Impact of the Okyereko irrigation project in Ghana on the risk of human malaria infection by Anopheles species (Diptera: Culicidae)

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 Anopheles gambiae Giles, then to sibling species by PCR and An. gambiae s.s. forms were further identified by restriction analysis. The parity of Anopheles mosquitoes were determined and ELISA-based methods used to determine Plasmodium falciparum sporozoite infections and host blood meal source. More Anopheles mosquitoes were obtained at Okyereko. Members of the An. Gambiae complex were dominant at Okyereko and An. funestus at Bewadze. Within the An. gambiae complex, only An. gambiae s.s. 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 An. funestus than An. gambiae at both sites, but were 17.5-fold lower at Okyereko. At Okyereko, the four infective An. gambiae s.s. were found to be the M form while at Bewadze three of the four infective An. gambiae s.s. 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 An. funestus and An. gambiae s.s. S form.