A comparison of indoor and outdoor malaria vectors in a longitudinal cohort study in Webuye, Western Kenya

Abstract Background Malaria remains a major public health challenge in Kenya, with ~ 75% of the population is at risk. Despite the decline in malaria prevalence in children associated with sustained coverage of Insecticide Treated Nets in Kenya, this gain has not been uniform and there is a great deal of heterogeneity in transmission including areas with much higher transmission than the national average. This calls for a closer look at vector control to understand where it is failing. The main malaria vectors in Webuye are Anopheles gambiae s.s, An. arabiensis and An. funestus which are all known to be anthropophagic and endophagic. However, An. arabiensis has in recent times been reported to shift towards exophagic and zoophillic behavior which has been implicated in maintaining residual transmission. Despite scale-up of LLINs with PBO in the region, malaria burden remains high. Among the factors potentially sustaining transmission may be increased exophilic and exophagic vectors, thus reducing the efficacy of LLINs. We compared indoor and outdoor malaria vector diversity, P. falciparum infection, and measured the blood meal sources for the outdoor vectors. Methods For 12 months, collection of adult Anopheles mosquitoes by aspiration was done weekly in buildings with human sleeping spaces (indoor), non-human dwellings and suspected mosquito resting areas (outdoor). Mosquitoes were identified morphologically and laboratory analysis was conducted by PCR. Results Out of the 46,502 female mosquitoes collected,1,878 (4%) were anophelines. A total of 1,641 (87%) of the anophelines were collected indoors and 237 (13%) outdoors. The highest proportion of mosquitoes were collected between April and June. Ten different Anopheles species were identified indoors while six were identified outdoors. Anopheles gambiae ss was the most abundant vector both indoors and outdoors. A high proportion of the outdoor female Anopheles had bloodmeals from animals, only 3% fed on humans and all were negative for Plasmodium sporozoites. The sporozoite rate for indoor collection was 4.8%. Conclusion There was no evidence that outdoor resting mosquitoes are contributing to malaria transmission. However, the presence of several other species besides the traditional vectors poses a threat of cryptic species sustaining transmission and the ability to feed on animals implicated in sustaining vector population.