Effect of short-term heating on bioefficacy of deltamethrin-coated long-lasting insecticidal nets

Abstract We recently reported that long-lasting insecticidal nets (LLINs) distributed in Papua New Guinea (PNG) between 2013 and 2019, exhibited severely diminished efficacy to knock down and kill susceptible Anopheles mosquitoes. This coincided with a rise in malaria observed in PNG since 2015. Here we show that LLIN bioefficacy is increased by heating LLINs prior to WHO cone bioassays. Unused LLINs with low bioefficacy, delivered to PNG in 2019, were heated to 120°C for 5 minutes. Cone bioassays were performed before and at 1 h, 7 days and 30 days after heating. This led to a significant increase in 24 h mortality (17% to 61%) and 60 min knock down (31% to 72%). The effect was sustained over 30 days. Bioassays are crucial in quality assurance of LLIN products. Our findings indicate that bioassay results can easily be manipulated. This may have implications for quality assurance procedures used to assess LLINs.