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Vector Surveillance and Resistance Monitoring

Vector Surveillance and Resistance Monitoring

176 - The effect of dual-active-ingredient, long-lasting insecticidal nets on insecticide resistance in malaria vectors in Benin: results of a 3-year, three-arm, cluster-randomised, controlled trial

Wednesday, March 5, 2025
3:15 PM – 3:25 PM AST
Location: 208 A
Abstract: Insecticide resistance among malaria vector species now occurs in 84 malaria-endemic countries and territories worldwide. Novel vector-control interventions, including long-lasting insecticidal nets (LLINs) that incorporate new active ingredients with distinct modes of action, are urgently needed to delay the spread of resistance and to alleviate reversals in malaria-control gains. We assessed the longitudinal effect of two dual-active-ingredient LLINs on insecticide resistance during a cluster-randomised controlled trial (cRCT) in Benin. This 3-year, three-arm, cRCT was conducted in three districts in southern Benin, to compare the effects of LLINs containing chlorfenapyr–pyrethroid or pyriproxyfen–pyrethroid with LLINs containing pyrethroid-only. In 19,292 mosquitoes (Anopheles gambiae sensu lato) collected over 36 months, we measured longitudinal phenotypic insecticide resistance profiles using bioassays and genotypic resistance profiles using quantitative, real-time, reverse transcriptase PCR of metabolic resistance genes in two clusters per trial group. In all three trial groups, a significant effect of LLINs on insecticide resistance selection was evident, with the median lethal dose (LD50) of α-cypermethrin approximately halving between baseline and 12 months post-LLIN distribution. Over the subsequent 2 years, the LD50 of α-cypermethrin increased past baseline values in all three trial groups. We observed minimal reductions in chlorfenapyr susceptibility and variable but significant reductions in fertility after pyriproxyfen exposure, with an overall trend of increasing susceptibility across trial years. Several metabolic genes were implicated in resistance selection, including CYP6P4, which encodes an enzyme known to metabolise pyriproxyfen in vitro, and CYP6P3 and CYP9K1, both of which encode enzymes that are involved in pro-insecticide activation. After 24 months, chlorfenapyr–pyrethroid LLINs no longer mitigated pyrethroid resistance selection. This finding raises issues for current net-procurement schedules, which are based on an operational net lifespan of 3 years. Knowledge of the effects of novel LLINs on insecticide-resistance selection is crucial for the pragmatic design of prospective resistance-management strategies.