Leishmania & Trypanosoma
Microbiology · Parasitology · lean revision notes
Leishmania & Trypanosoma
The haemoflagellates of the family Trypanosomatidae are kinetoplast-bearing protozoa (order Kinetoplastida) responsible for kala-azar, oriental sore, African sleeping sickness and Chagas disease. They are united by a single flagellum, a DNA-rich kinetoplast, and the existence of distinct morphological stages — a perennial favourite for image-based NEET PG questions.
Classification & the kinetoplast concept
Both genera belong to the family Trypanosomatidae, characterised by the presence of a kinetoplast — a specialised region of the single mitochondrion packed with extranuclear DNA (kDNA arranged as catenated minicircles and maxicircles). The kinetoplast lies near the basal body from which the flagellum arises and is a Giemsa-staining, deeply basophilic structure that helps identify these parasites.
Four morphological forms occur across the life cycles (you must be able to tell them apart on a slide):
| Stage | Flagellum | Kinetoplast position | Undulating membrane | Seen in |
|---|---|---|---|---|
| Amastigote (Leishman-Donovan / LD body) | Absent (intracellular) | Anterior, rod-like | Absent | Leishmania, T. cruzi (in tissues) |
| Promastigote | Anterior, free | Anterior end | Absent | Leishmania (sandfly gut, culture) |
| Epimastigote | Anterior | Just anterior to nucleus | Short | Trypanosoma (vector gut, culture) |
| Trypomastigote | Posterior, runs full length | Posterior end | Full-length, prominent | Trypanosoma (blood) |
High-yield: Leishmania exists as only two forms — amastigote (in human macrophages) and promastigote (in the sandfly). Trypanosoma passes through all four forms.
Mnemonic for stages by kinetoplast position (A-P-E-T → anterior to posterior): Amastigote (no flagellum) → Promastigote (anterior) → Epimastigote (near nucleus) → Trypomastigote (posterior). The kinetoplast marches from front to back.
LEISHMANIA
Species, vector & disease
All Leishmania are transmitted by the bite of female sandflies: Phlebotomus in the Old World (India, Africa, Mediterranean) and Lutzomyia in the New World. The parasite is an obligate intracellular organism of the reticulo-endothelial system (macrophages).
| Species | Disease | Geography | Reservoir |
|---|---|---|---|
| L. donovani | Visceral leishmaniasis (kala-azar) | India (anthroponotic), East Africa | Humans (India) |
| L. infantum / chagasi | Visceral (infantile) | Mediterranean, S. America | Dogs |
| L. tropica | Cutaneous — dry/urban oriental sore | Old World cities | Humans, dogs |
| L. major | Cutaneous — wet/rural | Rural Middle East, Africa | Rodents |
| L. braziliensis | Mucocutaneous (espundia) | Central/South America | Forest rodents |
| L. mexicana | Cutaneous — chiclero ulcer | Central America | Rodents |
High-yield: Indian kala-azar (L. donovani) is anthroponotic — humans are the only reservoir, the vector is Phlebotomus argentipes, and there is no animal reservoir. This makes elimination feasible and is the basis of India's kala-azar elimination programme.
Life cycle
Sandfly bite (promastigote injected) → phagocytosed by macrophage → transforms to amastigote → multiplies by binary fission, ruptures cell → infects new macrophages → sandfly takes blood meal, ingests amastigotes → transform to promastigotes in gut → migrate to proboscis (metacyclic) → next bite.
Visceral leishmaniasis (Kala-azar)
Clinical features:
- Long incubation (2–6 months), insidious onset.
- Massive splenomegaly (often the most striking sign) > hepatomegaly, fever (classically double-rise / dromedary twice-daily spikes), pancytopenia.
- Hyperpigmentation of skin — "kala-azar" literally means black fever (darkening of hands, feet, abdomen, forehead).
- Pancytopenia → anaemia, recurrent infections, bleeding.
- Massive polyclonal hypergammaglobulinaemia (IgG) with reversed albumin:globulin ratio — basis of the aldehyde test.
- Post-kala-azar dermal leishmaniasis (PKDL): hypopigmented macules/nodules appearing months–years after treatment (India 6 months–3 years). PKDL patients are a parasite reservoir and drive transmission.
High-yield: PKDL is the human reservoir that perpetuates anthroponotic transmission — eliminating PKDL is central to control. In India it follows treated VL; in Sudan it can be concurrent.
Diagnosis — investigation of choice:
- Demonstration of LD bodies (amastigotes) in splenic aspirate = highest sensitivity (~95%) and the gold standard, but carries bleeding risk. Bone marrow and lymph node aspirates are safer alternatives.
- rK39 immunochromatographic strip test — rapid, sensitive, the field test of choice for Indian VL; detects antibody to a kinesin antigen.
- Aldehyde (Napier's formol-gel) test — non-specific; serum + formalin → gelation and opacification within minutes due to raised IgG. Cheap screening, not confirmatory.
- Culture: NNN (Novy-MacNeal-Nicolle) medium → promastigotes.
