Is mosquito larval source management appropriate for reducing malaria in areas of extensive flooding in The Gambia? A cross-over intervention trial

Abstract

Larviciding to control malaria was assessed in rural areas with extensive seasonal flooding. Larval and adult mosquitoes and malaria incidence were surveyed routinely in four 100-km(2) areas either side of the Gambia River. Baseline data were collected in 2005. Microbial larvicide was applied to all water bodies by hand application with water-dispersible granular formulations and corn granules weekly from May to November in two areas in 2006 and in the other two areas in 2007 in a cross-over design. The intervention was associated with a reduction in habitats with late stage anopheline larvae and an 88% reduction in larval densities (P < 0.001). The effect of the intervention on mosquito densities was not pronounced and was confounded by the distance of villages to the major breeding sites and year (P = 0.002). There was no reduction in clinical malaria or anemia. Ground applications of non-residual larvicides with simple equipment are not effective in riverine areas with extensive flooding, where many habitats are poorly demarcated, highly mobile, and inaccessible on foot.

Figure 1.

Study area in The Gambia. Larval control was applied in each zone over the area enclosed by the two broken lines furthest from the center of each zone and the study villages are enclosed by the two broken lines nearest the center of each zone.

Figure 2.

Trial profile, The Gambia.

Figure 3.

Rainfall during the study period, The Gambia.

Figure 4.

Seasonal abundance of larval and adult Anopheles gambiae during the study, The Gambia. Gray bars represent periods of larvicide application.

Figure 5.

Impact of larval control on female Anopheles gambiae s.l. densities stratified by intervention year and distance of the villages to the nearest riverine floodplain, The Gambia.