Evaluation in test huts of the protective effects of untreated mosquito nets against Anopheles gambiae s.s. bites

The protection against insect bites afforded by untreated mosquito bed nets was studied at the field stations of Yaokoffikro and M'bé in Ivory Coast. We tested damaged mosquito nets (tear holes covering 0.8% of the total area) in Yaokoffikro, undamaged mosquito nets and huts without mosquito nets in M'bé. The blood feeding rate of Anopheles gambiae was 70% lower in huts with undamaged mosquito nets than in huts with no mosquito net or a damaged mosquito net. The natural exophilic behavior of the mosquito was increased and mosquito mortality reached 7.4%. The blood feeding rate of An. gambiae was 83% in huts with no mosquito net and 68% in huts with damaged mosquito nets. Exophilic behavior was normal (25%) and overall mortality was 5%. Immediate mortality of An. gambiae was 54% with undamaged mosquito nets and only 17% in huts with no mosquito net. This difference may reflect the state of nutrition of the mosquitoes: 86% of the dead mosquitoes found in huts with undamaged mosquito nets had not fed, versus only 24% in huts with no mosquito net. The unfed mosquitoes may be considered to have died from starvation. Intact mosquito nets conferred some protection against An. gambiae. This protection was not total but was "better than nothing", with 25% of mosquitoes blood feeding effectively even in the presence of an intact mosquito net. This protection reduces the probability of survival of the females. In contrast, torn mosquito nets trap the mosquitoes that enter, then allowing more than two thirds of the mosquitoes to take a blood meal on the sleeper.