Studies on malaria and on A. b. balabacensis and A. minimus responses to DDT spraying were conducted in a forested hilly area in northern Thailand. In a first phase, base-line data were collected from July 1970 to March 1972. In a second phase, the study area received five round of DDT spraying over a period of two years and at the same time all malaria infections received radical treatment. During this two-year period of field operations, entomological and epidemiological observations were continued. The studies carried out in the second phase, showed that malaria transmission decreased under the applied optimum anti-malarial measures but was not interrupted. Human ecology and population movement inside the forest, especially during the dry season, contributed to a great extent to this result. The transmission occurring in the early part of the monsoon season clearly indicates the importance of the timing of DDT spraying. A. b. balabacensis appeared to be transmitting malaria all the year round in the deep forest but only in the monsoon season in the forest fringe. The vectorial capacity of both vectors was estimated separately for indoor and outdoor populations. The pre-spraying values obtained for A. b. balabacensis were much higher thaan for A. minimus. After DDT spraying A. b. balabacensis showed a decrease in vectorial capacity estimated at 31.5 times for the indoor population and 18 times for the outdoor population. A. minimus, on the other hand, showed a much smaller decrease, estimated at 6.8 and 1.9 times for the indoor and outdoor populations respectively.
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