Mosquitos and Malaria in the Kabaw and Kale Valleys, Burma
Authors: Macan T. T. 
Published: July 2009

(1) The Kabaw and Kale Valleys, separated by a low watershed and both draining through lateral gorges, have a combined length of some 250 miles and a width of between 4–10 miles. They lie at an altitude of approximately 500 feet above sea level.

(2) There is heavy rainfall during the months of June to September inclusive and little or no rain in the months November to April inclusive. The Kabaw Valley, the more northerly of the two, is forested and there are only small isolated clearings, but a big area in the centre of the Kale Valley has been cleared. Rice is the main crop grown.

(3) There appears to be no pre-war information about mosquitos and malaria. Information during the war was gathered by several different types of Unit engaged on different types of work but there is sufficient to give a fairly complete picture.

(4) The valleys are highly malarious throughout their length and breadth; only one village out of twenty-three examined had a spleen rate under 60 per cent.

(5) Transmission of malaria takes place throughout the year and is intense during the months May to December inclusive.

(6) 17 species of Anopheles are recorded. Common species during the rains are A. minimus, A. leucosphyrus, A. philippinensis, A. jeyporiensis, A. annularis, A. hyrcanus and A. vagus. A. minimus is only locally abundant at this time; A. leucosphyrus is widespread but confined to the forest.

(7) When the rain ceases, species such as leucosphyrus and vagus, which breed in pools and similar habitats, become reduced in numbers. Species such as philippinensis, jeyporiensis, annularis, hyrcanus and barbirostris, which breed in larger bodies of stagnant water such as rice fields, also become reduced in numbers during the dry season, though more slowly, because their breeding places disappear less rapidly. As the streams and rivers settle down, after the rains have stopped, conditions for A. minimus and A. maculatus become good and these are the only two common and widely distributed species in the dry season.

(8) During the dry season, sporozoite rates of A. minimus were 1/159=0·63 per cent, in October, 19/519=3·66 per cent, in December and 10/514=1·94 per cent. in May and it is an important vector and the only vector. A. maculatus does not apparently attack man and plays no part in malaria transmission. In September the sporozoite rate of A. minimus was only 1/332=0·31 per cent, and the important vector of the period of the rains appeared to be A. leucosphyrus with a sporozoite rate of 2/134=1·49 per cent.

(9) The status of A. leucosphyrus as a vector in the Indo-Burmese border region is discussed. There is good evidence that, under wartime conditions with troops operating in the forest, it is an important vector during the rains. Its significance in peacetime is less easy to assess; as only a few people live in the forest or at the forest's edge, its importance is probably not great. On the other hand it does not rest indoors, and, as in the past reliance has been placed on day catches, it may have been overlooked. There is, however, evidence that military operations in the forest produce conditions which lead to an increase in the numbers of this species.

(10) Adult collections were nearly all made at night in camps or villages. The species found in large numbers near man were A. minimus, A. leucosphyrus, A. philippinensis, A. jeyporiensis, A. annularis, and A. hyrcanus. A. minimus and A. leucosphyrus, the two definitely incriminated vectors, were the most successful at obtaining blood when the host, man, was protected by a mosquito net. The other four species, however, were sometimes filled with blood, undoubtedly of human origin and all may play some part in malaria transmission under war conditions. The chances of a specimen becoming infected were reduced under the conditions existing when the dissections were made and so moderately long series of negative dissections (Table XI) cannot be taken as conclusive evidence that any one species played no part in malaria transmission. In peacetime these four species may feed mainly on cattle, though a description is given of a night catch in a village where A. philippinensis was caught attacking man although cattle were available.

(11) In September very few of any of the eight common species were resting by day in the place where they fed at night and it is presumed that they spent the day in the forest. Unfortunately very few inhabited houses were visited and almost the only available catching places were Army tents and shelters. The results suggest, however, that all the species tend to rest in the forest rather than indoors. In December in the Kale Valley A. minimus rested in houses by day; in May in the Kabaw Valley it did not. It is believed that where, as in the Kabaw Valley, forest comes up to the edge of the village, A. minimus seeks shelter for the day in the forest, but that where, as in the Kale Valley, the village stands in a clearing, it rests by day in the houses near where it has fed. The facts observed, however, do not preclude the possibility of a seasonal change in resting habit. It is clear that A. minimus is not invariably a house rester and the importance of catching by night to gain a true picture of the mosquito population is stressed.

(12) A. hyrcanus was usually the first species to appear after dark, followed by A. philippinensis, A. leucosphyrus and A. minimus in that order, the last named being usually most abundant in the second half of the night. Time of greatest activity was, however, variable in all species. There were only a few isolated instances of Anophelines biting in the forest by day.

(13) A. hyrcanus was attracted to light.

(14) Larvae identified as A. pallidus with the aid of the standard Indian keys invariably proved to be A. philippinensis on emergence.

(15) The variation found in A. minimus was considerable and is briefly discussed. The only constant difference between A. minimus and A. aconitus was found to be the flavescence on the proboscis.