Resistance to pyrethroids in Anopheles gambiae s.l. from the Kilombero Valley, Tanzania: synergists, oxidases and susceptibility to malaria parasites (Plasmodium falciparum)

Vector resistance to insecticides, particularly pyrethroids, is an impediment to malaria control. However, the effects of metabolic insecticide resistance mechanisms on Plasmodium falciparum infection in mosquitoes remain poorly understood. We used the synergist, piperonyl butoxide (PBO) to demonstrate a major role for oxidases in pyrethroid-resistant, blood fed, wild-caught An. gambiae s.l., from the Kilombero valley, Tanzania and further investigated the relationship between CYP4G16 expression (one of the two genes overexpressed in resistant Anopheles mosquitoes) and sporozoite copy number. Blood fed-wild, caught adult An. gambiae s.l. (F0) were allowed to lay eggs. The resulting F1 generation was used for susceptibility-testing using WHO methods, and resistance was confirmed against permethrin, deltamethrin and lambdacyhalothrin (26%-86% mortality). Mosquitoes were fully susceptible (100% mortality) to bendiocarb and pirimiphos methyl. The addition of PBO to the pyrethroid assays fully restored susceptibility. After they had laid eggs, the F0 adults were used to characterise parasite infection and resistance gene expression, both using qPCR. The CYP4G16 gene copy number was significantly higher in Plasmodium infected mosquitoes than their uninfected counterparts (Mann-Whitney, p < 0.0001). However, there was no relationship between CYP4G16 gene copy number and P. falciparum sporozoite copy number (Pearson's r = 0.06361, 95% CI). This study suggests that pyrethroids-treated nets combined with PBO may help overcome major oxidative resistance mechanisms. It is also notable that these oxidative mechanisms are associated with increased Plasmodium infection in mosquitoes.