Biology subjects

Mechan, F.

Publications and source records attributed to Mechan, F..

6 recordsLinked to original sources

bioRxiv2026

Multi-site phenotypic characterisation of Kisumu colonies for comparison of insecticide susceptibility between testing centres

Testing of insecticides and insecticide-based vector control tools relies on the use of bioassays whereby insects are exposed and mortality, or another relevant entomological endpoint, is measured. To be able to compare results between products and across time and space standard laboratory strains are used. Insecticide-susceptible strains are often used as comparators in studies involving insecticide-resistant populations. Kisumu is a long-established, widely-used insecticide-susceptible strain of Anopheles gambiae. In this study, 16 facilities across different countries exposed their Kisumu colonies aged 3-5 days to four insecticides (permethrin, alpha-cypermethrin, DDT and pirimiphos-methyl) at a range of concentrations in WHO tube assays to assess susceptibility. Insecticide-treated filter papers were made at the Liverpool School of Tropical Medicine and sent to testing centres, along with a standard protocol and testing conditions requirements, to standardise testing as much as possible. A first observation was that DDT results were affected by the use of Risella oil, the WHO-recommended carrier oil but immiscible in acetone, leading to uneven coating of papers and increased variability in bioassay results (40x variance between colonies). Bioassay data generated by the testing facilities suggest a wide range of susceptibility to different insecticides between colonies of Kisumu: LC50 values between colonies differed, though not significantly, by up to 4x for permethrin, 6x for pirimiphos-methyl, and 15x for alpha-cypermethrin. Some colonies are classified as insecticide resistant based on the WHO definition of <98% mortality on exposure to a discriminating concentration. The assumption that Kisumu is a standard susceptible strain, consistent between facilities is challenged, which has implications when comparing results between studies. We propose genotypic analysis of the colonies to identify signs of resistance markers indicative of contamination from resistant strains or wild mosquitoes, and generation of guidance to support the maintenance and monitoring of susceptible mosquito strains.

zoology↗

bioRxiv2026

Multi-centre laboratory study to determine discriminating concentrations for broflanilide and isocycloseram resistance monitoring in mosquitoes

1In 2024-2025 a multi-centre study involving seven international laboratories was conducted with the support of the World Health Organization (WHO). The aim of the study was to establish and validate discriminating concentrations (DCs) in WHO bottle bioassays for monitoring susceptibility to broflanilide and isocycloseram in Anopheles gambiae s.s., An. funestus, An. stephensi and Aedes aegypti. The following values are recommended for adoption as DCs for broflanilide: 10 {micro}g/bottle for An. gambiae and Ae. aegypti, 15 {micro}g/bottle for An. funestus, and 25 {micro}g/bottle for An. stephensi. The recommended DCs for isocycloseram are 15 {micro}g/bottle for Ae. aegypti, 30 {micro}g/bottle for An. gambiae, 50 {micro}g/bottle for An. stephensi, and 60 {micro}g/bottle for An. funestus. Based on the experiences of conducting this study, which represents the application of a generic protocol for establishing discriminating concentrations produced by the WHO, technical recommendations are made on the generation and analysis of DC data for insecticides in future.

zoology↗

bioRxiv2025

Refining the resistance-to-damage (RD) score to predict operational Insecticide-Treated Net lifespan and identify paths to innovation

