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Bed nets treated with insecticidal pyrethroids and indoor residual spraying (IRS) have been cornerstones of malaria control in Africa. Resistance of mosquitoes to pyrethroids is increasing, diminishing the effectiveness of treated bed nets. To assess the effectiveness of new malaria prevention agents, investigators conducted a four-group, cluster-randomized, controlled trial in 48 clusters from 40 villages in Tanzania. Clusters received standard long-lasting pyrethroid-treated bed nets only, bed nets treated with both a pyrethroid and piperonyl butoxide (PBO), standard bed nets plus IRS, or PBO bed nets plus IRS. PBO acts synergistically with the pyrethroid, preventing its detoxification in the mosquito. IRS was done with a recently developed long-lasting formulation of insecticidal organophosphate, pirimiphos-methyl.
At baseline, malaria prevalence in children in the study area was 65%. Prevalence of malaria infection in children aged 6 months to 14 years, the primary study outcome, was lower in groups with PBO long-lasting insecticidal nets than in those with standard nets 9 months after baseline (29% vs. 42%). Malaria prevalence was lower in the IRS groups than in the non-IRS groups (28% vs. 44%). The effect of PBO-treated nets decreased over time but was still apparent at 21 months (45% infection rate with PBO nets vs. 62% with standard nets). Addition of IRS to PBO long-lasting nets did not significantly improve protection.
Protopopoff N et al. Effectiveness of a long-lasting piperonyl butoxide-treated insecticidal net and indoor residual spray interventions, separately and together, against malaria transmitted by pyrethroid-resistant mosquitoes: A cluster, randomised controlled, two-by-two factorial design trial. Lancet 2018 Apr 11; 391:1577. (https://doi.org/10.1016/S0140-6736(18)30427-6)
Killeen GF and Ranson H.Insecticide-resistant malaria vectors must be tackled. Lancet 2018 Apr 11; 391:1551. (https://doi.org/10.1016/S0140-6736(18)30844-4)
Comment
Incorporating PBO into treatment of insecticidal nets significantly improves community protection in areas with high-level pyrethroid resistance. The impact of indoor residual spraying can be more rapid and might be favored during malaria epidemics. Based on this trial, WHO recently recommended using dual-treated nets in areas with pyrethroid resistance. Editorialists observe that the PBO nets are likely more expensive than standard nets but less costly than indoor spraying. They chide decision makers for being slow to recommend using these nets, which apparently received interim WHO approval a decade ago. Meanwhile, children continue to die from malaria.