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Background  The widespread use of indoor-based malaria vector control interventions has been shown to alter the behaviour of vectors in Africa. There is an increasing concern that such changes could sustain residual transmission. This study was conducted to assess vector species composition, feeding behaviour and their contribution to indoor and outdoor malaria transmission in western Kenya.    Methods    Anopheles  mosquito collections were carried out from September 2015 to April 2016 in Ahero and Iguhu sites, western Kenya using CDC light traps (indoor and outdoor), pyrethrum spray catches (PSCs) (indoor) and pit shelters (outdoor). Species within  Anopheles gambiae  s.l. and  Anopheles funestus  s.l. were identified using polymerase chain reaction (PCR). Enzyme-linked immunosorbent assay (ELISA) was used to determine mosquito blood meal sources and sporozoite infections.    Results  A total of 10,864 female  Anopheles  mosquitoes comprising  An. gambiae  s.l. (71.4%),  An. funestus  s.l. (12.3%),  Anopheles coustani  (9.2%) and  Anopheles pharoensis  (7.1%) were collected. The majority (61.8%) of the anopheline mosquitoes were collected outdoors. PCR result (n = 581) revealed that 98.9%  An. arabiensis  and 1.1%  An. gambiae  s.s. constituted  An. gambiae  s.l. in Ahero while this was 87%  An. gambiae  s.s. and 13%  An. arabiensis  in Iguhu. Of the 108  An. funestus  s.l. analysed by PCR, 98.1% belonged to  An. funestus  s.s. and 1.9% to  Anopheles leesoni . The human blood index (HBI) and bovine blood index (BBI) of  An. arabiensis  was 2.5 and 73.1%, respectively.  Anopheles gambiae  s.s. had HBI and BBI of 50 and 28%, respectively. The HBI and BBI of  An. funestus  was 60 and 22.3%, respectively. Forage ratio estimate revealed that  An. arabiensis  preferred to feed on cattle,  An. gambiae  s.s. showed preference for both human and cattle, while  An. funestus  preferred human over other hosts. In Ahero, the sporozoite rates for  An. arabiensis  and  An. funestus  were 0.16 and 1.8%, respectively, whereas in Iguhu, the sporozoite rates for  An. gambiae  s.s. and  An. funestus  were 2.3 and 2.4%, respectively. In Ahero, the estimated indoor and outdoor entomological inoculation rate (EIR) was 108.6 infective bites/person/year (79.0 from  An. funestus  and 29.6 from  An. arabiensis ) and 43.5 infective bites/person/year (27.9 from  An. arabiensis  and 15.6 from  An. funestus ), respectively. In Iguhu, the estimated indoor and outdoor EIR was 24.5 infective bites/person/year (18.8 from  An. gambiae  s.s. and 5.7 from  An. funestus ) and 5.5 infective bites/person/year (all from  An. gambiae  s.s.), respectively.    Conclusion    Anopheles gambiae  s.s. showed an increasing tendency to feed on cattle.  Anopheles arabiensis  was highly zoophagic, whereas  An. funestus  showed anthropophagic behaviour. While the majority of malaria transmission occurred indoor, the magnitude of outdoor transmission was considerably high. Additional control tools that complement the existing interventions are required to control residual transmission.

Indoor and outdoor malaria vector surveillance in western Kenya: implications for better understanding of residual transmission