Balantidium coli is a parasitic species of ciliate alveolates that causes the disease balantidiasis. It is the only member of the phylum Ciliophora known to be pathogenic to humans, In addition, B. coli is known to infect multiple species of mammal including cattle, camels, sheep, buffalo, and rodents, as well as some birds and marine mammals in rare cases. B. coli has a world wide distribution, but tends to be spread more in humid and moist environments.
Morphology
thumb|Balantidium coli as seen in a wet mount of a stool specimen. The organism is surrounded by cilia.
Balantidium coli has two developmental stages, a trophozoite stage and a cyst stage. In trophozoites, the two nuclei are visible. The macronucleus is long and sausage-shaped, and the spherical micronucleus is nested next to it, often hidden by the macronucleus. The opening, known as the peristome, at the pointed anterior end leads to the cytostome, or the mouth. Cysts are smaller than trophozoites and are round and have a tough, heavy cyst wall made of one or two layers. Usually only the macronucleus and sometimes cilia and contractile vacuoles are visible in the cyst, however, both nuclei are present because nuclear multiplication does not occur when the organism is a cyst. Living trophozoites and cysts are yellowish or greenish in color.
Transmission
Balantidium is the only ciliated protozoan known to infect humans. Balantidiasis is a zoonotic disease and is acquired by humans via the fecal–oral route from the normal host, the domestic pig, where it is asymptomatic. Contaminated water is the most common mechanism of transmission.
Role in disease
Balantidium coli lives in the cecum and colon of humans, pigs, rats, and other mammals. It is not readily transmissible from one species of host to another because it requires a period of time to adjust to the symbiotic flora of the new host. Once it has adapted to a host species, the protozoan can become a serious pathogen, especially in humans. Trophozoites multiply and encyst due to the dehydration of feces. The disease poses a problem mostly in developing countries, where water sources may be contaminated with swine or human feces.