Exploiting a xenosurveillance approach on non-vector mosquitoes to detect malaria pathogens in Djibouti city

INTRODUCTION: Entomological surveillance is broadly applied to detect pathogens in mosquito vectors and estimate the risk of exposure to mosquito-borne diseases. Irrespective to vector competence, bloodfed mosquitoes could also be used as “biological syringes” to highlight pathogen circulation in human and animal populations (xenosurveillance) (Grubaugh et al., 2015. PLoS Negl Trop Dis, 9:e0003628). In this study, a xenosurveillance approach was applied in Djibouti to detect malaria parasites in bloodfed non-vector mosquitoes. After the introduction of the Asian vector, Anopheles stephensi, an increase in malaria cases is occurring in this area (Seyfarth et al., 2019. Parasitol Res, 118:725-32). The implementation of surveillance strategies is essential to prevent malaria outbreaks and a xenosurveillance approach could enhance early pathogen detection in areas where Anopheles vectors might be at low densities. MATERIALS AND METHODS: From January to February 2020, 11 sticky resting box (SRb) (Pombi et al., 2014. Parasit vectors, 7:247) and 12 bG-sentinel (bG-S) traps modified with a sugar feeding system (Manzi et al., 2023. Sci Rep, 13:12840) were deployed in Djibouti City. SRbs were serviced weekly, while bG-S worked for four consecutive days. Collected mosquitoes were morphologically identified to define species and gonotrophic stage. All abdomens from fed culicine females (e. g., Culex and Aedes spp.) were analysed through DNA extraction (Rider et al., 2012. Malar J, 11:193) and PCR to define the blood meal host (Kent and Norris, 2005. Am J Trop Med Hyg, 73:336-42); additionally Anopheles females were subjected to DNA extraction from head and thorax. DNA extracted from each mosquito genus was further processed to detect plasmodium spp. through PCR and sequencing (Calzetta et al., 2018. Med vet Entomol, 32:372-77). RESULTS AND CONCLUSIONS: Overall, 14,378 mosquitoes were sampled during the study period; of these the 92.5% was collected with bG-S traps. Culicinae represented almost all of the total sample and included Cx. quinquefasciatus (96.7%), Ae. aegypti (2.6%) and Cx. sitiens (0.2%). Collected Anophelinae were An. stephensi (0.5%) and An. dthali (0.1%). The blood meal source was successfully identified in 26.4% of sample showing blood evidence (N: 500) and 46.9% of these fed on human hosts. No malaria parasites were detected in Anopheles species, which could be explained by the low number of collected females (N: 36). Conversely, p. falciparum was detected in different dates (31thJan, 15th and 20th Feb) from six human-fed culicine mosquitoes (Cx. quinquefasciatus: 3 and Ae. aegypti: 3). According to our findings, molecular detection of pathogens in blood fed non-vector mosquitoes make malaria surveillance more feasible at low vector density. Enhancing surveillance is needed to reduce malaria burden and a xenosurveillance approach could be more effective in a low transmission context, such as in areas of new introduction or where eradication plans occur.