- Montenegro (leishmanin) skin test: NEGATIVE in active VL (anergy), becomes positive after cure; positive in cutaneous forms.
High-yield: NNN medium grows the promastigote form. The leishmanin (Montenegro) test is negative in active kala-azar and positive in cutaneous leishmaniasis and post-treatment.
Management — drug of choice:
- Liposomal amphotericin B is the drug of choice in India (single-dose regimens used in elimination programmes) — preferred because of widespread antimony resistance in Bihar.
- Miltefosine — the first effective oral anti-leishmanial; teratogenic (contraindicated in pregnancy), used in combination/short regimens.
- Paromomycin (aminoglycoside, IM), pentavalent antimonials (sodium stibogluconate) — antimonials now largely abandoned in India due to resistance.
Cutaneous & mucocutaneous leishmaniasis
- Oriental sore (L. tropica/major): self-limiting papule → nodule → painless ulcer with raised indurated margins on exposed skin; heals with a depressed scar over months. Leishmanin test positive.
- Espundia (L. braziliensis): mucocutaneous — destructive metastatic lesions of the naso-oral mucosa, disfiguring.
- DCL (diffuse cutaneous): anergic, lepromatous-like nodules; leishmanin negative.
TRYPANOSOMA
Two clinically distinct groups: African (sleeping sickness) by tsetse fly, and American (Chagas) by reduviid bug. The blood form is the trypomastigote (the "C/S-shaped" flagellate with an undulating membrane on a blood film).
| Feature | African (T. brucei) | American (T. cruzi) |
|---|---|---|
| Disease | Sleeping sickness | Chagas disease |
| Vector | Tsetse fly (Glossina) | Reduviid / triatomine "kissing" bug |
| Transmission | Inoculative (bite, saliva) | Contaminative (bug faeces rubbed in) |
| Tissue form | Stays extracellular (trypomastigote) | Amastigote intracellular (heart, gut) |
| Key sign | Winterbottom's sign, sleeping | Romaña's sign, cardiomyopathy |
| Diagnosis | Blood/CSF/lymph node smear | Xenodiagnosis, smear (acute) |
African trypanosomiasis (Sleeping sickness)
Two subspecies, both spread by Glossina (tsetse fly):
| T. b. gambiense | T. b. rhodesiense | |
|---|---|---|
| Region | West/Central Africa | East Africa |
| Course | Chronic (months–years) | Acute (weeks–months) |
| Reservoir | Humans | Game animals, cattle (zoonotic) |
| Vector | Glossina palpalis (riverine) | Glossina morsitans (savanna) |
Clinical course (staged):
- Trypanosomal chancre at bite site.
- Haemolymphatic stage: intermittent fever, Winterbottom's sign (enlarged posterior cervical lymph nodes), Kerandel's sign (delayed deep hyperaesthesia).
- Meningoencephalitic (CNS) stage: daytime somnolence, reversed sleep cycle, tremor, coma — the "sleeping sickness."
Immune evasion is by antigenic variation of the variant surface glycoprotein (VSG) → recurrent parasitaemia waves; this defeats vaccine development.
High-yield: Winterbottom's sign (posterior cervical lymphadenopathy) is classic for T. b. gambiense. CNS staging requires CSF examination (cell count, trypomastigotes) and dictates drug choice.
Diagnosis: trypomastigotes in chancre fluid → blood → lymph node aspirate → CSF (depending on stage). CSF examination is mandatory for staging. Card agglutination test (CATT) screens for gambiense.
Management:
- Early (haemolymphatic): Suramin (rhodesiense) or pentamidine (gambiense).
- Late (CNS): Melarsoprol (arsenical, crosses BBB, causes reactive encephalopathy) or eflornithine (gambiense; "resurrection drug"). Newer oral fexinidazole treats both stages of gambiense.
High-yield: Drug choice depends on stage + species. CNS disease needs a BBB-penetrating drug (melarsoprol/eflornithine); peripheral disease uses suramin/pentamidine which do not cross into CNS.
American trypanosomiasis (Chagas disease)
Caused by T. cruzi, transmitted by the reduviid (triatomine "kissing") bug of genera Triatoma, Rhodnius, Panstrongylus. The bug defecates while feeding; metacyclic trypomastigotes in faeces are rubbed into the bite wound or conjunctiva — contaminative transmission. Also via blood transfusion, transplacental, and oral (contaminated sugarcane/açaí juice).
In tissues, T. cruzi multiplies as amastigotes (especially cardiac muscle, GI smooth muscle, neurons), unlike African forms which remain extracellular trypomastigotes in blood. The blood trypomastigote is C- or U-shaped with a large posterior kinetoplast.
Clinical features:
- Acute phase: Romaña's sign — unilateral painless periorbital/eyelid oedema with conjunctivitis at the conjunctival entry site; or a chagoma (indurated skin lesion) at cutaneous entry. Fever, lymphadenopathy, hepatosplenomegaly; myocarditis/meningoencephalitis can be fatal in children.
- Indeterminate phase: asymptomatic, seropositive, may last years.
