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Microbiology · Intestinal and urogenital protozoa

Balantioides coli !

  • Parasite
  • For completeness

The only ciliate that infects humans. It comes from water or food contaminated with pig feces and can cause dysentery.

Updated 2026-11-01

Also called: Balantidium coli (old name)

At a glance

Shape
Oval trophozoite covered in cilia, with a kidney-shaped macronucleus
Family
Balantidiidae
Type
Intestinal ciliated protozoan (trophozoite and cyst)
Stain
Direct stool exam (visible without stain because of its size)
Size
Trophozoite about 40 to 200 µm; cyst about 50 to 70 µm

Morphology

  • Large trophozoite covered in cilia, with a cell mouth (cytostome).
  • Kidney-shaped macronucleus and a small micronucleus.
  • Round, thick-walled cyst.
  • The largest protozoan seen in human stool.

Transmission

  • Fecal-oral: water or food contaminated with cysts, mainly from pig feces.
  • More common where pigs are raised and sanitation is poor.

Reservoir: Pigs (main); Humans, rodents and primates

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. Cysts are swallowed with contaminated water or food.
  2. Trophozoites are released in the small intestine and move to the colon.
  3. They multiply by binary fission and may exchange genetic material (conjugation).
  4. Some invade the colonic wall and cause ulcers.
  5. Trophozoites encyst and the cysts leave in the feces.

Virulence factors

  • Hyaluronidase and ciliary motility that help it invade the mucosa.
  • Colonic ulcers similar to those of amoebiasis.

Diseases it causes

  • Balantidiasis: from silent infection to colitis with diarrhea or dysentery; rarely perforation or extraintestinal disease.

Clinical picture

  • Most people have no symptoms.
  • Intermittent chronic diarrhea or dysentery with abdominal pain.
  • Severe forms in malnourished or immunocompromised people.

Diagnosis

  • Fresh stool exam: large trophozoites with fast ciliary motion; examined promptly because they die outside the colon.
  • Serial samples, since shedding is intermittent.
  • Colonic biopsy at colonoscopy.

Treatment

Tetracyclines are the classic option; metronidazole or iodoquinol are alternatives.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Keeping pig farming away from water sources and homes.
  • Safe water, handwashing and excreta management.

Vaccine: There is no vaccine.

Test yourself

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

What is unique about Balantioides coli among human parasites?

It is the only ciliated protozoan that infects humans and the largest one seen in stool.

What is its main reservoir?

The pig; infection is more common where pigs are raised and sanitation is lacking.

What illness does it cause when it invades the colon?

Colitis with diarrhea or dysentery and ulcers, resembling intestinal amoebiasis.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Balantidiasis (CDC (DPDx), 2024); Parasitosis humanas (6.ª ed.), Botero D, Restrepo M (Corporación para Investigaciones Biológicas (CIB), 2019).

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Representative model, for educational purposes only.