CDC bottle bioassays

In 2016 and 2017, pyrethroid resistance intensity was determined using Centers for Disease Control and Prevention (CDC) bottle bioassays, which involved coating 250 ml glass bottles with 1×, 2×, 5× and 10× the diagnostic concentration of permethrin (21.5, 43, 107.5 and 215 µg active ingredient (ai)/bottle) and deltamethrin (12.5, 25, 62.5, 125 µg ai/bottle) [10, 11]. For all bottle bioassay tests, a mouth aspirator was used to introduce 20–25 female An. gambiae (s.l.) aged 3–5 days into each bottle, with four bottles tested per insecticide dose. In 2018, a comparison of the CDC bottle bioassay and the World Health Organization (WHO) tube test for pyrethroid intensity was conducted in two sites using permethrin and deltamethrin.

Chlorfenapyr susceptibility was determined using bottles treated with 100 µg ai/bottle. At the time of testing there was no published guidance regarding chlorfenapyr susceptibility procedures or diagnostic concentrations, while work coordinated by the WHO was ongoing to develop a suitable procedure. The dose of 100 µg ai/bottle was used as an interim diagnostic concentration based on unpublished data shared between the manufacturer, BASF, PMI and CDC. Wild-caught An. gambiae (s.l.) from Djenné, Mopti, Bandiagara and Bankass were tested for susceptibility to chlorfenapyr. Testing was conducted simultaneously with a susceptible insectary strain (An. coluzzii Ngousso).

A bottle coated with 1 ml acetone was used as a control. Vials of permethrin and deltamethrin technical grade active ingredient (TGAI) were provided by CDC at pre-measured concentrations for intensity tests. A vial of chlorfenapyr TGAI was provided by BASF. The diagnostic time was 30 min for pyrethroids, based on CDC guidelines [12]. Chlorfenapyr tests were conducted for the diagnostic time of 60 min, with mortality recorded every 24 h for 72 h.