct extra dark band, the remaining 95 per cent. being indistinguishable from typical gambiae. In this case we mdst rely entirely on egg characters before we can have any idea at all about the relative abundance of melas and garnbiae in houses. , This was one of the first things to be tackled in Lagos. From January, 1945, onwards several catching stations were selected in and around Lagos, and at intervals mosquitoes were collected by hand-catching from these village and town houses. Of the total collected a sample of anything up to fifty females were put aside, one mosquito to a cage, and they were identified by the eggs they laid during the following 2 or 3 days. As melas and gambiae lay equally well in the laboratory we could work out fairly accurately the number of melas and gamblae in the catch. From these records we now have for the first time some idea of the incidence of the two species in several different localities round Lagos, and the relative abundance month by month right through the season, based on about 15,000 adults collected, and about 7,000 egg batches identified. This is rather a laborious method, but there is no other way of acquiring such figures at the moment, and the results amply repay time and labour.

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For example, in one village surrounded by hundreds of acres of mangrove and brackish swamp, we find that at certain times of the year most of the mosquitoes in the houses are not melas, as we might expect, but gambiae, sometimes originating from a single freshwater borrow pit beside the village. Lagos town itself is situated on an island on which there are no longer brackish swamps, but just three-quarters of a mile across the harbour there is a great production of melas for many months of the year from the extensive tidal swamps. Yet 96 per cent. of the house catch in Lagos is gambiae which originate almost entirely from local rain-filled breeding places. Similarly, just about a mile or two inland from the melas infested coastal swamps one finds villages in which the population is entirely gambiae. In Freetown estuary, too, egg identification has revealed sudden changes in the mosquito population which might otherwise have escaped our notice. Wellington village, where much of our work on breeding habits of rnelas was carried out, is dominated by melas for the greater part of the year. In the early rains in June there is a great peak of mosquito production corresponding to intense breeding of melas in the adjacent Avicennia orchard. Egg identification, however, showed that at this peak period, which only lasts a few weeks, about half the mosquitoes in the houses are gam biae, originating from numerous rain-filled pools in among the village houses. As results of control measures in all these areas become more and more evident, it will be increasingly important to know the identity of mosquitoes still occurring in small numbers in houses, so that time and money are n o t wasted in controlling the wrong breeding places. There is a great need for more information of this kind on the coast, and it can only be done b y routine egg identification of mosquitoes from representative catching stations. For example, we have no idea of the relative abundance of melas and ¢arnbiae in the other two colonial capitals, Accra and Bathurst. It is possible that they may both be affected by extensive tidal swamps nearby, or it is possible that local breeding of .¢ambiae in freshwater pools is the main concern. Until such information is forthcoming we have to rely on conjecture, which is definitely not conducive to sound malaria control. We have already referred to the records of BARBERand OLL','GER (1931) who found infections among melanic coastal forms of garnbiae, presumably melas, in the Lagos area. In Sierra Leone,toO,BLAC~CLOCKand WILSON (1941) found four sporozoite infections in seventy-four "var. rnelas" on the Bullom shore, and they considered that the high infection rate both of malaria parasites and microfilariae was of considerable importance for West Africa. On the other hand there were reports from East Africa that MACKAY(1938) found no infections in " vat. rnelas " from Dar-es-Salaam. In this connection I might point out that nothing was known at that time about the identity of melas in East Africa, and as far as I know the situation is equally vague today. Once the identity of rnelas, as distinct from gambiae, had been established

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it was felt that a new series of dissections of material identified both by palpal banding and egg characters was desirable. As a result we have the records of 1,000 dissections carried out by No. 5 Malaria Field Laboratory under Dr. TREDRE, on material from Freetown estuary. These rnelas showed a sporozoite rate of 4"2 per cent. for the whole season. This is m u c h lower t h a n the usual figures for gambiae f r o m this area (GORDON et al., 1932), but the figures were not strictly comparable as "the records were, from different localities. This question was taken up in more detail recently in the Lagos area where all identifications were based on egg characters. There we have records from m a n y localities at all months of the year ; but what is more important, we were fortunate enough to find the type of village which we had looked for in vain near Freetown, namely one in which both rnelas and gambiae occurred together in numbers in the village houses t h r o u g h o u t most of the year. We could now test the infectivity of garnbiae and melas when they were exposed to chances of infection as equal as it is possible to find in nature. T h e results showed that under those equal conditions gambiae, with a mean sporozoite rate of 11"1 per cent., is nearly three times as efficient a vector as rnelas, with a mean rate of 4"5 per cent. These consistent differences were also shown in all other catching stations, the combined figures for sporozoite rate being I0"0 per cent. for gambiae, and 3"5 per cent. for rnelas. Furthermore, at certain times of the year the sporozoire rates of small samples of gambiae reached heights never approached by melas. T h e highest figure for any m o n t h was a sporozoite rate of 8 per cent. for melas, compared with the remarkable figure, of 29 per cent. for garnbiae. T h i s latter figure is just a little short *of the " epidemic " infection found in gambiae after its introduction to Brazil, and also agrees closely with BARBER'S findings in those localities in which he, unknowingly, was working with pure garnbiae. Although gambiae remains unique in this respect, it does not alter the fact that melas is a pretty efficient vector, too, and that the two of t h e m form a very formidable partnership on the West African coast at least. As has been emphasized earlier in this section, the recognition of the parts played by rnelas and gambiae in malaria transmission i n any locality in West Africa is an essential basis for sound control policy. The relative importance of the two varies greatly from place to place, even between one end of a town and the other; and so far it is only in' the Lagos area and in Freetown estuary that we have the necessary information. As we shall now see the breeding places and control of the two species are so strikingly different that effective control against one species alone m a y leave the problem of the other practically untouched. BREEDING PLACES OF Anopheles rneIas. I shall deal first with melas, as it is with this anopheline that most of our knowledge has only recently been acquired.

