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©2013 UpToDate ® Print Email Balantidium coli life cycle Cysts are the parasite stage responsible for transmission of balantidiasis (1) . The host most often acquires the cyst through ingestion of contaminated food or water (2) . Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine (3) . 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 (4) . Trophozoites undergo encystation to produce infective cysts (5) . Some trophozoites invade the wall of the colon and multiply. Some return to the lumen and disintegrate. Mature cysts are passed with feces (1) . Reproduced from: Centers for Disease Control and Prevention. Parasites and Health: Balantidiasis. Available at: file://www.dpd.cdc.gov/dpdx/HTML/Balantidiasis.htm .