Analysis of the 24-Hour Biting Patterns and Human Exposures to Malaria Vectors in South-Eastern Tanzania

Abstract

Background Afrotropical malaria vectors are generally believed to bite nocturnally, leading to the predominant use of Insecticide-Treated Nets (ITNs), which target indoor, nighttime-biting mosquitoes. This focus is reinforced by biases in entomological surveys, which largely overlook daytime mosquito activity. However, recent evidence challenges this paradigm, showing that Anopheles biting can extend way into the daytime, coinciding with human activities at dawn, daytime, and evenings, suggesting a broader risk spectrum and potential protection gaps. This study therefore investigated the diurnal and nocturnal biting patterns of the malaria vectors, Anopheles arabiensis and Anopheles funestus in south-eastern Tanzania, to better understand the scope of residual transmission and inform strategies for improved control. Methods Host-seeking mosquitoes were collected hourly using miniaturized double net traps, indoors and outdoors over 24-hour periods between June 2023 and February 2024. Concurrently, human activities indoors and outdoors were monitored half-hourly to correlate with mosquito collections. A structured questionnaire was used to assess household members’ knowledge, perceptions, and experiences regarding exposure to mosquito bites during both nighttime and daytime. Results Nocturnal biting by An. arabiensis peaked between 7pm and 11pm, while An. funestus peaked later, between 1am and 3am. Daytime biting accounted for 15.03% of An. gambiae catches, with peaks around 7-11am and after 4pm; and 14.15% of An. funestus catches, peaking around mid-mornings, from 10am-12pm. Night-time exposure to An. arabiensis was greater outdoors (54.5%), while daytime exposure was greater indoors (80.4%). For An. funestus, higher exposure was observed indoors both at nighttime (57.1%) and daytime (69%). Plasmodium falciparum sporozoites were detected in both day-biting and night-biting An. arabiensis. Common daytime activities potentially exposing residents during peak biting hours included household chores, eating, sleeping (including due to sickness), resting under shades or verandas, and kids playing. From evenings onwards, exposures coincided with resting, socializing before bedtime, and kids playing. Nearly all survey respondents (95.6%) reported experiencing daytime mosquito bites, but only 28% believed malaria was transmissible diurnally. Conclusion This study updates our understanding of malaria vector biting patterns in south-eastern Tanzania, revealing considerable additional risk in the mornings, daytime, and evenings. Consequently, ITNs, which primarily target nocturnal mosquitoes, may have larger protection gaps than previously thought. Complementary strategies are needed to holistically suppress vectors regardless of biting patterns (e.g. using larval source management) and to extend personal protection limits (e.g. using repellents). Additionally, community engagement and education on mosquito activity and protective measures could significantly reduce malaria transmission risk.