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The capacity for " var. melas " to breed in brackish water in coastal swamps has been known for many years. BARBERand OLINGER in 1930 found it breeding among paspalum, which is a coarse sea grass covering great stretches of tidal swamp near Lagos. In Freetown the first brackish water breeding place was also found in a marshy patch of paspalum and other grass in an area outside the mangroves and only flooded at spring tides (RIBBANnS, 1944). But brackish swamps of this kind are scarce in Freetown estuary where most of the great area between low tide mark and high spring tide mark is covered with dense mangrove. And yet in those pure mangroves areas, with few open spaces of paspalum swamp, we were finding high catches of melas in the village houses. In the one known breeding place described above, which lies just west of Freetown, larvae were never found extending from the sea grass into the adjacent mangrove. As this was regarded as a typical melas breeding place the search for similar types of swamp went on for months, with completely negative results. Finally we found a few larvae in a part of the mangrove belt where the mangrove appeared to grow in rather a different way from usual, spaced out on very flat and fairly firm ground, rather like little apple trees. To cut a long story short these characteristic " orchards " as we called them belonged to those of the black mangrove, Avicennia, while the great bulk of the mangrove belt is composed of the more familiar red mangrove, Rhizophora, with its stilt roots, aerial shoots, and dense tangled growth. Dense breeding o f rnelas was soon found to be confined to those Avicennia orchards, whereas the great stretches of pure Rhizophora, and their fringes of paspalum, were harmless. (MuIRHEADTHOMSON,1945). These Avicennia orchards are only flooded by the, spring tides, and vary greatly in size, anything up to 50 acres. At certain times enormous numbers of melas larvae may be found scattered right through the orchard. Now it was clear that the presence of one of these great orchards near the anchorage or seaplane base might nullify the anti-malaria work going on ashore. Searching for the breeding grounds by exploring on foot in the mangroves is a tedious job, especially with the prospect of about 20 miles of coastline covered with mangrove anything up to 5 miles deep. When viewed from high ground the great mangrove belt appears a uniform green in the rainy season, but in the dry season the Avicennia orchards can be picked out by the light green colour of their foliage, compared with the various darker sl-/ades of Rhizophora. Aerial reconnaissance was then tried, and new orchards were seen in this way, ahhough the method had its drawbacks in' that it was very difficult to get one's bearings even looking ou~ of a comparatively slowmoving Swordfish plane. Finally, the method of aerial photography was tried. The R.A.F. had already a fairly complete mosaic of the estuary, made up of vertical aerial photographs, but in this mosaic it. was very difficult to distinguish the differences in foliage texture and shade. But when the orginal prints were enlarged

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twice everything stood out clearly, and we could pick out and define the Avicennia orchards right round the estuary. In this way we saw that the total area of Avicennia orchard was small compared with the harmless Rhizophora, and later work showed that only about six of these were of first importance as breeding places. Now these orchards are only flooded by the spring tides covering them twice a day for 4 or 5 days each fortnight, or each month in the dry weather, and as we might expect it is the periodic flooding with salt water that produces the breeding places. But what was entirely unexpected was that the great crop of melas breeding starts almost as soon as the flat orchard is flooded by the first high spring tide, and that the larvae spend most of their 5 or 6 days larval life being flooded by the spring tides twice a day, often to a depth of 1 or 2 feet. When the spring tides subside, and the brackish pools are left undisturbed, the great crop of larvae are already full grown and pupating. If the same pools are visited a day or two later it may be almost impossible to find larvae. Conditions in all the Avicennia orchards in Freetown estuary are almost identical in relation to flooding by spring tides, and as a result for 2 or 3 days each fortnight, or each month in the dry season, there is an enormous peak output of melas from all the breeding grounds. Fortunately, the tidal movements and h e i g h t s in Freetown estuary are accurately known, and as a result we could bring our survey methods to an unusual pitch of refinement. Sitting in comparative comfort in the laboratory, with tide tables in one hand and the series of vertical aerial photographs in the other, it was possible to tell weeks in advance the exact day and hour on which an unexplored Avicennia orchard would have to be visited in order to determine whether or not it was an important breeding ground. If we went there on the wrong day we might find nothing, even in an important orchard, or we might find the place dry. Even on the very best day, if we visited the place an hour or two too early or too late we might find the place completely flooded by the high @ring tide so that the larvae are widely distri buted and difficult to detect, or we might find the tide so low that: there was no way of reaching the breeding ground by canoe up the narrow creeks through the tangled belt of Rhizophora. That clear cut picture unfortunately only applies, as far as we know at present, to Freetown estuary with its profuse growth of mangroves, and great deposits of silt where the two large rivers flow into the estuary. In the low-lying lagoon area of Lagos we find rather a different picture. There are few rivers bringing down silty water to the lagoon, the mangrove belt is much scantier, and the inter-tidal zone is characteri:~ed by great stretches of paspalum sea grass. We find one or two Avicennia orchards like those of Freetown estuary, but we find rnelas breeding grounds in such a wide variety of places that Avicknnia is no longer a guiding light. Furthermore,

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tidal movements in the lagoon area are extremely variable, and their heights actually unpredictable. In Gold Coast, again, we find places like Accra where the coastal configuration is rather different, with melas breeding among the paspalum at the edges of small lagoons which may or may not be open to the sea at all times, and in which there may be no clear mangrove belt. The true estuarine conditions found in Freetown estuary may possibly be duplicated in other parts of West Africa such as the Niger delta, but it is already clear that there is such a range of conditions that the findings in one part of the coast, no matter how clear cut and conclusive they are, may actually be quite misleading if applied directly to some other part. One other point about the breeding habits of meIas is worth noting; that is, its almost complete restriction to the tidal belt. When the egg differences were discovered, the distribution of A. melas and A. gambiae breeding places in these coastal areas was studied by collecting eggs from a wide range of potential breeding places. In this way it was found that even in places where melas is enormously abundant, it makes use of freshwater pools only to a negligible extent. We have found a few'eggs in rain-filled pools in Freetown itself, and under very similar conditions in Lagos town. But for all practical purposes melas is rigidly confined to the area below high spring tide mark. C O N T R O L OF

A . melas

W e have seen that in Freetown estuary dense breeding of rnelas is mainly confined to about half a dozen Avicennia orchards, whose total area is not more than 200 to 300 acres. This concentrated and clearly defined nature of the breeding grounds suggested that their control was a practical possibility. These breeding grounds tend to dry out at intervals during the neap tide periods, but drainage alone does not control breeding, because as we have seen, most of the larval development takes place while the ground is still being flooded twice a day by the spring tides. When the spring tide period is over, and the drains start to act, the larvae are already full grown, or pupated, and aduhs are produced before the ground dries up. About the end of 1942 the idea of keeping the high spring tides out by bunding the breeding grounds arose quite independently in Freetown and in Lagos, and despite the fundamental similarity of the problems the development of this method during the following 2 or 3 years pursued an independent course in the two colonies. The idea of reclaiming a tidal swamp by building a bund or dyke to keep out the high spring tides, and draining off the impounded water at low tide through automatic or hand-operated sluice-gates, is certainly not a new one. As far as West Africa goes I believe it was first used about 1939 or 1940 for reclaiming tidal swamps' for rice cultivation in Sierra Leone. II'l that colony the malaria control models were based on this, and the presence of an Irrigation and Drainage Department

