Predicting productive larval habitats for anapheles gambiae in lowland Western Kenya
Mutuku, Francis M.
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Larval habitats of the anopheline vectors of human malaria form an integral part of transmission because they produce the adult female (i.e., vector) stage. In this study, conducted in a region holoendemic for Plasmodium falciparum malaria in lowland, western Kenya, location of aquatic habitats of larvae of various species of mosquitoes in the genus Anopheles were studied. The overall goal of the study was to predict location of productive An. gambiae s.l larval habitats in the Asembo study area. Two larval habitat surveys, one for wet and dry season each were carried out. Habitat characterization and the density of Anopheles larvae were determined during these surveys. A supervised land cover classification was developed from high resolution IKONOS satellite data and extensive ground sampling. The study area was divided into 4 x 4 meter cells, numbering 646,416. Cells with and without larval habitats were typified according to their land cover, Normalized difference vegetative index (NDVI) at radius of 60m, distance of the nearest stream, elevation and the interaction between land cover and distance of the nearest stream using logistic regression. A total of 1,198 habitats and 19,776 Anopheles larvae of 9 species were quantified in samples from a rainy season and 184 habitats and 582 larvae from a dry season. Anopheles gambiae s.l. was the dominant species. The results showed that anopheline larvae generally occurred most frequently in water pools that were shallow, unshaded with turbid (with suspended soil particles) water and with little or no aquatic vegetation and more often located close to streams on agricultural lands. Malaria vectors, An. gambiae s.1 larval habitats were significantly different from those of other non-vector anophelines. Larval habitats characteristics and spatial distribution of the sibling species in the An. gambiae complex were similar. Compared to An. gambiae s.l larval habitats, other anophelines larval habitats were deeper and closer to streams and with more aquatic vegetation. Agricultural land cover were positively associated with the presence of larval habitats, and were located relatively close to stream channels; whilst nonagricultural land cover were negatively associated with the presence of larval habitats and were more distant from stream channels. IKONOS imagery was not useful in direct detection of larval habitats (mostly because the habitats were small), but was useful in their localization through statistical association with land cover. The modelling outcomes demonstrated that areas that were close to streams, were at relatively low elevation and had agricultural land cover features had high probability of Anopheles mosquito larval habitats being located on them. In conclusions, more than a half of the anopheline larval mosquito population in Asembo study area comprised of An. gambiae s.l. It was clearly demonstrated that high resolution satellite images could be utilized in identifying high probability sites for the location of potential larval habitats such as those areas covered by maize fields and pasturelands, but may not be suited to directly detect specific water bodies which serve as potential larval habitats. The resulting continuous distribution maps from the modelling process showing the likely form of distribution of larval habitats in the Asembo study area can greatly compliment the existing larval ground surveys by helping direct vector control to areas that have high habitat suitability which comprise about '/3 of the total study area. The results of this study provide essential information for habitat-based control programs that aim at targeting productive malaria vector habitats