Differential effects of dual and synergist-based insecticide-treated bed nets on pyrethroid resistance and knockdown mutation dynamics in Anopheles gambiae s.l. populations in south- western Burkina Faso

Background The introduction of next-generation insecticide-treated nets (ITNs) in Burkina Faso aims to mitigate pyrethroid resistance in malaria vectors. This study evaluated the impact of different ITN types on phenotypic resistance and kdr mutation frequencies in Anopheles gambiae s.l. populations across three health districts over three years. Methods Annual mosquito collections were conducted in Banfora (where pyrethroid chlorfenapyr nets had been distributed), Gaoua (pyrethroids only ITNs), and Orodara (Pyretrhroid-PBO ITNs). Two populations were analysed: adult females collected directly from the field and those reared from field-collected larvae. WHO susceptibility bioassays measured 24-hour mortality after exposure to 1x, 5x, and 10x concentrations of deltamethrin and alphacypermethrin, with and without pre-exposure to piperonyl butoxide. Frequencies of kdr mutations L1014F and L1014S were determined by PCR. Results High-intensity resistance was observed in each study district, with mortality consistently below 45% and not reaching WHO thresholds even at 10x doses. PBO increased mortality, indicating metabolic resistance, but failed to restore full susceptibility. L1014F predominated across all districts, years, and mosquito populations. L1014S remained low and variable. Pyr-only nets were associated with rising L1014F frequencies and lower mortality in resistance assays. Pyr-CFR nets improved mortality in resistance assays without increasing kdr prevalence. Pyr-PBO nets showed partial and inconsistent efficacy, with mosquitoes having mixed patterns in resistance assays. Similar patterns between field and lab-reared populations were observed. Conclusion ITN type strongly influenced resistance dynamics. Dual-AI nets, particularly Pyr-CFR, appear more effective in managing resistance. Integrated resistance management combining ITN rotation, routine monitoring, and complementary interventions is essential to preserve vector control efficacy.