Pyrethroid resistance management — DTM&H MCQ

Correct answer: E — Nets treated with pyrethroid plus piperonyl butoxide (PBO)

The correct answer is E, nets treated with pyrethroid plus piperonyl butoxide (PBO). PBO is a synergist that inhibits mosquito cytochrome P450 monooxygenase enzymes, the enzymes responsible for metabolic detoxification of pyrethroids, thereby restoring insecticidal potency in vector populations where resistance is driven by this mechanism. WHO has issued a conditional recommendation for deployment of pyrethroid-PBO nets instead of pyrethroid-only nets in areas with confirmed, intermediate-level pyrethroid resistance conferred at least in part by a monooxygenase-based mechanism, based on cluster-randomised trial evidence of additional public health value. This matches the scenario of declining pyrethroid efficacy in a central African control programme, making PBO nets the logical next-generation net to deploy while newer dual active-ingredient nets are scaled up. Why the other options are wrong: A. Untreated nets with smaller mesh size: these provide only a mechanical barrier with no chemical killing effect on mosquitoes, so they do nothing to address resistance-driven loss of insecticidal efficacy and reduce overall vector control impact. B. Nets treated with DDT alone: DDT is not used to impregnate bed nets in any WHO-recommended net class; it is historically used for indoor residual spraying, and cross-resistance with pyrethroids (shared sodium channel target site) limits its usefulness against pyrethroid-resistant vectors anyway. C. Nets treated with organophosphates only: organophosphates are not a WHO-prequalified net treatment class; they carry higher mammalian toxicity concerns for prolonged skin contact and are used in residual spraying, not netting. D. Nets with repellent-impregnated fabric without insecticide: repellents may reduce landing rates but lack the lethal, population-suppressing effect of insecticide, so they cannot substitute for an insecticidal net in a resistance-management strategy. Key point: PBO acts as a synergist restoring pyrethroid killing power against metabolically resistant Anopheles, which is why WHO conditionally recommends pyrethroid-PBO nets over pyrethroid-only ITNs where monooxygenase-mediated resistance is confirmed.

Reference: WHO Guidelines for Malaria Vector Control, Recommendations on core interventions, NCBI Bookshelf (updated 2023): https://www.ncbi.nlm.nih.gov/books/NBK538119/