Mode of Transmission of Entamoeba histolytica

Primary Route: Fecal-Oral

Infective Form: The Mature Cyst

• Only the cyst is infective - not the trophozoite
• Cysts are quadrinucleate (4 nuclei) and are passed in the feces of infected individuals
• Cysts can survive in the external environment for weeks to months, especially in damp conditions and temperatures between -5°C and 40°C - Fishman's Pulmonary Diseases, p. 1082
• Trophozoites, by contrast, do NOT survive outside the human host and are therefore not infectious by ingestion

Specific Mechanisms of Spread

Life Cycle After Ingestion

1. Ingested cysts pass through the stomach (acid-resistant wall protects them)
2. In the small intestine, each cyst excysts and forms 8 daughter trophozoites (via metacystic division)
3. Trophozoites migrate to the large intestine, where they may:
   • Remain as commensals (asymptomatic in ~90% of cases)
   • Invade the mucosa, causing amoebic colitis/dysentery
   • Enter the portal circulation and reach the liver (amebic liver abscess)
4. In the colon, trophozoites encyst again and are excreted in feces, completing the cycle - Fishman's Pulmonary Diseases, p. 1082

High-Risk Groups

• People in developing countries with poor sanitation
• Travelers to endemic regions (India, Africa, Mexico, Central/South America)
• Institutionalized individuals
• MSM (men who have sex with men)
• Immigrants from endemic areas - Rosen's Emergency Medicine, p. 3211

Key Points for Exam

• Infective stage = mature quadrinucleate cyst (ingested)
• Diagnostic stage = trophozoite or cyst (passed in stool)
• E. histolytica is classified as a Category B biodefense pathogen (CDC/NIAID) because of its low infectious dose, environmental stability, resistance to chlorine, and ease of spread through food/water contamination
• Entamoeba dispar is morphologically identical but non-pathogenic and 10x more prevalent; differentiation requires antigen, PCR, or immunologic testing - Fishman's Pulmonary Diseases, p. 1080

Pathogenicity of Entamoeba histolytica

Fig. 230-1 Life cycle of E. histolytica - Harrison's Principles of Internal Medicine, 22nd Ed.

Key Principle

Step-by-Step Mechanism of Pathogenicity

Step 1 - Adherence: Gal/GalNAc Lectin

• The trophozoite surface carries a galactose and N-acetyl-D-galactosamine (Gal/GalNAc)-specific lectin
• This lectin binds to carbohydrate determinants on colonic epithelial cells and mucus
• Adherence is the first and essential step - without it, no invasion occurs
• The Gal/GalNAc lectin is also directly linked to virulence: ingestion of enteropathogenic bacteria (e.g., EPEC, Shigella) increases its expression, while commensal E. coli decreases it - Harrison's, p. 2243

Step 2 - Mucus Degradation

• After attachment, trophozoites release glycosidases and proteases that degrade mucus polymers
• This strips away the protective mucous layer overlying the epithelium
• E. histolytica encodes at least 44 cysteine proteinase genes involved in:
  • Degrading colonic mucin glycoproteins
  • Digesting extracellular matrix components (collagen, elastin, villin)
  • Digesting hemoglobin
  • Inactivating IL-18 (an anti-parasitic cytokine)
  • Cleaving IgA and IgG antibodies, neutralizing host defenses
  • Degrading complement anaphylatoxins C3a and C5a - Goldman-Cecil, p. 3302; Harrison's, p. 2238

Step 3 - Contact-Dependent Cytotoxicity (Three Mechanisms)

• Goldman-Cecil, p. 3302; Harrison's, p. 2240

Step 4 - Contact-Independent Cytotoxicity

• Trophozoites produce prostaglandin E2 and other inflammatory mediators
• These increase ion permeability of intercellular tight junctions, disrupting epithelial barrier integrity - Harrison's, p. 2240

Step 5 - Phagocytosis of Host Cell Corpses

• After killing cells, trophozoites phagocytose the apoptotic corpses using multiple ligands including the Gal/GalNAc lectin, a phosphatidylserine receptor, serine-rich E. histolytica protein (SREHP), and collectins
• This phagocytic step is required for colonic infection to progress - Goldman-Cecil, p. 3302

Step 6 - Invasion and Ulcer Formation

• Trophozoites penetrate the colonic mucosa, producing characteristic flask-shaped ulcers (narrow neck, broad base in submucosa)
• Ulcers may coalesce, causing hemorrhage, perforation, or toxic megacolon in severe cases

Step 7 - Evasion of Host Immunity

• Antioxidant defense: E. histolytica lacks glutathione but uses a thioredoxin-thioredoxin reductase system to resist reactive oxygen and nitrogen species (ROS/RNS) from host phagocytes - Harrison's, p. 2242
  • This is also the target of metronidazole (inhibits this antioxidant system)
• Complement resistance: Cysteine proteases cleave C3a and C5a, preventing complement-mediated lysis
• Antibody degradation: IgA and IgG are proteolytically degraded
• Cathelicidin resistance: Amebic cysteine proteinases digest antimicrobial peptides produced by the host intestine

Step 8 - Hepatic (Extraintestinal) Invasion

• Trophozoites penetrate the intestinal wall and enter portal venous circulation
• Reach the liver, causing inflammation, necrosis, and formation of a hepatic "abscess" (actually a necrotic cavity, not a true pus-filled abscess)
• The right hepatic lobe is most commonly affected (due to portal blood flow patterns)
• Invasive disease is 10x more common in men than women - attributed to differences in complement-mediated killing and testosterone effects on IFN-γ secretion - Harrison's, p. 2222

Role of Gut Microbiome in Pathogenicity

• Protective bacteria (Ruminococcaceae, Coriobacteriaceae, Clostridiaceae) are present in asymptomatic carriers but absent in invasive disease
• Pathogenic gut flora (Shigella, EPEC) upregulate Gal/GalNAc lectin expression, worsening invasive capacity
• High Prevotella copri abundance in infants correlates with diarrhea during infection - Harrison's, p. 2248

Immune Response Summary

• Goldman-Cecil, p. 3303; Harrison's, p. 2251

Summary of Virulence Factors