Susceptibility of An. gambiae s.l. from Côte d’Ivoire to Insecticides used on Insecticide-Treated Nets: Evaluating the Additional Entomological Impact of Piperonyl Butoxide and Chlorfenapyr

Abstract

Background : 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, limiting options for insecticide resistance management due to the paucity of alternative insecticides. Two types of insecticide-treated nets (ITNs), ITNs with pyrethroids and the synergist piperonyl butoxide (PBO), and Interceptor ® G2 nets, a net treated with a combination of chlorfenapyr and alpha-cypermethrin, are believed to help in the control of pyrethroid-resistant mosquitoes. Methods : The susceptibility of An. gambiae s.l. to pyrethroid insecticides with and without pre-exposure to PBO as well as to chlorfenapyr was investigated in fifteen sites across the country. Susceptibility tests were conducted on 2- to 4-day old adult female An. gambiae s.l. reared from larval collections. The resistance status, intensity, and effects of PBO on mortality after exposure to different concentrations of deltamethrin, permethrin and alpha-cypermethrin were determined using WHO susceptibility test kits. In the absence of a WHO-recommended standard protocol for chlorfenapyr, two interim doses (100 and 200 µg/bottle) were used to test the susceptibility of mosquitoes using the CDC bottle assay method. Results : Pre-exposure to PBO did not result in full restoration of susceptibility to any of the three pyrethroids for the An. gambiae s.l. populations from any of the sites surveyed. However, PBO pre-exposure did increase mortality for all three pyrethroids, particularly deltamethrin (from 4.4% to 48.9%). Anopheles gambiae s.l. from only one site (Bettie) were susceptible to chlorfenapyr at the dose of 100 µg active ingredient (a.i.)/bottle. Even at the dose of 200 µg (a.i.)/bottle, susceptibility was only recorded in 10 of the 15 sites.Conclusion : Low mosquito mortality was found 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, noting that bioassays are conducted in controlled conditions and may not fully represent field efficacy.

Reduced long-lasting insecticidal net efficacy and pyrethroid insecticide resistance are associated with over-expression ofCYP6P4, CYP6P3andCYP6Z1in populations ofAnopheles coluzziifrom South-East Côte d’Ivoire

“…However, in Côte d'Ivoire, reported LLIN usage has been low, requiring additional behavioural interventions [2,34]. Our findings of high mosquito mortality following exposure to clothianidin and chlorfenapyr and improved vector susceptibility with PBO treatment (on both LLINs and in resistance bioassays), are consistent with data from other sentinel sites across Côte d'Ivoire [16,35,36], and strongly support the deployment of vector control interventions incorporating these new active ingredients.…”

Evidence supporting deployment of next generation insecticide treated nets in Burkina Faso: bioassays with either chlorfenapyr or piperonyl butoxide increase mortality of pyrethroid-resistant Anopheles gambiae s.l.

“…To supplement universal coverage with pyrethroid ITNs, which are deployed via mass campaigns every three years, U.S. While there is a good prospect that PBO plus deltamethrin ITNs will provide greater control than pyrethroid ITNs in most locations of Burkina Faso in the short term, there are already examples in parts of Côte d'Ivoire [37] and…”