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).
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 | Caecum, ascending colon; liver | Flask-shaped ulcers; liver abscess | Endemic regions with poor sanitation |
| Giardia duodenalis | Duodenum and jejunum | Little or none; villous blunting in heavy infection | Children, travellers, campers, IgA deficiency |
| Cryptosporidium | 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.
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.
Connections
Microorganisms3
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 (Centers for Disease Control and Prevention, 2024); About Crypto Infections (Centers for Disease Control and Prevention, 2024); Amebic Liver Abscess (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.