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with experience of empoldering seemed to offer a flying start to melas control. However, for various reasons, the progress in bunding has been disappointingly slow in Freetown estuary, and it is to Lagos that we must turn our attention to realize the enormous possibilities of this method of control. In the Lagos area bunding has been carried out by Dr. GILROY and his staff to an extent and with a thoroughness that forms a perfect working model for all malariologists who are likely to encounter this type of problem in any part of the world (GILROYand CHWATT, 1945). In Lagos there is no question of clearly defined breeding grounds of limited area to be reclaimed. The aim here has been the comprehensive, reclamation by bunding of the whole inter-tidal area with the exception of the outer fringe of mangrove lining the creeks and the lagoon. The very small tidal fluctuation in the Lagos lagoon, usually just a foot or two, has imposed less strain on the bunds, but at the same time made it more difficult to run off the impounded water from the flat swampland. As a result all sluice-gates are hand operated, the sills of thetide-gates are set 3 feet below mean sea level, and the internal drains are wide enough and deep enough to be of the nature of canals rather than drains. They always contain salt water, continuous with the lagoon water when the sluice is open, and clean edges. When well established they remain completely free of melas breeding. It is not just a matter of concentrating breeding into a small area, it is complete elimination of vast breeding grounds of melas. The technique in Lagos has been perfected to such a degree by Dr. GILROY that I have seen apparently hopeless quagmires of brackish swamp and mangrove dried out in a matter of weeks. (In the last 4 years the total area of tidal swamp reclaimed in this way is about 5 square miles..) To return to Freetown estuary there is an enormous tidal range, about 12 to 15 feet, compared with Lagos. This range should greatly facilitate running off the impounded water at low tide from the drains behind the bund, but at the same time the height of the spring tides, and their frequency, imposes a severe strain on the earth bunds, which evidently need to be constructed on a larger scale than in Lagos. Furthermore, the Freetown orchards, although of limited extent, are exposed to a much heavier rainfall than Lagos, to a heavy run-off of water from the hills of the dry land, and to heavy seepages from the dry land at swamp edge. That is the sort of range of conditions one is likely to meet in the West Coast, and it indicates, I think, that while the principle of bunding has immense possibilities in eradicating melas from coastal towns and seaports, it may. have to be modified considerably to suit local conditions. From what I have seen it is a method which seems to have great possibilities in the Gambia, near Bathurst. In the Gold Coast, as we have seen, the problem is mainly one of melas breeding in numerous small lagoons with ill-defined outlets on a surf-pounded beach. Those who are familiar with the problem in that part do not advocate bunding, and the best method of dealing with such places has not yet been worked out. (BRucE WILSON, 1946).

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BREEDING PLACES AND CONTROL OF

A.

gambiae.

The advances in knowledge about garnbiae in West Africa in the last few years have naturally not been quite so striking as with melas. Gambiae as such has been familiar to generations of malario]ogists and it remains probably the most dangerous anopheline in the world. In some ways it proves to be a remarkably versatile adversary, while in other ways it appears to be a narrow specialist, with well defined likes and dislikes, unable to maintain itself unless things are just so, As an example of its versatility we have sonle observations on the effect of artificial flushing of partly dried stream beds to eliminate gambiae breeding in poolsl In the rocky colony of Sierra Leone there are some streams which provide ideal breeding places for gambiae in the dry weather, when the water is reduced to shallow sunlit pools on the rocky bed. Artificial flushing of these stream beds, a control method which has proved very effective against such stream breeding mosquitoes as A. minimus and A. fluviatilis in India, seemed to be an ideal method for this particular problem. As a result dams and sluice gates were installed at the upper parts of some of these streams early in the war. One of the first things I was shown in Freetown was this flush in operation, and I was much impressed. A turbulent flood was released which swept this rocky stream bed from bank to bank. It appeared as if nothing could survive such a deluge, and the flush seemed to satisfy all requirements. Unfortunately, when the flood subsided after an hour or so, we still found large numbers, of full grown gambiae larvae and pupae in the stream bed. After a time we found out that what was happening was this: The first flush washed the gambiae larvae out of their pools to the slack water at the edge, where the larvae submerged and evidently lay qu'iet at the bottom. In fact, we could show in the laboratory that garnbiae larvae can sur-vive complete immersion for 4 hours. When the flush subsides the larvae come to the surface, and follow the slowly receding water back to the central pools. When the flush was stepped up to three times a week, with a double flush imposed from an additional sluice, another complication appeared. The arid bed and banks of the stream started sprouting a heavy growth of vegetation with tufts of grass in mid stream, all of which afforded additional harbourages for gambia~ larvae. In a canalized bed the results of flushing would probably be much more effective, but it all shows how extraordinarily careful and critical one must be in applying to one kind of mosquito measures which have been established by observations on a different kind of mosquito. As for gambiae's specialist habits, what is always most striking is not so much the places where garnbiae does breed, as the great variety of surface water in which it does not breed. Even the ideal breeding place associated with garnbiae, the small sunlit puddle or pool, may be curiously negative. In some of the laterite flats near Free/own, an immense number of such

R.

C. M U I R H E A D T H O M S O N

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pools may "exist for weeks on end in the rainy season, and yet the proportion breeding gam biae may be as low as 1 in 50. But in the odd pool, or group of puddles, which seem to satisfy all requirements, gambiae breeding seems to run riot, and hundreds of larvae and pupae may be taken in a few square feet of surface water. The existence of these odd pools swarming with larvae has been one of Freetown's problems since the time of Ross. (Ross et al., 1902; Ross, 1902). In Lagos, too, I have seen a single flooded vegetable garden in the heart of the densely packed town, whose output was worked out at about 50,000 female garnbiae per day for several weeks on end. This feature of gambiae breeding is one that crops up on the whole coast, and probably in the whole of West Africa .In this respect I think that during the last few years there has been an increasing realization that the most rapid and striking reduction irt the numbers of garnbiae can be brought about, not by large and costly drainage schemes, but by the so-called temporary measures, systematic search for, treatment or elimination of the small pools and puddles which form the major breeding places. There are many places where breeding grounds have been greatly reduced or eliminated by well-planned drainage schemes ; there are places like Lagos Island where improved drainage, and the slow and inconspicuous process of sand filling over many years, have produced a tropical swamp town almost completely free of garnbiae breeding places. But unless these schemes are coupled with an efficient organization for routine inspection and treatment of casual pools, their good effects may be completely nullified. These remarks naturally do not apply to rnelas, whose control, as we have seen, is an entirely different problem, in which large scale bunding or other engineering schemes is the only answer. Of the measures directed against the adult mosquito, regular house-spraying with pyrethrum in kerosene has been used a great deal in recent years, particularly in Gold Coast (EDDEY, 1944). The use of spray-killing alone has been advocated as an effective means of controlling anopheles in West Africa, a suggestion which seems to he supported by the undoubted success of spraykilling against gambiae in South Africa, and against culicifacies in India. In villages near Lagos. however, we have been unable, even by intensive spraying 4 to 6 days a week, to bring about a substantial reduction either in the number of anopheles, or in their infectivity; probably due to the finding that in houses which are being frequently sprayed, more and more mosquitoes tend to leave the house after feeding, and rest outside, thus escaping further exposures to. day-time spraying (ML'IRHEADTHOMSON, 1947). However disappointing the results obtained by this measure alone, it may still be possible that when it is combined with vigorous anti-larval work, spray-killing might provide an additional effective blow against the few remaining adults. In this respect it is important to remember that the Rockefeller Foundation and the Brazilian Health Authorities in their outstandingly successfu!

