Impact of the okyereko irrigation project in Ghana on the risk of human malaria infection by anopheles species (diptera: Culicidae).

Abstract

The impact of an irrigation scheme on malaria transmission in coastal savanna was studied. Adult mosquitoes (Diptera : Culicidae) were sampled using human landing and pyrethrum spray catches at Okyereko (irrigated village) and at Bewadze (non-irrigated, 9.5 km away) during the dry season of 2002/2003. Each mosquito was first identified morphologically as <I>Anopheles gambiae</I> Giles, then to sibling species by PCR and <I>An. gambiae s.s.</I> forms were further identified by restriction analysis. The parity of <I>Anopheles</I> mosquitoes were determined and ELISA-based methods used to determine <I>Plasmodium falciparum</I> sporozoite infections and host blood meal source. More <I>Anopheles</I> mosquitoes were obtained at Okyereko. Members of the <I>An. Gambiae</I> complex were dominant at Okyereko and <I>An. funestus</I> at Bewadze. Within the <I>An. gambiae</I> complex, only <I>An. gambiae s.s.</I> was identified, and the M form constituted 91.7 % at Okyereko and 66.7 % at Bewadze. The biting rate was higher at Okyereko, but parous rates were similar in both villages. The infection rates were 2.5-fold higher in <I>An. funestus</I> than <I>An. gambiae</I> at both sites, but were 17.5-fold lower at Okyereko. At Okyereko, the four infective <I>An. gambiae s.s.</I> were found to be the M form while at Bewadze three of the four infective <I>An. gambiae s.s.</I> were found to be the S form and one was the M form. The entomological inoculation rate was also significantly lower at Okyereko. The human blood index was 84.1 % at Okyereko and 77.1 % and Bewadze. Malaria risk was therefore lower at the irrigated village and the likely reason is the low numbers of <I>An. funestus</I> and <I>An. gambiae s.s.</I> S form.