Question
Describe the etiopathogenesis and lab diagnosis of kala- azar. (PTO)
Answer
Kala-azar (Visceral Leishmaniasis)
Etiopathogenesis: caused by the protozoan parasite Leishmania donovani (in the Indian subcontinent), transmitted by the bite of the female sandfly (Phlebotomus argentipes in India). When an infected sandfly takes a blood meal, it injects promastigotes into the host’s skin; these are phagocytosed by host macrophages, within which they transform into the amastigote form and multiply. Infected macrophages rupture, releasing amastigotes that infect further macrophages, and the infection disseminates via the bloodstream to the reticuloendothelial system — particularly the spleen, liver, and bone marrow — causing progressive organ infiltration/enlargement, bone marrow suppression, and a marked polyclonal immune activation with hypergammaglobulinaemia. The resulting clinical picture includes prolonged irregular fever, progressive hepatosplenomegaly (splenomegaly typically more prominent), pancytopenia, weight loss, and characteristic darkening/hyperpigmentation of the skin (the literal meaning of “kala-azar,” Hindi for “black fever”).
Laboratory diagnosis:
- Demonstration of LD (Leishman-Donovan) bodies (amastigotes) in splenic aspirate (the most sensitive method, though carries a small risk of haemorrhage given the enlarged, friable spleen) or bone marrow aspirate (a safer alternative, though somewhat less sensitive) — stained with Giemsa/Leishman stain, showing intracellular round-to-oval amastigotes within macrophages.
- Serology — the rK39 rapid immunochromatographic strip test is the standard rapid, field-deployable test (detecting antibody against a recombinant L. donovani antigen), widely used in India’s kala-azar elimination programme; Direct Agglutination Test (DAT) and ELISA are also used.
- Culture — on NNN (Novy-MacNeal-Nicolle) medium, growing the motile promastigote form; sensitive but slow (days to weeks) and requires specialized facilities, not used for routine rapid diagnosis.
- PCR — highly sensitive and specific molecular method, increasingly used, particularly for cases with low parasite burden or for post-treatment monitoring.
- Non-specific supportive laboratory findings: pancytopenia, markedly elevated ESR, and hypergammaglobulinaemia with an inverted albumin-globulin ratio.
- Aldehyde (formol-gel) test — a classical, simple bedside test demonstrating hypergammaglobulinaemia (gelation of serum on adding formalin), now largely of historical interest.