Amoebiasis / Entamoeba histolytica

Classification

Important distinction: What was once called "E. histolytica" morphologically actually encompasses four species: pathogenic E. histolytica and non-pathogenic E. dispar, E. moshkovskii, and E. bangladeshi - all morphologically indistinguishable; differentiation requires molecular methods.

1. Morphology

A. Trophozoite (Vegetative form)

• Size: 12-50 µm (invasive forms typically >20 µm; non-pathogenic forms 15-20 µm)
• Motility: Actively motile by unidirectional pseudopodial movement - a hyaline pseudopod is extended, then the rest of the cell is drawn forward ("snail-like")
• Cytoplasm: Two zones:
  • Ectoplasm (outer): Clear, hyaline
  • Endoplasm (inner): Granular; may contain ingested red blood cells (pathognomonic - seen only in E. histolytica, never in E. coli or other commensals)
• Nucleus: Single; nuclear membrane lined by fine, regularly distributed chromatin granules (uniform peripheral chromatin); karyosome (endosome) is small and central
• Key feature: Erythrophagocytosis (RBCs in cytoplasm) is the only pathognomonic feature in stool specimens

B. Cyst (Infective form)

• Size: 10-20 µm
• Shape: Spherical, with a refractile wall
• Nuclei: 1 (uninucleate, immature) → 2 → 4 nuclei (mature, quadrinucleate - infective stage). Each nucleus has the same fine peripheral chromatin and central karyosome as the trophozoite
• Chromatoid bars: Cigar-shaped masses of ribonucleoprotein with smooth, rounded ends (this distinguishes from E. coli cysts, which have splintered/frayed chromatoid bars)
• Glycogen vacuole: Present in early cysts; disappears as maturation progresses
• Found only in lumen of colon and mushy/formed feces

Comparison: E. histolytica vs. E. coli

2. Life Cycle

Step-by-step:

1. Ingestion of mature quadrinucleate cysts - via contaminated food/water (fecal-oral route). Trophozoites cannot survive in the external environment or gastric acid, so cysts are the infective stage.
2. Excystation - Cysts pass through the stomach; exposure to gastric acid stimulates release of the pathogenic trophozoite in the duodenum. Each quadrinucleate cyst produces 8 trophozoites (4 nuclei → 8 cells by binary fission).
3. Colonization of large intestine - Trophozoites descend to the large intestine (primarily caecum and ascending colon, then rectosigmoid). They dwell in the colon, multiply by binary fission.
4. Two possible outcomes:
   • Non-invasive: Trophozoites remain in the lumen → encyst → cysts passed in formed stool → environmental contamination → transmission
   • Invasive: Trophozoites invade the bowel wall → ulceration → may enter portal circulation → liver (most common extraintestinal site), then lungs, brain, heart, skin
5. Encystment - When conditions become unfavorable (temperature drop, desiccation), trophozoites retract pseudopodia, become spherical, and encyst (uninucleate → quadrinucleate cyst)
6. Excretion - Cysts excreted in stool; carriers can shed up to 1.5 × 10⁷ cysts/day. Cysts survive in feces, water, and soil for several days at low temperature/moisture.

Cyst survival: Survive in moist conditions for days to weeks; resistant to chlorination at standard water treatment doses; killed by drying, heating to ~55°C, or freezing. Sand filtration and boiling are effective.

3. Pathogenesis

Mechanisms of tissue invasion

1. Galactose-inhibitable adherence protein (lectin) - Trophozoites attach to colonic epithelial cells via a Gal/GalNAc-specific lectin on their surface. This attachment is required for cytolysis to occur.
2. Cytotoxin production - After attachment, trophozoites secrete toxins that cause lytic necrosis (not inflammatory necrosis initially). This causes a lethal alteration in host cell membrane permeability → irreversible rise in intracellular calcium → cell death.
3. Lysing of immune cells - Trophozoites lyse colonic epithelial cells, neutrophils, lymphocytes, and monocytes. Release of toxic neutrophil constituents after neutrophil lysis amplifies tissue destruction.
4. Anaerobic preference - Amebae are killed by ambient oxygen concentrations; they thrive in low-oxygen environments (colonic lumen, abscess cavities).

