Malaria vector densities in transmission and non-transmission areas during 23 years and land use in Chiang Mai province, Northern Thailand

Accumulated anopheline mosquito collection protocols from Chiang Mai province, northern Thailand were compiled and analyzed with the objective to describe and compare the variation in density of malaria vectors between two areas: transmission area (TA) and non-transmission area (NTA), and between two periods: 1977 to 1989 and 1990 to 1999. We hypothesized that the reasons for differences in transmission between areas were caused by higher or increasing densities of malaria vectors in TA and lower or decreasing densities in NTA. We further hypothesized that low vector densities in NTA could be caused by changes in land use, specifically increased landscape diversity and forest fragmentation. Our results indicate that the vector situation in Chiang Mai province is very complex. There were no significant differences in density of the main malaria vectors, Anopheles minimus sensu latu and A. dirus s.l. between areas (TA and NTA). However, the change in density of A. minimus s.l. from the first to the second period was significantly different; TA having an increasing trend and NTA a decreasing trend. Furthermore, the reduction of malaria vectors in the NTA, especially A. minimus s.l. may have been a result of increased landscape diversity and forest fragmentation. There were minor landscape changes in the TA and large landscape changes in the NTA. It was concluded that changes in malaria mosquito densities and land use are important for explaining differences in malaria transmission between areas, but that other factors such as human movement and behavior, and socioeconomic conditions probably also play a role in the transmission of malaria in Chiang Mai province. Die gesammelten Aufzeichnungen zu Anophelesmucken-Erfassungen wurden mit dem Ziel zusammengestellt, die Variation der Dichte der Malariavektoren zwischen zwei Gebieten, einem Ubertragungsgebiet (TA) und einem Nicht-Ubertragungsgebiet (NTA), sowie zwischen den Zeitraumen 1977 bis 1989 und 1990 bis 1999 zu beschreiben und zu vergleichen. Wir stellten die Hypothese auf, dass die Unterschiede in der Ubertragung zwischen den Gebieten durch die hohere oder zunehmende Dichte der Malariavektoren in den TA und niedrigere oder abnehmende Dichte in den NTA verursacht wurden. Wir vermuteten zudem, dass die niedrige Vektorendichte in den NTA durch Veranderungen der Landnutzung, insbesondere durch eine zunehmende Landschaftsdiversitat