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Entamoeba histolytica - High-Yield Points for 2nd BDS Microbiology

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1. Classification (Taxonomy)

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2. Morphology - Two Key Forms

A. Trophozoite (Vegetative Form) - Invasive form

• Size: 12-60 µm (average 15-30 µm)
• Only form found in tissues
• Cytoplasm has two zones: hyaline ectoplasm (outer) + granular endoplasm (inner)
• PATHOGNOMONIC FEATURE: Contains ingested red blood cells (erythrophagocytosis) in cytoplasm
• Single nucleus with a central, small karyosome + fine, evenly distributed peripheral chromatin
• Motility: Unidirectional, rapid movement with pseudopodia (finger-like)
• Found in: liquid/diarrheal stools; survives only a few hours outside host

B. Cyst (Infective Form) - Transmission form

• Size: 10-20 µm
• Up to 4 nuclei (mature quadrinucleate cyst is infective)
• Contains chromatoid bars with rounded/blunt ends (important to distinguish from E. coli - which has splintered/frayed ends)
• May contain glycogen vacuole (disappears in mature cyst)
• Found in: formed/solid stools; resistant to gastric acid, chlorination

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3. Life Cycle

Ingestion of mature quadrinucleate CYST (fecal-oral route)
        ↓
Excystation in duodenum (gastric acid stimulates release)
        ↓
TROPHOZOITES released → multiply in large intestine
        ↓
Either: 
(a) Invade colonic epithelium → Disease (amebiasis)
(b) Remain in lumen → form PRE-CYST → mature CYST
        ↓
Cysts passed in formed feces → Environmental contamination
        ↓
Re-ingestion (fecal-oral route) completes cycle

• Definitive host: Humans (only natural host)
• No intermediate host
• Infective stage: Mature quadrinucleate cyst
• Diagnostic stage: Trophozoite (diarrheal stool) or Cyst (formed stool)

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4. Pathogenesis - "Flask-Shaped Ulcer" (High-Yield!)

• Cysts excyst in duodenum → trophozoites invade large intestine (especially cecum, ascending colon, sigmoid, rectum)
• Trophozoites attach via galactose-inhibitable adherence lectin (Gal/GalNAc lectin)
• Cause lytic necrosis of colonic epithelial cells by:
  • Cytotoxin production
  • Lethal alteration of host cell membrane permeability
  • Irreversible rise in intracellular calcium
• Classic lesion: Flask-shaped ulcer - small point of entry, narrow neck, expanded necrotic submucosa
• Lysis of neutrophils → toxic contents released → further tissue destruction
• Ameboma: inflammatory granulomatous mass in intestinal wall (can mimic carcinoma)

Extraintestinal Spread:

• Via portal circulation → Liver (most common - right lobe affected)
• Amebic liver abscess: contains "anchovy paste/chocolate sauce" material (sterile, necrotic)
• Less common: lungs, brain, spleen, pericardium

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5. Clinical Features

Intestinal Amebiasis:

• Majority are asymptomatic carriers (most carry non-pathogenic E. dispar)
• Symptomatic: crampy abdominal pain, diarrhea, dysentery
• Amoebic dysentery: bloody mucoid stools ("red-currant jelly" stool), tenesmus
• No fever typically (unlike bacillary dysentery)

Amebic Liver Abscess (Extraintestinal):

• Right hypochondriac pain, hepatomegaly
• Elevation of right dome of diaphragm

• 50% of liver abscess patients give NO history of intestinal symptoms

• Diagnosed by USG/CT/MRI

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6. Laboratory Diagnosis

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7. Differentiation: E. histolytica vs E. coli (Very Exam-Frequent!)

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8. E. histolytica vs E. dispar (Important!)

• Morphologically identical - cannot be differentiated by microscopy
• E. dispar is the non-pathogenic, more common commensal form
• Differentiated by: isoenzyme analysis (zymodeme), monoclonal antibody-based stool antigen tests, PCR
• Only E. histolytica contains erythrocytes in trophozoite cytoplasm

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9. Treatment

• Note: Metronidazole alone does NOT eradicate luminal cysts - always follow with a luminal agent.

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10. Epidemiology

• Worldwide distribution; highest in tropical/subtropical regions with poor sanitation
• Prevalence 10-15% in endemic areas; up to 50% in some zones
• ~50 million cases/year globally; up to 100,000 deaths/year
• 2nd most common cause of death from parasitic disease (after malaria)
• Transmission: fecal-oral (contaminated water, food, vegetables)
• Mechanical vectors: flies, cockroaches
• High-risk groups: institutionalized patients, homosexual men (oral-anal practices), travelers to endemic areas

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11. Quick-Recall Points for MCQs

• Infective form = mature quadrinucleate cyst
• Diagnostic form in tissue = trophozoite
• RBCs in cytoplasm = pathognomonic of E. histolytica
• Ulcer shape = Flask-shaped
• Liver abscess contents = "Anchovy paste" (chocolate sauce colored)
• Most common site of liver abscess = Right lobe
• Drug for invasive = Metronidazole; drug for luminal/cyst passer = Diloxanide furoate
• Non-pathogenic lookalike = E. dispar
• Chromatoid bars in E. histolytica = rounded ends (vs splintered in E. coli)

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Leishmania donovani - High-Yield Points for 2nd BDS Microbiology

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1. Classification (Taxonomy)

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2. Historical Facts (MCQ-Favourite!)

