The physical and insecticidal durability of two dual active ingredient nets (Interceptor® G2 and Royal Guard®) in Benin, West Africa; results for a durability study embedded in a cluster randomised controlled trial

Background: Studies evaluating the physical and insecticidal durability of dual active ingredient (AI) insecticide-treated nets (ITNs) are essential for making programmatic decisions for their deployment. We performed a prospective study embedded in a cluster randomised controlled trial (cRCT) to evaluate the attrition, fabric integrity, and insecticidal durability of Interceptor® G2 (alpha-cypermethrin-chlorfenapyr) and Royal Guard® (alphacypermethrin-pyriproxyfen), compared to Interceptor® (alpha-cypermethrin) in Benin. Methods A total of 2,428 study nets in 1,093 randomly selected households in 5 clusters per arm of cRCT were monitored for ITN attrition and fabric integrity every 6–12 months post-distribution. Householders were further surveyed to investigate non-study net use and their preference for ITN fabric types used in the study nets. A second cohort 120 nets per ITN type withdrawn every 12 months were assessed for chemical content and insecticidal activity in laboratory bioassays. Alpha-cypermethrin bioefficacy was investigated in WHO cone bioassays and tunnel tests using susceptible Anopheles gambiae Kisumu while chlorfenapyr and pyriproxyfen bioefficacy were investigated using the pyrethroid resistant An coluzzii Akron strain. Chlorfenapyr bioefficacy was assessed in tunnel tests while pyriproxyfen activity was assessed in cone bioassays as the reduction in fertility of blood-fed survivors using ovary dissection. Bioefficacy was expressed as the proportion of ITNs passing predetermined criteria i.e knock-down ≥ 95% or 24hrs mortality ≥ 80% or reduction in fertility ≥ 50%. Results Overall ITN survivorship was 52% at 24 months and reduced to 15% at 36 months. Median ITN survival time was lower with Royal Guard® relative to Interceptor® (1.6 years vs . 2.3 years HR = 1.49, 95% CI = 1.36–1.66, p < 0.001) and Interceptor® G2 (1.6 years vs 2.1 years; HR = 1.33, 95% CI = 1.20–1.47, p < 0.001). Householders overwhelmingly preferred polyester nets relative to polyethylene nets (96%) and more Royal Guard® nets were replaced with spare polyester nets from previous campaigns. All Royal Guard® nets passed efficacy criteria for alphacypermethrin at all time points (100%) while ITN pass rates after 24 months had reduced to < 40% for pyriproxyfen and chlorfenapyr. The chemical content analysis showed a higher loss rate of the non-pyrethroid insecticides relative to the pyrethroids in each dual AI ITN; 74% vs 47% for Royal Guard® and 85% vs 63% for Interceptor® G2 at 36 months. Conclusion The median ITN survival time for Interceptor® G2 (2.1 years) and Royal Guard® (1.6 years) in Benin is substantially lower than the 3 years. Royal Guard® nets were discarded more quickly by householders partly due to their low preference for polyethylene nets. The insecticidal activity of the non-pyrethroid insecticides in both dual AI ITNs was short-lived compared to alpha-cypermethrin. The results corroborate the findings from the cRCT in Benin.