Determination of the discriminating concentration of chlorfenapyr (pyrrole) and Anopheles gambiae s.l. susceptibility testing in preparation for distribution of Interceptor® G2 insecticide-treated nets
Authors: Oxborough Richard Martin , Seyoum Aklilu , Yihdego Yemane , Chabi Joseph , Wat’senga Francis , Agossa Fiacre R. , Coleman Sylvester , Musa Samdi Lazarus , Faye Ousmane , Okia Michael , Bayoh Mohamed , Alyko Evelyne , Rakotoson Jean-Desire , Masendu Hieronymo , Sovi Arthur , Gadiaga Libasse , Abong’o Bernard , Opondo Kevin , Baber Ibrahima , Dabire Roch , Gnanguenon Virgile , Yohannes Gedeon , Varela Kenyssony , Fondjo Etienne , Carlson Jenny , Armistead Jennifer S. , Dengela Dereje 
Published: May 2021

Abstract
BackgroundFollowing agricultural use and large-scale distribution of insecticide treated nets (ITNs), malaria vector resistance to pyrethroids is widespread in sub-Saharan Africa. Interceptor® G2 is a new dual active ingredient (AI) ITN treated with alpha-cypermethrin and chlorfenapyr for the control of pyrethroid-resistant malaria vectors. In anticipation of these new nets being more widely distributed, testing was conducted to develop a chlorfenapyr susceptibility bioassay protocol and gather susceptibility information. MethodsBottle bioassay tests were conducted using five concentrations of chlorfenapyr at 12.5, 25, 50, 100 and 200µg AI/bottle in ten countries in sub-Saharan Africa using 13,639 wild collected An. gambiae s.l. (56 vector populations per dose) and 4,494 pyrethroid susceptible insectary mosquitoes from 8 colonized strains. In parallel, susceptibility tests were conducted using a provisional discriminating concentration of 100µg AI/bottle in 16 countries using 23,422 wild collected pyrethroid resistant An. gambiae s.l. (259 vector populations). Exposure time was 60 minutes, with mortality recorded at 24, 48 and 72 hours after exposure. ResultsMedian mortality rates (up to 72h after exposure) of insectary colony mosquitoes was 100% at all five concentrations tested, but the lowest dose to kill all mosquitoes tested was 50µg AI/bottle. The median 72h mortality of wild An. gambiae s.l. in 10 countries was 71.5%, 90.5%, 96.5%, 100% and 100% at concentrations of 12.5, 25, 50, 100 and 200µg AI/bottle, respectively. Log-probit analysis of the five concentrations tested determined that the LC95 of wild An. gambiae s.l. was 67.9µg AI/bottle (95% CI: 48.8-119.5). The discriminating concentration of 203.8µg AI/bottle (95% CI: 146-359) was calculated by multiplying the LC95 by three. However, the difference in mortality between 100 and 200µg AI/bottle was minimal and large-scale testing using 100µg AI/bottle with wild An. gambiae s.l. in 16 countries showed that this concentration was generally suitable, with a median mortality rate of 100% at 72h.ConclusionsThis study determined that 200µg AI/bottle chlorfenapyr in bottle bioassays is the most suitable discriminating concentration for monitoring susceptibility of wild An. gambiae s.l., using mortality recorded up to 72h. Testing in 16 countries in sub-Saharan Africa demonstrated vector susceptibility to chlorfenapyr, including mosquitoes with multiple resistance mechanisms to pyrethroids.