Abstract
Malaria control programs implementing Long-Lasting Insecticidal Nets (LLINs) are encouraged to conduct field monitoring of nets’ survival, fabric integrity and insecticidal bio-efficacy. The reference method for testing the insecticide activity of LLINs needs 100 two-to-five-day-old female mosquitoes per net, which is highly resource-intensive. We aimed at identifying an alternative protocol, using fewer mosquitos, while ensuring a precision in the main indicator of ±5 percentage points (pp). We compared different laboratory methods against the probability of the LLIN to fail the test as determined by a hierarchical Bayesian model. When using 50 mosquitoes per LLIN and considering mortality only instead of mortality or knock-down as validity criteria, the average error in the measure of the proportion of nets considered as valid was 0.40 pp. The 95% confidence interval of this value never exceed 5 pp when the number of LLIN tested was ≥40. This method slightly outperforms the current recommendations. As a conclusion, testing the bio-efficacy of LLINs with half as many mosquitoes provides a valid evaluation of the proportion of valid LLINs. This approach could increase entomology labs’ testing capacity and decrease costs, with no impact in the decision process for public health purposes.
Highlights
Mass distribution of Long Lasting Insecticidal Nets (LLIN) is the vector control intervention most used against malaria in sub-Saharan Africa, covering 44% of the population at risk, far ahead of indoor residual spraying, which covers only 7%1
Over the 12 scenarios, the average accuracy varied between 88.0% and 97.7%, the average specificity between 89.7% and 98.4%, the average sensitivity between 74.8% and 95.4%, and the average absolute difference between observed and estimated proportion of valid LLINs varied between 0.0 pp and 4.4 pp (Figs 2–4 and Table 2); average refers to the mean throughout the manuscript
The results showed that using only two cones per side of LLIN instead of four, and considering mortality only instead of mortality or knocked down (KD) as validity criteria, the mean error in the measure of the proportion of nets considered as valid was less than 0.5 pp, and when testing at least 40 LLINs, it is very unlikely that this indicator deviates more than 5 pp from its expected value
Summary
Mass distribution of Long Lasting Insecticidal Nets (LLIN) is the vector control intervention most used against malaria in sub-Saharan Africa, covering 44% of the population at risk, far ahead of indoor residual spraying, which covers only 7%1. Changes to any of these three parameters are expected to affect the operational bio-efficacy of LLINs. Malaria control programs and donors more and more want to test the LLINs for durability and insecticide bio-efficacy, in order to evaluate their effectiveness, before and after their distribution to the population. A batch of nets (30–50 nets per brand/product) passes the phase III if 80% of the batch are valid 3 years after distribution These guidelines provide no guidance for the interpretation of this key indicator - the proportion of valid LLINs - for observational effectiveness studies (phase IV). A LLIN needs 100 susceptible, two- to five-day-old female mosquitoes to be tested Applying this protocol to a large number of LLINs requires considerable logistics and a high-performance entomology lab in order to breed and grow enough mosquitoes. If the number of mosquitoes needed to test the bio-efficacy of LLINs could be reduced, the human and animal resources, the duration, and the costs of an evaluation would be reduced in equal proportion
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