524

MALARIA VECTORS IN WEST AFRICA

campaign against gambiae in Brazil, emphasized the importance of attacking both the larvae and the adult mosquito. In this report the emphasis has been on gambiae and rnelas. N o mention has yet been made of funestus, which is garnbiae's able partner in crime over so much of tropical Africa. Actually, in the West African coastal strip, on which the colonial capitals are situated, funestus is only of local importance. Where it does appear it has not been neglected. For example in Kissy village, now almost a suburb of Freetown, fuuestus used to be as important as garnbiae (GORDON, et al., 1932), but in the last few years, as a result of measures recommended by BLACKLOCKand WILSON (1942) funestus has now dwindled to practically nothing. Funestus was also the subject of some of the earliest field experiments on the larvicidal effect of D D T carried out by Professor BUXTON (1945) near Takoradi over 2 years ago; and RIBBArCDS(1946) has also added to our knowledge of this species by observations in Gold Coast on the effect of bush clearing, moonlight, and other factors on nocturnal activity. There are odd localities, too, where A. nili has been shown to be an important vector. But in general gambiae and "rnelas have received the lion's share of attention, and rightly so. DDT

IN M A L A R I A CONTROL.

N o discussion on recent knowledge of malaria control would be complete without at least passing reference to DDT. D D T needs no introduction here; the main features have been fully dealt with by Professor BUXTON (1945) in his address to this society and in the discussion that followed. While its remarkable insecticidal properties are firmly established, there are one or two instances where it has been acclaimed in a way hardly justified by the available facts. As a larvicide there is no doubt that it is a most valuable weapon. A report from Freetown by Dr. WALTON says that with 2 oz. D D T per acre, using a watery dilution of a kerosene soap-izal emulsion, a complete kill of anopheline and culicine larvae is obtained. To achieve the same results with paris green costs three to four times as much, and about twenty-five times anaore in the case of malariol. In this way D D T is certainly going to improve control without necessarily revolutionizing it. Now we come to the question of D D T as a residual insecticide in houses. The commonly accepted idea is that treatment of the inner walls and ceilings of village houses will make them lethal to mosquitoes for two or possibly more months. This conclusion is supported by reports from many countries, gnd dealing with different anophelines. If this were indeed true, then it would undoubtedly open up entirely new vistas of mosquito eradication and malaria control.

R. C. MUIRHEAD THOMSON

525

Unfortunately, in some recent work in Lagos, we have disclosed unforeseen snags which may greatly limit the value of this insecticide against mosquitoes. This is briefly the story. When one sprays the walls of village houses with D D T in kerosene, there is the most dramatic fall in the number of mosquitoes caught resting there by day, and this may continue for several weeks or even months. That is the sort of experiment that has been done in many different parts of the world, and many people have taken that as sufficient proof of mosquito reduction. In Lagos, after a careful study of the normal house-haunting habits of garnbiae and melas, we found that although many gambiae and melas remain in dark houses after feeding at night, others leave the house at dawn and rest outside. When they leave the house at any time between dusk and dawn they are attracted by the faint light coming in at windows and other openings from outside. We designed a simple trap based on this attraction to faint light in such a way that all mosquitoes leaving the house were trapped in a small detachable window cage. By this means we were able to show that while a house treated with D D T in kerosene remains free of resting anopheles for weeks afterwards, as early as the 4th day after treatment mosquitoes were entering the hut, feeding on the occupants, and leaving the hut after feeding. Of those mosquitoes escaping from the treated hut in this way, and caught in the window cage, there was no appreciable mortality in the following 48 hours. Within a week of treatment large numbers of mosquitoes were feeding in the hut every night, all of them leaving afterwards, so that nothing was found in the hut on the following day. (MuIRHEAD THOMSON, 1947). Presumably what is happening is that when blood-fed anopheles want to settle on the walls after feeding, they are irritated by slight contact with the treated surface, and quickly leave" the hut before they have absorbed a lethal dose of the insecticide. We could find no evidence that mosquitoes were succumbing to the effects of insecticide inside the hut. In another experiment, an isolated village had all the houses treated with D D T in kerosene, but we continued to End blood-fed anopheles in outside resting places beside the village, and among those, mosquitoes with gland infections. This irritant quality of the D D T in kerosene, which prevents it exercising its full lethal action, may possibly be overcome by using D D T in other forms, or it may be influenced by the type of house or wall surface. But at present it may prove a serious obstacle, and there is certainly no cause as yet for undue optimism, in, West African conditions at least, about revolutionary results of residual spraying with D D T in kerosene. We cannot say yet whether or not these findings have any bearing at all on the use of D D T against mosquitoes in houses in other parts of the world: but it is fairly clear that the dramatic fall in house catch following treatment with D D T as a residual spray in houses, cannot be accepted 'as definite proof

526

MALARIA

VECTORS IN \VEST AFRICA

that mosquitoes are being killed. The methods used in West Africa might be applied with advantage elsewhere to follow up the possibility of mosquitoes feeding in treated houses, and leaving unharmed. If there are any differences of opinion at this stage about the residual effects of D D T on mosquitoes in houses, I think that the field entomologists themselves must share the blame. There is little doubt that the sudden advent of DDT, with its remarkable residual insecticidal properties caught us unprepared. Our knowledge about mosquito movements in houses was not nearly adequate enough to provide fool-proof field tests. The work in Lagos has shown that without a sound knowledge of the mosquito's normal behaviour, it may be very difficult to interpret accurately the results of field tests. The work is really just beginning, and it is still just as necessary as ever to approach the subject with the same caution exercised by Professor BUXTON in his address to this society nearly 2 years ago. ANOPHELINE REDUCTION AND MALARIA CONTROL.

