Durability of Yorkool® long-lasting insecticidal nets in Benin: temporal evolution of deltamethrin concentration assessed using portable C-Vue HPLC chromatography during household use

The effectiveness of long-lasting insecticidal nets (LLINs) depends on both chemical durability and sustained biological efficacy under operational conditions. However, insecticide depletion over time does not always correspond directly to loss of entomological effectiveness. This study assessed the relationship between deltamethrin content and biological efficacy of Yorkool ® LLINs over 18 months of household use in Benin. A longitudinal study was conducted in southern Benin to monitor Yorkool ® LLINs at baseline (T0), 7 months (T7), 12 months (T12), and 18 months (T18). Household practice data were collected and analyzed at all time points except T0. However, chemical and biological evaluations were performed only at T0, T12, and T18. Deltamethrin surface concentrations were quantified using a portable C-Vue HPLC device and expressed in mg/m 2 . Biological efficacy was assessed using WHO cone bioassays with susceptible Anopheles gambiae Kisumu mosquitoes, measuring knock-down at 60 min and mortality at 24 h. Chemical and biological trends were analyzed comparatively. Mean deltamethrin concentration declined progressively from approximately 55 mg/m 2 at T0 to 50.7 mg/m 2 at T12 and 45.3 mg/m 2 at T18. Despite this chemical depletion, cone bioassays showed consistently high knock-down and mortality rates, with most nets meeting WHO criteria for optimal or minimal effectiveness. The relationship between insecticide concentration and mosquito mortality was positive but non-linear, suggesting a biological threshold effect. Yorkool ® LLINs maintained high biological efficacy during the first 18 months of use despite moderate chemical depletion. These findings suggest that loss of insecticide content does not immediately translate into loss of functional performance. However, the absence of follow-up up to 36 months and the use of a susceptible mosquito strain limit extrapolation to full operational lifespan and resistance settings. Extended durability studies are needed to define the functional failure threshold of LLINs.