Medicine & USMLE

Entamoeba histolytica

  1. Giardia lamblia
  2. Toxoplasma gondii
  3. Entamoeba histolytica
  4. Cryptosporidium
  5. Naegleria fowleri
  6. Trypanosoma brucei
  7. Plasmodium Overview
  8. Plasmodium Disease (Malaria)
  9. Babesia
  10. Trypanosoma cruzi
  11. Leishmania
  12. Trichomonas vaginalis
  13. Enterobius vermicularis (pinworm)
  14. Ascaris lumbricoides (giant roundworm)
  15. Strongyloides stercoralis (threadworm)
  16. Ancylostoma and Necator
  17. Trichinella spiralis
  18. Trichuris trichiura (whipworm)
  19. Toxocara canis
  20. Onchocerca volvulus
  21. Loa loa
  22. Wuchereria bancrofti
  23. Taenia solium
  24. Diphyllobothrium latum
  25. Echinococcus granulosus
  26. Schistosoma
  27. Clonorchis sinensis
  28. Sarcoptes scabiei (Scabies)
  29. Pediculus humanis and Phthirus pubis (Lice)


Entamoeba histolytica is a parasitic amoeba that is transmitted through the fecal-oral route, usually by ingesting contaminated water or food. The parasite may rarely be transmitted sexually between men who have sex with men. While most infections are asymptomatic, symptomatic infections result in bloody diarrhea, known as dysentery. An infection with entamoeba can be complicated by spread to the liver, where the amoeba forms an abscess filled with an anchovy paste like exudate. An infection with entamoeba is diagnosed through a stool ova and parasite exam, which reveal entamoeba trophozoites with engulfed red blood cells inside, or by way of a colonoscopy that reveals flask-shaped ulcers. Finally, metronidazole is the standard treatment option for Entamoeba, although luminal agents like paromomycin may also be used to clear intraintestinal cysts.

Key Points

  • Characteristics
    • Parasitic amoeba with two forms
      • Cyst (infectious form- dormant)
      • Trophozoite (invasive form- activated/feeding stage)
    • Transmission
      • Fecal-oral transmission
        • Ingestion of cysts in contaminated food/water
      • Sexually transmissible
        • Men who have sex with men (MSM)
    • Presentation: Amebiasis
      • Bloody diarrhea (dysentery)
      • Liver abscess
        • Anchovy paste”-like brown exudate on needle aspiration
    • Diagnosis
      • Stool O&P
        • Trophozoites and cysts seen in stool
          • May have up to 4 nuclei
        • Trophozoites have engulfed RBCs in their cytoplasm
      • Colonoscopy shows flask-shaped colonic ulcers
    • Treatment
      • Metronidazole
      • Paromomycin, diiodohydroxyquin, diloxanide