Metabolic resistance of Anopheles funestus to pyrethroids and organochlorines in Toumbokro, rural Côte d’Ivoire

Insecticide resistance in malaria vectors threatens control efforts. Anopheles funestus Giles s.l. increasingly contributes to malaria transmission in Côte d'Ivoire especially in central and western regions, yet resistance data remain poorly known. This study assessed the susceptibility of An. funestus s.s. from Toumbokro to major insecticides and identified underlying metabolic resistance mechanisms. Blood-fed An. funestus s.l. females were collected indoors with an electric aspirator and induced to oviposit. F1 progeny from PCR-confirmed An. funestus s.s. were exposed to permethrin (0.75%), deltamethrin (0.05%), lambda-cyhalothrin (0.05%), and DDT (4%) in WHO tube assays. Synergist assays with piperonyl butoxide (PBO) were used to evaluate enzyme-mediated resistance, and biochemical assays were used to measure activity of detoxification enzymes. Anopheles funestus s.s. showed resistance to all insecticides, with mortality rates of 78% (permethrin), 76% (deltamethrin), 44% (lambda-cyhalothrin), and 72% (DDT). Pre-exposure to PBO fully restored susceptibility to pyrethroids (100% mortality), implicating metabolic resistance, but only partially restored susceptibility to DDT (82%). Biochemical assays confirmed significantly greater cytochrome P450 monooxygenase activity in An. funestus s.s. (0.00144 nmol/mg of protein) compared with the susceptible Kisumu strain (0.00085 nmol/mg of protein). An. funestus s.s. from Toumbokro in central Côte d'Ivoire is resistant to pyrethroids and DDT, mainly via metabolic mechanisms. This resistance likely sustains its role in malaria transmission despite widespread pyrethroid-treated long-lasting insecticidal nets (LLIN) use suggesting a need for PBO-based or dual-active ingredient LLINs in vector control strategies.