Does washing insecticide-treated nets 20 times for experimental hut evaluations provide a suitable proxy for their end-of-life performance under household conditions?

Background: Insecticide-treated nets (ITNs) are washed 20 times as part of experimental hut trials to simulate the loss of active ingredient (AI) occurring over their intended 3-year lifespan and estimate insecticidal durability. The ability of the 20-wash method to predict the end-of-life performance of ITNs has not been empirically validated.

Methods: We performed an experimental hut trial to compare the efficacy of new ITNs unwashed and washed 20 times to field-aged ITNs withdrawn from households 3 years post-distribution against a pyrethroid-resistant vector population in Covè, Benin. Four products from pyrethroid-only (Interceptor), pyrethroid-piperonyl butoxide (PermaNet 3.0), pyrethroid-pyriproxyfen (Royal Guard) and pyrethroid-chlorfenapyr (Interceptor G2) ITN types were tested. Net pieces were tested in bioassays and sent for chemical analysis to assess differences in surface AI bioavailability and total chemical content between washed and field-aged nets. Susceptibility bioassays were also performed to assess insecticide resistance in the Covè vector population.

Results: Mosquito mortality in experimental huts was similar or slightly higher with field-aged nets than washed nets with Interceptor (11% vs. 10%, p = 0.339, OR = 1.19, 95% CIs [0.84, 1.69]), PermaNet 3.0 (12% vs. 18%, p < 0.001, OR = 1.78, 95% CIs [1.34, 2.38]) and Royal Guard (9% vs. 14%, p = 0.076, OR = 1.33, 95% CIs: [0.97, 1.83]). Likewise, field-aged Royal Guard induced a similar reduction in fertility to washed Royal Guard (22% vs. 29%, p = 0.066). In contrast, mortality was significantly lower with field-aged nets Interceptor G2 compared to washed nets (54% vs. 19%, p < 0.001, OR = 0.18, 95% CIs [0.14, 0.24]). Blood-feeding inhibition was higher with field-aged nets than washed nets across all ITN types. Retention of non-pyrethroid AIs was lower than for the pyrethroid, particularly with field-aged nets (PermaNet 3.0 (roof): 25% vs. 68%, p < 0.001, Royal Guard: 27% vs. 53%, p < 0.001, Interceptor G2: 14% vs. 39%, p < 0.001).

Conclusions: In this setting, the 20-wash method provided a suitable proxy for the end-of-life killing and sterilising performance of Interceptor, PermaNet 3.0 and Royal Guard® in experimental huts. In contrast, washing overestimated the end-of-life performance of Interceptor G2 for mortality and underestimated the personal protection of all field-aged ITNs.