Dose-Effects of Bednets Impregnated with a Natural Pyrethrins Formulation: Persistence, Feeding Inhibition, and Wash Resistance against Anopheles gambiae s.s

Background. This study evaluated dose-response, persistence, irritancy and wash resistance of mosquito nets treated with a synergized natural pyrethrins-formulation against Anopheles gambiae s.s. Methods. Multi-filament polyester netting was impregnated to deposit doses of pyrethrins equivalent to 75 mg/m2, 100 mg/m2, 250 mg/m2, 500 mg/m2, and 1000 mg/m2. Dose-response, persistence, and wash resistance were determined by a 3-minute cone bioassay on three-day-old female A. gambiae s.s. and residual-insecticide persistence was assessed in modified WHO bioassay tunnels and HPLC, respectively. Results. Interaction between dose and time post-exposure to treated net on knockdown (KD) and mortality of mosquitoes was significant (P = 0.0001). Significant likelihood ratio (χ2 = 33.23, P = 0.00266) confirmed post-exposure time effect on KD. Unwashed pyrethrins-impregnated nets at 500 mg/m2 had consistent KD and mortality of > 90% for six months. Nets washed 1–3 times achieved > 85% KD and mortality. A reduced efficacy was observed after wash 4 (P = 0.0001), with a 46% knockdown and 42% mortality. Conclusion. Bio-efficacy to assess KD on Anopheles could be standardized at 15 minutes post-exposure. Pyrethrinsformulation is suitable for nets impregnation due to its persistence (> 6 months) and allows for three washes without compromising bio-efficacy. Further work on improvement of the formulation to allow use of pyrethrins in long life nets is recommended.