Balantidium coli

Balantidium coli is a ciliated protozoan that infects the digestive tract. It is the largest protozoan and the only ciliated parasite that infects humans. Infection is not common and only a total of 1000 cases were reported prior to 1980. This intestinal parasite is the cause of Balantidiasis also known as Balantidiosis.

Balantidiasis was discovered in 1857 when Malmsten reported the first case at Stockholm. For more information on its discovery, please see References section for journal article.

Balantidium coli is most commonly found in areas of poor sanitation as well as areas where people come in close contact with pigs, sheep, and goats since the disease is usually spread by water or food contaminated with animal feces. Infections are usually asymptomatic; however, people with other concomitant infections are more likely to develop symptoms such as diarrhea (watery, bloody, mucoid), nausea, vomiting, abdominal pain, anorexia, headache, weight loss.

There are three types of manifestations of Balantidium coli infection:


The morphology of the parasite differs depending on the stage of its life cycle. As a cyst, the parasite is able to infect other hosts and survive outside of a host due to its protective outer coating. However, the parasite does not have the ability to move. Cysts are round and grow to 55um in length. As a trophozoite, the parasite is motile and can produce disease, but does not have the ability to survive outside a host. The trophozoites can grow to about 50-100um in length and 40-70um in width. The cilia are visible on the surface of the trophozoite as seen above, while the cysts are encased in a thick protective shell and the cilia can only be seen in the newly encysted organisms. The parasite also contains a visible bean shaped macronucleus, which can be seen in the Life Cycle section. The micronucleus is spherical and resides near the macronucleus.

Pictures - Top: Trophozoite - arrow points at cytostome or mouth, Bottom: Unstained Cyst


Parasite created by:
Nathaniel Hsu, Spring 2006,
Stanford University
Parasites & Pestilence: Infectious Public Health Challenges
Prof. D. Scott Smith,

Last Update: May 24, 2006