Pathogenesis of Entamoeba histolytica (Amebiasis)

1. The Organism - Quick Recap

• Cyst (10-20 µm): Infective form, quadrinucleate, has chitin wall, resistant to gastric acid and chlorination. Has rounded-end chromatoid bars (key ID feature).
• Trophozoite (10-60 µm): The ONLY form present in tissues, responsible for all pathology. Has ingested RBCs in cytoplasm - pathognomonic finding.

Note: E. histolytica is morphologically identical to non-pathogenic E. dispar, E. moshkovskii, and E. bangladeshi - molecular methods are needed for differentiation.

2. Life Cycle and Entry

1. Ingestion - Cysts are swallowed in fecally contaminated food/water
2. Excystation - Cysts pass through the stomach (gastric acid actually triggers excystation), then in the duodenum/small intestine the quadrinucleate cyst undergoes nuclear and cytoplasmic division → 8 trophozoites per cyst
3. Colonization - Trophozoites travel to the large intestine (cecum and ascending colon most commonly) and multiply under anaerobic conditions (they are obligate fermenters of glucose; they lack mitochondria and Krebs cycle enzymes)

3. Pathogenesis - Stepwise Invasion (The Core Topic)

Step 1: Adherence to Colonic Mucus

• Trophozoites adhere to colonic mucins via the Gal/GalNAc lectin (galactose/N-acetylgalactosamine-inhibitable adherence lectin)
• This lectin binding is essential for subsequent cytolysis - without contact, no killing occurs
• The lectin is also important for immune evasion and liver abscess formation

Step 2: Mucus Degradation

• Secreted amebic cysteine proteinases (especially cysteine protease 5/EhCP5) degrade the polymeric structure of colonic mucin
• This breaks the protective mucus barrier and enables direct contact with the underlying colonic epithelium

Step 3: Contact-Dependent Killing of Epithelial Cells

Step 4: Inflammatory Amplification (Host Damage)

• Amebic cysteine proteinases activate pre-IL-1β → IL-1β in neighboring epithelial cells
• IL-1β activates NF-κB, triggering a pro-inflammatory cytokine cascade
• This recruits neutrophils to the site
• Trophozoites then lyse the neutrophils, releasing toxic neutrophil granule contents - this causes further, amplified tissue damage (essentially the ameba turns the host's own defenses against the host)

Step 5: Tissue Invasion and Flask-Shaped Ulcer Formation

• After epithelial destruction, trophozoites degrade the extracellular matrix via proteinases and invadopodia (similar to metastatic cancer cell invasion structures)
• Trophozoites accumulate above the muscularis mucosae - downward invasion is often halted here for unclear reasons
• They then spread laterally, undermining the overlying mucosa
• This produces the classic "flask-shaped ulcer": narrow neck through the mucosa, expanding into a broad necrotic area in the submucosa
• The ulcers have a pinhead-sized opening, small neck, and large base - undermining large mucosal surfaces
• In established ulcers, few inflammatory cells are seen (immune evasion is highly effective)

4. Virulence Factors Summary Table

5. Extraintestinal Spread

1. Liver (most common) - ~40% of amebic dysentery cases develop liver abscesses
   • Amebic liver abscesses: >10 cm, "anchovy paste" contents, scant inflammatory reaction at margins, shaggy fibrinous lining
   • Central liquefaction of tissue, minimal mononuclear cell infiltrate
2. Lung - by direct extension from liver (transdiaphragmatic) or hematogenous spread
3. Brain - hematogenous spread (rare)
4. Pericardium/Heart - direct extension (rare)
5. Kidney - hematogenous (rare)

Key note: Amebae survive only in environments with low oxygen pressure (anaerobic). Ambient oxygen concentrations kill them - this is why they favor mucosal crypts and anaerobic tissue planes.

6. Host Factors that Determine Invasion

• Number of cysts ingested
• Pathogenic capacity of the strain (genetic polymorphisms in the E. histolytica genome correlate with virulence)
• Host factors: gut motility, immune competence, HLA class II alleles (susceptibility is linked to specific HLA types), leptin receptor mutation affecting epithelial cell signaling
• Enteric bacterial flora: certain bacteria enhance amebic growth

7. Immune Response and Evasion

• Children with fecal anti-amebic lectin IgA have short-lived protection against re-infection
• Amebic cysteine proteinases cleave IgA - a direct immune evasion mechanism

• 90% of colonized persons spontaneously clear infection within a year

• The parasite kills neutrophils and lymphocytes, releasing their toxic contents to cause more damage while eliminating the immune response

8. Pathology - What You'll See

9. Treatment Basis (Biochemical Rationale)

• Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Ch. 113
• Medical Microbiology, 9e (Murray), Ch. 72
• Robbins, Cotran & Kumar Pathologic Basis of Disease
• Jawetz, Melnick & Adelberg's Medical Microbiology, 28e, Ch. 46