In this mainly entomological report no mention has been made of the effects of all these measures on the incidence of malaria, particularly among the Services, during the last 5 or 6 years. This itself is a very big subject, and one which I am not competent to review or discuss." I understand also that this aspect of the problem will be dealt with fully in a forthcoming publication by Dr. TREI)RE. We usually refer to our measures to reduce anopheles as anti-malarial. But in West Africa we do not know at what point the two terms become more or less synonymous. Everything depends on whose malaria we are trying to reduce, non-immune European, or indigenous African. In this connection I should like to mention some interesting work that has been going on in Freetown for the last 3 years under Dr. Tt:RXER and Dr. WALTON (TuRy~R and WALTON, 1946; WALTON, 1947). They have been concerned with finding the relation between the great reduction in anopheles which they have brought about in Freetown over that period, the extent to which malaria transmission has been reduced, and the extent to which malaria rate and the blood picture of the local African population has changed. Noteworthy is the regular blood examination, every month where possible, of infants from birth onwards to 18 months, so that exact data are beingobtained about the frequency of new infections and their seasonal distribution. It is obvious that a great deal more information of this kind is required. Even the entomological specialist, narrow though his aims and outlook may be, can hardly help noticing that in these coastal areas at least the indigenous African seems hardly aware of malaria as such. I do not think anyone is very sure about the extent to which malaria contributes to the ill-health of the adult West Africa; and one naturally comes up against the controversial issue as to how far and to what extent measures against anopheles in West Africa

DISCUSSION

527

should be extended b e y o n d the large towns and ports a n d m a i n centres of n o n - i m m u n e population. It seems to m e t h a t before we talk loosely a b o u t rural malaria control in W e s t Africa it would be a reasonab]e idea to find out first exactly h o w i m p o r t a n t .rural m a l a r i a really is in that p a r t of the world. REFERENCES. BARBER, M. A. & OLINCFm, M . T . (1931). Ann. trop. Med. Parasit., 25, 461. BLACKLC~CK, D. B. (1941). Malaria in and around Freetown Harbour. Unpublished report. ---& WILSON, C. (1941). Ann. trop. Med. Parasit., 35, 37. • (1942). Ibid., 36, 182. - - - - - - &(1944). Report o7l a Mission to West Africa, March to April, 1944. Unpublished report. BUXTON, P . A . (1945). Bull. ent. Res., 36, 2, 121. . (1945). Trans. R. Soc. trop. Med. Hyg., 38, 367. EDDEY, L . G . (1944). Ibid., 38, i67. EVANS, A . M . (1931). Ann. trop. Med. Parasit., 25, 443. • (1938). Ivlosquitoes of the Ethiopian Region. II. Anophelini. London : British Museum (Natural History). GmRoY, A. B. & CHWATT, L . J . (19.45). Ann. trop. IVied. Parasit., 38, 189. GORDON, R. M., HICKS, E. P., DAWY, T. H., & WATSON, M. (1932). Ibid., 26, 237. MACKAY, R. (1938). Second (Final) Report of the Malaria Unit, Dar-es-Salaam. Dar-esSalaam : Government Printer. MA'rTINCLEY, P. F. (1944). Ann. trop. Med. Parasit., 38, 189. RmBANDS, C . R . (1944). Ann. trop. Med. Parasit., 38, 2, 85. (1944). Bull ent. Res,, 35, 271. (1946). Ibid., 36, 395. . (1946). Ibid., 37, 33. ROBERTSON, J. D. (quoted by MATTINCLEY, 1944). ROSS, R., ANNETT, H. E. & AUSTEN, E . E . (1902). Report of the Malaria Expedition to West Africa. Mem. Liverpool Sch. Trop. Med. 2. Ross, R. (1902). Mosquito Brigades and how to Organize Them. London : G. Philip & Son. SOP~R, F. & WILSON, D. B. (1943). Anopheles gambiae in Brazil, 1930 to 1940. New York : Rockefeller Foundation• THOMSON, R. C. MUmHEAD. (1945). Bull. eAt. Res., 36, 2, 185. • (1947). Ibid. In the press• TREDRe, R. F. (1946). Ann. trop. Med. Parasit. In the press• TURNER, J. & WALTON, G . A . (1946). Report on Malaria in Freetown and District. Sierra Leone : Government Printer. WALTON, G. A. (personal conununication). WILSON, D. BRUCE. (1946). Report on the Control of Malaria in the Accra, Takoradi, and Sekondi areas• Gold Coast : Government Printer•

DISCUSSION. Professor P. A. B u x t o n : MR. PRESIDENT, I should like first to t h a n k our speaker and congratulate h i m on having delivered an admirable p a p e r which will be of great service to us, and for having spoken so briefly. Dr. MUIRHEAD THOMSON has delivered the goods, closed and sat down in 45 minutes, a m o s t unusual accomplishment. T h e r e is truly a remarkable a m o u n t of i n f o r m a t i o n that has come in f r o m West Africa during the last 4 or 5 years.

528

DISCUSSION

I have been frequently in that country, and know the amount of hard work needed to produce this communication and summarize it. One point of considerable interest is that after THOMSON'S first period of service in Assam, the Colonial Office was moved to offer him an appointment in West Africa, with the general mandate that he should go there to study the biology of gambiae, not to control it, but to contribute to our knowledge of that one species of insect. He has been there 4 or 5 years concentrating on gambiae and melas ; and very clearly this policy of concentrating on the main vector but leaving the man to develop the subject in his own way was sound. The decision showed vision on the part of the Colonial Office, and has in fact proved sensible and wise. In the paper there are a great many things that are distinct and valuable contributions to the subject. First of all Dr. MUIRHEAD THOMSON'S own discovery that you can separate melas and gambiae apparently every time by the character of the eggs. Every previously known character of adult and larvae were less satisfactory. Having found this point of distinction, another very valuable advance was the discovery of the association of melas breeding with the orchards of Avicennia in Freetown estuary. If you read that story at length you will find it a fascinating account of most careful natural history. The distribution of the different sorts of mangrove depends very precisely on their position on the very gradual slope in the estuary. Just a bit down the hill you wiU have the red mangrove, Rhizophora, harmless from the mosquito point of view, and a little up the slope you may have the Avicennia. The elaborate story of the relation of the mosquitoes not only to the mangrove but to a particular point in the tidal "cycle is a convincingly worked out story. A third point that will stand out for a long time in the paper, and in the work done in West Africa, is the doubt thrown on D D T for this particular purpose against gambiae under West African conditions. It is noteworthy that within the last year Dr. MUIRHEnO THOMSON, and at least one other penetrating worker, have begun to feel considerable doubt as to whether D D T is as effective in killing adult mosquitoes in houses as has been believed. I agree that Dr. THOMSON'S full paper shows that under his conditions D D T is acting more as an irritant than as an insecticide. It is driving mosquitoes out of the house, not to die but to live, and some to develop sporozoites, which after all is the one thing we want them not to do. They are living long enough to become harmful, and that is a very disappointing and disturbing thing. It seems to me that most other users of D D T have failed to look far enough, or deep enough, and have been satisfied with the more superficial observation that after putting D D T on the wall you did not find mosquitoes there for 3 months. There is, however, evidence for the sort of anopheles found in the Mediterranean and in the southern parts of North America, that they sit on walls treated with D I ) T and remain long enough to take up a lethal dose. Perhaps there is a difference between the different species of Anopheles into which further and fuller enquiry will have to be made.

