balantidiasis /bal′əntidī″əsis/ , an infection caused by ingestion of cysts of the protozoan Balantidium coli, the largest human protozoan. Pigs are the animal reservoir. In some cases, the organism is a harmless inhabitant of the large intestine, but infection with B. coli usually causes diarrhea. Infrequently the infection progresses, and the protozoan invades the intestinal wall and produces ulcers or abscesses, which may cause dysentery and death. Human infections, a rare event in industrialized countries, are usually acquired by ingestion of food or water contaminated by mammal feces. Diagnosis is made by identification of trophozoites in the stool or in sampled colonic tissue. Tetracycline, iodoquinol, or metronidazole is usually prescribed to treat the infection.