Loiasis — DTM&H MCQ

Correct answer: B — Chrysops deerfly

The correct answer is B, Chrysops deerfly. This vignette describes loiasis: migratory Calabar swellings of the limbs, a worm crossing the conjunctiva (African eye worm), and sheathed microfilariae with diurnal periodicity on a daytime blood film, all classic for Loa loa acquired in Central/West Africa. Loa loa is transmitted by the day-biting Chrysops deerfly (mango fly), and its microfilariae circulate in peripheral blood during daylight hours, exactly matching the daytime film finding described. The geographic history (Cameroon, rainforest belt) and the combination of subcutaneous migration plus subconjunctival worm are pathognomonic for this vector-parasite pairing. Why the other options are wrong: C. Culex mosquito: this is the vector for Wuchereria bancrofti and Brugia species causing lymphatic filariasis, which presents with lymphoedema/elephantiasis and nocturnally periodic sheathed microfilariae, not Calabar swellings or eye worm. A. Simulium blackfly: this transmits Onchocerca volvulus, causing onchocerciasis with skin nodules, pruritus and river blindness; microfilariae are found in skin snips, not in blood films, and there is no eye-crossing worm or Calabar swelling. D. Triatomine bug: this is the vector for Trypanosoma cruzi (Chagas disease), a trypanosome, not a filarial nematode, and it does not produce microfilaraemia or subcutaneous swellings. E. Sandfly: sandflies transmit Leishmania species and cause cutaneous, mucocutaneous or visceral leishmaniasis, entirely unrelated to filarial disease or microfilaraemia. Key point: daytime sheathed microfilariae plus migrating conjunctival worm and transient limb swellings equal loiasis, transmitted by Chrysops deerflies, in contrast to nocturnal periodicity (Wuchereria/Brugia via Culex) or skin-snip diagnosis (Onchocerca via Simulium).

Reference: Manson's Tropical Diseases, 24th Edition (2024), Filarial infections chapter: Loa loa is transmitted by the day-biting Chrysops fly in the rainforests of Central and West Africa, producing Calabar swellings and subconjunctival eye worm migration (via ScienceDirect Topics summary, https://www.sciencedirect.com/topics/medicine-and-dentistry/loa-loa-filariasis)