The spread of pyrethroid-resistant mosquitoes and their predicted malaria burden across the Democratic Republic of the Congo

Malaria control in the Democratic Republic of the Congo (DRC) is increasingly challenged by pyrethroid resistance in mosquito populations, although its epidemiological consequences remain uncertain. Entomological data from discriminating-dose bioassays conducted on wild mosquitoes were collated to characterise temporal changes in pyrethroid resistance since 2009. These estimates were incorporated into a mechanistic malaria transmission model to quantify how resistance may have reduced the effectiveness of insecticide-treated nets (ITNs) and affected malaria burden. Model projections were compared with malaria prevalence estimates from the 2013-2014 and 2023-2024 Demographic and Health Surveys, which were used as external comparisons rather than for model fitting. Counterfactual scenarios were generated in which mosquitoes remained fully susceptible to pyrethroids throughout the study period. The model estimated that pyrethroid resistance increased substantially over time and reduced the protection provided by pyrethroid-only ITNs. Nationally, approximately 8.6% of malaria clinical cases, corresponding to 33.6 million additional cases, and approximately 76,000 additional deaths between 2009 and 2024 were attributable to pyrethroid resistance in the modelled counterfactual comparison. The introduction of pyrethroid-PBO ITNs from 2019 partially restored the protection lost to pyrethroid resistance. These findings highlight the potential epidemiological consequences of pyrethroid resistance while also emphasising the uncertainty arising from sparse resistance surveillance and limited epidemiological observations. Improved entomological surveillance and continued deployment of more effective vector-control tools will be important for malaria control in the DRC.