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

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 Anopheles gambiae sensu lato (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. 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 where the host-seeking behaviours, which include free-flying activity are known to enhance pro-insecticide chlorfenapyr intoxication to mosquitoes.

Can pyrethroid-piperonyl butoxide (PBO) nets reduce the efficacy of indoor residual spraying with pirimiphos-methyl against pyrethroid-resistant malaria vectors?

“…In our study, the pyrethroid-PBO ITNs when applied alone in a hut induced low vector mosquito mortality (22-26%) compared to what has been observed in hut trials against the same vector population with another type of novel dual ITN (71-76%) [35,36]. Similar levels of hut mortality rates with pyrethroid-PBO ITNs (<30%) and failure of PBO to fully restore pyrethroid susceptibility in bioassays has also been reported from studies in Burkina Faso [37], Cameroon [38], Cô ' v [39] and Senegal [40]. This may indicate the presence of complex resistance mechanisms in the West African region unaffected by PBO.…”

Determination of the discriminating concentration of chlorfenapyr (pyrrole) and Anopheles gambiae sensu lato susceptibility testing in preparation for distribution of Interceptor® G2 insecticide-treated nets

“…gambiae s.l. in five of 15 sites, with possible resistance recorded in Bouaké, Gagnoa, Nassian, Sakassou, and San Pédro [ 44 ]. This could be a sign of chlorfenapyr resistance in some parts of Côte d’Ivoire, but further phenotypic and genotypic data should be collected to confirm this finding.…”

Entomological Indicators of Malaria Transmission Prior to a Cluster Randomised Controlled Trial of a “Lethal House Lure” Intervention in Central Côte D’Ivoire

“…gambiae than An. coluzzii as evidenced elsewhere in Côte d'Ivoire [7,36] and other parts in sub-Saharan Africa [22,37,38].…”

“…However, despite these considerable efforts to reduce transmission, malaria remains one of the major causes of morbidity and mortality in sub Saharan Africa [1,3]. Vector control relies on a handful of insecticides used for indoor residual spraying (IRS) and treatment of longlasting insecticidal nets (LLINs) and insecticide resistance has been widely detected in malaria vectors across the continent [4][5][6][7]. The situation is particularly worrying with an increase in intensity and mechanisms of insecticide resistance detected over time [7,8].…”

“…Vector control relies on a handful of insecticides used for indoor residual spraying (IRS) and treatment of longlasting insecticidal nets (LLINs) and insecticide resistance has been widely detected in malaria vectors across the continent [4][5][6][7]. The situation is particularly worrying with an increase in intensity and mechanisms of insecticide resistance detected over time [7,8]. Therefore there is a pressing need for effective, sustainable tools or strategies for malaria control.…”