Exploiting a xenosurveillance approach to detect malaria parasites in Djibouti City

Entomological surveillance of anopheline mosquitoes is traditionally applied to estimate human exposure to malaria parasites. However, blood-fed non-vector mosquitoes could also be used as ‘biological syringes’ to highlight pathogen circulation in host population (xenosurveillance). In this study, a xenosurveillance approach was adopted to detect human plasmodia in human-fed mosquitoes collected in Djibouti City (Djibouti). The study area is historically hypo-endemic with unstable malaria transmission, primarily linked to imported cases. In the last two decades, major efforts to eradicate malaria was supported by the National Malaria Control Programme, but a rise in human cases was reported after the introduction of the Asian vector Anopheles stephensi. To investigate the feasibility of xenosurveillance, a mosquito sampling was carried out from January to February 2020 using sticky resting box (SRB) and BG-Sentinel traps modified with a sugar feeding system (BG-FTA). Collected specimens were morphologically identified to define species and gonotrophic stage. DNA was extracted from abdomens of fed mosquitoes (e.g. Culex and Aedes spp.) to identify the blood meal host through PCR; additionally, Anopheles females were subjected to DNA extraction from head and thorax. DNA extracted from both anopheline and human-fed culicine was further processed with PCR and sequencing to detect Plasmodium spp. Overall, 14,378 specimens were reported, with the 92.5% of total sample collected using the BG-FTA traps. Culicinae represented almost all of the total sample and included Culex quinquefasciatus (96.7%), Aedes aegypti (2.6%) and Cx. sitiens (0.2%); the only anopheline species were An. stephensi (0.5%) and An. dthali (0.1%). No malaria parasites were detected in Anopheles specimens, which could be explained by the low number of collected females (n=36). Conversely, P. falciparum was detected in different dates (31st January, 15th and 20th February) from six human-fed mosquitoes (Cx. quinquefasciatus, n=3, and Ae. aegypti, n=3). According to our findings, molecular detection of pathogens in blood fed non-vector mosquitoes make malaria surveillance most feasible at low vector density. Enhancing surveillance is needed to reduce malaria burden and a xenosurveillance approach could be most effective in eradication contexts and newly introduction areas.