# Entamoeba histolytica

> Entamoeba histolytica for NEET-PG Microbiology, covering amoebic dysentery, liver abscess, trophozoite and cyst morphology, E. dispar and drug treatment.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/entamoeba-histolytica
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Markell and Voge's Medical Parasitology; Harrison's Principles of Internal Medicine
- How to cite: "Entamoeba histolytica", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/entamoeba-histolytica

## Direct answer

Entamoeba histolytica is a protozoan that causes amoebic colitis and liver abscess, identified by trophozoites containing ingested red blood cells and mature cysts with four nuclei. It is morphologically identical to the non-pathogenic Entamoeba dispar, so the two are separated by antigen, PCR or isoenzyme tests. Metronidazole is the drug of choice for invasive disease, and a luminal agent must follow to eradicate the cyst stage.

## What you must remember

- **Trophozoite:** the invasive form contains ingested red cells, a key microscopic clue, and lives in the colonic mucosa.
- **Mature cyst:** four nuclei and cigar-shaped chromatoid bodies with rounded ends, while immature cysts have fewer nuclei.
- **Amoebic dysentery:** bloody mucoid stools with flask-shaped ulcers in the caecum and rectum, undermined at their edges.
- **Amoebic liver abscess:** usually solitary and in the right lobe, with anchovy-sauce pus, and it may occur without diarrhoea.
- **Entamoeba dispar:** non-pathogenic and identical on light microscopy, separated by PCR, isoenzyme analysis or antigen tests.
- **Tissue amoebicide:** metronidazole or tinidazole for invasive disease.
- **Luminal amoebicide:** diloxanide furoate, paromomycin or iodoquinol to clear cysts from the bowel and prevent relapse.

## Bloody diarrhoea after a trip

A young man returns from a trip to the plains of northern India with bloody mucoid diarrhoea, fever and a tender, enlarged liver.

1. **Look for the trophozoite:** a fresh stool or ulcer scraping is examined for red-cell-containing trophozoites, and the specimen must be examined quickly because trophozoites lyse.
2. **Check for cysts:** stool microscopy showing four-nucleate cysts confirms intestinal infection, although cysts are not always shed in acute dysentery.
3. **Image the liver:** ultrasonography shows a solitary right-lobe abscess, and aspiration is considered for large or threatened collections.
4. **Send serology and antigen tests:** serology is positive in most patients with liver abscess and helps where tissue is scarce, while antigen tests distinguish E. histolytica from E. dispar in stool.
5. **Start tissue amoebicide:** metronidazole treats invasive disease, and the full course is completed before the luminal agent is added.
6. **Eradicate the lumen:** a luminal agent such as diloxanide furoate clears cysts, because metronidazole alone does not eliminate them from the bowel.

Each step separates invasive disease from carriage, so the treatment covers both the tissue and the lumen.

## Where the exam tries to catch you

Entamoeba questions test the morphology, the species split and the two-drug logic of treatment.

- **Trap:** labelling every cyst-bearing stool as E. histolytica — **Correct:** E. dispar looks identical on microscopy and is non-pathogenic, so PCR or antigen tests are needed to separate them.
- **Trap:** stopping treatment after metronidazole — **Correct:** a luminal agent is required to clear cysts and prevent relapse or ongoing transmission.
- **Trap:** attributing a right-lobe liver abscess to bacteria by default — **Correct:** a solitary right-lobe abscess with anchovy-sauce pus points to an amoebic cause, although pyogenic abscess must still be excluded.

## Frequently asked questions

### How does Entamoeba histolytica differ from Entamoeba dispar?
Entamoeba histolytica and Entamoeba dispar look identical under light microscopy, but only E. histolytica is pathogenic. They are separated by PCR, isoenzyme analysis or stool antigen tests. Identifying the species matters because only E. histolytica infection needs treatment.

### What is the drug of choice for amoebic liver abscess?
Metronidazole is the drug of choice for amoebic liver abscess, given as a tissue amoebicide. It must be followed by a luminal agent such as diloxanide furoate or paromomycin to clear intestinal cysts. Aspiration may be added for large or threatened abscesses.

### What does the pus of an amoebic liver abscess look like?
Amoebic liver abscess typically contains thick, brownish necrotic pus, described as anchovy-sauce pus. The pus may be sterile on routine bacterial culture. Its appearance is a classic viva fact, but it cannot replace serology or imaging.

### How many nuclei does a mature Entamoeba histolytica cyst contain?
A mature Entamoeba histolytica cyst contains four nuclei, while immature cysts contain fewer. Chromatoid bodies are also seen in the cytoplasm of mature cysts. Counting nuclei is a standard microscopy question in NEET-PG examinations.