A survey of indoor and outdoor Anopheles density, species composition and circumsporozoite rate of malaria vectors on the Bijagós Archipelago, Guinea-Bissau

Abstract

Background: The malaria-endemic Bijagós Archipelago is situated 50 km off the coast of mainland Guinea-Bissau. It is a seasonal malaria transmission setting, with insecticide-treated bed nets as its primary control strategy. Little is known about the vector diversity and behaviour across the Archipelago. Methods: In 2019, a survey took place on 16 of the inhabited islands across the Archipelago. Adult mosquitoes were collected using odour-baited outdoor light traps and indoor light traps at houses selected at random. Larval surveys were conducted for each village sampled. Anopheles adults caught were morphologically identified and a sub-sample was analysed to identify species within the Anopheles gambiae complex using RFLP-PCR. Sporozoite positivity was detected within a sub-sample by CSP-ELISA. Results: Anopheles gambiae sensu lato was present on all islands sampled. Anopheles density varied between islands, with densities ranging from 0.0 to 98.7 per trapping night from indoor traps and 0.1–165.2 per trapping night from outdoor traps. Anopheles melas was the most commonly observed species, accounting for 85.2% of all Anopheles caught from both indoor and outdoor light traps. A high level of hybridization between An. gambiae sensu stricto and Anopheles coluzzii was seen on some islands across the Archipelago. The overall sporozoite rate was 0.86% (0.2% for indoor traps; 1.4% for outdoor traps). Conclusions: Species within An. gambiae s.l. are the primary vectors on the Bijagós. Anopheles melas may contribute to transmission throughout the year in the Bijagós. The vector species composition, abundance and infection rates uncovered in this study are useful for informing tailored, effective vector control programme in the Bijagos.