BackgroundInsecticide Treated Nets (ITNs) vary greatly in their physical characteristics yet the relationship with field durability remains poorly understood. To address this, we refined the existing resistance-to-damage (RD) metric for greater predictive power in estimating operational lifespan and to identify key avenues of innovation. MethodsThe original resistance-to-damage (oRD) score was calculated by comparing four physical attributes; snag strength, bursting strength, hole enlargement resistance and abrasion resistance; against aspirational values previously determined to provide resistance to routine damage. Here, the weighting of each is improved to provide a better prediction of field life, resulting in a weighted resistance-to-damage (wRD) score. Durability data from 37 field sites (across 14 countries) involving 10,000 campaign ITNs was used to quantify the relationship between physical characteristics and field life. Bivariable linear regression models were used to assess the correlation between RD scores and ITN survival time. A Cox regression survival analysis was applied to examine the impact of the RD score on physical survival, adjusting for net care and environment variables (Risk Index), evaluating the risk of ITN failure from the time they were first hung. ResultsThe bivariate analysis, not adjusting for differences between sites, shows that the wRD was a better predictor for net lifespan than oRD (wRD r2= 0.07 oRD r2= 0.01). In the multivariate analysis, adjusting for risk index, the wRD model was able to explain more than 70% of variability in net survival (wRD r2 = 0.74). Consequently, when comparing nets with different wRD scores in the same site, a 10-point increase in wRD score was associated with a 3.9-month gain in median survival (p=0.016). Increasing wRD from 35 to 70, the range of current products, increases median field life by 13.65 months (95% CI: 3.03-24.16). An ITN product with an wRD score of 68 is associated with a median lifespan of 2.7 years. The increase in predictive power of the wRD score came primarily from increasing the weighting of the hole enlargement resistance variable, which is a strong differentiator between ITN products. In contrast, abrasion-resistance was a negligible contributor to predictive power. DiscussionThe wRD score more accurately predicts ITN survival in the field than the oRD score. These findings highlight that resistance to hole enlargement is the primary differentiating factor in predicting field life, with some products performing far better than others. Snag and bursting strength remain essential characteristics yet are similarly low compared to aspirational values across available ITN products. All ITNs WHO prequalified in 2020 perform poorly on snag strength, indicating this as a core bottleneck in achieving longer operational ITN lifespan. Consequently, resistance to snagging and hole enlargement should be keys targets for future innovation to enhance the physical durability of ITNs.

ecology↗

bioRxiv2024

Influence of mould growth and outdoor exposure on the efficacy of attractive targeted sugar baits in western kenya.

IntroductionAttractive targeted sugar baits (ATSBs) are effective against Anopheles mosquitoes in semiarid climates with low humidity. High humidity, however, promotes growth of moulds on the surface of ATSBs. The impact of mould on ATSB efficacy against malaria vectors remains unknown. This study explored how mould growth affects the performance of ATSB version 1.2 by comparing mouldy stations from exposed environments to non-mouldy stations from protected settings through laboratory bioassays with the local malaria vector, Anopheles arabiensis. MethodsOne hundred ATSB stations were deployed in Asembo, Rarieda-Subcounty, Siaya County, with six samples (three mouldy from exposed locations and three non-mouldy from protected locations) collected monthly for laboratory bioassays. These were tested alongside three new laboratory-kept ATSBs and two negative controls (water only and 77% sugar solution with water) to assess mosquito feeding and mortality over 48 hours. ResultsThis study found that after 12 months of outdoor exposure, the mouldiest ATSBs from exposed locations showed a non-significant reduction in Anopheles arabiensis feeding rates compared to the least mouldy ATSBs from protected locations 57.42% (95% CI: 45.64-68.85) vs. 74.40% (95% CI: 64.56-82.50), respectively (P =0.062). Mosquito mortality significantly declined on mouldy ATSBs compared to laboratory controls (95% CI: 92.23-97.48) vs. 98.70% (95% CI: 97.87-99.30) respectively (P = 0.002). In contrast, protected (non-mouldy) ATSBs showed only a slight reduction in mortality compared to controls 95.94% (95% CI: 90.42-97.46) vs. 98.91% (95% CI: 97.67-99.60) respectively (P = 0.009). ConclusionThis study provides evidence that environmental exposure post-deployment slightly reduced the efficacy of ATSBs in controlling Anopheles arabiensis, particularly beyond the recommended 6-month period. Although mould may have contributed to this reduction over 12 months, no significant difference was found between mouldy and non-mouldy ATSBs. However, mould invasion and community concerns highlight the need to replace mouldy stations to maintain effectiveness and safety.

