Early-evening indoor and outdoor foraging by major malaria vectors in Nchelenge, Zambia

Despite a decade of indoor residual spraying, malaria remains holoendemic in Nchelenge District in northern Zambia. This study examined the hypothesis that the intransigence of malaria in Nchelenge District is due in part to early evening and outdoor foraging of local anopheline species. Three replicate collections were performed at 24 households using CDC light traps over 6 weeks during the dry season of 2019. Traps were set indoors, in animal pens, and in outdoor gathering spaces from 16:00-22:00, and overnight indoors in all households during the third replicate. Twelve anopheline species were collected in early-evening traps. The major vector, Anopheles funestus, dominated collections (n = 1,865, 68.2%) and were captured primarily indoors (n = 1,370, 73.4%). Specimens with detectable human blood and Plasmodium falciparum positive specimens were found in all three trap types, indicating that infected anopheline foraging occurred before 22:00. EIR estimates for early-evening anophelines varied spatially and by species and were highest among indoor caught An. funestus at inland locations (18.74 bites/person/6months). There was little difference in anopheline abundance, blooded rates, and sporozoite infection rate when evening and overnight traps were compared, indicating there may be similar anopheline activity before and after 22:00. Overall, this study adds to the growing body of evidence that malaria transmission is not limited to late-night, indoor settings in Nchelenge District, Zambia.