[Balantidium coli]

Causal Agents

Balantidium (=Neobalantidium) (=Balantioides) coli, a large ciliated protozoan, is the only ciliate known to be capable of infecting humans. It is often associated with swine, the primary reservoir host. Recent molecular analyses have suggested the need for taxonomic revision, and it is now sometimes referred to as Neobalantidium coli or Balantioides coli, although this nomenclature has neither been resolved nor widely adopted in the medical community.

Life Cycle

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


Swine are the primary reservoir hosts. Humans can also be reservoirs, and other potential animal hosts include rodents and nonhuman primates.

Geographic Distribution

Balantidium coli occurs worldwide. Because pigs are the primary reservoir, human infections occur more frequently in areas where pigs are raised and sanitation is inadequate.

Clinical Presentation

Most cases are asymptomatic. Clinical manifestations, when present, may be acute or chronic with abdominal symptoms. Complications of associated diarrhea or dysentery can occur in protracted infections. Symptoms may be severe or fatal in debilitated/immunocompromised persons.

Extraintestinal infection is rare but potentially serious and typically occurs secondary to intestinal infection. Peritonitis and liver abscesses have been noted following intestinal perforation or rupture of fulminant colonic ulcers. Invasion of urogenital tract may be caused by contamination from the anal region or through fistulae caused by severe infection.

Balantidium coli cysts in wet mounts.

Both Balantidium coli trophozoites and cysts may be shed in stools. Both stages may occur in diarrheal stools, and usually only cysts are observed formed stools. 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 trophozoites can also invade tissue.


Figure A: B. coli cyst in a wet mount, unstained.
Figure B: B. coli cyst (dart) and trophozoite (arrow) in the same field, from a primate fecal sample.
Balantidium coli trophozoites.
trophozoite in a primate fecal sample
Figure A: B. coli trophozoite in a primate fecal sample. Note the prominent groove of the cytostome (arrow), cilia (darts), and starch granules within the cell.
Figure D: 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 B: B. coli trophozoite in a wet mount, 500× magnification. Note the visible cilia on the cell surface.
Figure E: B. coli trophozoite in a Mann's hematoxylin stained smear, 500× magnification. Note the cytosome (black arrow) and the bean shaped macronucleus.
Figure C: B. coli trophozoite under differential interference contract (DIC) microscopy, 500× magnification
Balantidium coli trophozoites in intestinal tissue, stained with hematoxylin and eosin (H&E)
Figure A: B. coli trophozoites in colon tissue stained with hematoxylin and eosin (H&E) at 200x magnification.
Figure B: B. coli trophozoites in colon tissue stained with hematoxylin and eosin (H&E) at 400x magnification.

Laboratory Diagnosis

Diagnosis is based on detection of trophozoites in stool samples from symptomatic patients or in tissue collected during endoscopy. Cysts are less frequently encountered, and are most likely to be recovered from formed stool. 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; concentration via sedimentation or flotation can increase the probability of recovery. Lugol’s iodine is sometimes used for staining, but may obscure internal morphological features.

More on: Morphologic comparison with other intestinal parasites

Laboratory Safety

Standard protocols  apply for the processing of stool samples. Precautions (e.g., PPE, use of a biosafety cabinet) should be taken to avoid accidental exposure to cysts in unfixed stool specimens, as these may be potentially infectious to laboratory personnel.

Suggested Reading

Schuster, F.L. and Ramirez-Avila, L., 2008. Current world status of Balantidium coli. Clinical Microbiology Reviews, 21 (4), pp.626–638.

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