A functional durability framework for insecticide-treated nets (ITNs): integrating fabric and insecticide durability through mathematical modelling of longitudinal field data from Tanzania

Abstract Background Insecticide-treated nets (ITNs) are assumed to remain functional for three-years. However, products within the same class differ in fabric durability and persistence of bioavailable insecticide. Understanding the relative contributions of both factors to ITN efficacy under real-use conditions is essential for optimising product design. Methods Physical and chemical decay were assessed using pyrethroid ITNs collected from a three-year longitudinal field trial in Tanzania. The effects of insecticide content, hole location, and holed surface area on 24h-mortality and blood-feeding of laboratory-reared pyrethroid-susceptible Anopheles gambiae sensu stricto mosquitoes were evaluated using the Ifakara Ambient Chamber Test (I-ACT) bioassay. Entomological effects were analysed using a differential equation model for mosquito mortality and feeding, with a hyperbolic function for holed area and a saturating function for insecticide content, allowing separation of barrier and insecticidal effects. Results Fabric integrity and insecticide content declined over time, with marked variation between brands. Approximately 50% of fabric damage occurred in the bottom quadrant that was typically tucked under mattresses and contributed minimally to mosquito entry. As nets aged, increasing holed area led to higher blood-feeding success among attacking mosquitoes, driven primarily by large holes in lower side panels above the mattress, representing 27% of holed surface area accessible to mosquitoes. Despite this, residual insecticidal activity remained sufficient to maintain low mosquito survival. The model showed excellent fit and enabled independent estimation of barrier and insecticidal effects. Differences in mosquito mortality between brands were largely explained by linear scaling of insecticide concentration, with a shared parameter describing the concentration required to achieve half-maximal effect. Overall, estimated entomological effectiveness declined only modestly with net age. Conclusion Functional durability of ITNs can be understood as the outcome of two analytically separable processes: physical degradation of the fabric and loss of bioavailable insecticide. I-ACT enabled precise measurement of both effects. Even heavily damaged nets retained substantial effectiveness when insecticidal activity remained high and nets continued to be used. This framework provides a practical means for comparing ITN products and strengthening durability monitoring. Future ITN design should prioritise insecticidal performance to kill mosquitoes, while recognising that fabric construction influences both fabric durability and user acceptability. These fabric related characteristics are important to maximise retention, sustained use and ensure cost-effective public health impact.