Investigating multi-resistance mechanism of Anopheles gambiae s.l to insecticides in Osun State, Nigeria

The increasing resistance to pyrethroids and other insecticide classes by Anopheles mosquitoes continues to threaten the effectiveness of core vector-control interventions and risks reversing hard-won gains. This study investigated resistance mechanisms in Anopheles gambiae s.l. populations in two urban areas (Olorunsogo and Freedom Park) in Osogbo, Osun State. Larvae were collected from the field and reared to adults for WHO bioassay insecticide susceptibility testing using different doses of pyrethroids and an organophosphate. Synergist assays following WHO procedures assessed the involvement of monooxygenases in resistance. Sibling species were identified by Polymerase Chain Reaction (PCR) - Intentional Mismatch Primer- and allelespecific PCR was used to detect the L1014F Knockdown resistance (Kdr) mutation. Metabolic enzyme activities were measured by biochemical assays. High levels of pyrethroid resistance were observed at both locations, with variation between insecticides. Preexposure to PBO produced differing recoveries in mortality: PBO+permethrin yielded 96% mortality in Olorunsogo and 75% in Freedom Park, while PBO+deltamethrin produced 99% and 87%, respectively. Kdr allele frequencies were low, at 0.84 and 0.86 in Olorunsogo and Freedom Park. Monooxygenase activity was very low in resistant populations (P = 0.000), whereas glutathione Stransferases were significantly overexpressed (59.52 ± 1.44; P = 0.001). βesterase activity was also elevated (18.86; P = 0.000). The findings reveal multiple resistance mechanisms in urban Osun State populations of An. coluzzii, including metabolic detoxification and targetsite mutation. Therefore, proactive, evidencebased resistance management strategies are urgently needed to restore and sustain the efficacy of current vectorcontrol tools and strategies in the state.