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.

Abstract

BACKGROUND:

Following 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.

METHODS:

Bottle bioassay tests were conducted using five concentrations of chlorfenapyr at 12.5, 25, 50, 100, and 200 µg AI/bottle in 10 countries in sub-Saharan Africa using 13,639 wild-collected Anopheles gambiae sensu lato (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 100 µg AI/bottle in 16 countries using 23,422 wild-collected, pyrethroid-resistant An. gambiae s.l. (259 vector populations). Exposure time was 60 min, with mortality recorded at 24, 48 and 72 h after exposure.

RESULTS:

Median mortality rates (up to 72 h after exposure) of insectary colony mosquitoes was 100% at all five concentrations tested, but the lowest dose to kill all mosquitoes tested was 50 µg AI/bottle. The median 72-h 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 200 µg AI/bottle, respectively. Log-probit analysis of the five concentrations tested determined that the LC95 of wild An. gambiae s.l. was 67.9 µg AI/bottle (95% CI: 48.8-119.5). The discriminating concentration of 203.8 µg AI/bottle (95% CI: 146-359) was calculated by multiplying the LC95 by three. However, the difference in mortality between 100 and 200 µg AI/bottle was minimal and large-scale testing using 100 µg 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 72 h.

CONCLUSIONS:

This study determined that 100 or 200 µg AI/bottle chlorfenapyr in bottle bioassays are suitable discriminating concentrations for monitoring susceptibility of wild An. gambiae s.l., using mortality recorded up to 72 h. Testing in 16 countries in sub-Saharan Africa demonstrated vector susceptibility to chlorfenapyr, including mosquitoes with multiple resistance mechanisms to pyrethroids.

• Authors: Richard M Oxborough, Aklilu Seyoum, Yemane Yihdego, Joseph Chabi, Francis Wat'senga, Fiacre R Agossa, Sylvester Coleman, Samdi Lazarus Musa, Ousmane Faye, Michael Okia, Mohamed Bayoh, Evelyne Alyko, Jean-Desire Rakotoson, Hieronymo Masendu, Arthur Sovi, Libasse Gadiaga, Bernard Abong'o, Kevin Opondo, Ibrahima Baber, Roch Dabire, Virgile Gnanguenon, Gedeon Yohannes, Kenyssony Varela, Etienne Fondjo, Jenny Carlson, Jennifer S Armistead, Dereje Dengela
• Journal: Malaria journal (Malar J) Vol. 20 Issue 1 Pg. 316 (Jul 14 2021) ISSN: 1475-2875 [Electronic] England
• PMID: 34261475 (Publication Type: Journal Article)
• Copyright: © 2021. The Author(s).

Chemical References

• Insecticides
• Pyrethrins
• chlorfenapyr

Topics

• Animals
• Anopheles (drug effects)
• Dose-Response Relationship, Drug
• Insecticide Resistance
• Insecticide-Treated Bednets
• Insecticides (pharmacology)
• Pyrethrins (pharmacology)