Daytime and public space exposure to Anopheles funestus bites in Western Province, Zambia: implications for malaria surveillance and control

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BACKGROUND: Western Province, Zambia experiences persistent and residual malaria transmission despite high household coverage of core vector control interventions. Standard vector surveillance, conducted overnight at households, indicates that the dominant malaria vector Anopheles funestus sensu stricto (s.s.) bites opportunistically both indoors and outdoors, and remains active throughout the night and into the late morning after the sun rises. This suggests that the full extent of community exposure to An. funestus s.s. bites may not be well characterized in Zambia. This study piloted an expanded vector surveillance approach to capture 24 -hour biting patterns at households and in public spaces, including schools and markets, where core interventions offer limited protection. METHODS: Monthly mosquito collections were made in two rural villages and two peri-urban neighborhood-based clusters in Western Province, Zambia, from January to April 2024. Paired indoor-outdoor human landing catches were implemented over 24-hour periods. A total of 9600 collection hours were performed at randomly selected households, with 768 additional collection hours performed equally across two school and two market buildings. RESULTS: A total of 2305 female Anopheles mosquitoes from 11 morphologically differentiated species were collected, with An. funestus sensu lato (s.l.) the most abundant (41%, 942). Aggregated across all hours of the day and all locations there was an overall average of 6.9 An. funestus s.l. bites/day (b/d) (95% CI 5.0-8.8), with comparable rates indoors (3.52 b/d) and outdoors (3.37 b/d). Similar rates occurred at home (2.27 b/d), school (2.38 b/d), and market (2.25 b/d). While 87.6% of bites (6.0 b/d) occurred overnight (1800-0600 h), 12.1% (0.83 b/d) occurred during daylight hours between 0600-1100 h. CONCLUSION: Results document significant exposure to An. funestus bites both indoors and outdoors, at home as well as in public spaces such as schools and markets, and late in the morning until 11:00 h in Western Province, Zambia. The flexible blood-feeding behaviours exhibited by this dominant malaria vector highlight important operational gaps in the protection offered by current vector control strategies that are deployed primarily indoors and/or during nighttime hours. Vector surveillance efforts should be extended to better characterize the full scope of transmission risk throughout the community and guide the development of new approaches to target transmission occurring outdoors, during the daytime, and in public spaces away from the home.

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