Which is the largest intestinal protozoa found in humans?

Balantidium coli is the only ciliated protozoan that causes human disease (i.e., diarrhea) and its length is up to 200 micro meter, making it the largest of the human intestinal protozoa. Domestic animals, especially pigs, are the main reservoir for the organism, and humans are infected after ingesting the cysts in food or water contaminated with animal or human feces. The trophozoites excyst in the small intestine, travel to the colon, and, by burrowing into the wall, cause an ulcer similar to that of Entamoeba histolytica. However, unlike the case with E. histolytica, extraintestinal lesions do not occur. Most infected individuals are asymptomatic. Diagnosis is made by finding large ciliated trophozoites or large cysts with a characteristic V-shaped nucleus in the stool. There are no serologic tests. The treatment of choice is tetracycline. Prevention consists of avoiding contamination of food and water by domestic animal feces. Ref: Levinson W. (2012). Chapter 53. Minor Protozoan Pathogens. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.