Limited impact of Long lasting insecticide treated nets (LLINs) on malaria transmission in a rural village of Burkina Faso: the role of mosquito behavioral plasticity

Background. Long Lasting Insecticide treated bednets (LLINs) are the main malaria vector control strategy, with 3-year mass distribution campaigns recommended by WHO. Nevertheless, reduced LLINs efficacy is occurring in some Sub-Saharan countries (e.g. Burkina Faso) where the annual malaria incidence remains still high. Among many causal factors accounting for this scenario, a crucial role is played by insecticide resistance and changing in mosquito biting behavior, parameters that the PhD project attempts to investigate. Thanks to entomological surveys conducted in Goden village (Burkina Faso) a year after LLINs introduction (occurred in 2010) and during subsequent mass distribution campaigns, we observed: i) decrease of mosquito human-feeding in favor of animals; ii) altered biting time diverted from the typical mid-night peak. These evidences might explain the high mosquito infectivity rates (5.8%-6.6%) constantly observed through ten years in Goden despite LLINs usage. We here further investigate mosquito behavioral changes in response of LLINs, comparing data from recent samplings with those conducted previously in the village. Methods. During 2019 rainy season, indoor-outdoor collections of host-seeking mosquitoes were performed by: i) BG-traps with BG-lure and fermenting yeast, while a man was sleeping under LLIN indoors; ii) Human Landing Catches (HLC) in absence of LLIN. The sampling protocols followed the same condition of BG trapping and HLC collections conducted, respectively, in 2011 and 2015. Results. In 2019, Anopheles gambiae s.l. mosquitoes were collected more outdoors than indoors by BG-traps (31% IN vs 69% OUT; t=-5.85; p<0.005; N=902), conversely 65% of mosquitoes were collected by BG-traps indoors in 2011 (t=9.18; p<0.0001; N=418). HLC in 2019, collected A. gambiae s.l. without differences between indoors and outdoors (U=729; p=0.31; N=2,616), confirming results obtained in 2015 (U=0.52; p=0.74; N=1,944). Conclusions. The increased vector outdoor biting observed in 2019 is mainly driven by diversion-effect exerted by LLINs. A strong exophagy is not visible in local vector population if humans are readily accessible, as observed for HLC collections in 2019 and 2015, suggesting an opportunistic biting behavior not genetically selected by the continued presence of LLINs in Goden. The high endophagy reported in 2011 might be due to a limited repellent effect of LLIN brand used at that time in the village. Overall our results highlight an inner behavioral plasticity of vector population, which can promptly react to bednets pressure adopting reversible changes in biting behavior. Such plasticity could be causative of LLIN limited impact in Goden and should be accounted for future malaria control strategies.