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Lymphatic filariasis
A 38-year-old man from a coastal endemic district has progressive swelling of the left leg with thickened skin, and has had recurrent episodes of fever with a tender red streak up the inner thigh. Which is the most appropriate way to demonstrate the parasite?
- correctA. Peripheral blood smear collected between 10 pm and 2 am
- B. Peripheral blood smear collected in the early morning
- C. Skin snip from the affected calf
- D. Bone marrow aspiration
Explanation
Key fact
Wuchereria bancrofti microfilariae show nocturnal periodicity - they are in the peripheral blood only around midnight.
Peripheral blood smear between 10 pm and 2 am. The microfilariae are sequestered in the lungs by day and appear in the blood at night.
Why
Lymphoedema with recurrent lymphangitis in an endemic area is bancroftian filariasis.
Microfilariae time their appearance to when the vector mosquito bites.
Culex quinquefasciatus feeds at night, so periodicity is nocturnal.
A daytime smear is commonly negative even in heavy infection.
The sheathed microfilaria with no nuclei in the tail tip identifies W. bancrofti.
Microfilariae appear when the vector bites
Why not the others
- B. Peripheral blood smear collected in the early morning — will usually be negative. Only the Pacific strain shows diurnal subperiodicity.
- C. Skin snip from the affected calf — is the test for Onchocerca volvulus, which lives in the skin and causes river blindness, not lymphoedema.
- D. Bone marrow aspiration — is for visceral leishmaniasis. Filarial worms do not reside in marrow.
Remember
The parasite keeps the mosquito's hours. Night-biting vector, night-circulating microfilaria.
Go deeper
Antigen detection by immunochromatographic card test works at any time of day and has largely replaced timed smears in programme settings.
Adult worms are seen as the filarial dance sign on scrotal ultrasound.
Established elephantiasis is caused by the host's inflammatory response and does not reverse with diethylcarbamazine, which kills microfilariae.
Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed. (Universities Press, 2020); Park K. Park's Textbook of Preventive and Social Medicine, 28th ed. (Bhanot, 2025).