Onchocerciasis — DTM&H MCQ

Correct answer: E — Simulium blackfly

The correct answer is E, Simulium blackfly. This vector breeds in fast flowing, well oxygenated rivers, exactly the exposure described in a farmer working a river valley in Ghana, and transmits the filarial nematode Onchocerca volvulus. Adult worms produce microfilariae that migrate through the dermis and eye, accounting for the chronic pruritic dermatitis, subcutaneous nodules (onchocercomata) and punctate keratitis of ocular onchocerciasis. Diagnosis by skin snips demonstrating microfilariae is the classic confirmatory test, and the Simulium life cycle in rapid rivers is the epidemiological hallmark linking geography to vector. This combination of river valley exposure, skin snip microfilariae and eye disease is pathognomonic for onchocerciasis and its blackfly vector. Why the other options are wrong: A. Anopheles mosquito: this transmits malaria (Plasmodium species) and also acts as vector for some lymphatic filariae, but not Onchocerca volvulus, and does not explain skin snip positive microfilariae or river dwelling exposure. C. Chrysops deer fly: this is the vector for Loa loa, causing Calabar swellings and eye worm migrating across the conjunctiva, not skin nodules or skin snip microfilariae, and breeds in rainforest, not fast rivers. D. Culex mosquito: this transmits Wuchereria bancrofti and Japanese encephalitis; lymphatic filariasis causes lymphoedema and blood microfilariae on night blood films, not skin snip microfilariae or keratitis. B. Tsetse fly: this transmits African trypanosomiasis (sleeping sickness), presenting with fever, lymphadenopathy and later neurological decline, not dermatitis with skin snip microfilariae. Key point: fast flowing river exposure plus skin snip microfilariae plus eye disease equals onchocerciasis transmitted by Simulium blackfly, distinguishing it from other filarial and vector borne tropical diseases.

Reference: World Health Organization, Onchocerciasis (river blindness) fact sheet, 2024, https://www.who.int/news-room/fact-sheets/detail/onchocerciasis