DISCUSSION

529

Dr. C. R. Ribbands : Mr. PRESIDENT, there is very little that one can add to the excellent paper that Dr. MUIRHEAD THOMSON has given us. I think the thing that must be interesting us all at the moment is not the biology of melas, fascinating as it is, but Dr.-MUIRHEAD THOMSON'S most important work on the effect of D D T on gambiae, and his critical comments on the effects of pyrethrum spraying. In this connection the first thing I would like to say concerns repellency. Dr. MUIRHEADTHOMSON found in West Africa that after pyrethrum spraying mosquitoes came into the room that had been sprayed and then left again. In somewhat similar work, conducted in Assam, I found that minimus were not there after pyrethrum spraying, and further work in the evening showed that they did not come in at all. It seems to me that it all depends on whether you offer them an alternative place or not. In my experiment the mosquitoes had the alternative of going into the adjacent houses which were not pyrethrum-sprayed, and they took it ; whereas in Dr. MUIRHEADTHOMSON'S case after spraying they had no choice, and so they went into the sprayed houses to feed, and afterwards came out again. I think that a rather important point when considering the protection of individual houses. One often finds places where there is a mixed population of immune local people and non-immune whites, and one is very much interested in protecting the small local population of whites. In that case it is quite possible that one can get some definite malaria protection by spraying individual houses before or during the evening in order to get the effect of rcpellency. Dr. MUIaHEAD THOMSON'S fundamental work with D D T in the field seems to lead to the conclusion that residual sprays of D D T are almost useless against .4. gambiae, but those of us who have worked with it elsewhere are not quite so pessimistic. I agree that repellency of some sort plays a big part in the effect of D D T . I found, in Assam, that treatment of whole tea estates with doses adequate to eliminate mosquito catches often yielded no apparent malaria reduction. That tends to confirm Dr. MUIRHEAD TaOMSON'S point of view. But my heaviest treatments with D D T did seem to produce good effects. I think that the possibility of getting malarial control with residual D D T is reduced, but not necessarily eliminated, by the repellency. It may be possible to apply a dose so big that mosquitoes alighting will immediately absorb a lethal dose. If that is possible we may still get malaria control by D D T with massive doses, although it may be a failure in small doses. The trouble is that massive doses are expensive, and if we have to use massive doses of D D T to get malaria control we shall have to specialize and put it only in certain parts of rooms or in places where mosquitoes are most likely to alight, instead of spilling it all over the walls and ceilings according to the present standard method. Dr. J. W . Scharff : I desire also to pay hearty tribute to the vivid eloquence of the speaker. The remarks I wish to make must inevitably be coloured with memories of similar work carried out in Malaya. I am particularly pleased to

530

DISCUSSION

hear how it has been possible to amalgamate agricultural land improvement with the anti-larval work in some of the anti-malarial coastal drainage measures described by Dr. MUIRHE~a~ THOMSON. Before the war in Malaya practically all rural anti-malarial projects, including those along the coast, had been associated with land reclamation and were carried out in such a way as to provide additional acreage for crops. If the question of extending anti-larval measures into the rural areas of West Africa comes to be seriously considered it should, I submit, be done with very close regard for any opportunity this may offer for the extension of land cultivation and crop production. It had been found in Malaya that relatively inexpensive anti-malaria drainage schemes could be designed to facilitate agriculture. The modern generation of malaria field workers should appreciate the necessity of growing food as well as "of arresting disease. Present-day needs have shown how necessary it is that these subjects should be closely allied. On the question of malaria immunity, though one has often heard the view expressed that such an immunity amongst African natives implies no crippling effect among the infected community, yet experiences in other equally malarious countries, where the difficulties and tragedies entailed in the build-up of a solid immunity may have been more closely observed, suggest that a light-hearted acceptance of the burden of malaria, even in rural Africa, may not be wholly justified, when " health " is the final goal. I should like to congratulate Dr. THOMSON on his choice of the term " direct," in place of the much misunderstood epithet of " temporary," used in relation to anti-malaria measures. In Malaya we were driven to the use of the term " semi-permanent " when we were in fact employing methods which were direct and permanent, but which were not so expensive as brick and masonry, to which the name " permanent " is generally assigned. I find it difficult to understand the failure of sluicing mentioned in the paper now under discussion. Is it not possible that the larvae were breeding in the impounded water above the dam, as was described by Dr. P. F. RUSSELL in the Philippines ? Then there is a point, which I understand will be further elaborated by Professor WILLIAMSON, and which I believe to be of much ,ignificance: Dr. THOMSON spoke of the remarkable specificity of certain of the breeding haunts of the larvae of A . gambia and this contrasts sharply with what one has previously heard of the almost universal nature of its breeding habits. This point of specificity in the breeding places of the malaria-carrying anopheline is one of great importance to field workers and is one wherein much more detailed chemical, geological and biological investigation should be done to determine the underlying characters of the favoured breeding places. The sensitivity of larvae to soil conditions underneath their breeding ,water is a direction in which one may hope to enhance the value and scope of malaria field surveys. Malayan experience points to the fact that the character of the soil and possibly of the soil bacteria circulating in water in contact with such

DISCUSSION

531

soil, may have some effect in determining whether the anopheles which breeds there, is likely to become readily infected with malaria. Finally with regard to D D T , I had the opportunity to assist in some of the early experiments on air and ground spraying with D D T during the war years in India and Burma. In the course of this investigation we tried to determine the relative values of indoor and outdoor spraying under a variety of conditions, but the report of these trials remains buried in the secret archives of South-East Asia Command. The R.A.F. Station at Cuttock, near Calcutta, lent itself remarkably well for some of these tests. On the outskirts of the aerodrome there were some huts identical in character and well stocked each night with an unusually wide range of different species of Anopheles. In the particular experiment I have in mind, one hut was sprayed with D D T in kerosene at the rate of 20 mg. per square foot and another remained unsprayed. Both the treated and the untreated huts were closed with nets every morning after sunrise and counts were made of the mosquitoes caught over a period of 2 months, during the height of the malaria season. In the course of this trial n o live mosquitoes were caught in the hut treated with D D T . The condition of the building made it possible to collect dead mosquitoes found lying on the cement floor as well as on the window ledges and hanging on cobwebs. The findings were remarkable not only because of the long lasting effect and the wholesale slaughter of these insects, but also because of the unending supply of dead anophelines which could be identified, thus revealing the number of different species which had succumbed to the effects of the poison. The total number of the dead catch was on some occasions higher than the numbers caught alive in the untreated hut ; care was taken that none was left uncaught. Many of the anophelines not ordinarily house-haunters were found dead and from this it was assumed that, after feeding upon the animals stabled in these huts, they must have been impelled by reason of their engorgement to rest on the DDT-treated walls to extrude the blood plasma before flying away and that the period of resting was long enough for them to acquire a fatal dose of poison. This was the sort of evidence upon which the comforting conclusion was reached that indoor spraying with D D T would probably be successful elsewhere if applied with the same thoroughness as was done in the experiment. It is therefore to be hoped that more detailed study of the value of indoor spraying in West Africa may be made to determine the precise conditions under which the elusive malaria vector, the habits of which have been so charmingly described by our speaker this evening, may be killed by D D T . Professor K. B. W i l l i a m s o n : I have listened with great interest to Dr. MUIRHEAD THOMSON'S account of his illuminating investigations on the anopheline vectors of malaria in West Africa, and I should like to refer very briefly to a question that concerns this and all similar investigations, namely, what makes one species of mosquito at one time and place an effective carrier of human

