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Microbiology · Intestinal and urogenital protozoa

Entamoeba histolytica !

  • Parasite
  • Exam priority: high

An amoeba caught from water or food contaminated with feces. It can cause bloody dysentery and liver abscesses.

Updated 2026-11-01

Also called: amoeba, amebiasis agent

At a glance

Shape
Trophozoite with pseudopods; round cyst with up to four nuclei
Family
Entamoebidae
Type
Intestinal amoeboid protozoan (trophozoite and cyst)
Stain
Direct exam with iodine and saline; trichrome
Size
Trophozoite about 10 to 60 µm; cyst about 10 to 15 µm

Morphology

  • Trophozoite with one nucleus, fine peripheral chromatin and a small central karyosome.
  • May contain ingested red blood cells (erythrophagocytosis), a strong but not exclusive clue.
  • Mature cyst with four nuclei and chromatoid bodies with rounded ends.
  • Under the microscope it is identical to E. dispar and E. moshkovskii, which are non- or weakly pathogenic.

Transmission

  • Fecal-oral: water, food or hands contaminated with cysts.
  • Oral-anal sexual contact.
  • Asymptomatic carriers shed cysts and sustain transmission.

Reservoir: Humans (asymptomatic carriers)

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. Mature four-nucleus cysts are swallowed in contaminated water or food.
  2. In the small intestine the cyst opens and releases trophozoites.
  3. Trophozoites move to the colon and multiply by binary fission.
  4. They may stay in the lumen (carrier), invade the mucosa (colitis) or travel via the portal vein to the liver.
  5. In the colon, trophozoites encyst and cysts leave in feces, surviving for weeks.

Virulence factors

  • Gal/GalNAc lectin: adhesion to the colonic mucosa.
  • Amoebapores: punch holes in cells and kill them on contact.
  • Cysteine proteases: break down extracellular matrix and mucus, aiding invasion.
  • Phagocytosis of red cells and dead cells.

Diseases it causes

Clinical picture

  • Most infected people have no symptoms.
  • Colitis: abdominal pain, tenesmus and stools with mucus and blood of gradual onset; flask-shaped ulcers on histology.
  • Liver abscess: fever, right upper quadrant pain and tender hepatomegaly, often without diarrhea at the time.
  • Rare complications: rupture into pleura, pericardium or peritoneum; brain abscess; perianal skin lesions.

Diagnosis

  • Serial stool exams (several samples on different days): cysts or trophozoites, but they cannot separate E. histolytica from E. dispar.
  • Specific stool antigen or PCR: separate E. histolytica from non-pathogenic species.
  • Serology: positive in most liver abscesses; can stay positive for years.
  • Liver ultrasound or CT for abscess; aspiration is rarely needed.

Treatment

Two steps: a tissue amoebicide (nitroimidazole: metronidazole or tinidazole) for invasive disease, followed by a luminal agent (paromomycin, among others) to clear colonic cysts. Asymptomatic carriers of confirmed E. histolytica receive the luminal agent only. Antidiarrheals are avoided.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Safe drinking water: boiled, filtered or chlorinated.
  • Handwashing after using the toilet and before preparing or eating food.
  • Basic sanitation and proper excreta disposal.
  • Washing and disinfecting raw fruit and vegetables.

Vaccine: There is no vaccine.

Test yourself

Think of the answer, then tap the question to see it.

How many nuclei does the mature E. histolytica cyst have?

Four. An Entamoeba cyst with more than four points to E. coli, a commensal.

Why does a stool report of «E. histolytica/E. dispar» not confirm amoebiasis?

Because they look identical; only a specific antigen test or PCR confirms the pathogenic species.

What is the most common extraintestinal manifestation of E. histolytica?

Amoebic liver abscess, usually single and in the right lobe.

Why is a luminal amoebicide given after metronidazole?

Because the nitroimidazole works in tissue but does not reliably clear cysts in the colonic lumen, which cause relapse and spread.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Amebiasis (CDC, 2024); Amebiasis | Amebiasis | CDC (CDC, 2024); Amebiasis: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.