Outdoor feeding of malaria vectors on Bioko Island, Equatorial Guinea, continues to increase in response to indoor-based vector control

Indoor residual spraying has been implemented on Bioko Island under the Bioko Island Malaria Control Project since 2004. In 2009 human landing collections (HLCs) were started as part of the vector monitoring. This revealed that the major remaining vector on the island, Anopheles gambiae M form, a species that is considered primarily an indoor feeder, predominantly fed outdoors. Limited data was available from before 2009, but it was concluded that An. gambiae likely switched to outdoor feeding as a result of five years of indoor-based vector control. Monthly indoor and outdoor HLCs were conducted on Bioko on a continuous basis since 2009. Here we present data for 2009-2014 from four collection sites (Arena Blanca, Balboa, Biabia, and Mongola) that yielded a large number of mosquitoes. In all four sites outdoor biting rates continued to increase for An. gambiae M form, as well as for the other malaria vector on the island, An. melas, resulting in outdoor biting rates as high as 80%. Analyses of biting rates before and after spray rounds indicate that insecticide repellency is an unlikely explanation for the observed shift in behavior. We therefore conclude that the indoor application of insecticides has continued to exert considerable selection pressure on malaria mosquito populations resulting in a further shift towards outdoor feeding behavior. While the epidemiological impact of this behavioral shift depends on human behavior, these data do raise serious concerns about the future effectiveness of indoor based vector control on the island.