Balantidium coli: Morphology, Transmission, Laboratory diagnosis and Treatment

· Parasitology

Balantidium coli is a protozoan parasite belonging to the phylum Ciliophora, the members of which possess cilia in at least one stage of their life cycles. The ciliophora also have two different types of nuclei, one macronucleus and one or more micronuclei.

It has both the trophozoite and cyst forms as part of its life cycle. The cyst form is the infective stage. After ingestion of the cysts and excystation, trophozoites are formed which secrete hyaluronidase, that aids in the invasion of the tissue. The trophozoite stage is oval in structure somewhat pear shaped and covered with cilia. It is easily seen in wet mount preparations under low-power magnification. The cytoplasm contains both a macronucleus and a micronucleus, in addition to two contractile vacuoles that may harbor debris such as cell fragments and ingested bacteria.

Motile trophozoites can be seen in fresh wet preparations, but the specimen must be observed soon after collection. Cyst formation takes place as the trophozoite moves down the large intestine.

Pigs and chimpanzees act as an animal reservoir for Balantidium coli. It is the only pathogenic ciliate and the largest pathogenic protozoan known to infect humans. Transmission occurs by fecal-oral route following ingestion of the cysts in contaminated food or water. Infection is more common in warmer climates and in areas where humans are in close contact with pigs.

Infection with Balantidium coli is most often asymptomatic. However, symptomatic infection can occur, resulting in bouts of dysentery similar to amebiasis. In addition, colitis caused by Balantidium coli is often indistinguishable from that caused by Entamoeba histolytica. Symptoms typically include diarrhea, nausea, vomiting, headache, and anorexia. Fluid loss can be dramatic, as seen in some patients with cryptosporidiosis. The organism can invade the submucosa of the large bowel, and ulcerative abscesses and hemorrhagic lesions can occur. The shallow ulcers and submucosal lesions that result from invasion are prone to secondary infection by bacteria and can be problematic for the patient. Death due to invasive B. coli infection has been reported. Even though the organism can invade the underlying mucosa, it does not typically spread to other organs.


Differential characteristics of trophozoites and cysts of Balantidium coli


  • Shape and size: Ovoid with tapering anterior end; 50–100 um long, 40–70 um wide
  • Motility: Rotary, boring; may be rapid
  • Nuclei: 1 large kidney-bean-shaped macronucleus may be visible in unstained preparation; 1 small round micronucleus adjacent to macronucleus, difficult to see
  • Cytoplasm: May be vacuolated; may contain ingested bacteria and debris; anterior cytostome
  • Cilia: Body surface covered with longitudinal rows of cilia; longer near cytostome

Note: It may be confused with helminth eggs or debris on a permanent-stained smear, concentration or sedimentation examination is recommended.


  • Shape and size: Spherical or oval; 50–70 _m in diam
  • Nuclei: 1 large macronucleus, 1 micronucleus, difficult to see
  • Cytoplasm: Vacuoles are visible in young cysts; in older cysts, internal structure appears granular
  • Cilia: Difficult to see within the thick cyst wall

Laboratory diagnosis

Either microscopic examination for ova and cyst of feces or histologic examination of intestinal biopsy specimens establishes the diagnosis of Balantidium coli infections. The diagnosis can be established only by demonstrating the presence of trophozoites in stool or tissue specimens. It is very easy to identify these organisms in wet preparations and concentrated stool samples.

It can be challenging to identify Balantidium coli from trichrome-stained permanent smears because the organisms are so large and have a tendency to overstain. This makes the organism less discernible and increases the chance of misidentification.


The treatment of choice for B. coli infection is tetracycline, although it is considered an investigational drug. Metronidazole and iodoquinol can be used as therapeutic alternatives.

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