Natural and anthropogenic environmental factors associated with the spatial heterogeneity of adult <italic>Anopheles gambiae</italic> and <italic>An. funestus</italic> in western Kenya.

Abstract

Differences in house-level <italic>Anopheles</italic> densities (HLAD) are likely to affect the intensity of <italic>Plasmodium</italic> transmission and malaria. Spatial patterns of house-level <italic>An. gambiae</italic> and <italic>An. funestus</italic> densities in a 70 km<super>2</super> area of western Kenya with year-round, intense <italic>Plasmodium</italic> transmission were characterized during one wet and one dry season to examine their spatial correlation structure and to test for global spatial autocorrelation. Traditional measures of spatial dispersion (<italic>k</italic> of the negative binomial distribution, Taylor's power law, Lloyd's index of mean crowding, Iwao's index of patchiness) and spatial statistics were used. Results indicated that both mosquito species had indices and parameters characteristic of non-random distributions and aggregated populations. To understand environmental factors associated with the HLAD spatial heterogeneity, vegetation indexes (from Landsat TM satellite images), house construction, house occupant characteristics, and geographical context (e.g. house density, distance to water bodies) were evaluated and multivariable linear models developed. Results for the wet season indicated that lakeshore distance and land cover characteristics were associated with variation in HLAD. In the dry season, geographical context variables, as well as land cover, were associated with HLAD, but the direction of this association differed by species. Models with interaction terms were significant and explained up to 39% of HLAD's variance. Spatial statistical methods were used to examine HLAD clustering patterns across time and season. Overlap was detected between high density or hot spot clusters of <italic>An. gambiae</italic> and <italic>An. funestus </italic> for both wet seasons that were studied. In the dry season, however, there were few hot spots and no overlap for either species. The low density or cold spot clusters, were similar for both species in both seasons. These findings provide new insight into how <italic>An. gambiae</italic> and <italic>An. funestus</italic> abundances are associated with ecological spatial heterogeneity, and how this might affect sampling programs, measures of population size, and interpretation of population and species dynamics. Results suggest different complex dynamics underlying each <italic>An. gambiae </italic> and <italic>An. funestus</italic> house-level densities. Sampling frames focused on individual or groups of houses may be more appropriate for classifying malaria exposure risk than those based on village-level estimates.