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[Microbiology](https://durgical.com/en/microbiology/) · [Intestinal and urogenital protozoa](https://durgical.com/en/microbiology/groups/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

## Key points

- Four-nucleus cyst; trophozoite with ingested red cells.
- Dysentery of gradual onset and right-lobe liver abscess.
- Microscopy cannot tell E. histolytica from E. dispar: use antigen or PCR.
- Two-step treatment: metronidazole or tinidazole, then a luminal agent such as paromomycin.
- Serology is most useful for liver abscess.

## 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

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

- Intestinal amoebiasis: from silent carriage to amoebic colitis with dysentery, toxic megacolon, perforation or ameboma. [Intestinal amoebiasis (amoebic dysentery) →](https://durgical.com/en/conditions/intestinal-amoebiasis/)
- Amoebic liver abscess: the most common extraintestinal form; right lobe, «anchovy paste» contents. [Amoebic liver abscess →](https://durgical.com/en/conditions/amoebic-liver-abscess/)

## 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:

- [Metronidazole](https://durgical.com/en/pharmacology/metronidazole/)
- [Tinidazole](https://durgical.com/en/pharmacology/tinidazole/)

## 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.

## In Colombia

Amoebiasis is common where sanitation is poor. Colombian stool reports often read «E. histolytica/E. dispar complex»; that alone does not confirm the pathogenic species, so asymptomatic people are not treated without an antigen test or PCR confirming E. histolytica.

## Memory aid

«Histolytica dissolves tissue»: it eats red cells, digs flask ulcers and climbs to the liver; treatment has two stops, «tissue then lumen».

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/entamoeba-histolytica/)

## Related

- [Commensal intestinal amoebae (Entamoeba coli, E. dispar, Endolimax nana and others)](https://durgical.com/en/microbiology/commensal-intestinal-amoebae/)
- [Giardia duodenalis](https://durgical.com/en/microbiology/giardia-duodenalis/)
- [Balantioides coli](https://durgical.com/en/microbiology/balantioides-coli/)

## Related conditions

- [Intestinal amoebiasis (amoebic dysentery)](https://durgical.com/en/conditions/intestinal-amoebiasis/)
- [Amoebic liver abscess](https://durgical.com/en/conditions/amoebic-liver-abscess/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Intestinal and urogenital protozoa](https://durgical.com/en/microbiology/groups/intestinal-and-urogenital-protozoa/)

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](https://www.cdc.gov/dpdx/amebiasis/index.html) (CDC, 2024); [Amebiasis | Amebiasis | CDC](https://www.cdc.gov/amebiasis/about/index.html) (CDC, 2024); [Amebiasis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000298.htm) (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/entamoeba-histolytica/