Household washing agents and insecticide-treated net bioefficacy: evidence from a rapid review and laboratory evaluation

Introduction. Household washing of insecticide-treated nets (ITNs) influences their insecticidal durability. The World Health Organization (WHO) recommends the Collaborative International Pesticides Analytical Council (CIPAC) washing agent for standardised laboratory evaluation of ITNs. However, evidence on whether CIPAC reflects household washing practices remains limited. This study synthesised evidence on household ITN washing agents used across Africa and compared mosquito mortality outcomes after laboratory washing with agents commonly used in Tanzania versus CIPAC.

Materials and Methods. A rapid review of studies reporting ITN washing practices in Africa was conducted to identify commonly used household washing agents. An exploratory laboratory study was then performed using an alpha-cypermethrin-incorporated ITN washed with Jamaa®, Family®, OMO®, water only, or CIPAC washing agent across 0–20 wash cycles. Bioefficacy was assessed using WHO cone bioassays against pyrethroid-susceptible An. gambiae and resistant An. arabiensis. Binomial logistic regression assessed the effect of washing agent on mortality, adjusting for wash number.

Results. Thirty-four studies (30,887 household responses) were included in the rapid review. Bar soaps accounted for 59.0% of reported household washing agents. Mortality did not differ significantly between household washing agents and CIPAC for either mosquito strain (all p > 0.05). Water-only washing resulted in significantly higher mortality than CIPAC among susceptible (OR = 60.1, 95% CI: 35.1-103.1; p < 0.001) and resistant mosquitoes (OR = 2.0, 95% CI: 1.4-2.8; p < 0.001). Mortality declined significantly with increasing wash number regardless of washing agent.

Conclusions. Bar soaps were the most frequently reported agents used for ITN washing across the settings in Africa included in the review. No significant differences in mosquito mortality were observed between household washing agents and CIPAC in this exploratory laboratory study. These data provide preliminary support for CIPAC as a proxy for household soap where feasible, but further confirmatory studies with a wider range of ITNs and settings are needed.