Loiasis — DTM&H MCQ

Correct answer: A — Loa loa

The correct answer is A, Loa loa. This filarial nematode is endemic to Central and West African rainforest belts including Cameroon and is transmitted by Chrysops (deer) flies. The clinical triad in the stem, transient Calabar swellings (episodic non-pitting angioedema typically of the hands and forearms), subconjunctival migration of the adult worm across the eye, and sheathed microfilariae detected on a daytime blood film, is pathognomonic for loiasis. Loa loa exhibits diurnal periodicity, so microfilariae peak in peripheral blood around midday, which is why the daytime film is diagnostic. Diagnosis is confirmed by this combination of exposure history, eosinophilia, and microscopy, and quantifying microfilarial density is essential before treatment because of the risk of encephalopathy with diethylcarbamazine or ivermectin at high loads. Why the other options are wrong: D. Wuchereria bancrofti: causes lymphatic filariasis with nocturnal periodicity, so microfilariae are found on night blood films, not daytime films, and it presents with lymphoedema or hydrocele rather than Calabar swellings or eye worm. C. Brugia malayi: also causes nocturnally periodic lymphatic filariasis in South and Southeast Asia, not Cameroon, and produces lymphadenitis and lymphoedema, not migratory eye worm. E. Onchocerca volvulus: causes onchocerciasis with pruritic skin nodules and river blindness; microfilariae are found in skin snips, not peripheral blood, because adults reside in subcutaneous nodules. B. Mansonella perstans: produces non-periodic, unsheathed microfilariae in blood and is usually asymptomatic or causes mild pruritus, without Calabar swellings or eye worm migration. Key point: Sheathed, diurnally periodic microfilariae plus Calabar swellings and subconjunctival eye worm in a returning traveller from Central Africa equals Loa loa.

Reference: Diagnosis, management and prevention of loiasis: guideline of the German Society for Tropical Medicine, Travel Medicine, and Global Health (DTG), 2024 - PMC (https://pmc.ncbi.nlm.nih.gov/articles/PMC12137371/); consistent with UK tropical medicine practice (Manson's Tropical Diseases, 24th ed.)