Determinants of locally acquired malaria infections in Zanzibar: a cross-sectional study

Background Zanzibar has made significant progress in malaria control reaching a population prevalence of around 1% with program-led interventions. Nevertheless, malaria infections persist in people with no recent travel history implying locally acquired infections. Understanding risk factors of local transmission is crucial to refine elimination strategies. Methods From May 2017 to October 2019, a rolling cross-sectional survey was conducted in five Zanzibar districts, linked to routine malaria surveillance procedures. The study involved testing all individuals living in households of clinical malaria cases (index cases) routinely detected at a health facility, and a sample of neighboring households using malaria rapid diagnostic tests and qPCR. Information on socio-demographic and household characteristics, recent travel and bed net use were collected during interviews. Results Among 17,891 surveyed individuals, 15,151 (85%) had not travelled outside Zanzibar in the last sixty days. Of these, 7286 were tested for malaria by qPCR and 2.6% [95% confidence interval ( CI ): 2–3%] tested positive. Members of index households were more likely to be infected (adjusted odds ratio [ aOR ] = 14.9, 95% CI 9.7–23.0) than neighbours, as were individuals aged 5–15 and 16–25 years compared to older age groups ( aOR  = 4.0, 95% CI 2.5–6.5 and aOR  = 2.0, 95% CI 1.3–3.1, respectively). Infections were more likely in Unguja residents ( aOR  = 1.3 95% CI 0.9–2.0), in people living in houses with no windows ( aOR  = 2.1, 95% CI 1.4–3.1), in those with no bed net ( aOR  = 2.3, 95% CI 1.4–3.1), those going to sleep early and in people living in areas with a higher local index of wetness. Conclusion Combatting locally acquired malaria in Zanzibar requires interventions and strategies that promote uptake of existing interventions such as bed nets, housing improvements, and the identification and targeting of individuals at higher risk of infection.