Insufficient duration of insecticidal efficacy of Yahe insecticide-treated nets in Papua New Guinea

Background: Insecticide-treated nets (ITNs) are the backbone of antimalarial vector control in Papua New Guinea (PNG). Over recent years it was recognised that the quality and performance of ITNs has been decreasing, which is likely contributing to the stagnation in the malaria control effort in PNG. Here, we present results from the first 24 months of a durability study with the ITN product Yahe LN® in PNG. Methods: The durability study was conducted in four villages on the northern coast of PNG, in an area with high malaria transmission, following WHO-recommended methodologies adapted to the local scenario. A cohort of individually identifiable Yahe ITNs were distributed by the PNG National Malaria Control Program from October to December 2021. Insecticidal efficacy of the ITNs was tested using cone bioassays at baseline and at 6 months intervals, alongside evaluation of physical integrity and the proportion of ITNs lost to follow-up. Questionnaires were used to collect information on ITN end user behaviour such as the frequency of use and washing. Results: We observed gradual uptake and replacement of previous campaign nets by the communities such that at 6 months 45% of all newly distributed nets were in use in their designated households. Insecticidal efficacy of the Yahe nets, expressed as the percent 24h mortality of fully pyrethroid susceptible colony mosquitoes, decreased from 91% to 45% within the first 6 months of distribution, even though >90% of study nets had never been washed. Insecticidal efficacy decreased further to <20% at 24 months. ITNs accumulated physical damage (holes) at a rate similar to previous studies, and 35% were classified as too torn by 24 months. ITNs were lost to follow-up such that 61% of campaign nets were still present at 24 months. Wash resistance assays indicated rapid reduction in insecticidal performance with each consecutive wash such that 24h mortality was below 20% after 10 washes. Discussion and Conclusion: Yahe ITNs are not performing as per label claim in an area with fully pyrethroid susceptible vectors and should be investigated more widely and in other settings. The mass distribution of low and variable quality ITN products is one of the major ongoing challenges for global malaria control in the last decade.