Leishmania donovani
Kinetoplastid protozoan with two morphological stages: flagellated promastigotes in the sandfly vector; round, non-motile amastigotes replicating exclusively in mammalian macrophage phagolysosomes. gp63 zinc metalloprotease cleaves complement C3b → iC3b, diverting entry to the CR3 non-inflammatory receptor; LPG transiently delays phagosome maturation during the critical promastigote-to-amastigote differentiation window; FeSOD scavenges NADPH oxidase-generated superoxide. IL-10 induction (ERK/CREB axis) creates an immunosuppressive autocrine loop blocking IL-12 and TNF-α. gp63 additionally cleaves STAT1 (blunting IFN-γ signalling) and PKC-α/β. Amastigotes disseminate hematogenously to spleen, liver (Kupffer cells), and bone marrow, producing massive organomegaly, hypersplenism-driven pancytopenia, and cachexia. The A2 protein determines viscerotropism. Without treatment: 100% fatal within 2 years.
Classification & Structure
| Taxonomy | Kinetoplastea; Trypanosomatida; Leishmania (donovani complex: L. donovani — Indian subcontinent + East Africa; L. infantum/chagasi — Mediterranean + Latin America) |
| Promastigote | 10–20 µm elongated; anterior flagellum; dense LPG coat; infective metacyclic form shorter and stouter (7–10 µm); inoculated by sandfly bite into dermis |
| Amastigote | 2–4 µm round/oval; rudimentary internal flagellum; kinetoplast (large mitochondrial kDNA network) visible; replicates by binary fission in phagolysosome; pH 5.0–5.5 optimum |
| gp63 (Leishmanolysin) | 63 kDa zinc metalloprotease; GPI-anchored; ~500,000 copies/metacyclic promastigote; cleaves C3b→iC3b (CR3 uptake), C5 (prevents MAC), PKC-α/β, STAT1, NF-κB p65, antibody Fc regions |
| LPG (Lipophosphoglycan) | GPI-anchored galactosyl-mannose phosphate polymer; major surface glycolipid; inhibits PKC-α; scavenges ROS; transiently delays Rab5→Rab7 phagosome maturation by ~20–30 min (differentiation window) |
| FeSOD | Iron-superoxide dismutase; unique Fe³+ cofactor (mammals use Mn-SOD or Cu/Zn-SOD); scavenges superoxide (O₂⁻) generated by host NADPH oxidase; essential phagolysosome survival factor |
| A2 protein | Amastigote-specific stress protein; required for hepatic and splenic persistence; distinguishes viscerotropic (L. donovani, L. infantum) from cutaneotropic (L. major) species |
| Kinetoplast / kDNA | Concatenated network of thousands of minicircles (~1 kb) + ~25 maxicircles (~22 kb); minicircles encode guide RNAs for RNA editing of maxicircle transcripts; target of diagnostic kDNA PCR |
Infection Mechanism & Pathogenesis
1 · Sandfly inoculation and neutrophil Trojan horse
Female Phlebotomus argentipes (India/Bangladesh) or P. orientalis (East Africa) regurgitates metacyclic promastigotes with immunosuppressive saliva (adenosine, PGE2) into dermis. Complement is activated but gp63 converts C3b → iC3b; iC3b-opsonised promastigotes engage CR3 on neutrophils and macrophages — a non-inflammatory uptake receptor that does not trigger the oxidative burst. Promastigotes are first phagocytosed by neutrophils, which undergo apoptosis within hours; apoptotic neutrophils are engulfed by arriving macrophages (efferocytosis) — delivering intact promastigotes in a non-inflammatory context that delays effective killing. This "Trojan horse" mechanism is a key early immune evasion step.
2 · Phagosome maturation arrest and differentiation
LPG inhibits PKC-α → slows PI3-kinase recruitment → delays Rab5→Rab7 transition → phagolysosome maturation delayed by ~20–30 minutes. During this critical window, promastigotes differentiate into amastigotes: HSP70/HSP83 upregulated (acid-stable chaperones); surface protein repertoire shifts; metabolic enzymes become acid-optimised. Differentiated amastigotes are paradoxically well-adapted to the phagolysosomal environment: they replicate optimally at pH 5.0–5.5 and use FeSOD, cysteine proteases (CPA, CPB), and acid-stable metabolic enzymes to thrive in what should be a lethal compartment.
