Entamoeba histolytica

Aka: Entamoeba histolytica, E. histolytica, Amebiasis, Amoebiasis, Amebic Dysentery, Amoebic Dysentery, Ameboma, Amoeba Histolytica Dysentery, Amebic Liver Abscess, Amoebic Liver Abscess

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II. Epidemiology

  1. Prevalence: 10% worldwide
  2. Asymptomatic cyst carriage in 90% cases
  3. Symptomatic cases per year: 50 million worldwide
  4. Fatalities per year: 100,000

III. Risk factors

  1. Mental health institutions (High Prevalence)
  2. Crowded living conditions
  3. Poor sanitation
  4. Travel to endemic areas
    1. Asia
    2. Africa
    3. Latin America

IV. Pathophysiology

  1. Two forms
    1. Cyst (12 um diameter): Spheres with up to 4 nucleii
      1. Divides into trophozoites in Small Intestine
      2. Cysts can survive weeks in moist environment
    2. Trophozoite (25 um long)
      1. Contains 1 nucleus and ingested RBCs
      2. Moves via finger-like pseudopods toward colon
      3. Some trophozoites transform into cysts
  2. Results in enterocolitis
    1. Intraluminal disease
      1. Profuse Diarrhea with malabsorption
      2. Ulceration of colon and terminal ilium
      3. Intestinal bleeding
    2. Systemic dissemination
      1. Liver Abscess
      2. Lung Abscess
      3. Brain Abscess
  3. Transmission via fecal-oral route
    1. See Waterborne Illness
    2. See Foodborne Illness
    3. Food preparation contaminated by poor hygiene
    4. Human waste used for crop fertilization
    5. Oral-anal sex

V. Symptoms

  1. Acute
    1. Fulminant onset
    2. Cramping, moderate to severe Abdominal Pain
    3. Bloody, profuse Diarrhea
    4. Mucus in stools
    5. Tenesmus
    6. Malaise
  2. Chronic
    1. Normal stools alternate with symptomatic phase

VI. Signs

  1. Acute
    1. Fever
    2. Diffuse abdominal tenderness
    3. Dehydration
    4. Weight loss
  2. Chronic
    1. Fever
    2. Tenderness and cramping of cecum and ascending colon
  3. Liver Abscess (within 5 months of onset)
    1. Fever (10-15 of cases)
    2. RUQ Abdominal Pain or liver tenderness
    3. Liver friction rub if Liver Abscess present
    4. Diarrhea (33% of cases)

VII. Differential Diagnosis

  1. See Waterborne Illness
  2. See Foodborne Illness
  3. Appendicitis
  4. Inflammatory Bowel Disease (especially Crohn's Disease)

VIII. Complications

  1. Ameboma growth into intestinal lumen
    1. Risk of Bowel Obstruction
    2. Risk of Intussusception
  2. Toxic Megacolon
  3. Pneumatosis coli
  4. Abscess formation
    1. Lung Abscess
    2. Brain Abscess
    3. Liver Abscess
      1. See signs above
      2. Risk of rupture
      3. Risk factors for complication
        1. Multiple cysts or cysts >10 cm in size
        2. Superior right liver lobe involvement
        3. Left liver lobe involvement
      4. Course
        1. Spontaneous resolution by 6 months in 66%
        2. Persist >1 year in 10%

IX. Labs

  1. Entamoeba histolytica by stool PCR (preferred)
  2. Entamoeba histolytica by stool Antigen testing
    1. Test Sensitivity: 87%
    2. Test Specificity: >90%
  3. Ova and Parasite exam (3 samples required)
    1. Precaution: Microscopy alone does not distinguish E histolytica from the benign E. dispar
    2. Fresh Stool Exam with Microscopy and gross exam
    3. Motile or encysted organisms
    4. Watery stool with mucus or blood
  4. Liver Function Tests
    1. Alkaline Phosphatase increased in 75% of cases
    2. Serum Aminotransaferases (AST, ALT) increased in 50% of cases
    3. Serum Bilirubin is typically normal
  5. Other stool tests
    1. Fecal Leukocytes positive
    2. Occult blood positive
    3. Fecal Eosinophilia (Charcot-Leyden crystals present)

X. Diagnostic Testing

  1. Endoscopy
    1. Mimics Crohn's Disease
    2. Colonic ulcerations
      1. Discrete ulcers of variable depth in right colon
      2. Exudative hyperemic ulcers with small Hemorrhages
  2. Biopsy
    1. Intramural trophozoites at edge of ulceration

XI. Imaging

  1. Barium Enema may show Ameboma
    1. Irregular barium distribution in ascending colon
  2. Right Upper Quadrant Ultrasound
    1. Hepatic Abscess (oval hypoechoic cyst)

XII. Management: Asymptomatic cysts

  1. Preferred agents
    1. Paromomycin 25-35 mg/kg/day orally divided three times daily for 7 days or
    2. Iodoquinol (Yodoxin) 650 mg orally three times daily for 20 days
  2. Alternative agent
    1. Diloxanide furoate (Furamide) 500 mg orally three times daily for 10 days

XIII. Management: Diarrhea or mild Dysentery

  1. Requires combined use of both tissue and luminal agent
  2. Tissue agents for trophozoites (choose one)
    1. Metronidazole (Flagyl) 500 to 750 mg orally three times daily for 7-10 days OR
    2. Tindazole 2 g orally daily for 3 days
  3. Luminal agents for cysts (choose one) - start after tissue agent course is completed
    1. Paromomycin 25-35 mg/kg/day orally divided three times daily for 7 days OR
    2. Iodoquinol (Yodoxin) 650 mg orally three times daily for 20 days

XIV. Management: Severe disease (e.g. Liver Abscess)

  1. Requires combined use of both tissue and luminal agent
  2. Tissue agents for trophozoites (choose one)
    1. Metronidazole (Flagyl) 750 mg IV three times daily for 10 days or
    2. Tindazole 2 g orally daily for 5 days
  3. Luminal agents for cysts (choose one) - start after tissue agent course is completed
    1. Paromomycin 25-35 mg/kg/day orally divided three times daily for 7 days or
    2. Iodoquinol (Yodoxin) 650 mg orally three times daily for 20 days

XV. References

  1. Gilbert (2015) Sanford Guide to Antimicrobials, accessed on IOS app 5/11/2016
  2. Kucik (2004) Am Fam Physician 69(5):1161-8 [PubMed]
  3. Petri (1999) Clin Infect Dis 29:1117-25 [PubMed]

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