Reduced Efficacy of Piperonyl Butoxide Based Long-Lasting Insecticide-Treated Nets Against Malaria Vectors in Western Kenya.

In response to rising insecticide resistance among malaria vector species, countries are implementing long-lasting insecticide treated nets (LLINs) coformulated with piperonyl butoxide (PBO), which can restore susceptibility to pyrethroid-based nets. In mid-2021, Kenya National Malaria Program rolled out PBO-LLINs in Bungoma County. We evaluated the susceptibility of wild-collected vectors to new versus used PBO-LLINs using the WHO cone bioassays. Results were confirmed with the WHO susceptibility assay. A total of 2,020 female Anopheles were tested, of which 1,480 were wild-collected and 540 were a Kilifi-susceptible strain. Mortality after exposure to new PBO-LLINs was only 68% in the wild-collected mosquitoes and as low as 34% when mosquitoes were exposed to used, 24-month old PBO-LLINs. Anopheles arabiensis was the main vector species that survived exposure. Susceptibility tests confirmed very low mortality after exposure to permethrin with or without PBO. Reduced sensitivity can be attributed partly to the aging of the net but also to very high levels of resistance in the local vector population.