3 · Suppression of macrophage killing machinery
LPG prevents PKC-mediated phosphorylation of p47phox → reduced NADPH oxidase complex assembly → diminished superoxide burst. gp63 inside the macrophage cleaves STAT1 (blocking IFN-γ signal transduction) and NF-κB p65 (suppressing pro-inflammatory transcription). ERK1/2 activation → CREB phosphorylation → IL-10 promoter activation → IL-10 autocrine/paracrine loop → suppresses IL-12 and TNF-α → maintains M2-like (anti-inflammatory) macrophage phenotype. iNOS is not induced; no nitric oxide to kill amastigotes.
4 · Visceral dissemination and organ damage
Amastigote-laden macrophages traffic via lymphatics and blood to: spleen (massive expansion of infected macrophages; germinal centre collapse → impaired B-cell responses; hypersplenism → pancytopenia); liver (Kupffer cell infection; hepatomegaly; hypoalbuminaemia from hepatic dysfunction; liver pathology partially reversible with treatment); bone marrow (macrophage infiltration → haemopoietic suppression → anaemia, thrombocytopenia, neutropenia). The A2 protein is required for hepatic/splenic persistence. Polyclonal B-cell activation produces hypergammaglobulinaemia — IgG and IgM rise dramatically but are not protective for VL; low albumin and high globulin produce a characteristic biochemical profile.
Host Immune Response
Disease Spectrum
| Feature / Manifestation | Clinical Detail |
|---|---|
| Incubation period | 2–6 months (range: weeks to >2 years); long incubation makes travel history critical |
| Fever | Prolonged, undulant; double daily fever spikes ("double quotidian"); duration months; relative bradycardia |
| Splenomegaly | Massive, progressive; may extend below umbilicus; can become largest organ by weight; hypersplenism → pancytopenia |
| Hepatomegaly | Less marked than splenomegaly; Kupffer cell infiltration; portal inflammation; hypoalbuminaemia; partially reversible |
| Skin darkening (kala-azar) | Hyperpigmentation of face, hands, feet; ACTH-like parasite products stimulate melanocytes; "kala-azar" = Hindi for "black fever" |
| Pancytopenia | Normocytic anaemia (haemolytic + hypersplenism + marrow suppression); thrombocytopenia (bleeding); neutropenia → secondary infections (common cause of death) |
| Post-Kala-azar Dermal Leishmaniasis (PKDL) | 5–10% (India) to ~50% (Sudan) post-treatment; macular → papular → nodular skin rash; residual amastigotes in dermis; important transmission reservoir; requires extended retreatment |
| HIV co-infection | Up to 35% of VL cases in Ethiopia; dramatically worse prognosis; high relapse rate; L-AmB preferred; secondary prophylaxis required |
Treatment & Prophylaxis
Liposomal Amphotericin B (L-AmB) — WHO Drug of Choice Globally
Single 10 mg/kg IV or 3–5 mg/kg × 3–5 doses; >95% cure rate in India; lipid formulation selectively targets macrophage-rich tissues (the parasite reservoir); far better safety than conventional AmB; expensive and requires refrigeration — significant barrier in endemic regions.
Miltefosine — Only Oral Treatment
2.5 mg/kg/day × 28 days orally; first effective oral treatment for VL; 94% cure rate in treatment-naïve patients in India; teratogenic (mandatory contraception required); emerging resistance (PKDL reservoir in India); still effective in Africa and Latin America.
Antimonials (SSG, Meglumine Antimoniate)
20 mg/kg/day IM × 28–30 days; effective in East Africa and Latin America; >60% clinical resistance in Bihar, India (renders these agents clinically useless there); cardiac toxicity (QTc prolongation), pancreatitis, painful injections.
Combination Therapy
L-AmB + miltefosine or SSG + paromomycin (East Africa): reduces treatment duration, prevents resistance emergence; used in WHO South Asian visceral leishmaniasis elimination campaigns targeting the 2030 zero-transmission goal.
Cross-Atlas Connections
References
- Kaye P, Scott P. Leishmaniasis: complexity at the host-pathogen interface. Nat Rev Microbiol. 2011;9(8):604-15. doi:10.1038/nrmicro2608 · PubMed 21747391
- Chappuis F, Sundar S, Hailu A, et al. Visceral leishmaniasis: what are the needs for diagnosis, treatment and control? Nat Rev Microbiol. 2007;5(11):873-82. doi:10.1038/nrmicro1748 · PubMed 17938629
- Bennett JE, Dolin R, Blaser MJ. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
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This entry covers Leishmania donovani biology, virulence mechanisms, and treatment. Vaccine development (L110f, ChAd63-KH), drug resistance mechanisms, and PKDL pathogenesis are planned expansions.