Coating formulation change leads to inferior bioefficacy of long-lasting insecticidal nets in Papua New Guinea

Introduction: Long lasting insecticidal nets (LLINs) play a key role in reducing malaria transmission in endemic countries. Previously, we demonstrated a substantial decrease in the bioefficacy of LLINs for malaria prevention delivered to Papua New Guinea (PNG) between 2013 and 2019. This coincided with a rise in malaria cases in the country. The present study was aimed at determining the underlying cause of the observed reduced bioefficacy of these LLINs. In particular, we tested the hypothesis that a change in the coating formulation of the respective LLIN product was the cause for the observed reduction in bioefficacy, and had led to completely altered product properties and performance. Methods A set of PermaNet® 2.0 LLIN samples (n = 12) manufactured between 2007 and 2019 was subjected to combustion ion chromatography in order to understand the chemistry of the LLIN polymer coating formulation. In addition, World Health Organization (WHO) LLIN wash tests and cone bioassays were conducted to further characterize the change in product performance that occurred between 2012 and 2013. Results High polymer fluorine content (3 to 4 g/kg) was measured in PermaNet® 2.0 manufactured up to 2012, whereas nets which were manufactured after 2012 did not contain polymer fluorine indicating a coating formulation change from polyfluorocarbon (PFC) to a non-PFC formulation. The coating formulation change as part of the manufacturing process thus directly resulted in the observed reduction in bioefficacy. In addition, the manufacturing change impacted wash resistance leading to a faster reduction in 24h mosquito mortality in the non-PFC coated product. Conclusion A change in coating formulation of PermaNet® 2.0 resulted in reduced product performance in PNG. Post-2012 PermaNet® 2.0 LLINs should not be considered to be the same product as PermaNet® 2.0 LLINs produced prior to and in 2012. Coating formulation changes should be validated not to impact LLIN product performance.