Entamoeba histolytica

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Key Points

  • Entamoeba histolytica
    • Characteristics
      • Parasitic amoeba with two forms
        • Cyst (infectious form)
        • Trophozoite (invasive form)
    • Transmission
      • Fecal-oral transmission
        • Ingestion of cysts in contaminated food/water
          • Usually in tropical countries with poor sanitation or in travelers
          • Small inoculum needed (1 cyst enough to cause disease)
    • Presentation: Amebiasis
      • Majority of cases (90%) are asymptomatic
      • Bloody diarrhea (dysentery) 
        • Presents with abdominal pain, stool with visible blood and mucus
        • Bloody due to intestinal invasion by trophozoite and resulting hemorrhage
        • Note: bloody helps differentiate vs. other parasitic diarrhea, e.g. Cryptosporidium
      • Liver abscess
        • Anchovy paste”-like brown exudate on needle aspiration
          • Mixture of acellular debris and necrotic hepatocytes
        • Presents with fever and RUQ abdominal pain/tenderness
        • May be visualized on abdominal ultrasound or CT
    • Diagnosis
      • Stool O&P (ova and parasite exam)
        • Specific use of stool microscopy; trophozoites and cysts seen in stool
          • May have up to 4 nuclei
        • Trophozoites have engulfed RBCs in their cytoplasm
      • Stool antigen tests (ELISA) or serum antibodies can also be used
      • Colonoscopy shows flask-shaped colonic ulcers
        • Flask-shape due to invasion and necrosis of intestinal wall
    • Treatment
      • Metronidazole
      • Paromomycin, diiodohydroxyquin, diloxanide
        • Luminal agents used to eliminate intraintestinal cysts
        • May be given as monotherapy for asymptomatic cyst passers
      • Needle aspiration/surgical resection for hepatic cysts