Onchocerciasis vector — DTM&H MCQ

Correct answer: E — Simulium blackfly

The correct answer is E, Simulium blackfly. Onchocerciasis (river blindness) is caused by Onchocerca volvulus, and it is transmitted to humans through exposure to repeated bites of infected blackflies of the genus Simulium, which breed in fast flowing, well oxygenated river water, explaining the geographic clustering near rivers in the stem. The larvae mature into adult worms that produce microfilariae migrating through the dermis, causing pruritic dermatitis, subcutaneous nodules (onchocercomata), and, when microfilariae invade the eye, progressive visual impairment and blindness. This triad of nodules, dermatitis and river-associated blindness in West African savannah settings is the classic exam pattern for onchocerciasis and its vector. Why the other options are wrong: A. Aedes mosquito: Aedes transmits dengue, yellow fever, chikungunya and Zika, arboviral diseases with febrile illness rather than chronic nodular dermatitis and river blindness. B. Culex mosquito: Culex transmits lymphatic filariasis (Wuchereria bancrofti) and Japanese encephalitis; lymphatic filariasis causes lymphoedema and hydrocele, not blindness or the described nodules. C. Tsetse fly: Tsetse transmits African trypanosomiasis (sleeping sickness), presenting with fever, lymphadenopathy and neurological decline, not river-associated dermatitis or blindness. D. Sandfly: Sandflies transmit leishmaniasis (cutaneous or visceral), producing ulcerating skin lesions or hepatosplenomegaly, with no link to fast-flowing rivers or ocular microfilarial disease. Key point: River blindness (onchocerciasis) is transmitted by Simulium blackflies breeding in fast-flowing rivers, uniquely linking the geographic and vector clue to nodular dermatitis and blindness.

Reference: WHO, Onchocerciasis (River Blindness) Fact Sheet, https://www.who.int/news-room/fact-sheets/detail/onchocerciasis