'532

DlSCUSSXOI~

malaria, but a poor carrier or absolute non-vector at other times, or in other places ? I believe it is customary to explain this fact by reference to " species sanitation " in its varietal aspect, the proof being considered complete when it is shown that certain varieties within the species are predominantly nonanthropophilic. The case of Anopheles maculipennis provides the best investigated example. It is obvious that the biting of man is as necessary for malaria transmission as turning the switch is for producing electric light. But this may not be the whole story. The mere making of contact will not ensure these results if there is no current to flow, nor parasite present to pass on. Some soils seem to bring this about by contaminating breeding waters, especially water-logged alluvial soils rich in rotting vegetation. Sir RONALDROSS counselled investigation of the relations of soil and malaria by modern methods, but I am not aware that much interest has been taken in the subject. Does contamination brought about by organically rich soil underlying stagnant breeding waters, or by excess of vegetation rotting within them, directly diminish or abolish the vectorial power of mosquitoes adapted to them, by rendering their tissues biochemically unsuitable for development of the malaria parasites ? Be that as it may, I think Dr. SCHARFF will bear out, and I believe Sir MALCOLM WATSON and Dr. BARROWMANwould if they were present, that Malayan experience shows that the incidence of malaria is largely determined by soil characteristics. Sir MALCOLM WATSON reached the conclusion that dangerous mosquitoes were absent from certain coastal ricefields because of the chemical composition of their water. And subsequent investigation showed that a large area free from Anopheles, and almost devoid of mosquito breeding, stagnated over stale waterlogged peat. And there is very little malaria in the western coastal rice belt of Malaya, the soil being an ill-drained alluvium rich in rotting vegetation. The coastal forests provide an intriguing exception, but the malaria rate rises steadily, and the soils get purer and lighter, and are better drained and carry purer water the higher one goes inland. This relationship is not by any means confined to Malaya. COVELL'Sworld lists contain too many examples of destructive vectors which breed in spring-fed seepages, clear streams, rock'-pools, etc., for the correlation between purity of soil and water and malaria to be entirely accidental. Examples could be multiplied. I learned in conversation with Dr. MOREAU of a locality in French Indo-China where A. vagus, elsewhere a harmless puddle, etc., breeder, had been proved to be a vector of malaria, and it turned out that it was breeding on a sandy river bank. Dr. MCARTHUR, who is present tonight, and who has made the important discovery that A. leucosphyrus is a virulent vector in British North Borneo, tells me that he inclines to the belief that there may be two varieties, of which one only breeds in spring and seepage-fed water, and carries malaria, while the other so far as is yet known does neither. As is well known, contaminated food and environment contribute flavour to fish, a liberal dressing of Worcester sauce being necessary to render palatable those caught in muddy waters. The same thing occurs in mosquitoes, as an

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observation made in my laboratory by ZAIN showed. An adult A. barbirostris, bred from a surviving full-grown larva kept for a few days in dark brown water in which excess of lawn clippings had been rotted, had its midgut and Malpighian tubes stained a deep black, presumably by chemical transformation of the refractory humic matter present in the water, under conditions of defective oxidation approximating to those in waterlogged organic soils. One hesitates to conclude that delicate malaria parasites could undergo development on tissues so highly charged. Much less contamination might suffice to prevent its completion. Only experiment can decide the issue, and in no controversial spirit I venture the suggestion that the problem of the ecological determinants of vectorial power merits investigation at the highest biochemical level by means of modern microchemical technique. If Dr. MUIRHEAD THOMSON,repeating experimental enterprise that reaped important discoveries from the miracle of his " aphrodisiac " lampshade, can find or make time to carry out such work, this Society and science will be yet further indebted to him. Dr. G. A. Walton : While in Freetown I had an ample opportunity of seeing the ways in which one found A. gambiae larvae in particular places. The soil may be mentioned. The whole of this area is derived from acid igneous rocks. The water is very acid, so much so that even when stream water in Freetown contains large quantities of ammonia the p H is still on the acid side. It would be very difficult to alter the soil because there is such a colossal acid reserve: nevertheless, I think an investigation into the reason why A. gambiae breeds in certain places is really called for. I wished when I was there that I could have the opportunity to do so ; but we were all very busy getting rid of A. gambiae itself. It is worth while mentioning that in my opinion this anopheles breeds in places that are definitely associated with human activity. It is not exactly very easy to explain that, but you may have a very large field with an enormous number of small pools in it ; and the ones that .4. gambiae chooses to lay eggs in are the ones recently formed by workmen who have been getting rock out of the ground. Another thing worth mentioning is that the .4. gambiae seems to show a preference for places where people wash in or cross streams. Also in swamps it chooses the places where people walk through the swamp. We have also found A. gambiae breeding in a font in a church I and in one or two other curious places. In general the impression is that some form of human inter-' ference definitely excites the A. gambiae to the place where it will lay eggs. It is a very interesting problem indeed, and I think something could be done there really valuable.

Dr. Edward Hindle : Mr. PRESIDENT, I should like to take the opportunity of saying what a very special pleasttre it has given me to be here this evening and listen to the address by my first Honours Graduate at Glasgow. It is also a pleasure to see the result of a zoological training in problems of this kind.

