Lymphatic filariasis — DTM&H MCQ

Correct answer: D — Culex mosquito

The correct answer is D, Culex mosquito. This man from West Africa has classic lymphatic filariasis caused by Wuchereria bancrofti, presenting with painless lymphoedema of the leg and hydrocoele, and midnight blood shows sheathed microfilariae that stain pink with Giemsa (in contrast to Loa loa, which stains blue). W. bancrofti microfilariae display nocturnal periodicity, peaking in peripheral blood overnight to coincide with the nocturnal biting habits of their principal vector, Culex quinquefasciatus, which is the main mosquito vector transmitting the parasite in many endemic regions including parts of West Africa. The combination of nocturnal sampling, sheathed pink-staining microfilariae, and genitourinary/limb lymphatic involvement is pathognomonic for bancroftian filariasis transmitted by Culex. Why the other options are wrong: C. Simulium blackfly: This is the vector for Onchocerca volvulus, which causes onchocerciasis (river blindness and skin nodules), not lymphatic disease, and its microfilariae are unsheathed and found in skin, not blood. A. Chrysops deerfly: This transmits Loa loa, producing Calabar swellings and eye worm; its microfilariae are sheathed but stain blue with Giemsa and show diurnal, not nocturnal, periodicity. E. Aedes mosquito: Aedes is the principal vector for Brugia malayi filariasis in parts of Asia and for arboviruses (dengue, Zika), not the main West African W. bancrofti vector. B. Phlebotomus sandfly: This transmits Leishmania species, causing cutaneous or visceral leishmaniasis, with no microfilaraemia at all. Key point: Sheathed, pink-staining microfilariae with nocturnal periodicity plus lymphoedema and hydrocoele signals W. bancrofti transmitted by Culex mosquitoes.

Reference: PAHO/WHO, Lymphatic Filariasis, https://www.paho.org/en/topics/lymphatic-filariasis