[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Gastrointestinal tract](https://durgical.com/en/general-pathology/areas/gi/) › Intestinal protozoa: amoebic flask ulcers, Giardia and Cryptosporidium

# Intestinal protozoa: amoebic flask ulcers, Giardia and Cryptosporidium

- Lesion

Single-celled parasites from dirty water either burrow into the colon (amoeba) or cling to the small-bowel lining (Giardia, Cryptosporidium).

Updated Nov 12, 2026

## Key points

- E. histolytica: cyst survives acid; trophozoites induce apoptosis, invade crypts and spread sideways in the submucosa: flask-shaped ulcers with a narrow neck and broad base; caecum and ascending colon most often.
- Trophozoites resemble macrophages but often contain ingested red cells.
- Amoebic liver abscess in a large share of amoebic dysentery: large, with scant inflammation and a shaggy fibrinous lining (anchovy-paste contents).
- Giardia: pear-shaped trophozoites with two nuclei on the villus surface; mucosa may look normal; worse with IgA deficiency or agammaglobulinaemia; variant surface protein switching.
- Cryptosporidium: small blue spheres that appear to sit on the brush border but are intracellular; terminal ileum and proximal colon; chlorine-resistant oocysts; chronic in AIDS.

## What it is

Three protozoa matter most. Entamoeba histolytica invades the colon and can spread to the liver; it is common in countries such as India, Mexico and Colombia. Giardia, the commonest parasitic pathogen of humans, clings to the duodenal brush border without invading. Cryptosporidium sits in a vacuole at the apex of enterocytes and causes watery diarrhoea, severe in people with immunodeficiency. Cysts or oocysts of all three resist chlorine or gastric acid.

## Step by step

- Cysts of E. histolytica survive the stomach and release trophozoites in the colon.
- Trophozoites attach to colonic epithelium and induce apoptosis.
- They invade crypts and then burrow laterally in the submucosa.
- Neutrophils arrive; the narrow mucosal opening and wide submucosal cavity form a flask ulcer.
- Trophozoites enter portal venules and reach the liver, forming an abscess.

## The three key intestinal protozoa

| Organism | Site | Lesion | Who is most at risk |
| --- | --- | --- | --- |
| [Entamoeba histolytica](https://durgical.com/en/conditions/intestinal-amoebiasis/) | Caecum, ascending colon; liver | Flask-shaped ulcers; liver abscess | Endemic regions with poor sanitation |
| [Giardia duodenalis](https://durgical.com/en/conditions/giardiasis/) | Duodenum and jejunum | Little or none; villous blunting in heavy infection | Children, travellers, campers, IgA deficiency |
| [Cryptosporidium](https://durgical.com/en/conditions/cryptosporidiosis-and-other-intestinal-coccidia/) | Terminal ileum, proximal colon, biliary tree | Apical organisms; villous atrophy in heavy infection | AIDS, malnourished children |

## Clinical notes

- Amoebiasis can cause necrotizing colitis and megacolon; metronidazole targets its fermentative metabolism.
- Giardia and Cryptosporidium are classic waterborne outbreak organisms because filtration, not chlorine, removes them.

## Memory aid

Amoeba digs a flask, Giardia hugs the villus, Crypto sits on the roof but lives inside.

## Test yourself

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

## Why are amoebic ulcers flask-shaped?

Trophozoites pass through a small mucosal opening and then spread laterally in the submucosa, creating a narrow neck and broad base.

## Which immune defect makes giardiasis more severe?

Deficient antibodies, especially secretory IgA deficiency or agammaglobulinaemia.

## Where does Cryptosporidium sit in relation to the enterocyte?

In a parasitophorous vacuole at the apex of the cell; it looks extracellular but is enclosed by host membrane.

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## Connections

## Related 5

- [Infectious enterocolitis](https://durgical.com/en/general-pathology/infectious-enterocolitis/)
- [Intestinal helminths](https://durgical.com/en/general-pathology/intestinal-helminths/)
- [Malabsorption and diarrhoea](https://durgical.com/en/general-pathology/malabsorption-and-diarrhoea/)
- [Liver infections: bacterial, fungal and parasitic](https://durgical.com/en/general-pathology/liver-infections-bacterial-fungal-and-parasitic/)
- [Patterns of necrosis: coagulative, liquefactive, caseous, fat, fibrinoid, gangrenous](https://durgical.com/en/general-pathology/patterns-of-necrosis-coagulative-liquefactive-caseous-fat-fibrinoid-gangrenous/)

## Related conditions 6

- [Intestinal amoebiasis (amoebic dysentery)](https://durgical.com/en/conditions/intestinal-amoebiasis/)
- [Amoebic liver abscess](https://durgical.com/en/conditions/amoebic-liver-abscess/)
- [Giardiasis](https://durgical.com/en/conditions/giardiasis/)
- [Cryptosporidiosis and other intestinal coccidia (Cyclospora, Cystoisospora)](https://durgical.com/en/conditions/cryptosporidiosis-and-other-intestinal-coccidia/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Bacterial infectious diarrhoea and dysentery](https://durgical.com/en/conditions/bacterial-infectious-diarrhoea-and-dysentery/)

## Microorganisms 3

- [Entamoeba histolytica](https://durgical.com/en/microbiology/entamoeba-histolytica/)
- [Giardia duodenalis](https://durgical.com/en/microbiology/giardia-duodenalis/)
- [Cryptosporidium spp.](https://durgical.com/en/microbiology/cryptosporidium-spp/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [About Giardia Infection](https://www.cdc.gov/giardia/about/index.html) (Centers for Disease Control and Prevention, 2024); [About Crypto Infections](https://www.cdc.gov/cryptosporidium/about/index.html) (Centers for Disease Control and Prevention, 2024); [Amebic Liver Abscess](https://www.ncbi.nlm.nih.gov/books/NBK430832/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/intestinal-protozoa-amoebic-flask-ulcers-giardia-and-cryptosporidium/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/intestinal-protozoa-amoebic-flask-ulcers-giardia-and-cryptosporidium/