• Discovered in 1903 simultaneously by:
  • William Boog Leishman - found parasite in spleen smears from soldiers in India
  • Charles Donovan - found it at Government General Hospital, Madras
• Hence named Leishman-Donovan (LD) bodies - the amastigote stage seen in tissue
• "Kala-azar" = Hindi for "black fever" (due to skin darkening)
• "Dum-Dum fever" - after Dum-Dum cantonment near Calcutta where early cases described

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3. Morphology - TWO Key Stages

A. Amastigote (Leishman-Donovan body) - in HUMANS

• Size: 2-3 µm, oval/round
• Non-flagellate (no external flagellum)
• Found intracellularly inside macrophages/reticuloendothelial cells
• On Leishman/Giemsa stain: "dot and dash" appearance
  • Dot = nucleus (round, red-purple)
  • Dash = kinetoplast (rod-shaped, dark-staining, next to nucleus)
• Found in: spleen, liver, bone marrow, lymph nodes

B. Promastigote (Leptomonad form) - in SANDFLY

• Size: 15-25 µm, elongated/spindle-shaped
• Has a free anterior flagellum (motile)
• Kinetoplast at anterior end
• Found in: sandfly gut + saliva (infective stage)
• Cultivated on NNN medium (Novy-MacNeal-Nicolle medium)

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4. Life Cycle

SANDFLY (Phlebotomus) - Intermediate/Vector Host
        |
Infected sandfly bites human → injects PROMASTIGOTES
        ↓
Promastigotes phagocytosed by macrophages/monocytes
        ↓
Convert to AMASTIGOTES inside macrophages
(lose flagellum, become intracellular)
        ↓
Amastigotes multiply by binary fission → fill cytoplasm
        ↓
Cell bursts → released amastigotes phagocytosed by new macrophages
        ↓
Spreads via reticuloendothelial system:
SPLEEN → LIVER → BONE MARROW → LYMPH NODES
        ↓
Uninfected sandfly bites infected human → ingests amastigotes
        ↓
In sandfly midgut: amastigotes → PROMASTIGOTES (with flagella)
        ↓
Migrate to sandfly proboscis → infective for next bite

• Definitive host: Humans
• Intermediate/Vector: Female sandfly (Phlebotomus spp.)
• Infective stage to humans: Promastigote
• Diagnostic stage in humans: Amastigote (LD body)

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5. Vector - Sandfly (Phlebotomus)

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6. Reservoirs

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7. Pathogenesis & Clinical Features - Visceral Leishmaniasis

Cardinal Features (HFAS mnemonic):

• Hepatosplenomegaly (massive splenomegaly - most prominent sign)
• Fever - intermittent, irregular, sometimes double-daily spike; 39-40°C
• Anemia + Agranulocytosis + thrombocytopenia (pancytopenia)
• Skin darkening (hyperpigmentation - especially forehead, temples, perioral, midabdomen) → "Kala-azar" = black fever

Progressive Disease:

• Emaciation, marked weight loss, weakness
• Epistaxis, purpura (due to thrombocytopenia)
• Susceptibility to secondary bacterial infections
• Oral ulcerations (cancrum oris/noma)
• FATAL if untreated - death ~2 years from onset

Post-Kala-Azar Dermal Leishmaniasis (PKDL):

• Occurs 1-2 years after apparent cure of kala-azar
• Especially in India
• Macular/maculopapular/nodular skin rash (face, trunk)
• Parasites abundant in skin lesions
• Patient becomes a reservoir for transmission
• Treated with prolonged antimony therapy

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8. Laboratory Diagnosis

Definitive Diagnosis:

Serological Tests:

Montenegro (Leishmanin) Skin Test:

• NEGATIVE during active kala-azar (anergy due to immunosuppression)
• Becomes positive after cure (indicates past exposure/immunity)
• Not useful for active diagnosis of visceral leishmaniasis

Other:

• Pancytopenia on CBC (anemia + leukopenia + thrombocytopenia)
• Hypergammaglobulinemia (polyclonal) + hypoalbuminemia (reversed A:G ratio)
• Elevated ESR, elevated serum bilirubin, abnormal liver function tests
• PCR: most sensitive and specific; used for species identification

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9. Treatment

India note: Widespread resistance to antimonials (sodium stibogluconate) → Amphotericin B / Miltefosine now preferred in India

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10. Comparison of Leishmaniasis Types (High-Yield Table)

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11. Quick-Recall Points for MCQs

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