Escalating pyrethroid resistance in Anopheles funestus increases Plasmodium transmission

Background: The swift rise in pyrethroid resistance among malaria vectors poses a significant challenge to global malaria control. Although vector control tools have advanced, limited understanding of the vectorial capacity of highly pyrethroid-resistant mosquitoes hinders accurate predictions of how new interventions will affect malaria transmission in these resilient populations. Consequently, further research is needed to elucidate how escalating resistance influences mosquito vectorial capacity and shapes malaria transmission dynamics. Methods: Wild Anopheles funestus s.l. mosquitoes were sampled from households in Adamaoua and Centre regions of Cameroon between February and September 2021 using a Prokopack aspirator. Phenotypic resistance to pyrethroids (permethrin, deltamethrin, alphacypermethrin) was assessed using WHO tube bioassays at discriminating concentration (DC), as well as at 5× and 10× DC. Plasmodium sporozoite infections were detected using TaqMan and nested PCR assays, and key resistance markers were genotyped by allele-specific PCR. Pearson’s chi-squared tests and odds ratio were applied to examine the associations between resistance phenotypes, genotypes, and infection status. Results : Mosquitoes from Cameroon displayed low mortality rates in Mibellon, with only 33% mortality at the diagnostic dose (DD) and less than 95% mortality at 5× and 10× DD. In contrast, Elende populations exhibited lower resistance intensity. Sporozoite infection rates were very high in both collection sites (8.8% and 6% in Mibellon and Elende, respectively). Mosquitoes killed by elevated doses of type II pyrethroids were significantly more infected with Plasmodium than survivors ( p <0.05). Additionally, those surviving the highest deltamethrin doses showed reduced vectorial capacity compared to those surviving the 1x diagnostic dose (OR=2.8; 95% CI=1.1–7.3; p =0.041). The persistence of elevated Plasmodium infection rates in mosquitoes surviving the highest permethrin concentrations underscores the direct impact of pyrethroid resistance escalation in driving malaria transmission. Genotypic analysis identified the L119F- GSTe2 mutation as associated with both resistance escalation and increased Plasmodium transmission whereas an opposite effect was observed for the P450 CYP9K1 with homozygote resistant 454A- CYP9K1 less infected than susceptible. Conclusions: These findings highlight the urgent need for effective resistance management strategies to combat escalating pyrethroid resistance, while supporting the sustained use of type II pyrethroids, which remain effective against key Plasmodium -transmitting populations.