The durability of pyrethroid LLINs with and without Piperonyl butoxide: results from a randomised control trial in Uganda.

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Background & MethodsThe use of long-lasting insecticidal nets (LLINs) in malaria control strategies is threatened by growing pyrethroid resistance in Anopheles vectors. To restore susceptibility, LLINs supplemented with the synergist piperonyl butoxide (PBO) have been developed. However, there is limited data on their operational lifespan. Here, the chemical integrity, bioefficacy and fabric integrity of two PBO-LLINs (OlysetPlus and PermaNet3.0) were compared with their pyrethroid-only equivalents (Olyset and PermaNet2.0) across two years of use.Nets used for 0, 12 and 25-months were provided from a cluster-randomised trial embedded into Uganda's national LLIN campaign. Active ingredients were quantified using high-performance liquid chromatography, bioefficacy assessed with WHO cone-bioassays against pyrethroid-resistant An. gambiae s.s., and physical integrity by measuring holes.Results & DiscussionThe pyrethroid content of all LLINs assessed was stable across the two-year period however the PBO content of OlysetPlus and PermaNet3.0 fell sharply, declining by 55% (p<0.001) and 58% (p<0.001) respectively. In bioassays, PermaNet3.0 achieved 100% mortality at baseline but declined to 46.25% (CI:38.21-54.28) after two years (p<0.001). Mortality with OlysetPlus was very low at all timepoints. The pyrethroid-only LLINs failed to kill at all timepoints. There was no difference in fabric integrity between LLIN products. Damage varied by housing type: mean damage was 443cm2 (CI:161-1258cm2) for thatched-roofed vs 32cm2 (CI:18-57cm2) for iron-roofed (p=0.003). These findings from Uganda indicate PBO content in these PBO-LLINs declines faster than pyrethroid content, accompanied by a reduction in bioefficacy. Additionally, housing improvement was associated with reduced physical damage which may limit accessibility to mosquitoes.