The impact of bund construction on the transmission of malaria in The Gambia

Abstract

Water impoundments are known to affect the risk of many vector-borne diseases. Here I examine the impact of bund (small embanlonents) construction on the transmission of malaria in The Gambia. I hypothesised that bund construction, designed to collect rainwater for irrigation and keeping saltwater out, would provide ideal breeding sites for Anopheles gambiae mosquitoes, the major vectors of malaria in Africa, and thereby increase malaria transmission. Mosquito larvae were sampled along transects in three different areas. Each transect lay perpendicular to a bund, including an area 100m either side of the bund. These were sampled for larvae using area samplers every three weeks from May to December 2005, including the period of annual rainfall from June to October. Adult mosquitoes were caught using three emergence traps which were positioned at each side of the bund in different water bodies (i.e. six at each transect, three either side of the bunds), within 50m left and right of each transect. During a Countrywide survey, 20 transects were sampled in different parts of The Gambia in September 2005, when mosquito numbers were relatively high. 10 were situated in parts of the river where it was salt and 10 were situated in parts of the river where the water was fresh. Mosquitoes were roughly twice as common in water on the landward side of the bunds, 68% of Anopheles larvae and 84% of all mosquito larvae, (OR) = 9.37, 95% confidence intervals (95% CIs = 2.26-38.79, P <0.002), compared with the riverside. Similarly 82% of Anophelme adults and 89% of all adult mosquitoes (OR = 2.59, 95% CIs = 1.23-5.48, Р - 0.013) were collected on the landward side. There is also an indication that this is also true for the countrywide survey in which more mosquitoes were collected from the landside of the bund (63% Anophelines, 65% Culicines), although this did not reach statistical significance. One possible explanation for these findings is that larvae on the riverside of the bunds are washed away with the tide and predated by the larger number of fish found there; These findings indicate that the construction of bunds for agricultural purposes increases mosquito larval density on the landward side by increasing the breeding ground for malaria vectors of the An. Gambiae complex. These constructions increase the number of vector mosquitoes emerging from the floodplains of the River Gambia, and thus are likely to increase the intensity of malaria transmission.