Behavioral patterns of humans and Anopheles arabiensis reveal outdoor malaria transmission risk at Pemba Island, Zanzibar.

BACKGROUND

Zanzibar has achieved and sustained very low malaria prevalence through high coverage of indoor residual spraying (IRS) and insecticide-treated nets (ITNs). However, residual transmission persists, particularly on Pemba Island. Increasing evidence suggests that the predominance of exophagic vectors and outdoor human activity patterns may reduce the effectiveness of indoor-focused interventions. This study aimed to quantify malaria transmission risk arising from the interaction between human behavior and mosquito biting activity indoors and outdoors on Pemba Island.

METHODS

From February 2024 to January 2025, adult malaria vectors were collected using human landing catches (HLCs) indoors and outdoors at sentinel sites across four districts of Pemba Island. Concurrent human behavior observations (HBOs) were conducted hourly to record location, activity, age, sex, and ITNs use within households. Human biting rates (HBRs) and behavior-adjusted biting rates were calculated to estimate human behavior-adjusted exposure and identify protection gaps by age and sex.

RESULTS

A total of 981 Anopheles mosquitoes were collected, of which 98% of An. gambiae s.l. were An. arabiensis. Mosquito collection was significantly higher outdoor than indoor (Relative rate = 4.37, 95% CI 3.61-5.29, p < 0.0001). Most bites occurred outdoors (≈82%), with peak biting in the early evening (19:00-20:00 h) for both indoor and outdoor. Across the population, approximately 85% of unprotected exposure occurred outdoors while individuals were awake. Children under five experienced relatively more indoor exposure, while school-aged children and adults were predominantly exposed outdoors. Men experienced higher outdoor exposure than women, whereas women had greater indoor exposure while awake. ITNs provided substantial protection for young children but prevented a declining proportion of exposure with increasing age, leaving substantial residual risk across all groups.

CONCLUSIONS

Residual malaria transmission on Pemba Island is driven primarily by outdoor exposure linked to human behaviour patterns and exophagic An. arabiensis. While ITNs continue to protect young children indoors, they do not address most exposure among older children and adults. Malaria elimination efforts in Zanzibar will require complementary interventions that target outdoor transmission and behavioral risk, alongside existing indoor-focused strategies.