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Balantidium coli is a large ciliated protozoan that lives as a parasite in the intestines of humans and other animals and is the cause of human balantidiasis. Balantidium coli has a worldwide distribution. Because pigs are an important animal reservoir, human infections are more common in areas where pigs are raised. Other potential animal reservoirs include rodents and non-human primates.
The cyst stage of the B. coli life cycle is responsible for transmission.The host most often acquires the cyst through ingestion of contaminated food or water. Following ingestion, excystation occurs in the small intestine and the trophozoites colonize the large intestine. The trophozoites reside in the lumen of the large intestine of humans and other animals, where they reproduce by binary fission, during which conjugation (an exchange of genetic material) may occur. Trophozoites undergo encystation to produce infective cysts. Some trophozoites invade the wall of the colon and multiply. Some return to the lumen and disintegrate. Mature cysts are passed with feces.