Human-biting activity and human exposure indices of major malaria vectors in Malawi

Olanga, E. A., Kambewa, E. A., Kaunde, N., Banda, J., Jones, C.M2,3., Reimer, L. and Mzilahowa, T.

Background: Malaria transmission persists in areas with wide coverage of treated bednets in Malawi. There is a need to better understand indoor and outdoor transmission and human behaviours that facilitate exposure to vector bites. This study sought to determine the biting patterns and human exposure indices of major malaria vectors in Malawi.

Methods: Mosquito and human behaviour surveys were carried out in villages in Chikwawa and Dedza districts. Mosquitoes were collected using Human Landing Catches (HLC) over a period of eight months between August 2019 to March 2020. HLC was conducted from 5pm till 11 am. Human behaviour data was collected during cross-sectional surveys performed once during the dry (October 2019) and rainy season (March 2020). Questionnaires were administered to participants to assess times and activities carried out indoors and outdoors.

Results: A total of 9,822 anophelines were caught across the study sites. Anopheles arabiensis (91%) was the major vector in Dedza and An. funestus (99%) in Chikwawa. The Human biting rate (HBR) of An. funestus was 3.2 bites/person/night in Chikwawa and An. gambiae 13.2 bites/person/night in Dedza. The biting activity of An. arabiensis was higher outdoors than indoors (OR=1.04, CI=[1.046-1.048], P<0.001). An. funestus was most likely to bite indoors compared to outdoors (OR=1.12, CI=[1.11-1.13], P<0.001). Taking into account human behaviour, exposure to An. funestus bites for non-bednet users mainly occurred indoors (95 – 99% in Chikwawa and 90 - 95% in Dedza) across seasons. Exposure to An. arabiensis bites indoors and when people were asleep was between 92% and 96% in both districts. Treated bednets were estimated to offer >70% protection against Anopheles bites for users (78% in Chikwawa and 74% in Dedza).

Conclusion: Malaria transmission primarily occurs indoors at night when people are asleep in Chikwawa and Dedza. Use of bednets should be prioritized in both districts.