Pyrethroid Resistance in Anopheles gambiae s.l. from Oyo State, Nigeria: Mechanisms and Implications for Malaria Control

Background: Insecticide resistance among malaria vectors threatens control strategies that are reliant on insecticidal interventions. This study assessed pyrethroid resistance in Anopheles gambiae s.l. populations across Ibadan North (IBN) and Ido LGAs, Oyo State, Nigeria, to elucidate resistance mechanisms and their implications for vector control.

Methods: Larvae and pupae from both LGAs were collected using the dipping method and reared to adulthood in the University of Ibadan Insectary. Molecular identification of An. gambiae complex members was conducted. Three- to five-day-old adult females (n=25/insecticide) were exposed to permethrin, deltamethrin, bendiocarb, and pirimiphos-methyl using WHO susceptibility tests. Synergist assays with Piperonyl Butoxide (PBO) were used to assess the metabolic resistance. kdr (L1014F/S) mutations were detected via allele-specific PCR (TaqMan) at the Nigerian Institute of Medical Research (NIMR), Lagos. Data were analyzed using descriptive statistics and ANOVA (α=0.05).

Results: An. coluzzii predominated in IBN (96.0%) compared with Ido (78.7%), while An. gambiae s.s. was higher in Ido (21.3%) than in IBN (3.3%). An. arabiensis was only recorded in IBN (0.7%). While bendiocarb and pirimiphos-methyl susceptibility was high (98.0 - 100.0%), pyrethroid resistance was widespread (25.0 - 95.0%). PBO restored deltamethrin susceptibility in IBN and one Ido community, suggesting cytochrome P450-mediated metabolic resistance. However, permethrin resistance persisted post-synergist exposure, indicating voltage-gated sodium channel mutations. Only kdr-w (L1014F) was detected, with the allelic frequencies nearing fixation (0.61 in Ido; 01 in IBN).

Conclusion: This study confirms significant pyrethroid resistance in An. gambiae s.l., driven by kdr-w (L1014F) mutations and metabolic resistance. Our findings highlight the urgent need for molecular surveillance and alternative vector control strategies to sustain malaria prevention efforts.