Influence of landscape on distribution of adult Anopheles spp. in lowland, western Kenya

Malaria risk to humans varies in a heterogeneous landscape due to the nonrandom distribution of adult female Anopheles and their associated infective bites. We quantified this variation in a 15 x 1 km transect in the Asembo Bay region of western Kenya. The study site was a rural area characterized by rolling terrain of moderate elevation variation (1150 to 1250 m) and occupied by subsistence agriculturists facing holoendemic Plasmodium falciparum malaria. All houses, roads, and streams (8 total) were mapped and incorporated into GIS layers. Adult Anopheles were sampled indoors using the pyrethrum knockdown method in 417 houses from 214 compounds. They were identified to species using both morphology and PCR and tested for malaria infection by the sporozoite ELISA method. The influence of topographic variables, including elevation, slope, flow accumulation, and distance of the sample points from streams were examined. 1,281 adult, female Anopheles were collected; most were An. funestus and An. arabiensis. The distribution was strongly aggregated and nonrandom. Regression analysis was used to quantify the relative contribution of each variable to the distribution.