Balantidium coli is the only ciliated protozoan and can be found in monkeys in the tropics. It is occasionally found in humans, particularly among institutionalized groups and those with poor hygiene. B. coli is relatively rare in the U.S.
Human infections can range from asymptomatic to severe symptoms such as diarrhea or dysentery, nausea, anorexia, insomnia, and muscle weakness. Diarrhea can persist for months, causing the patient to lose a large amount of fluid.
Like E. histolytica, B. coli can invade tissue and cause ulceration. Occasionally, the organism can invade other organs, such as the lungs and peritoneal area, causing symptoms of urinary tract infection, peritonitis, and vaginitis.
The life cycle is shown to the right, indicating that a person becomes infected by consuming food or water contaminated with the organism's cyst form. B. coli typically lives in the colon in the trophozoite form and can form cysts that are passed in the stool.
