Lymphatic filariasis — DTM&H MCQ

Correct answer: C — Mosquito vector, commonly Culex in urban bancroftian filariasis

The correct answer is C, mosquito vector, commonly Culex in urban bancroftian filariasis. Wuchereria bancrofti is exclusively transmitted by mosquitoes, and in coastal East African and urban settings the principal vector is Culex quinquefasciatus, which breeds prolifically in polluted urban water and drives the high antigen positivity, hydrocele and lymphoedema described. Sheathed microfilariae showing nocturnal periodicity on blood film is classic for W. bancrofti, reflecting the parasite's adaptation to the nocturnal biting behaviour of Culex. In rural African foci Anopheles can also transmit the parasite, but the urban, coastal Tanzanian picture described here is textbook Culex-driven bancroftian filariasis. Why the other options are wrong: B. Phlebotomine sandfly: this is the vector for leishmaniasis and sandfly fever, not for any filarial nematode producing sheathed microfilariae. E. Simulium blackfly: this transmits Onchocerca volvulus (river blindness), which causes unsheathed microfilariae found in skin snips, not blood films. D. Cyclops copepod: this is the intermediate host for Dracunculus medinensis (guinea worm), a tissue nematode acquired by ingestion, not an insect-transmitted blood filaria. A. Freshwater Bulinus snail: this is the intermediate host for Schistosoma haematobium, a trematode causing urinary schistosomiasis, unrelated to lymphatic filariasis or microfilaraemia. Key point: Nocturnal sheathed microfilariae plus hydrocele and lymphoedema in an African coastal or urban setting points to W. bancrofti transmitted by Culex mosquitoes, distinguishing it from the other vector-borne tropical diseases listed.

Reference: WHO, Lymphatic filariasis fact sheet (2024), https://www.who.int/news-room/fact-sheets/detail/lymphatic-filariasis