Question
Pathogenesis and laboratory diagnosis of filariasis
Answer
Pathogenesis of lymphatic filariasis (Wuchereria bancrofti, Brugia malayi):
Infective third-stage larvae are transmitted by mosquito bite (Culex for W. bancrofti; Mansonia/Anopheles for Brugia), migrating to and maturing within lymphatic vessels/lymph nodes into adult worms. Adult worms cause lymphatic dilatation, endothelial proliferation, and progressive lymphatic dysfunction, both from direct physical presence and from the host’s chronic inflammatory/immune response, with eosinophilic and granulomatous inflammation around dead/dying worms contributing significantly to lymphatic obstruction and fibrosis. Secondary bacterial/fungal infection of the affected limb further worsens tissue damage, driving progression to chronic lymphoedema and elephantiasis.
Clinical spectrum: asymptomatic microfilaraemia; acute adenolymphangitis; chronic lymphoedema/elephantiasis and hydrocele (in men); Tropical Pulmonary Eosinophilia (a hypersensitivity syndrome without demonstrable microfilaraemia).
Laboratory diagnosis:
- Peripheral blood smear microscopy — thick smear (Giemsa) for microfilariae, timed to nocturnal periodicity (blood collection around midnight); concentration techniques (Knott’s method, membrane filtration) improve sensitivity.
- Circulating filarial antigen (CFA) detection — immunochromatographic card test, detects adult worm antigen, not subject to periodicity constraints.
- Ultrasonography — can visualize live, motile adult worms within dilated lymphatics (“filarial dance sign”).
- Serology — supportive, limited specificity due to cross-reactivity with other helminths.
Public health significance: targeted for global elimination through the WHO’s Global Programme to Eliminate Lymphatic Filariasis, using mass drug administration alongside vector control.