Impact Of Permethrin-treated Bed Nets On Entomologic Indices In An Area Of Intense Year-round Malaria Transmission

Abstract: The effect of permethrin-treated bed nets (ITNs) on malaria vectors was studied as part of a large-scale, randomized, controlled trial in western Kenya. Indoor resting densities of fed Anopheles gambiae s.l. and An. funestus in intervention houses were 58.5% (P 0.010) and 94.5% (P 0.001) lower, respectively, compared with control houses. The sporozoite infection rate in An. gambiae s.l. was 0.8% in intervention areas compared with 3.4% (P 0.026) in control areas, while the sporozoite infection rates in An. funestus were not significantly different between the two areas. We estimated the overall transmission of Plasmodium falciparum in intervention areas to be 90% lower than in control areas. Permethrin resistance was not detected during the study period. As measured by densities of An. gambiae s.l., the efficacy of bed nets decreased if one or more residents did not sleep under a net or if bed nets had not been re-treated within six months. These results indicate that ITNs are optimally effective if used every night and if per- methrin is reapplied at least biannually. As part of the efficacy trial, we assessed some standard entomologicparameters: the number of indoor resting mos- quitoes, house exiting behavior, the proportion of mosquitoes infected with Plasmodium falciparum sporozoites, and insec- ticide resistance. We also assessed the effect of several human factors on the number of blood fed mosquitoes found resting indoors. The most important of these is adherence, which is whether the individual owning the net actually slept under a properly deployed net during the night before resting mos- quitoes were collected. We also measured the impact of the number of bed nets in houses and the time since re-treatment of nets with insecticide on numbers of fed mosquitoes.