Significant variations in tolerance to clothianidin and pirimiphos-methyl in and populations during a dramatic malaria resurgence despite sustained indoor residual spraying in Uganda.

A dramatic malaria resurgence occurred in areas of Uganda between 2020 and 2022 coincident with the switch to clothianidin-based formulations for indoor residual spraying. During the resurgence, numbers increased but when an alternative insecticide, pirimiphos methyl, was reintroduced in 2023, both malaria cases and mosquito density fell. In this study, we investigated possible causes of the resurgence by assessing; 1) whether sufficient quantities of insecticide were sprayed; 2) the residual insecticide bioefficacy against wild mosquitoes and; 3) the insecticide susceptibility of vector populations using standard test tube assays and wall cone assays.