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Redefining the Wash Resistance Index to predict insecticidal durability of mosquito nets

Ole Skovmand¹, Duoc M Dang¹, Trung Q Tran^2,3

¹VectorisIQ SARL, Castelnau le Lez, France.

Malariaworld Journal

|February 13, 2026

Abstract

Background:

Insecticide-treated nets (ITNs) are a core malaria vector control intervention recommended by the World Health Organization (WHO). To obtain WHO endorsement, ITNs must demonstrate sustained bio-efficacy after repeated washing through standardised laboratory and semi-field evaluations. Wash resistance (WR) is typically evaluated over 20 washes (WR20) to approximate three years of use. Routine WR20 testing is impractical for quality control. To address this, the Collaborative International Pesticides Analytical Council (CIPAC) introduced the Wash Retention Index (WRI), based on four consecutive daily washes. The WRI was defined as the fourth root of the ratio of insecticide content after four washes to the initial content and adopted as a quality-control specification. This article examines the WRI and the prediction of WR20 when including or excluding the first wash with the aim to define a WRI method that more accurately reflects WR20, while remaining practical. Because surface insecticide may increase during storage due to diffusion from the polymer, the effect of excluding the first wash from index calculations to reduce storage-related variability was assessed.

Materials And Methods:

WR data were extracted from WHO Pesticide Evaluation Scheme (WHOPES) reports, and WRI values calculated assuming 1-day washing intervals. Additional data were generated from polyester and polyethylene nets with insecticide content quantified.

Results:

When WR20 was regressed on WRI (including the first wash), the model fit was weaker and significant effects of insecticide chemistry and polymer were retained, indicating that WRI does not capture intrinsic wash-off kinetics. In contrast, new WR (excluding the first wash) explained more variance in WR20 (adjusted R²= 0.76 vs 0.71), with a slope approaching proportionality (0.66 vs 0.31) and with no residual effects of chemistry or polymer.

Conclusions:

Excluding the first wash from WRI improves prediction of WR20. First-wash loss reflects production and storage effects rather than intrinsic wash resistance. Wash resistance can be more intuitively expressed as the Wash-Off Rate Index (WORI), representing average wash-off between washes 1-5.

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