Pyrethroid resistance intensity and mechanisms in Anopheles gambiae sensu lato (Diptera: Culicidae) from Oyo State, Nigeria.
TL;DR
The findings demonstrate intense, spatially heterogeneous pyrethroid resistance driven by multiple mechanisms, with important implications for malaria vector control.
Abstract
Background
Pyrethroid resistance in Anopheles gambiae sensu lato (s.l.) is increasing across Nigeria and threatens the effectiveness of long-lasting insecticidal nets (LLINs) and other pyrethroid-based interventions across Nigeria. This study quantified resistance levels, intensity, and metabolic contributions in An. gambiae s.l. populations across ecologically distinct zones of Oyo State, southwestern Nigeria, to generate evidence for locally tailored vector control strategies.
Methods
Between May and September 2018, immature stages of Anopheles gambiae sensu lato were collected from six Local Government Areas in Oyo State, Nigeria, with 10-15 larval habitats sampled per LGA using standard dipping procedures. Specimens were reared to adulthood under controlled insectary conditions (27 ± 2 °C; 75-84% Relative Humidity) before testing. For each insecticide in each LGA, approximately 450-550 non-blood-fed female mosquitoes were evaluated in accordance with WHO testing procedures. Susceptibility to alpha-cypermethrin (12.5 μg/bottle), deltamethrin (12.5 μg/bottle), and permethrin (21.5 μg/bottle) was determined using the CDC bottle bioassay at diagnostic concentrations (1×), with standard replicate batches per assay. Populations exhibiting resistance at 1× were further examined using elevated concentrations (2×, 5×, and 10×) to quantify resistance intensity. The involvement of metabolic detoxification was assessed through the contribution of cytochrome P450-mediated resistance mechanisms.
Results
A total of 9000 female mosquitoes were exposed to WHO diagnostic (1×) and elevated (2 × -10×) concentrations of alpha-cypermethrin, deltamethrin, and permethrin, complemented with piperonyl butoxide (PBO) synergist assays. At diagnostic dose, resistance was widespread, with mortality ranging from 5% (Egbeda) to 65% (Akinyele) for permethrin, 38.8-97% for alpha-cypermethrin, and 23-78% for deltamethrin, indicating significant spatial heterogeneity. Permethrin exhibited the highest resistance intensity, with incomplete mortality recovery even at 10×, while alpha-cypermethrin and deltamethrin showed full susceptibility restoration at 5× and 2×, respectively. Statistical analysis confirmed significant variation among LGAs (χ2, P < 0.001). Knockdown responses were reduced in Atiba, Afijio, and Egbeda, where several populations failed to reach 95% knockdown within 60 min. PBO pre-exposure fully restored susceptibility to alpha-cypermethrin (100%) across resistant LGAs, while deltamethrin showed partial recovery. In contrast, permethrin mortality remained incomplete (47-94%) after PBO, indicating limited metabolic involvement and likely target-site resistance.
Conclusions
Overall, the findings demonstrate intense, spatially heterogeneous pyrethroid resistance driven by multiple mechanisms, with important implications for malaria vector control. While PBO-based LLINs may provide partial mitigation, they are unlikely to fully address the complexity of resistance observed across the study area. Consequently, sustained control in Oyo State will require complementary and alternative strategies, including next-generation dual-active-ingredient nets and other non-pyrethroid interventions.