Balantidium coli is a species of ciliate protozoan, and is the only one that is a pathogen of humans. It is carried by pigs, rats, primates (including humans), rats, horses, cattle and guinea pigs. It is transmitted within or between these species mostly via fecal transmission. Pigs are the most significant reservoir hosts, though they show few if any symptoms.

Cysts are the parasite stage responsible for transmission of balantidiasis . 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. Both cysts and trophozoites are identifiable by a large, "sausage shaped" macronucleus.

The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by binary fission, during which conjugation may occur . Trophozoites undergo encystation to produce infective cysts . Some trophozoites invade the wall of the colon and multiply. Some return to lumen and disintegrate. Mature cysts are passed with feces. Symptoms can be local due to involvement of the intestinal mucosa, or systemic in nature and include diarrhea. Balantidiasis can be treated by carbarsone , tetracycline, or diiodohydroxyquin .

Less than 1% of the human population is infected worldwide.

Geographic Distribution

Worldwide. Because pigs are an animal reservoir, human infections occur more frequently in areas where pigs are raised, and in the Philipines, Mexico, and South America.

Source : http://www.dpd.cdc.gov/dpdx/HTML/Balantidiasis.htm