Lymphatic filariasis — DTM&H MCQ

Correct answer: B — Night blood film or circulating filarial antigen testing

The correct answer is B, night blood film or circulating filarial antigen testing. Wuchereria bancrofti microfilariae typically show nocturnal periodicity, circulating in peripheral blood in highest numbers between around 22:00 and 02:00, so a midday sample is frequently negative even in true infection. Circulating filarial antigen (CFA) testing detects adult worm antigen rather than microfilariae, so it is not affected by this diurnal variation and remains positive throughout the day, making it the preferred confirmatory test when clinical suspicion is high and daytime films are negative. This combination of night-timed microscopy and antigen detection is the recommended approach for confirming bancroftian filariasis in a patient with compatible exposure (coastal East Africa) and clinical features (lymphangitis, recurrent fever, limb swelling progressing towards lymphoedema). Why the other options are wrong: A. Skin snip microscopy from the calf: this technique detects Onchocerca volvulus microfilariae in the dermis, not Wuchereria bancrofti, which are found in blood and lymphatics, not skin. D. Urine filtration for terminal-spined eggs: terminal-spined eggs are characteristic of Schistosoma haematobium, causing urinary schistosomiasis, an entirely different helminth with a different life cycle and clinical picture. C. Stool microscopy for Strongyloides larvae: Strongyloides stercoralis is diagnosed by finding larvae in stool, and while it can cause eosinophilia and cutaneous symptoms, it does not cause lymphangitis or lymphoedema and blood microfilariae are absent. E. Thick film during fever paroxysm alone: filarial fever paroxysms do not reliably coincide with peak microfilaraemia, which is governed by nocturnal periodicity rather than fever timing, so this would still risk missing a low or absent daytime count. Key point: because W. bancrofti microfilariae show nocturnal periodicity, a negative daytime film does not exclude infection, so repeat night bloods or antigen testing (unaffected by time of day) are required to confirm the diagnosis.

Reference: CDC DPDx, Lymphatic Filariasis: Diagnostic Findings (parasitology reference underpinning UK tropical medicine and DTMH teaching, consistent with Manson's Tropical Diseases), https://www.cdc.gov/dpdx/lymphaticfilariasis/index.html