Impact of insecticide resistance evolution on malaria vector control

BACKGROUND: Insecticide-treated nets (ITNs) are the staple of malaria vector control. Given their importance, insecticide resistance (IR) is extremely concerning. Understanding how ITNs simultaneously impact the evolution of IR and malaria control is of critical importance, especially with next-generation mixture ITNs (NGM-ITNs) becoming available. METHOD: We developed a mosquito population dynamics and genetics model to explore the simultaneous impact of ITNs on both malaria transmission (measured as vectorial capacity, an entomological measure of transmission) and IR evolution (resistance gene frequencies), and their interactions. We explored the long-term impact of interventions often neglected in other modelling exercises. We conducted three sets of simulations and sensitivity analyses. The first set investigated the impact of pyrethroid-only ITNs (PYR-ITNs) on IR evolution and its consequent impact on malaria vector control. The second set investigated the dual impact of NGM-ITNs in controlling malaria transmission and mitigating the evolution of resistance: IR management (IRM). The third set further considered net retention and insecticide decay. RESULTS: For both ITNs type, higher ITN coverage provided greater malaria vector control than lower coverage (of the same ITN type), even when considering IR evolution. In addition, NGM-ITNs were superior to PYR-ITNs for both malaria vector control and IRM. Even when NGM-ITNs were deployed at reduced coverage (up to 30%) than PYR-ITNs (to account for their higher procurement costs), their transmission control efficacy remains superior to PYR-ITNs by providing an additional IRM benefit through reduced selection pressures. Moreover, we found that insecticide selection on male mosquitoes may be an important consideration for malaria vector control loss. While male mosquitoes do not contribute to transmission, they propagate resistance genes, and their importance in this process is a knowledge gap. DISCUSSION: Our results highlight the need to consider IRM when evaluating NGM-ITNs, which is not adequately considered in the evaluation pipeline. Not accounting for IRM means new interventions are being undervalued, as a component of their long-term effectiveness is being overlooked.