Onchocerciasis ocular risk — DTM&H MCQ

Correct answer: E — Associated with higher risk of ocular complications

The correct answer is E, associated with higher risk of ocular complications. Adult Onchocerca volvulus worms coil within subcutaneous nodules and continuously release microfilariae that migrate through adjacent tissue; nodules on the head and upper body lie close to the eye, so microfilarial migration into the cornea, anterior chamber and posterior segment is more likely, driving punctate keratitis, sclerosing keratitis, iridocyclitis and river blindness. This is why nodule distribution is used clinically to stratify ocular risk, and why nodulectomy of head and neck nodules has been used as an adjunct to ivermectin mass drug administration in some programmes. Nodule site reflects the anatomical route of microfilarial spread, not infection duration. Why the other options are wrong: A, Indicates more recent infection: Nodule location depends on where the biting Simulium blackfly deposited infective larvae and where adult worms encapsulate, not on chronicity; duration is better judged from nodule number, size and skin snip microfilarial density. B, Indicates drug resistance: Ivermectin susceptibility is assessed by microfilarial repopulation rates after treatment and community response to mass drug administration, not by nodule site. C, Suggests co-infection with Loa loa: Loa loa is diagnosed from its own features (Calabar swellings, eyeworm migration, blood microfilariae with diurnal periodicity) confirmed by blood film or PCR; onchocercal nodule site has no bearing on coexisting filarial species. E, No clinical significance: Incorrect, as nodule site directly informs ocular risk stratification and management priorities, as above. Key point: Head and upper body onchocercal nodules carry higher ocular risk because microfilariae have a shorter migratory distance to reach the eye.

Reference: CDC Yellow Book 2024, Onchocerciasis (River Blindness): eye disease risk is greater with nodules located on the head and upper body due to proximity of migrating microfilariae to ocular tissues. https://wwwnc.cdc.gov/travel/yellowbook/2024/infections-diseases/onchocerciasis-river-blindness