Detection of reduced susceptibility of Anopheles Gambiae s.l.  to pirimiphos-methyl in Benin

Insecticide resistance in Anopheles gambiae s.l. threatens malaria vector control strategies in sub-Saharan Africa. Organophosphates such as pirimiphos-methyl have been deployed in Benin through Indoor Residual Spraying (IRS) since 2013 as alternatives to pyrethroids. However, no published evidence had documented resistance to this compound. This study provides the first confirmation of reduced susceptibility to pirimiphos-methyl in Benin and investigates potential resistance mechanisms. The study was conducted from January 2022 to December 2024. Larvae of An. gambiae s.l. were collected from 20 districts along a north-south transect of Benin and reared to adults under insectary conditions. Susceptibility tests were performed according to WHO protocols using 0.25% pirimiphos-methyl-impregnated papers, and 24-h mortality was recorded. Molecular assays were conducted for species identification and detection of the Ace-1R (G119S) mutation. A total of 1,744 females were tested. Full susceptibility was observed in eight districts. Suspected and confirmed resistance was detected in eight and four districts, respectively, including key IRS-treated areas. Molecular analysis identified An. coluzzii (47.7%), An. gambiae s.s. (47.9%), and An. arabiensis (4.4%). The Ace-1R mutation occurred at low frequencies (< 5%), suggesting a limited role of target-site resistance. This study provides the first evidence of pirimiphos-methyl resistance in An. gambiae s.l. populations in Benin. The findings underline the need for: (i) strengthened and geographically expanded resistance surveillance, (ii) rotation or replacement of pirimiphos-methyl with new chemistries such as clothianidin or chlorfenapyr, and (iii) reinforcement of integrated resistance management strategies to preserve the effectiveness of IRS and other vector control interventions. These recommendations are essential for supporting national malaria control programs in evidence-based decision-making.