Modelling the impact of larviciding as a supplementary malaria vector control intervention in rural south-eastern Tanzania: A district-level simulation study

Background Combining larviciding with insecticide treated nets (ITNs) can reduce malaria transmission, but most modelling analyses use generalized scenarios rather than local contexts. In Tanzania and other countries, larviciding is increasingly being prioritized in national strategies, with growing advocacy for its broader implementation, particularly in settings where outdoor or residual transmission persists, for strengthening integrated vector management and achieving sustained malaria reduction. District-specific modelling is therefore essential to capture variation in transmission ecology, seasonality, and varying coverage levels, providing evidence that is both rigorous and actionable for malaria control programs. Method The Vector Control Optimization Model (VCOM) was adapted and extended to incorporate local seasonality, simulating the impact of larviciding across a range of coverage levels combined with ITNs. The model was parameterized using district-level field-data on mosquito mortality collected before (2016-2017) and after (2019-2021) larviciding implementation. Mosquito mortality rates were estimated using Bayesian inference. Outcomes were evaluated specifically for Anopheles gambiae s.l. including annual entomological inoculation rates (EIR), mosquito density, and biting rate. Sensitivity analysis explored the influence of key parameters driving transmission in this scenario study. Results The immature mosquito mortality rate due to larviciding is estimated to be 61% based on field data. VCOM simulation showed that, at 80%, ITNs coverage, larviciding substantially reduced mosquito densities, human biting rates, and EIR. Specifically, combining ITNs at 80% and larviciding coverage ≥ 60% lowered EIR below 1, the threshold required to interrupt malaria transmission. Sensitivity analyses highlighted the high impact of targeting immature mosquitoes, suggesting larviciding can effectively complement ITNs to control vectors, including invasive species like An. stephensi, regardless of feeding preference, resting, and biting behaviors, which hinder the effectiveness of most vector control tools. Conclusion This study provides local evidence that larviciding is an effective complement to ITNs for interrupting malaria transmission. Implementation should leverage innovative approaches, such as drones for precise mapping and targeted application of biological larvicides, to maximize coverage, efficiency, and scalability for district-level malaria control and elimination.