Kala-Azar (Visceral Leishmaniasis) — Microbiology

Etiologic Agent

Kala-azar ("black fever," also called Dumdum fever) is caused by protozoan parasites of the genus Leishmania, primarily:

Leishmania donovani (Indian subcontinent, East Africa - anthroponotic)
Leishmania infantum (Mediterranean, Latin America - zoonotic, dogs as reservoir)

These are obligate intracellular parasites of the reticuloendothelial system (macrophages of spleen, liver, bone marrow, lymph nodes) - Sherris & Ryan's Medical Microbiology, p. 1794.

Morphology

Leishmania exists in two forms:

Amastigote - the intracellular, non-flagellated, oval form (2-5 µm) found in human macrophages. In stained smears these show a nucleus and a rod-shaped kinetoplast and are called Leishman-Donovan (LD) bodies.
Promastigote - the extracellular, flagellated, spindle-shaped form found in the sandfly gut and in culture.

Vector and Transmission

Transmitted by the bite of infected female phlebotomine sandflies (Phlebotomus in Old World, Lutzomyia in New World), which feed nocturnally. The sandfly ingests amastigotes with a blood meal; these transform into promastigotes, multiply in the gut, and migrate to the proboscis for transmission at the next bite.

Life Cycle / Pathogenesis

Promastigotes are injected into the skin along with salivary peptides that inactivate host macrophages.
Complement is activated (classical pathway for L. donovani), opsonizing promastigotes with C3, which mediates attachment to macrophage CR1/CR3 receptors.
After phagocytosis, promastigotes lose their flagella and transform into amastigotes within the phagolysosome.
Intracellular survival is mediated by surface lipophosphoglycan and abundant membrane-bound acid phosphatase, which inhibit the macrophage oxidative burst and inactivate lysosomal enzymes.
Amastigotes multiply, rupture the macrophage, and infect neighboring mononuclear cells - producing progressive dissemination to spleen, liver, and bone marrow (unlike cutaneous species, L. donovani poorly elicits a protective TH1 response and better tolerates 37°C core body temperature, favoring visceral spread).

(Sherris & Ryan's Medical Microbiology, pp. 1794-1796)

Immune Response

CD4+ TH1 cells secreting IFN-γ normally activate macrophages to kill amastigotes via nitric oxide. In visceral leishmaniasis, this TH1 response is markedly suppressed, allowing dissemination. Circulating antibodies develop but are non-protective and can cause immune-complex glomerulonephritis.

Laboratory Diagnosis

Microscopy: Demonstration of LD bodies (amastigotes) in Giemsa-stained smears/aspirates from spleen (most sensitive), bone marrow, or lymph node.
Culture: Aspirate inoculated onto NNN (Novy-MacNeal-Nicolle) media, where promastigotes develop.
Serology: rK39 rapid immunochromatographic strip test (widely used field test), direct agglutination test (DAT), ELISA.
Molecular: PCR for Leishmania DNA - highly sensitive/specific.
Ancillary findings: pancytopenia, hypergammaglobulinemia (polyclonal), reversed albumin:globulin ratio.

Treatment (noted in the same source)

Liposomal amphotericin B is first-line in many regions; pentavalent antimonials (sodium stibogluconate) are traditional therapy; miltefosine, an oral agent, is effective and safe for both cutaneous and visceral leishmaniasis. A late complication in some cured patients is post-kala-azar dermal leishmaniasis (PKDL), marked by hypopigmented/nodular skin lesions, associated with high IL-10 levels predicting risk.

Recent Evidence Check

A quick literature check for updates on visceral leishmaniasis diagnosis/treatment is warranted given ongoing drug development (e.g., miltefosine resistance, combination regimens). I did not run this check in this pass due to budget scope — let me know if you'd like me to pull recent systematic reviews/meta-analyses on VL treatment (e.g., miltefosine vs. amphotericin B efficacy) for a currency check.

Short note on superantigen microbiology saq

Finding Sources

Reading File

Superantigen - Short Note (Microbiology SAQ)

Definition: Superantigens are microbial exotoxins that activate T cells by binding directly to MHC class II molecules on antigen-presenting cells (outside the normal peptide-binding groove, without antigen processing) and cross-linking them to the Vβ region of the T-cell receptor. This bypasses normal antigen specificity.

Mechanism:

Bind directly to MHC class II molecules on APCs (no processing/presentation needed).
Cross-link MHC II to TCR Vβ chain outside the antigen-binding groove.
Act as polyclonal T-cell activators - stimulate up to 20-30% of the entire T-cell pool (vs. ~0.001% with conventional antigens).
Massive, non-specific release of cytokines - IL-1, IL-2, IL-6, TNF-α, IFN-γ ("cytokine storm").
This cytokine surge produces systemic effects: fever, hypotension, shock, capillary leak, and multi-organ dysfunction.

Examples:

Organism	Superantigen	Disease
Staphylococcus aureus	TSST-1	Toxic shock syndrome
S. aureus	Enterotoxins (SEA-SEE)	Food poisoning (preformed toxin ingestion - vomiting, diarrhea), toxic shock
Streptococcus pyogenes (Group A)	Streptococcal pyrogenic exotoxins (SpeA, SpeC)	Streptococcal toxic shock syndrome, scarlet fever

Clinical significance:

Toxic shock syndrome (staphylococcal - classically tampon-associated; streptococcal - often with severe soft tissue infection/necrotizing fasciitis).
Food poisoning when enterotoxin is ingested preformed in contaminated food (rapid onset vomiting/diarrhea, no fever typically).
Proposed role in exacerbating conditions like psoriasis and chronic rhinosinusitis/nasal polyposis via chronic immune activation.

Key distinguishing point (exam favorite): Unlike conventional antigens, superantigens do NOT require intracellular processing and do NOT bind in the MHC peptide groove - hence they activate T cells regardless of antigen specificity, causing a disproportionately massive cytokine release from a small toxin dose.

Sherris & Ryan's Medical Microbiology, 8th ed., p. 892-893

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