Larval surveys reveal breeding site preferences of malaria vector Anopheles spp. in Zanzibar City

Malaria is a serious illness that causes over 500,000 deaths annually worldwide, mainly affecting children under five years of age. Tanzania, in Africa, contributes to approximately 4% of those deaths. Malaria is caused by Plasmodium spp. parasites, and it is vectored by mosquitoes in the Anopheles genus. In Zanzibar City, the capital of the Zanzibar archipelago in Tanzania, the incidence of malaria has decreased over the past few decades due to standardized treatment protocols in hospitals and dispensaries and public health interventions targeting adult mosquitoes. However, the incidence remains between 1-2%, with an increasing trend observed over the past few years likely stemming from continued exchange of Plasmodium spp. from other malaria-endemic areas (including mainland Tanzania). While larvicides are a powerful tool to reduce vector populations, this control strategy relies on information on breeding sites and their productivity. In Zanzibar City, no larval surveys have been done in the last few years. Our aim was to characterize Anopheles spp. breeding sites in Zanzibar City during the rainy season. We first conducted systematic larval surveys across 16 semipermanent/permanent water bodies and 30 temporary water bodies. Then, we used Principal Component Analysis and logistic regression to model the effect of physical and chemical parameters, and rainfall on Anopheles presence. We found that Anopheles spp. utilize mostly concrete, semipermanent breeding sites with high levels of dissolved oxygen concentrations but can utilize natural sites after heavy rains. The logistic regression model incorporating rainfall was able to predict the presence of Anopheles larvae with a positive predictive power of 65.7% and a negative predictive power of 88.8%. The data from our study suggest that Anopheles spp. have not yet expanded to using more polluted breeding sites in Zanzibar City (as they have in some mainland locations). These results can inform targeted larvicidal strategies in Zanzibar City.