Blackfly larval control — DTM&H MCQ

Correct answer: D — Larviciding selected river breeding sites

E, larviciding selected river breeding sites, is correct because the stem describes Simulium (blackfly) larvae attaching to rocks in fast-flowing, well-oxygenated river water, which is the specific ecological niche these larvae require. Vector control for onchocerciasis has historically relied on targeted application of larvicide to identified Simulium breeding sites in rivers, as used by the Onchocerciasis Control Programme in West Africa, which achieved sustained suppression of blackfly populations through prolonged, targeted larviciding of Simulium breeding sites. This matches the vector biology given in the vignette exactly: fast flowing, rocky substrate river attachment sites are precisely where larvicide must be delivered to interrupt transmission. Mass ivermectin distribution remains the primary elimination tool, but larviciding is the correct vector control answer when the question specifically asks about interrupting the aquatic larval stage described. Why the other options are wrong: A. Treat freshwater snails: Snails are the intermediate host for Schistosoma in schistosomiasis, not for Onchocerca volvulus, so molluscicide treatment has no role here. C. Empty household containers: This targets Aedes mosquitoes that breed in small, still water containers (relevant to dengue or Zika control), not Simulium, which needs flowing river water, not stagnant domestic containers. B. Drain stagnant tyres: Stagnant water in tyres is a classic Aedes aegypti breeding site for dengue/Zika/chikungunya control, entirely unrelated to fast flowing river dwelling Simulium larvae. E. Spray cattle sheds for tsetse: Tsetse flies transmit African trypanosomiasis and rest in vegetation or animal shelters; they are not aquatic and this measure does not address Simulium breeding in rivers. Key point: Onchocerciasis vector control specifically targets Simulium larvae attached to rocks in fast flowing, oxygenated river water, making larviciding of identified river breeding sites the appropriate intervention, distinct from stagnant water measures used against mosquito vectors.

Reference: WHO/Onchocerciasis Control Programme vector control: Trends in Parasitology, 'Vector control and entomological capacity for onchocerciasis elimination' (2022), https://www.cell.com/trends/parasitology/fulltext/S1471-4922(22)00060-5