Abstract
BackgroundInsecticide-treated nets (ITNs) undergo a series of tests to obtain listing by World Health Organization (WHO) Prequalification. These tests characterize the bioefficacy, physical and chemical properties of the ITN. ITN procurers assume that product specifications relate to product performance. Here, ITN test methods and their underlying assumptions are discussed from the perspective of the ITN manufacturing process and product characteristics.MethodsData were extracted from WHO Pesticide Evaluation Scheme (WHOPES) meeting reports from 2003 to 2017, supplemented with additional chemical analysis to critically evaluate ITNs bioassays with a focus on sampling, washing and wash resistance, and bioefficacy testing. Production methods for ITNs and their impact on testing outcomes are described.Results and recommendationsITNs are not homogenous products. They vary within panels and between the sides and the roof. Running tests of wash resistance using a before/after tests on the same sample or band within a net reduces test variability. As mosquitoes frequently interact with ITN roofs, additional sampling of the roof when evaluating ITNs is advisable because in nets where roof and sides are of the same material, the contribution of roof sample (20–25%) to the average is less than the tolerance for the specification (25%). Mosquito mortality data cannot be reliably used to evaluate net surface concentration to determine regeneration time (RT) and resistance to washing as nets may regenerate beyond the insecticide concentrations needed to kill 100% of susceptible mosquitoes. Chemical assays to quantify surface concentration are needed. The Wash Resistance Index (WRI) averaged over the first four washes is only informative if the product has a log linear loss rate of insecticide. Using a WRI that excludes the first wash off gives more reliable results. Storage conditions used for product specifications are lower than those encountered under product shipping and storage that may exceed 50 °C, and should be reconsidered. Operational monitoring of new ITNs and linking observed product performance, such as bioefficacy after 2 or 3 years of use, with product characteristics, such as WRI, will aid the development of more robust test methods and product specifications for new products coming to market.
Highlights
Insecticide-treated nets (ITNs) undergo a series of tests to obtain listing by World Health Organization (WHO) Prequalification
To gain PQ listing, manufacturers generate a dossier of data from a series of bioefficacy performance tests that include laboratory studies and small scale field trials based on WHO Pesticide Evaluation Scheme (WHOPES) guidelines [2]
Coating technologies used in commercial nets are based on the mixing of insecticide particle suspensions in water with polymer suspension or a polymerizing suspension that glues the insecticide particles to the yarns, while optionally adding a thin film covering these particles to slow active ingredients (AIs) loss through evaporation, wash-off and abrasion
Summary
Insecticide-treated nets (ITNs) undergo a series of tests to obtain listing by World Health Organization (WHO) Prequalification. The WHOPES guidelines for evaluating long-lasting insecticidal nets (LLIN) were developed in 1999–2000 in a co-operation between the Montpellier WHO reference laboratory at the Institute of Research and Development (IRD) at the Laboratories des Insectes Nuisibles (LIN) and the WHO reference chemical laboratory in Gembloux, Belgium. Their aim was to (1) provide specific and standardized procedures for testing ITNs for malaria vector control, and (2) harmonize the testing procedures carried out to ensure conformity of data used for registration and labelling of ITNs [3].
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