Outdoor host-seeking behaviors among An. gambiae s.l. vectors observed after initiation of indoor residual spraying on Bioko Island, Equatorial Guinea

Background Indoor-based anti-vector interventions remain the preferred means of reducing transmission risk in malaria endemic areas around the world. Despite demonstrated success in reducing human-mosquito interactions, these methods are effective solely against endophilic vectors. It may be that outdoor locations also serve as an important venue of host-seeking by An. gambiae sensu lato mosquitoes where indoor vector control measures are employed. We describe here the host-seeking activity of anopheline mosquito vectors in three villages on Bioko Island, Equatorial Guinea when a ongoing IRS campaign has been underway since 2004. Methods We evaluated the venue and temporal characteristics of hosting-seeking by anopheline vectors in a hyperendemic setting. Using human landing collections conducted inside and outside homes in three villages over eight nights we performed time-segregated analyses of when and where these mosquitoes preferentially feed. Results Intense outdoor biting was observed by An. gambiae s.s. and An. melas vectors throughout the night, including times when people are active outdoors during the early evening and morning hours. We confirm that An. gambiae s.s. is the primary malaria vector present in the Punta Europa region and seeks hosts outdoors at least as much as it does indoors. Further, An. gambiae s.l. vectors are seeking hosts at times when people may not protected by bed nets or insecticide-treated surfaces in their homes. Conclusions Our findings confirm that An. gambiae s.l. mosquitoes seek hosts in outdoor venues as much as they do indoors in the Punta Europa region of Bioko Island. Routine human landing collections may be used to determine the venue and time in which risk of transmission is greatest. Indoor-based residual insecticide and bed net-based approaches have been proven effective against epidemiologically important, endophilic anopheline vectors. However where host-seeking and resting occur outdoors in significant proportions, indoor-based interventions are may not to be as effective in interrupting malaria transmission.