Insecticide resistance profile of the Anopheles gambiae s.l. mosquitoes in Nigeria: a 5-year malaria vector surveillance report

Abstract

Abstract Background Nigeria bears the highest malaria burden globally, with vector control strategies relying heavily on the use of Long-lasting Insecticidal nets (LLINs). However, the widespread emergence of insecticide resistance threatens the effectiveness of this intervention in the country. This study provides a comprehensive 5-year overview (2020–2024) of the resistance status and resistance mechanisms of Anopheles mosquitoes in Nigeria. Methods Immature stages of mosquitoes were collected from 72 local government areas (spanning 12 States), raised to adults and exposed to standard concentrations of permethrin, deltamethrin, alphacypermethrin, and pirimiphos-methyl insecticide papers according to WHO criteria for 5 years (2020–2024). Furthermore, molecular test and bioassays were used to determine the involvement of kdr-L995F mutation and oxidase-based metabolic enzymes in areas where resistance was confirmed. All mosquitoes tested were identified morphologically. Spatial and temporal patterns of insecticide susceptibility results were visualised using geospatial mapping tools in R (sf, tidyr, ggplot2). Results Members of the Anopheles gambiae complex accounted for 95% of all mosquitoes tested across the 72 LGAs, with Anopheles coluzzii comprising 79.8% of collections. Other species encountered included An. funestus, An. pretoriensis, An. malculipalpis, and An. nilli . Widespread and persistent resistance to pyrethroid insecticides was detected throughout the study period. Mortality to permethrin remained consistently low (0–75%), indicating high levels of resistance across most locations. Resistance to deltamethrin and alphacypermethrin was also widespread but generally less pronounced, with mortality ranging from 10 to 90% and 20–80%, respectively. Temporal trends revealed localized reductions in resistance intensity between 2022 and 2023, coinciding with periods of large-scale LLINs distribution in several States. Synergist bioassays demonstrated substantial involvement of oxidase-based metabolic mechanisms, suggesting that multiple resistance mechanisms are operating across Nigeria. The frequency of the kdr-w mutation ranged from 0.0 to 0.7 and generally increased between 2021 and 2023 before declining slightly in 2024. While kdr-w frequency was significantly associated with reduced mortality to deltamethrin (τ = − 0.179, p < 0.001), no corresponding association was observed for permethrin (τ ≈ 0, p = 0.998), and the mutation was not a significant predictor of pyrethroid mortality overall after accounting for insecticide type and spatial clustering (β = 10.50, p = 0.078). These findings indicate that the contribution of kdr-mediated resistance varies among pyrethroid compounds. Conclusion The predominance of Anopheles coluzzii in tested collections, coupled with pervasive pyrethroid resistance and widespread oxidase-based metabolic resistant mechanisms, poses significant challenges to malaria vector control efforts in Nigeria. These findings underscore the urgent need to prioritize efforts at managing metabolic resistance mechanisms through integrated vector control and further strengthen IRS with the use of non-pyrethroid insecticides. Enhanced surveillance, including molecular diagnostics and monitoring of agricultural pesticide use, will be critical for sustaining malaria control gains and informing adaptive, evidence-driven intervention planning in Nigeria.