Changes in malaria vector densities over a twenty-three year period in Mae Hong Son Province, northern Thailand.

Mae Hong Son Province in northwestern Thailand has a long history of malaria. During the last two decades the province has had one of the highest malaria incidences of all provinces in Thailand. Data were analyzed to determine whether the vector populations were stable or increasing during the last two decades and to determine the seasonal prevalence of the main vectors, and whether or not they were related to the malaria transmission peak, in the wet season. We compiled and analyzed accumulated entomological records from 1977 to 1999. The aim was to investigate long-term changes in mean densities of malaria vectors between two periods (1977-1989 and 1990-1999), and the differences in vector densities between two seasons (wet and dry). A total of 141,144 adult anophelines of 29 species were collected on indoor and outdoor human baits and animal baits during the study period. Of the main malaria vectors, the densities of Anopheles minimus s.l. and Anopheles maculatus complex increased significantly. Anopheles dirus s.l., however, was stable between the two periods. These vector populations were associated with consistently high malaria incidence in the province during the last two decades. An. minimus s.l. density was not significantly different between seasons. However, in the second period, both An. dirus s.l. and An. maculatus complex showed a tendency for higher wet season densities. This can explain the high malaria incidence in the rainy season in Mae Hong Son. Environmental and climatic factors seem to have been favorable for supporting a consistently high vector population in the province, and consequently a high malaria transmission rate during the period of study.