Larval ecology of malaria vectors and the impact of larviciding on malaria transmission in The Gambia

Abstract

The study reported in this thesis explored the ecology of aquatic stages of mosquitoes in the middle reaches of the Gambia River in order to assess the feasibility and impact of microbial larviciding on malaria transmission in large river ecosystems in sub- Saharan Africa. All accessible water bodies in four study zones covering 400 km(^2) were mapped and sampled for mosquitoes. Microbial larvicides were applied in the four zones in across-over design and the impact of larviciding on mosquito densities assessed. Anopheline and culicine mosquitoes were found in all sampled habitats, apart from those with moving water. Similarly, all habitats, except puddles and water channels, had similar larval and pupal densities. Anopheles gambiae sensu lato, the major malaria vector in Africa, exploited a wide range of habitats and despite a decrease in population density during the dry season, could be found in breeding sites throughout the year. Mosquitoes shared habitats with other invertebrates including their predators. A closer look at rice fields revealed that mosquitoes were abundant in rice fields closer to the landward edge of the floodplains where water is fresher and contains high quantifies of nutrients. Mosquitoes of The Gambia were highly susceptible to both Bacillus thuringiensis var. israelensis (Bti) and B. sphaericus microbials, however no residual activity against anopheline larvae was observed. The basic training of personnel in identification of habitats, calibration of application equipment and active larviciding proved to be successful. Routine larviciding was associated with > 91 % reducfion (p < 0.001) in anophelines late stage larval density and 72 % (p < 0.001) in culicines. Overall, larviciding was associated with a 28% (p = 0.005) reduction in the number of adult female Anopheles gambiae s.l. found indoors, although this rose to 42%, when the study zone with the greatest abundance of breeding sites was excluded from the analysis. No significant reduction in adult culicines was observed. Ground application of Bti in areas with extensive floodplains is unlikely to contribute to a substantial reduction in malaria transmission in The Gambia, therefore vector control in such areas should target adult mosquitoes.