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Mechan, Frank (2022) The durability of long-lasting insecticidal nets treated with and without piperonyl butoxide: implications for bioefficacy and personal protection, Thesis (Doctoral), Liverpool School of Tropical Medicine.
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F Mechan -Final Thesis.pdf - Accepted Version Download (22MB) | Preview |
Abstract
Long-lasting Insecticidal nets (LLINs) are a fundamental component of malaria control strategies, yet they accumulate physical damage and lose insecticide over time. However, the impact of this deterioration on bioefficacy and bloodfeeding is poorly described. There is growing evidence that the operational lifespan of LLINs is less than the expected three years in some settings. Consequently, there is a need to assess the durability of LLIN products to determine the appropriate timescale of distribution. The emergence of pyrethroid-resistance in sub-Saharan Africa has motivated the development of LLIN designs that contain the synergist piperonyl butoxide to restore susceptibility. The aim of this study was to quantify the durability of LLINs with and without piperonyl butoxide in operational conditions.
The nets assessed in this study were provided from collections performed as part of a randomised control trial to compare pyrethroid-only and pyrethroid-PBO nets. Each cluster received a pyrethroid-PBO net ( ‘Olyset Plus’ or ‘PermaNet 3.0’) or pyrethroid-only equivalent (‘Olyset Net’ or ‘PermaNet 2.0’). Samples were assessed at baseline, 12 months, and 25 months post distribution. The chemical content of pyrethroid and PBO of nets was assessed using high-performance liquid chromatography. The number, size, and location of holes on each net was assessed visually. Bioefficacy was assessed by exposing pyrethroid-resistant Anopheles to samples using WHO cone and wireball bioassays. To evaluate the impact of hole location on protective effect, behavioural experiments with free-flying pyrethroid-resistant Anopheles around human-occupied holed nets were conducted. Bloodfeeding success, 1hr knockdown, and 24hr mortality were compared for nets with no hole, hole in the top, or hole in the side.
Pyrethroid content remained relatively stable across timepoints. However, the PBO content of both Olyset Plus and PermaNet 3.0 declined over the same period, falling by 55% (P<0.001) and 58% (p<0.001) respectively after 25 months. Both PBO nets were highly effective against pyrethroid-resistant An. gambiae when new but declined over time. After 25 months, 24hr mortality was 22.92% for Olyset Plus and 46.6% for PermaNet 3.0. There was a strong correlation between PBO content and mortality. There was no difference in any physical durability metric between any of the LLIN products evaluated, at any timepoint. In behavioural assays, holes on the top of the net had a much greatly risk of bloodfeeding compared to holes on the side (30.65% compared to 4.13%, P=0.021) after one hour. There was no difference in bloodfeeding success between Olyset Plus and Olyset Net (p=0.076). Very few bloodfed mosquitoes survived the assay with Olyset Plus, with 96.1% of all bloodfed mosquitoes dying after 24 hours despite very low morality for those that did not bloodfeed (<5%). Finally, when attempting to escape the net after bloodfeeding, mosquitoes were twice as likely to get out of a net with a hole on the top than on the side.
These findings indicate that pyrethroid-PBO bed nets were highly effective against pyrethroid-resistant mosquitoes in Uganda when new but efficacy declined sharply over time. For both Olyset Plus and PermaNet 3.0, this rapid reduction in bioefficacy correlated with a steep decline in PBO content. Given that pyrethroid-PBO nets are becoming widespread, this finding is highly concerning and requires further investigation in other settings. Moreover, the rapid reduction in bioefficacy indicates a distribution cycle shorter than three years may be prudent. Physical integrity outcomes were very similar for the PBO nets and their pyrethroid-only equivalents. Current WHO durability assessment guidelines consider all holes equally when evaluating serviceability for use, yet here it was observed that holes on the top of the net were a 10x greatest risk for mosquito entry and bloodfeeding compared to the side. Consequently, guidelines for assessing survivability should be updated to appropriately weight holes on the top.
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