RAS question
Onchocerciasis is transmitted through the bites of which insect?
Correct answer: (D) Simulium blackfly.
Onchocerciasis is transmitted by repeated bites of infected Simulium blackflies.
Explanation
Onchocerciasis, also called river blindness, is not a generic insect-borne disease; its transmission is tied to infected blackflies of the genus Simulium. WHO states that the disease is caused by the filarial worm Onchocerca volvulus and spreads through repeated bites of infected Simulium blackflies. The river-blindness clue matters because these blackflies breed along fast-flowing rivers and streams, matching the standard exam explanation that places the vector near such water bodies. During a blood meal, a female blackfly can take up microfilariae from an infected person, after which the parasite develops further in the fly and is passed to another human host in later bites. Therefore, among the options, Simulium blackfly is the precise vector.
Why the other options are wrong
- (A) Sand fly is wrong because the verified WHO source names infected Simulium blackflies, not sand flies, as the vector for onchocerciasis.
- (B) Aedes mosquito is wrong because the transmission described here is through repeated bites of infected blackflies of Simulium spp., not mosquitoes.
- (C) Tsetse fly is wrong because the question asks for the insect that transmits onchocerciasis, and WHO specifically identifies infected Simulium blackflies.
Concept
This tests the vector-disease pairing within communicable diseases, a compact fact pattern that often appears in RAS Science and Technology MCQs. It recurs because the name river blindness links the disease to the breeding habitat of its vector near fast-flowing rivers and streams.