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[Balantidium coli]

Causal Agents

Balantidium coli, a large ciliated protozoan parasite.

Life Cycle

Life Cycle

Cysts are the parasite stage responsible for transmission of balantidiasis The number 1. The host most often acquires the cyst through ingestion of contaminated food or water The number 2. Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine The number 3. The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by binary fission, during which conjugation may occur The number 4. Trophozoites undergo encystation to produce infective cysts The number 5. Some trophozoites invade the wall of the colon and multiply. Some return to the lumen and disintegrate. Mature cysts are passed with feces. The number 1

Geographic Distribution

Worldwide. Because pigs are an animal reservoir, human infections occur more frequently in areas where pigs are raised. Other potential animal reservoirs include rodents and nonhuman primates.

Clinical Presentation

Most cases are asymptomatic. Clinical manifestations, when present, include persistent diarrhea, occasionally dysentery, abdominal pain, and weight loss. Symptoms can be severe in debilitated persons.

Balantidium coli cysts in wet mounts.


Both Balantidium coli trophozoites and cysts are found in stool. Trophozoites are characterized by: their large size (40 µm to 200 µm), the presence of cilia on the cell surface, a cytostome, and a bean shaped macronucleus which is often visible and a smaller, less conspicuous micronucleus. Cysts are seen less frequently and range in size from 50 µm to 70 µm. B. coli can also invade tissue.

Figure A: B. coli cyst in a wet mount, unstained.

Figure B: B. coli cyst in a wet mount, unstained.

Figure C: B. coli cyst in a wet mount, unstained.

B. coli trophozoites.

Figure A: B. coli trophozoite in a wet mount, 500× magnification. Note the visible cilia on the cell surface.

Figure B: B. coli trophozoite under differential interference contract (DIC) microscopy, 500× magnification

Figure C: B. coli trophozoite in a wet mount, 1000× magnification. Note the visible cilia on the cell surface. Image contributed by the Oregon Public Health Laboratory.

Figure D: B. coli trophozoite in a Mann's hematoxylin stained smear, 500× magnification. Note the cytosome (black arrow) and the bean shaped macronucleus.

Balantidium coli trophozoites in intestinal tissue, stained with hematoxylin and eosin (H&E)

Figure A: Balantidium coli trophozoites in colon tissue stained with hematoxylin and eosin (H&E)at 200x magnification.

Figure B: Balantidium coli trophozoites in colon tissue stained with hematoxylin and eosin (H&E) at 400x magnification.

Figure C: Balantidium coli trophozoites in tissue stained with H&E.

Laboratory Diagnosis

Diagnosis is based on detection of trophozoites in stool specimens or in tissue collected during endoscopy. Cysts are less frequently encountered. Balantidium coli is passed intermittently and once outside the colon is rapidly destroyed. Thus stool specimens should be collected repeatedly, and immediately examined or preserved to enhance detection of the parasite.

More on: Morphologic comparison with other intestinal parasites

Treatment Information

Treatment information for balantidiasis can be found at:

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit

  • Page last reviewed: December 4, 2017
  • Page last updated: December 4, 2017
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