Do bednets reduce malaria transmission by exophagic mosquitoes?

The impact of bednet coverage on malaria prevalence in a suburb of São Tomé was monitored, by passive case detection, over a three-year period (1997-1999), when bednet use increased from 20 to 74%. Malaria parasites were detected in 1651 (41.6%) of the 3967 slides taken during the study. All four human malaria parasites were seen, with Plasmodium falciparum being the predominant species (94.9% of positive slides). Prevalence of malaria among residents decreased, particularly in 1-4 year olds. In addition, there was a concomitant decrease in prevalence also among non-net users, suggesting a mass effect on transmission, even though the only vector in the area is largely exophagic and zoophilic.