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A genetic model of the effects of insecticide-treated bed nets on the evolution of insecticide-resistance.

ABSTRACT: BACKGROUND AND OBJECTIVES:The evolution of insecticide-resistance in malaria vectors is emerging as a serious challenge for the control of malaria. Modelling the spread of insecticide-resistance is an essential tool to understand the evolutionary pressures and dynamics caused by the application of insecticides. METHODOLOGY:We developed a population-genetic model of the spread of insecticide-resistance in a population of Anopheles vectors in response to insecticides used either as adulticides (focussing on insecticide-treated bed nets (ITNs)) or as larvicides (either for the control of malaria or, as an inadvertent side-product, in agriculture). RESULTS:We show that indoor use of insecticides leads to considerably less selection pressure than their use as larvicides, supporting the idea that most resistance of malaria vectors is due to the agricultural use of the insecticides that are also used for malaria control. The reasons for the relatively low selection pressure posed by adulticides are (i) that males are not affected by the ITNs and, in particular, (ii) that the insecticides are also repellents, keeping mosquitoes at bay from contacting the insecticide but also driving them to bite either people who do not use the insecticide or alternative hosts. CONCLUSION:We conclude by discussing the opposing public health benefits of high repellency at an epidemiological and an evolutionary timescale: whereas repellency is beneficial to delay the evolution of resistance, other models have shown that it decreases the population-level protection of the insecticide.

SUBMITTER: Birget PL

PROVIDER: S-EPMC4571732 | biostudies-literature | 2015 Aug

REPOSITORIES: biostudies-literature

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A genetic model of the effects of insecticide-treated bed nets on the evolution of insecticide-resistance.

Birget Philip L G PL Koella Jacob C JC

Evolution, medicine, and public health 20150829 1

<h4>Background and objectives</h4>The evolution of insecticide-resistance in malaria vectors is emerging as a serious challenge for the control of malaria. Modelling the spread of insecticide-resistance is an essential tool to understand the evolutionary pressures and dynamics caused by the application of insecticides.<h4>Methodology</h4>We developed a population-genetic model of the spread of insecticide-resistance in a population of Anopheles vectors in response to insecticides used either as ...[more]

PMID: 26320183

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Project description:BackgroundPyrethroid insecticide-treated bed nets (ITNs) help contribute to reducing malaria deaths in Africa, but their efficacy is threatened by insecticide resistance in some malaria mosquito vectors. We therefore assessed the evidence that resistance is attenuating the effect of ITNs on entomological outcomes.Methods and findingsWe included laboratory and field studies of African malaria vectors that measured resistance at the time of the study and used World Health Organization-recommended impregnation regimens. We reported mosquito mortality, blood feeding, induced exophily (premature exit of mosquitoes from the hut), deterrence, time to 50% or 95% knock-down, and percentage knock-down at 60 min. Publications were searched from 1 January 1980 to 31 December 2013 using MEDLINE, Cochrane Central Register of Controlled Trials, Science Citation Index Expanded, Social Sciences Citation Index, African Index Medicus, and CAB Abstracts. We stratified studies into three levels of insecticide resistance, and ITNs were compared with untreated bed nets (UTNs) using the risk difference (RD). Heterogeneity was explored visually and statistically. Included were 36 laboratory and 24 field studies, reported in 25 records. Studies tested and reported resistance inconsistently. Based on the meta-analytic results, the difference in mosquito mortality risk for ITNs compared to UTNs was lower in higher resistance categories. However, mortality risk was significantly higher for ITNs compared to UTNs regardless of resistance. For cone tests: low resistance, risk difference (RD) 0.86 (95% CI 0.72 to 1.01); moderate resistance, RD 0.71 (95% CI 0.53 to 0.88); high resistance, RD 0.56 (95% CI 0.17 to 0.95). For tunnel tests: low resistance, RD 0.74 (95% CI 0.61 to 0.87); moderate resistance, RD 0.50 (95% CI 0.40 to 0.60); high resistance, RD 0.39 (95% CI 0.24 to 0.54). For hut studies: low resistance, RD 0.56 (95% CI 0.43 to 0.68); moderate resistance, RD 0.39 (95% CI 0.16 to 0.61); high resistance, RD 0.35 (95% CI 0.27 to 0.43). However, with the exception of the moderate resistance category for tunnel tests, there was extremely high heterogeneity across studies in each resistance category (chi-squared test, p<0.00001, I² varied from 95% to 100%).ConclusionsThis meta-analysis found that ITNs are more effective than UTNs regardless of resistance. There appears to be a relationship between resistance and the RD for mosquito mortality in laboratory and field studies. However, the substantive heterogeneity in the studies' results and design may mask the true relationship between resistance and the RD, and the results need to be interpreted with caution. Our analysis suggests the potential for cumulative meta-analysis in entomological trials, but further field research in this area will require specialists in the field to work together to improve the quality of trials, and to standardise designs, assessment, and reporting of both resistance and entomological outcomes.

Project description:BACKGROUND:Expanding access to long-lasting insecticidal nets (LLINs) is difficult if one is limited to government and donor financial resources. Private commercial markets could play a larger role in the continuous distribution of LLINs by offering differentiated LLINs to middle-class Ghanaians. This population segment has disposable income and may be willing to pay for LLINs that meet their preferences. Measuring the willingness-to-pay (WTP) for LLINs with specialty features that appeal to middle-class Ghanaians could help malaria control programmes understand what is the potential for private markets to work alongside fully subsidized LLIN distribution channels to assist in spreading this commodity. METHODS:This study conducted a discrete choice experiment (DCE) including a real payment choice among a representative sample of 628 middle-income households living in Ashanti, Greater Accra, and Western regions in Ghana. The DCE presented 18 paired combinations of LLIN features and various prices. Respondents indicated which LLIN of each pair they preferred and whether they would purchase it. To validate stated willingness-to-pay, each participant was given a cash payment of $14.30 (GHS 65) that they could either keep or immediately spend on one of the LLIN products. RESULTS:The households' average probability of purchasing a LLIN with specialty features was 43.8% (S.D. 0.07) and WTP was $7.48 (GHS34.0). The preferred LLIN features were conical or rectangular one-point-hang shape, queen size, and zipper entry. The average WTP for a LLIN with all the preferred features was $18.48 (GHS 84). In a scenario with the private LLIN market, the public sector outlay could be reduced by 39% and private LLIN sales would generate $8.1 million ($311 per every 100 households) in revenue in the study area that would support jobs for Ghanaian retailers, distributors, and importers of LLINs. CONCLUSION:Results support a scenario in which commercial markets for LLINs could play a significant role in improving access to LLINs for middle-income Ghanaians. Manufacturers interested could offer LLIN designs with features that are most highly valued among middle-income households in Ghana and maintain a retail price that could yield sufficient economic returns.

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A genetic model of the effects of insecticide-treated bed nets on the evolution of insecticide-resistance.

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A genetic model of the effects of insecticide-treated bed nets on the evolution of insecticide-resistance.

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