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Dr. MUIRHEADTHOMSONhas brought to our notice several observations of great interest in general biology apart from their special application in the problem that he has attempted to solve. There are one or two questions I should like to ask him. One is whether any observations have been made to determine the factors governing the selection by the mosquito of any particular site for the laying of its eggs. I mention this because another of my students, Mr. STUART, studied the egg laying of different species of Chironomids breeding on the shores of Millport, which are extraordinarily particular in their selection of pools. These pools contain varying mixtures of fresh water and sea water, even though they may be only a few inches apart, and you get larvae of one species of Chironomid in pools of a particular salinity, whilst adjacent pools of different salinity contain larvae of other species. It is not a question of survival, because you can take larvae from one pool and put them into another of different salinity, and they will survive and complete their development. What causes each particular pool to contain only certain species was not determined, but evidence was obtained that the females selected pools according to the salinity of the water. It was not a question of any difference in the pH value. With reference to DDT, the thing to study would seem to be the tarsi of anopheles, since this is the part of the body which comes in contact with the insecticide. I imagine that the structure of these tarsi must differ considerably, and perhaps the thickness of cuticle or some such factor might help to account for the varying results obtained with different species. In conclusion, I should like to express my thanks to the lecturer for a very interesting paper. Dr. John MeArthur: I have no real contribution to make to this very delightful paper, but our observations in Borneo suggest that results there with D D T would be parallel to those of Dr. THOMSON in Africa. The Japanese occupation of Borneo has prevented our trying out spraying of walls there with D D T , but, while this of course must be a matter of experiment, it is difficult to believe that it would be lethal to our vector mosquito, which enters the houses late at night, feeds on the sleeping natives, and returns to the jungle again apparently without resting to any extent on the walls. Dr. R. Ford Tredre : I should like to say what a pleasure it is to me to have heard Dr. MUIR~EAD THOMSON speak to this Society because, directly and indirectly, I have been associated with him during the time he has been doing this work. When in the R.A.M.C. I was posted as a malariologist to West Africa, and reached Freetown within a couple of months of Dr. MUIRHEAD THOMSON'S arrival. I.may say that I went to West Africa with the idea that malaria control from the military aspect merely amounted to control of freshwater gambiae, but when I got there I was rather surprised to find that this brackish water form might be of importance. There was no reliable information obtainable in Freetown as to the importance of melas as a vector of malaria along the shores of the estuary. Major RmBANDS, unfortunately for me, was

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transferred to another theatre of war almost immediately and that left me without an entomologist. The malariologist is very much at sea without his entomologist, but I soon made contact with Dr. MUIRHEAD THOMSON. I found that he was already, within a period of 6 weeks, on the track of this differentiation of melas from gambiae, and as a result of his very clever work we were able to estimate the density of melas in relation to the total gambiae population in any one village with which the Services were in contact. Along the southern border of the estuary from west to east there were some six villages. In number one we found the population almost entirely melas. Number two (Freetown) and number three (Kissy) were practically free from melas. Number four was practically entirely melas and so it went on. All measures of control in the field had been directed against fresh-water gambiae only, so that quite obviously at that time, it was 4 years ago, Dr. MUIRHEAD THOMSON'S work was most valuable. At about this time GILROY, working in Lagos, had commenced " b u n d " construction, which is still in progress and has had dramatic results in the reduction of the anopheline population. It remains to be seen what effect this reduction in anopheline population will have on malaria incidence.

The President (Dr. C. M~ Wenyon) : If no one else wishes to put questions to Dr. MUIRHEADTHOMSON it remains for me to ask him to reply to those which have been raised ; but before he does so I should like to ask him two for my own information. He has stated that Anopheles gambiae showed a higher infection rate with malaria than did the variety melas. Is that difference due to differences in the habits of the two mosquitoes, or is it that the one is more readily infected than the other ? The other question relates to Anopheles gambiae in Brazil. Has it been shown that the important Brazilian strain was a pure A. gambiae, or was there any admixture with the variety melas ? Dr. Muirhead Thomson (in reply): I do not think it will be possible to reply to all questions individually as it would take too long. With regard to the question that has just been asked by the PRESIDENT, as to whether there is any difference between the habits of melas and gambiae to account for their different infectivity, we have some evidence in Lagos that melas feeds on other animals to a greater extent than gambiae. As to experimental infections, the work of J. D. ROBERTSON a few years ago showed that under experimental conditions gambiae and melaswere about equally susceptible. The questions put forward by Professor BUXTON, Dr. SCHAl~F and Dr. RmBANDS about D D T can, I think, be answered all together. The work in West Africa has disclosed some urdoreseen snags. No doubt there will be more snags, but I must emphasize that the results as yet only apply to West Africa, and the particular type of village house there. The methods of investigation on the other hand might b~ applied with advantage elsewhere. I must not forget to add, in reply to Dr. SCHARFF'Squestion, that we could find no evidence at all of any anopheles being killed by D D T inside the houses, although I am

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aware that in other countries dead mosquitoes have been found in treated houses. The success of such control measures may depend entirely on differences between species and differences in housing. We are only at the beginning of the problem, and if we approach it with a more thorough preliminary knowledge of the mosquito's normal behaviour it should be easier to interpret some of the field experiments. Dr. RIBBANDSraised a point about the mosquito's reaction to pyrethrum when an alternative untreated house is available. I have some unpublished material about this which I don't think he has seen. In these experiments a group of isolated houses were all sprayed with pyrethrum, so that the mosquitoes had no choice of feeding in untreated houses. Under those conditions just as many mosquitoes as usual fed in the sprayed houses. But if some houses were left untreated, so that mosquitoes had a choice of feeding in sprayed or unsprayed houses, there was then a definite fall in the number entering and feeding in the hut sprayed with pyrethrum. In field experiments the results may depend a great deal on whether such a choice of blood meals in untreated houses is available or not. There is the question by Dr. WALTON and Professor WILLIAMSON about the composition of the water in the breeding place. This is a very important point and one which I hope to look into. I have had some experience of investigating this kind of problem in Assam, and think that research of that kind has to be done on a long-term policy, but results in the end are well worth while. The main reason for not tackling this question in West Africa is that we have been fully occupied with more immediate practical concerns, but the problem will have to be tackled some day. This selection of some types of water for breeding purposes, and avoidance of others, is not entirely determined by the female mosquito. We may find only one pool in fifty with plenty of eggs present, but that particular pool may not turn out to be the most favourable for breeding, as it may dry up or the eggs be washed away. When we discovered the egg differences between melas and gambiae, we followed their distribution by collecting eggs from fresh- and salt-water pools, and found melas almost entirely confined to the tidal belt as far as egg laying was concerned. We have odd records from fresh-water pools, but melas has a decided preference for salt water. Gambiae shows an equally distinct preference for fresh water, but there is no evidence yet to show why gambiae selects one particular type of fresh water and not another. We have some ideas about this, but nothing definite so far, and hope to tackle it seriously some day. The President : It only remains for me to thank Dr. MUIRHEAD THOMSON for his very interesting paper. The success of the evening is I think very largely the result of Dr. THOMSON'S having given us, as Professor BUXTON has pointed out, so much information in the space of three-quarters of an hour, an experience which is rather unusual in this Society. It is a precedent which deserves encouragement. Thank you, Dr. MUIRHEAD THOMSON.