zoology↗

bioRxiv2022

Behaviour of pyrethroid-resistant Anopheles gambiae at the interface of two dual active-ingredient bed nets, assessed by room-scale infrared video tracking

BackgroundThe success of Insecticide Treated Bednets (ITNs) for malaria vector control in Africa relies on the behaviour of the major malaria vectors, Anopheles species. Research into mosquito behavioural traits influencing the performance of ITNs has focused largely on time or location of biting. Here we investigated less tractable behaviours including timings of net contact, willingness to refeed and longevity post exposure to two next-generation nets, PermaNet(R) 3.0 (P3) and Interceptor(R) G2 (IG2) in comparison with a standard pyrethroid only net (Olyset (OL)) and an untreated net. MethodsSusceptible and resistant Anopheles gambiae mosquitoes were exposed to the nets with a human volunteer host in a room scale assay. Mosquito movements were tracked for two hours using an infrared video system, collecting flight trajectory, spatial position and net contact data. Post-assay, mosquitoes were monitored for a range of sublethal insecticide effects. ResultsOL, P3 and IG2 all killed over 90% of susceptible mosquitoes 24 hours after exposure, but this effect was not seen with resistant mosquitoes where mortality ranged from 16% to 72%. Total mosquito activity was higher around untreated nets than ITNs. There was no difference in total activity, the number, or duration, of net contact, between any mosquito strain, with similar behaviours recorded in susceptible and resistant strains at all ITNs. Net contact was focussed predominantly on the roof for all bednets. We observed a steep decay in activity for both susceptible strains when P3 and OL were present and with IG2 for one of the two susceptible strains. All treated nets reduced the willingness of resistant strains to re-feed when offered blood one-hour post-exposure, with a more pronounced effect seen with P3 and OL than IG2. ConclusionResults indicate that the effects of ITNs on mosquito behaviour are consistent, with no major differences in responses between strains of different pyrethroid susceptibility.

animal behavior and cognition↗

bioRxiv2022

The durability of long-lasting insecticidal nets treated with and without piperonyl butoxide (PBO) in Uganda.

Long Lasting Insecticidal Nets (LLINs) supplemented with the synergist piperonyl butoxide have been developed in response to growing pyrethroid resistance however their durability in the field remains poorly described. A pragmatic cluster-randomised trial was embedded into Ugandas 2017-2018 distribution to compare the durability of LLINs with and without PBO A total of 104 clusters were included with each receiving one of four LLIN products, two with pyrethroid+PBO (Olyset Plus and PermaNet 3.0) and two pyrethroid-only (Olyset Net and PermaNet 2.0). Nets were sampled at baseline, 12, and 25 months post-distribution to assess physical condition, chemical content, and bioefficacy. Physical condition was quantified using proportionate Hole Index and chemical content measured using high-performance liquid chromatography. Bioefficacy was assessed with three-minute WHO Cone and Wireball assays using pyrethroid-resistant An. gambiae, with 1hr knockdown and 24hr mortality recorded. There was no difference in physical durability between LLIN products assessed (p=0.644). The pyrethroid content of all products remained relatively stable across timepoints but PBO content declined by 55% (p<0.001) and 58% (p<0.001) for Olyset Plus and PermaNet 3.0 respectively. Both PBO LLINs were highly effective against pyrethroid-resistant mosquitoes when new, knocking down all mosquitoes. However, bioefficacy declined over time with Olyset Plus knocking down 45.72% (95% CI: 22.84-68.62, p=0.021) and Permanet 3.0 knocking down 78.57% (95% CI: 63.57-93.58, p<0.001) after 25 months. Here we demonstrate that both Olyset Plus and PermaNet 3.0 are as durable as their pyrethroid-only equivalents and had superior bioefficacy against pyrethroid-resistant An. gambiae. However, the superiority of PBO-LLINs decreased with operational use, correlating with a reduction in total PBO content. This decline in bioefficacy after just two years is concerning and there is an urgent need to assess the durability of PBO LLINs in other settings.

zoology↗