Insecticide-impregnated bed nets reduce malaria transmission in rural Zanzibar

There has been concern that impregnated bed nets are an insufficiently powerful method to control malaria in areas with very high perennial transmission, as in the humid lowland parts of tropical Africa. We carried out a 'cross-over' trial among children under 5 years of age in 2 villages in rural Zanzibar. In 1989, one village was supplied with newly permethrin-impregnated bed nets whereas the other served as unprotected control. In 1992, when those nets had lost their insecticidal activity and were badly torn, the village which had previously been the control was given newly impregnated bed nets. Each time, reinfection with Plasmodium falciparum was measured after initially clearing the parasites by administering a therapeutic dose of sulfadoxine/pyrimethamine. The introduction of bed nets led to a 74-78% reduction in the weekly rate of reinfection with malaria parasites, in all age groups. The nets apparently also affected perceived clinical symptoms, haemoglobin levels, and the mosquito sporozoite rate.