Entamoeba histolytica: trophozoite invasion and bloody diarrhoea
Entamoeba histolytica is a single-celled parasite whose trophozoites, its active feeding form, invade and ulcerate the colonic mucosa (the lining of the large bowel), typically causing bloody diarrhoea when symptomatic.
Remember
Cysts are the carriers, but it is the trophozoites that invade and ulcerate the colonic mucosa, so symptomatic infection typically gives bloody diarrhoea (dysentery) rather than watery diarrhoea from a toxin.
Entamoeba histolytica is an amoeba, a single-celled parasite. Infection starts with contaminated food or water. It exists in two forms.
The cyst is the tough form that survives outside the body and is swallowed to start the infection. The cyst then opens and releases the amoeba, which becomes a trophozoite, the active, feeding form.
Trophozoites stick to the cells lining the colon and kill them. They also release proteases, enzymes that break down proteins, and these digest the tissue between cells. So they invade the colonic mucosa, which is the lining of the large bowel, causing inflammation and ulceration. Because the lining is ulcerated, blood and mucus appear in the stool, and this picture is called dysentery. On biopsy the ulcers are typically flask-shaped, like a bottle under the microscope.
Most infected people have no symptoms, passing cysts in the stool. Trophozoites can also be carried to the liver in the portal circulation (the vessels bringing blood from the gut), forming an abscess, a pus-filled cavity that causes fever.
Key facts
- Cyst
- the tough, infective form that is swallowed in contaminated food or water; it does not invade tissue.
- Trophozoite
- the active, feeding form that invades the colonic mucosa.
- Site
- the large bowel, where trophozoites invade and typically produce flask-shaped ulcers. By contrast, Giardia sits on the small-bowel surface and causes malabsorption (poor absorption of nutrients).
- Stool
- diarrhoea with blood and mucus (dysentery), although most infected people have no symptoms.
- Cycle
- after some trophozoites turn back into cysts, these leave in the faeces (stool) and the cycle continues.
- Beyond the bowel
- trophozoites can reach the liver in the portal circulation and cause an abscess.
Exam traps
- A toxin acting in the small bowel, causing watery diarrhoea as in cholera, does not explain Entamoeba histolytica disease, where live trophozoites damage the colon directly.
- Bloody diarrhoea alone does not point to an amoeba, because Shigella, a bacterium, also invades the cells lining the colon, multiplies and forms abscesses in the bowel wall. The amoeba is a single-celled parasite that kills lining cells and digests tissue with proteases.
- In intestinal infections, villous atrophy (a flattening of the finger-like folds, the villi, that absorb nutrients) with malabsorption points to Giardia, whose trophozoites attach to the upper small bowel. Entamoeba histolytica affects the large bowel and produces dysentery, not malabsorption.
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