Insecticide-treated materials, mosquito adaptation and mass effect: entomological observations after five years of vector control in Burkina Faso

Insecticide-treated bednets and curtains have been shown to be successful i n reducing malaria transmission and child mortality in Africa over periods of up to 2 years. A major concern relating to this approach is that, in tim e, it will be compromised by the selection of mosquito genotypes that are r esistant at the biochemical or behavioural level. We report entomological d ata from a large area in Burkina Faso where insecticide-treated curtains ha ve been in use for up to 5 years. Longitudinal indoor and outdoor CDC light -trap catches were performed in 4 sentinel villages. In addition cross-sect ional surveys using indoor spray catches and outdoor CDC light-trap catches were performed each September in a larger number of villages, including 8 located outside the intervention area. We found no evidence of the selectio n of mosquito phenotypes that might compromise the intervention. Indoor and outdoor vector densities remained very low after 5 years of intervention, both compared with pre-intervention levels and with concurrent levels outsi de the intervention area. We found no evidence of a switch to outdoor rathe r than indoor biting. The proportion of blood meals taken on humans may hav e decreased but our data are inconclusive on this point. We observed higher vector densities and sporozoite rates at the periphery of the intervention zone than at the centre, which may reflect re-invasion of peripheral villa ges by mosquitoes from outside the intervention area. In 'real life' progra mmes, with perhaps patchy, less than optimal coverage, the protection again st malaria transmission provided to individuals using insecticide-treated m aterials may be less than that achieved in the randomized controlled trials which demonstrated an impact of insecticide-treated materials on child mor tality.