Malaria entomological inoculation rate in a village of Burkina Faso reveals high transmission risk both indoors and outdoors despite the large coverage of LLINs

Introduction – Long lasting insecticide treated bednets (LLINs) are considered among the most effective strategies in malaria vector control, leading to the prevention of 68% of the malaria cases in Africa in 15 years (Bhatt et al., 2015, Nature 526:207-211). Despite this success, the effectiveness of LLINs in sub-Saharan Africa seems to be heterogeneous, since in some hyperendemic countries the annual incidence and the entomological infection rates are still very high (Killeen et al., 2014, Mal. J. 13:330). In Burkina Faso the raising of LLIN coverage from 20% to 70% led to a significant reduction of malaria prevalence, without significantly affecting annual incidence. We here assessed the risk of malaria transmission in a LLIN-protected village of Burkina Faso where in previous surveys it has been detected a high SR rate (A. coluzzii, 7.6% in 2011, 9.3% in 2012) despite the high bednet coverage. Materials and methods – Host-seeking mosquitoes were collected in November 2015 by Human Landing Catch, both indoors and outdoors, in the village of Goden (Ouagadougou area). Collected mosquitoes were morphologically identified and Anopheles gambiae s.l. specimens were subsequently identified per species by PCR (Santolamazza et al., 2008, Mal. J. 7:163). Head+thorax of females were analysed for Plasmodium sporozoite presence by nested-PCR (Calzetta et al., 2018, Med Vet Entomol, in press). Results and conclusions – Among 695 out of 1955 Anopheles gambiae complex specimens (A. coluzzii 55%, A. arabiensis 44%, A. gambiae 1%) analysed so far for Plasmodium infection, the SR is 6.2% (6.9% A. coluzzii, 5.1% A. arabiensis), without significant differences between indoors and outdoors (6.6% and 5.7%, respectively; Chi-square= 0.27 P= 0.6). Considering an average of 83.3 host-seeking mosquitoes per person per night, the entomological inoculation rate (EIR) in the village can be estimated to 5.1 infective bites both indoors and outdoors. This results highlights that, despite the individual protection given by LLINs to the inhabitants from several years, the mosquito population in the area is still stably highly infected and present at high densities. This lead to a non-negligible risk of malaria transmission in the village indoors for people sleeping unprotected by a bednet and for all the population that exposed to mosquito bites outdoors.