Importance of low dispersion of Anopheles gambiae (Diptera: Culicidae) on malaria transmission in hilly towns in south Cameroon.

The dispersion of Anopheles gambiae Giles s.l. from larval breeding places was examined in the towns of Yaounde and Edea, Cameroon. The study areas are characterized by hilly reliefs of approximately 40 m and by larval breeding places in the swampy valleys. Mosquito density was estimated by night-biting indoor catches carried out during 1 yr in three districts, with four collecting houses in each, along a transect from 0 (at the valley bottom) to 350 m (at the hilltop). In each district, the human-biting rate decreased gradually from valley bottom to hilltop. In two districts, the observed densities were zero at 200 and 250 m from breeding places. In the third district where the habitat is less dense, there was a 50% reduction in biting mosquitoes at 350 m from valley bottom. Our studies confirm that dense habitat in the urban area is associated with a low degree of vector dispersion. It emphasizes that slope is another factor which limits dispersion. These findings explain why malaria transmission is heterogenous (range, 0-45 infective bites per man per year) inside the districts and occurs mainly around swamps in the valleys. These data are important when considering the uncontrolled urbanization which is occurring in the undeveloped areas in the valleys.