Paper II
Question
Laboratory diagnosis of lymphatic filariasis
Answer
Laboratory diagnosis of lymphatic filariasis (Wuchereria bancrofti, Brugia malayi):
- Peripheral blood smear microscopy — thick smear (Giemsa stain) for microfilariae, timed to nocturnal periodicity (blood collection around midnight) for most W. bancrofti strains, matching the night-biting habit of the Culex vector; thin smear for species identification (nuclear arrangement, presence/shape of sheath).
- Concentration techniques — Knott’s concentration method (using formalin to lyse RBCs and concentrate microfilariae) or membrane filtration, improving sensitivity for low-density microfilaraemia.
- Circulating filarial antigen (CFA) detection — an immunochromatographic card test detecting adult worm antigen, not subject to periodicity constraints (testable at any time of day), increasingly the preferred field/programmatic diagnostic tool.
- Ultrasonography — can directly visualize live, motile adult worms within dilated lymphatics (“filarial dance sign”), particularly useful in the scrotal region in men.
- Serology (antibody detection) — supportive, but limited specificity due to cross-reactivity with other helminth infections.
- Eosinophilia — a common but non-specific supportive laboratory finding.
Special note: Tropical Pulmonary Eosinophilia, a hypersensitivity manifestation of filarial infection, shows marked peripheral eosinophilia and elevated filarial antibody titres but no demonstrable microfilaraemia in blood — an “occult” filarial syndrome requiring a different diagnostic approach (antibody testing, response to DEC) than standard blood smear microscopy.