- Chronic phase:
- Dilated cardiomyopathy with conduction defects (right bundle branch block, apical aneurysm, arrhythmias, sudden death) — leading cause of death.
- Megaoesophagus (dysphagia) and megacolon (constipation) from destruction of the myenteric (Auerbach's) plexus.
High-yield: Romaña's sign = unilateral periorbital oedema in acute Chagas. Chronic Chagas → cardiomyopathy + megaoesophagus + megacolon from autonomic ganglion destruction.
Diagnosis:
- Acute: trypomastigotes on Giemsa-stained blood smear (high parasitaemia).
- Chronic: low parasitaemia → serology (≥2 tests) and xenodiagnosis (allow clean lab-reared reduviid bugs to feed on patient, then examine bug gut for parasites) or PCR.
Management — drug of choice: Nifurtimox or Benznidazole. Both are most effective in the acute phase; chronic organ damage is largely irreversible (manage heart failure, pacemaker, surgery for megacolon).
High-yield: Benznidazole/nifurtimox work best in acute Chagas; chronic cardiomyopathy responds poorly — emphasising early treatment and vector/blood-bank control.
Complications (cross-summary)
- Kala-azar: PKDL, secondary bacterial/TB infections, bleeding, cachexia, death if untreated (>90% mortality untreated). VL is an AIDS-defining opportunistic infection.
- Cutaneous: disfiguring scars; mucocutaneous espundia → midface destruction.
- African trypanosomiasis: fatal meningoencephalitis; rhodesiense can cause acute myocarditis.
- Chagas: sudden cardiac death, thromboembolism from apical aneurysm, volvulus from megacolon.
Key differentials
- Kala-azar vs other causes of fever + massive splenomegaly: malaria, chronic myeloid leukaemia, myelofibrosis, tropical splenomegaly, schistosomiasis, lymphoma. The combination of fever + huge spleen + pancytopenia + hypergammaglobulinaemia in an endemic area points to VL.
- LD bodies vs Histoplasma vs Toxoplasma on tissue smear: LD bodies are intracytoplasmic amastigotes with a nucleus + rod-shaped kinetoplast (the kinetoplast distinguishes them from Histoplasma yeasts and Toxoplasma).
- Chagas cardiomyopathy vs other DCM: apical aneurysm + RBBB + serology in a Latin American patient.
- Romaña vs orbital cellulitis / nephrotic oedema: unilateral, painless, with conjunctival entry point.
High-yield image fact: On Giemsa smear, the LD body shows a dot-like nucleus plus a smaller bar-shaped kinetoplast ("double-dot"); this and the absent flagellum distinguish it from look-alike intracellular fungi.
Recently asked / exam angle
- Image-based: identify amastigote (LD body) in bone marrow/splenic aspirate; identify the trypomastigote on a peripheral smear and name the vector.
- One-liner matches: Winterbottom's sign → T. b. gambiense; Romaña's sign → acute Chagas; chiclero ulcer → L. mexicana; espundia → L. braziliensis.
- Drug DOC: liposomal amphotericin B for Indian kala-azar; melarsoprol for CNS sleeping sickness; benznidazole for Chagas.
- Vector pairs: Phlebotomus → Leishmania; Glossina (tsetse) → African; Reduviid/triatomine bug → Chagas.
- Diagnostic tests: rK39 strip and aldehyde test for VL; xenodiagnosis for chronic Chagas; NNN medium grows promastigotes; leishmanin test negative in active VL.
- Concept: transmission type — African is inoculative, Chagas is contaminative (bug faeces).
- Pathophysiology link: megacolon/megaoesophagus due to Auerbach's plexus destruction; antigenic variation via VSG in African trypanosomes.
Rapid revision
- Kinetoplast = extranuclear mitochondrial DNA defining family Trypanosomatidae.
- Leishmania = amastigote (human macrophage) + promastigote (sandfly/NNN culture) only.
- Indian kala-azar = L. donovani, vector Phlebotomus argentipes, anthroponotic (no animal reservoir).
- VL = massive splenomegaly + double-rise fever + pancytopenia + hypergammaglobulinaemia + skin darkening.
- Splenic aspirate is the most sensitive test (gold standard); rK39 is the field test; aldehyde test detects high IgG.
- Leishmanin (Montenegro) test is NEGATIVE in active VL, positive in cutaneous disease/after cure.
- DOC for Indian kala-azar = liposomal amphotericin B; miltefosine is oral but teratogenic.
- PKDL is the human reservoir maintaining transmission.
- African sleeping sickness → tsetse fly (Glossina); Winterbottom's sign = posterior cervical nodes (gambiense, chronic).
- CNS-stage African disease needs melarsoprol or eflornithine; staging requires CSF.
- Chagas → reduviid bug (contaminative, faeces); Romaña's sign (acute), cardiomyopathy + megaviscera (chronic) via T. cruzi amastigotes.
- Chagas DOC = benznidazole/nifurtimox, effective mainly in the acute phase; xenodiagnosis for chronic cases.