Insecticide resistance mechanisms — DTM&H MCQ
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Correct answer: C — Pyrethroids and DDT
The correct answer is C, pyrethroids and DDT. Kdr (knockdown resistance) mutations occur in the insect voltage-gated sodium channel (VGSC) gene, the shared target site of pyrethroids and DDT, and these point mutations (classically at the L1014 locus) reduce channel sensitivity to both compounds. Because pyrethroids and DDT share this identical mode of action (sodium channel modulation), a single kdr mutation confers cross-resistance across both classes simultaneously, which is why kdr surveillance is a key entomological indicator for pyrethroid-based nets and IRS programmes. In this scenario the district is using a carbamate for IRS, so rising kdr frequencies in An. gambiae have no bearing on the current spray programme, but they matter greatly for any parallel pyrethroid-treated net distribution, since carbamates act via a different target (acetylcholinesterase) unaffected by kdr. Why the other options are wrong: A. Organophosphates: these are acetylcholinesterase inhibitors, a completely different target site from the VGSC; kdr mutations do not alter organophosphate binding or efficacy. B. Carbamates: also acetylcholinesterase inhibitors; resistance to carbamates arises from ace-1 target-site mutations or metabolic detoxification, not kdr. E. Neonicotinoids: these act on nicotinic acetylcholine receptors, an unrelated target unaffected by sodium channel mutations; resistance here is driven by distinct receptor mutations or metabolic mechanisms. D. Insect growth regulators: these disrupt chitin synthesis or juvenile hormone pathways during larval development, entirely unrelated to adult neuronal sodium channels, so kdr is irrelevant to this class. Key point: kdr mutations alter the voltage-gated sodium channel, the shared target of pyrethroids and DDT, producing cross-resistance to both but leaving acetylcholinesterase- and receptor-targeted classes (carbamates, organophosphates, neonicotinoids) and growth regulators unaffected.
Reference: WHO Global Plan for Insecticide Resistance Management in malaria vectors (GPIRM); Mnzava et al., Malaria Journal 2015: 'sodium channel modulators (pyrethroids, DDT) and acetylcholinesterase inhibitors (carbamates, organophosphates)' are the relevant insecticide mode-of-action classes for IRS resistance management, https://link.springer.com/content/pdf/10.1186/s12936-015-0693-4.pdf