Susceptibility of Anopheles gambiae From Côte D’Ivoire to Insecticides Used on Insecticide-Treated Nets: Evaluating the Additional Entomological Impact of Piperonyl Butoxide and Chlorfenapyr

Pyrethroid-treated mosquito nets are currently the mainstay of vector control in Côte d’Ivoire. However, resistance to pyrethroids has been reported across the country. Two types of insecticide-treated nets (ITNs), ITNs with pyrethroids and the synergist piperonyl butoxide (PBO), and Interceptor®G2 nets treated with a combination of chlorfenapyr and alpha-cypermethrin, may help control pyrethroid-resistant mosquitoes.

The authors investigated the susceptibility of Anopheles gambiae sensu lato (s.l.) to pyrethroid insecticides with and without pre-exposure to PBO and to chlorfenapyr in 15 sites. Susceptibility tests were conducted on 2- to 4-day old adult female An. gambiae s.l. reared from larval collections. We used World Health Organization (WHO) susceptibility test kits to determine the resistance status, intensity, and effects of PBO on mortality after exposure to different concentrations of deltamethrin, permethrin and alpha-cypermethrin. In the absence of a WHO-recommended standard protocol for chlorfenapyr, we used the Centers for Disease Control and Prevention (CDC) bottle assay method.

The study found low mosquito mortality for pyrethroids alone, and while PBO increased mortality, it did not restore full susceptibility. The vector was not fully susceptible to chlorfenapyr in one third of the sites tested. However, vector susceptibility to chlorfenapyr seems to be considerably higher than for pyrethroids alone or with PBO. These data should be used cautiously when making ITN procurement decisions. The bioassays are conducted in controlled conditions and may not fully represent field efficacy where host-seeking behaviors can enhance pro-insecticide chlorfenapyr intoxication to mosquitoes.