Abstract
BackgroundThe effectiveness of long-lasting insecticidal-treated nets (LLINs) and indoor residual spraying (IRS) for malaria control is threatened by resistance to commonly used pyrethroid insecticides. Rotations, mosaics, combinations, or mixtures of insecticides from different complementary classes are recommended by the World Health Organization (WHO) for mitigating against resistance, but many of the alternatives to pyrethroids are prohibitively expensive to apply in large national IRS campaigns. Recent evaluations of window screens and eave baffles (WSEBs) treated with pirimiphos-methyl (PM), to selectively target insecticides inside houses, demonstrated malaria vector mortality rates equivalent or superior to IRS. However, the durability of efficacy when co-applied with polyacrylate-binding agents (BA) remains to be established. This study evaluated whether WSEBs, co-treated with PM and BA have comparable wash resistance to LLINs and might therefore remain insecticidal for years rather than months.MethodsWHO-recommended wire ball assays of insecticidal efficacy were applied to polyester netting treated with or without BA plus 1 or 2 g/sq m PM. They were then tested for insecticidal efficacy using fully susceptible insectary-reared Anopheles gambiae mosquitoes, following 0, 5, 10, 15, then 20 washes as per WHO-recommended protocols for accelerated ageing of LLINs. This was followed by a small-scale field trial in experimental huts to measure malaria vector mortality achieved by polyester netting WSEBs treated with BA and 2 g/sq m PM after 0, 10 and then 20 standardized washes, alongside recently applied IRS using PM.ResultsCo-treatment with BA and either dosage of PM remained insecticidal over 20 washes in the laboratory. In experimental huts, WSEBs treated with PM plus BA consistently killed similar proportions of Anopheles arabiensis mosquitoes to PM-IRS (both consistently ≥ 94%), even after 20 washes.ConclusionCo-treating WSEBs with both PM and BA results in wash-resistant insecticidal activity comparable with LLINs. Insecticide treatments for WSEBs may potentially last for years rather than months, therefore, reducing insecticide consumption by an order of magnitude relative to IRS. However, durability of WSEBs will still have to be assessed in real houses under representative field conditions of exposure to wear and tear, sunlight and rain.
Highlights
The effectiveness of long-lasting insecticidal-treated nets (LLINs) and indoor residual spraying (IRS) for malaria control is threatened by resistance to commonly used pyrethroid insecticides
Overall, long-lasting insecticidal nets (LLINs) supplemented with window screens and eave baffles (WSEBs), co-treated with PM and binding agents (BA), killed mosquitoes in at least equal proportions to LLINs supplemented with conventional IRS using the same organophosphate active ingredient, and at least matched the World Health Organ‐ ization (WHO) wash-resistance requirements for pyrethroid-treatment of LLINs
While this study is limited in that it did not use a complete Latin Squares design, because IRS was sprayed in two huts and cannot be rotated like LLINs and WSEBS, mortality is a robust binary outcome less likely to vary with hut because these were of a standardized design [25, 26] and any variations in vector density will be reflected in both the nominator and the denominator
Summary
The effectiveness of long-lasting insecticidal-treated nets (LLINs) and indoor residual spraying (IRS) for malaria control is threatened by resistance to commonly used pyrethroid insecticides. Mosaics, combina‐ tions, or mixtures of insecticides from different complementary classes are recommended by the World Health Organ‐ ization (WHO) for mitigating against resistance, but many of the alternatives to pyrethroids are prohibitively expensive to apply in large national IRS campaigns. The effectiveness of IRS and LLINs are threatened by insecticide resistance against the four insecticide classes (carbamates, pyrethroids, organo-chlorines, organophosphates) that already have full recommendations from the World Health Organization (WHO) [4,5,6,7]. The current global plan for insecticide resistance management (GPIRM) recommends the use of mixtures of insecticides with complimentary mode of actions for LLINs [4] some of which have recently been developed [8]. Many countries have developed insecticide resistance management plans aligned with the GPIRM, but few of them have implemented such approaches in practice, mostly because they are too expensive to implement across national scales [9,10,11]
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