Habitat characterization for malaria vector mosquito larvae in Gamo Gofa, Ethiopia.

Abstract: Malaria is a widespread parasitic disease in developing countries of the tropics and subtropics, infecting approximately 200 million people and causing over half a million deaths every year. The disease is caused by the protozoan Plasmodium and is transferred to humans through infective bites from female mosquitoes of the genus Anopheles. In order to reduce malaria transmission, measures of larval control have been implemented throughout the tropics. This includes usage of larvicides, source reduction by environmental or physiochemical manipulation as well elimination of larval habitats. The purpose of this study was to evaluate differences in occurrence and densities of anopheline larvae by investigating the environmental characteristics of their habitat. The study was conducted in the Gamo Gofa Zone, Great Rift Valley, Ethiopia where a total of 26 sampling sites were chosen for larval sampling. Each sampling site was characterized according to a protocol and sampled for water chemistry analysis. Environmental variables studied include water depth, habitat size, distance to nearest dwelling, land use within a 10 m and 100 m from the sampling site and number of domestic animals within a 100 m. Physiochemical variables include water temperature, pH, electric conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), turbidity and phosphate. Larval sampling was conducted on each site using a soup ladle dipper. The occurrence of anopheline larvae was statistically analysed using multiple logistic regressions, while using linear regression for analysing larval abundance at positive sites. Larval sampling resulted in a total of 1245 mosquito larvae, 567 anopheline and 678 non-anopheline. Of the anopheline larvae, 118 were analysed morphologically by microscopy which resulted in 117 belonging to An. gambiae complex and one An. garnhami. Of the 26 sites investigated, 16 were positive for anopheline larvae. All sampled river fringes and flood pools were positive for anopheline larvae whereas none were found in irrigation channels. Negative correlation for anopheline larval occurrence was obtained for both water depth and percentage of tall vegetation within 10 m radius of the sampling area. Anopheline larval abundance was only correlated, positively, with water temperature. The study concludes that water depth, temperature and percentage of tall riparian vegetation are important factors to consider when designing a control program for anopheline larvae. One should be aware of the fact that clearing riparian forest and other tall vegetation is likely to improve growing conditions for anopheline larvae. Furthermore, different habitat classes were either exclusively positive or negative for anopheline larvae, irrigation channels in the area not being suitable larval habitats during the time of measurements.  The authors suggest that more studies are needed, preferably on a larger set of sampling sites and over a longer period. Keywords: Anopheles, Ethiopia, larval habitats, water quality, environmental variables, Gamo Gofa zone.