Abstract
BackgroundA previous cohort study in Malawi showed that users of new insecticide-treated bed nets (ITNs) were significantly protected against malaria compared to non-users, despite moderate levels of pyrethroid resistance among the primary mosquito vectors. The present study investigated whether ITNs that were 1–2 years old continued to protect users in the same area with moderate pyrethroid resistance.MethodsOne year following a baseline cross-sectional malaria parasitaemia prevalence survey and universal distribution of deltamethrin ITNs (May 2012), a fixed cohort of 1223 children aged 6–59 months was enrolled (April 2013). Children were tested for parasitaemia at monthly scheduled visits and at unscheduled sick visits from May to December 2013 using rapid diagnostic tests. ITN use the prior night and the condition of ITNs (based on presence of holes) was assessed by caregiver self-report. The incidence rate ratio (RR) comparing malaria infection among users and non-users of ITNs was modelled using generalized estimating equations adjusting for potential confounders and accounting for repeated measures on each child. The protective efficacy (PE) of ITN use was calculated as 1 − RR.ResultsIn this cohort, self-reported ITN use remained consistently high (> 95%) over the study period. Although users of ITNs were slightly more protected compared to non-users of ITNs, the difference in incidence of infection was not statistically significant (RR 0.83, 95% confidence interval [CI] 0.54–1.27). Among ITN users, malaria incidence was significantly lower in users of ITNs with no holes (of any size) compared to users of ITNs with ≥ 1 hole (RR 0.82, 95% CI 0.69–0.98).ConclusionsThere was no significant PE of using 1–2 year-old ITNs on the incidence of malaria in children in an area of moderate pyrethroid resistance, but among ITN users, the authors found increased protection by ITNs with no holes compared to ITNs with holes. Given the moderate levels of pyrethroid resistance in the primary malaria vector and recent evidence of added benefits of ITNs with synergists or non-pyrethroid insecticides, next-generation ITNs may be a useful strategy to address pyrethroid resistance and should be further explored in Malawi.
Highlights
A previous cohort study in Malawi showed that users of new insecticide-treated bed nets (ITNs) were significantly protected against malaria compared to non-users, despite moderate levels of pyrethroid resistance among the primary mosquito vectors
Coverage and use of insecticide‐treated bed nets in the study area As measured in the censuses conducted in the study area in households with and without enrolled children, the proportion of households with at least one ITN per two residents doubled following the ITN distribution campaigns from 23% in February 2012 to 56% in November 2012
ITN use among all residents increased from 58% to 70% in February 2012 and November 2012, respectively
Summary
A previous cohort study in Malawi showed that users of new insecticide-treated bed nets (ITNs) were significantly protected against malaria compared to non-users, despite moderate levels of pyrethroid resistance among the primary mosquito vectors. Resistance can occur due to upregulation of metabolic enzymes, which promote faster clearance of the insecticide, and/or mutations in target site genes, such as the knockdown resistance gene (kdr), which limit the binding of the insecticide to initiate paralysis Either of these mechanisms leads to reduced deterrence and mortality, which could compromise the mass killing effect of ITNs. ITNs may continue to provide partial protection through other mechanisms. Epidemiologic evidence from studies in Benin [5], Côte d’Ivoire [6], Malawi [7], Kenya [8], and a multi-country study [9] have shown that ITN use remains protective against malaria infection even in areas with moderate-to-high levels of pyrethroid resistance It is unclear whether ITNs will continue to be effective in areas with pyrethroid resistance as the nets develop holes or tears over time and the insecticide content on the net declines
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