Insecticide resistance classification — DTM&H MCQ

Correct answer: D — Confirmed resistance

The correct answer is D, confirmed resistance. Under WHO susceptibility test criteria for standard bioassays, 24-hour mortality of 98 to 100% confirms susceptibility, 90 to 97% suggests possible resistance requiring further confirmatory testing, and below 90% confirms resistance in the population. At 72% mortality, this Senegalese An. gambiae population falls well below the 90% cut-off, so no further confirmatory testing is required to call this resistant. This result has direct programmatic implications: WHO guidance states that confirmed pyrethroid resistance should prompt consideration of alternative products such as PBO-synergist nets or dual active-ingredient nets, or review of indoor residual spraying insecticide choice, since deltamethrin alone will underperform in vector control at this population. Why the other options are wrong: A, Fully susceptible: this requires mortality of 98 to 100%; 72% is far below this threshold, so susceptibility cannot be inferred. B, Possible resistance requiring confirmation: this band applies only to mortality of 90 to 97%, where the result is borderline and repeat testing is mandated; 72% is well outside this range and is diagnostic on its own. D, Tolerance but not resistance: WHO bioassay classification has no intermediate category of tolerance separate from resistance; mortality below 90% is classified as resistance, not a softer descriptor. E, Invalid test requiring repetition: invalidity relates to control mortality exceeding 20% (or 5 to 20% requiring Abbott correction) or inadequate sample size, not to a low mortality result itself, which is a valid and interpretable finding. Key point: mortality below 90% at 24 hours in a WHO bioassay is confirmed resistance with no further testing needed, unlike the 90 to 97% band which mandates repeat confirmatory assays.

Reference: WHO, Test procedures for insecticide resistance monitoring in malaria vector mosquitoes (2016), Section 3.6: mortality 98-100% = susceptible, 90-97% = possible resistance requiring confirmation, <90% = confirmed resistance. https://apmen.org/sites/default/files/all_resources/Test%20procedure%20for%20insecticide%20resistance%20monitoring%20in%20malaria%20vector%20mosquitoes%20WHO%202016.pdf