Pathological lesions

• Initial lesion: Pinhead-sized ulcer with raised edges in the mucosa; mucus, necrotic cells, and amebae exude
• Trophozoites multiply above the muscularis mucosae, spreading laterally → undermine mucosa
• Classic "flask-shaped" (collar-button) ulcer: Narrow neck through mucosa leading to an expanded necrotic area in the submucosa - small point of entry, large necrotic base. This is the hallmark lesion of amoebiasis.
• Bacterial superinfection occurs in larger, established ulcers
• Coalescence of ulcers → large denuded mucosal areas
• Ameboma (amoebic granuloma): Granulomatous tumorlike mass on intestinal wall, can obstruct the lumen and mimic carcinoma - produces "napkin ring lesion"
• Perforation into peritoneal cavity is possible

• Trophozoites penetrate the muscularis, enter mesenteric venules → portal blood → liver
• Amebic liver abscess (ALA): Most common extraintestinal form (~5% of intestinal cases). Predominantly right lobe, usually single. Aspirate is classically "anchovy sauce" / chocolate-colored pus (liquefied hepatic parenchyma). Trophozoites found at the margin of abscess, not in the center.
• Hematogenous/contiguous spread → lungs (rupture across diaphragm), brain, pericardium, skin (perianal ulcers)

• Number of amebae ingested
• Pathogenic capacity of the strain (zymodeme)
• Host immunity and gut motility
• Enteric bacteria (enhance amebic growth)

4. Laboratory Diagnosis

A. Microscopy (Stool Examination)

• Gold standard for intestinal amoebiasis
• Examine at least 3 stool specimens on separate days (parasites not homogeneously distributed)
• Fresh liquid/semi-liquid stools → look for motile trophozoites (survive only a few hours outside body; must be examined immediately)
• Formed stools → cysts (more stable)

• Trophozoites show rapid, progressive, unidirectional motility via hyaline pseudopodia; sharp ectoplasm-endoplasm demarcation

• Best method for detailed morphologic identification
• Shows nuclear detail: uniform peripheral chromatin + central karyosome
• Ingested RBCs in trophozoites = pathognomonic for invasive E. histolytica

Note: E. histolytica is morphologically indistinguishable from E. dispar, E. moshkovskii, and E. bangladeshi - RBC ingestion is the only microscopic pathognomonic feature.

B. Culture

• Robinson's medium / Jones' medium / Boeck-Drbohlav medium
• Not widely used for diagnosis; mainly for research
• Useful for zymodeme analysis

C. Antigen Detection (EIA/ELISA)

• Enzyme immunoassay (EIA) on stool specimens
• Highly specific; can differentiate E. histolytica from E. dispar
• Commercially available (e.g., E. histolytica II ELISA, TechLab)
• Sensitivity ~85-90%, specificity ~99%
• Preferred over microscopy for species-level diagnosis

D. Serology (Antibody Detection)

• Detects host anti-amebic IgG antibodies
• Most useful for extraintestinal amoebiasis (ALA)
• ~95% of ALA patients are seropositive
• ~70% seropositive in active intestinal infection
• ~10% seropositive in asymptomatic carriers
• Tests: Indirect hemagglutination (IHA) - most sensitive; ELISA; counterimmunoelectrophoresis (CIE); indirect fluorescent antibody (IFA); latex agglutination
• Limitation: Titers persist months-to-years after treatment, so cannot distinguish current from past infection; less useful in endemic areas

E. Molecular Methods (PCR / DNA probes)

• Most sensitive and specific method
• Can definitively distinguish E. histolytica from E. dispar and other species
• FDA-approved multiplex panels: BioFire FilmArray GI Panel, BD MAX Enteric Parasite Panel, xTag GI Pathogen Panel
• Useful when serology/antigen tests are inconclusive

F. Aspiration of Liver Abscess

• Aspirate is chocolate-brown ("anchovy sauce") pus
• Trophozoites more likely in last aspirated material (at abscess wall margin)
• Direct microscopy or permanently stained slides
• PCR amplification of amebic 16S rRNA from aspirate is highly reliable

G. Imaging

• Ultrasound, CT scan, MRI of liver: Show abscess (usually single, right lobe)
• Cannot distinguish amebic from pyogenic abscess on imaging alone - requires serology

Summary Table: Lab Diagnosis

Epidemiology (Quick Review)

• Worldwide distribution; highest in tropical/subtropical regions with poor sanitation
• Global: ~50 million infected; ~100,000 deaths/year
• India: ~15% prevalence (range 3.6-47.4%)
• Incubation period: 2-4 weeks (can be longer)
• Transmission: fecal-oral (contaminated water/food), oral-anal sexual contact, mechanical vectors (flies, cockroaches)
• Cysts resist standard chlorination; sand filtration and boiling are effective

Treatment (Brief)

• Invasive/systemic disease: Metronidazole or tinidazole (systemic penetration) → followed by luminal agent
• Luminal agents (to eradicate cysts/asymptomatic carriage): Diloxanide furoate, paromomycin, iodoquinol
• Treatment of asymptomatic carriers prevents environmental shedding