Residual malaria transmission — DTM&H MCQ
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Correct answer: D — Outdoor biting behaviour reduces impact of indoor interventions
The correct answer is D, outdoor biting behaviour reduces impact of indoor interventions. Bednets and indoor residual spraying (IRS) only protect against mosquitoes that feed or rest indoors at night; when vectors bite early in the evening, outdoors, before people retire under nets, this exposure window is entirely missed by both interventions. This is the well described phenomenon of residual malaria transmission, where human and vector behaviours (forest-fringe exposure, early evening outdoor activity, exophagic and crepuscular biting) sustain transmission despite good indoor coverage. WHO's technical guidance on residual transmission explicitly identifies feeding on humans when they are active and unprotected outdoors as a key behaviour limiting the achievable impact of ITNs and IRS. The scenario stem (outdoor biting, forest-fringe, early evening, limited IRS effect) is the classic exam fingerprint for this mechanism rather than any drug or vaccine failure. Why the other options are wrong: E. Bednets increase Anopheles breeding: nets have no plausible effect on larval habitat or oviposition; they act purely as a physical/insecticidal barrier to adult biting, so they cannot increase breeding. C. Chloroquine resistance causes mosquito survival: chloroquine is an antimalarial drug acting on the Plasmodium parasite in humans, not an insecticide; parasite drug resistance has no bearing on mosquito survival against insecticides. A. Snail control has replaced vector control: snails are the intermediate host for schistosomiasis, not malaria; this option confuses vector control programmes for entirely different diseases. B. Human vaccination has selected outdoor mosquitoes: no licensed malaria vaccine exerts selection pressure on mosquito biting site preference; vaccine-induced immunity targets the parasite, not vector behaviour. Key point: indoor tools (ITNs/IRS) cannot interrupt transmission driven by outdoor, early-evening biting, which is the hallmark of residual malaria transmission requiring supplementary strategies.
Reference: World Health Organization, WHO/HTM/GMP/MPAC/2014.5, Control of residual malaria parasite transmission, 2014 (https://www.ivcc.com/wp-content/uploads/2019/07/technical-note-control-of-residual-malaria-parasite-transmission-sep14.pdf)