Ciliates – Balantidium coli

The Ciliates are a large group of protozoans that are characterized by a having numerous hair-like organelles on their surface called cilia. They vary in size greatly from one species to another (10um to 4mm).

The most clinically important Ciliate in humans is Balantidium coli and is described below.

Disease / Pathogenesis

Balantidium coli produces balantidiasis. The disease can mimic the presentation of Entamoeba histolytica as it can be very aggressively invasive, invading the wall of the colon. Symptoms include a chronic diarrhea, or a diarrhea that can be bloody. The invasion of the colon will cause colitis with deep ulcerations. These ulcerations can lead to perforation of the intestine. Other symptoms include abdominal pain, nausea, and foul breath.

Location in the Host

Balantidium coli is located in the colon of the infected host.

Geographic Distribution

Balantidium coli can be found in temperate and warm regions. Primates in tropical areas are frequently infected. In temperate regions pigs are infected and can serve as a source of infections.

Life Cycle

Image courtesy of CDC

Morphology & Diagnosis

Balantidium coli is a very large organism by comparison to the amoeba. Trophozoites are large and oval with an average size of 50-100u by 40-70-u wide. The trophozoite is characteristically surrounded by cilia. The cilia can at times be difficult to see if they are short.

Balantidium coli is very motile and has a rotary or boring motility. The organism can move quickly through the field of view when seen under the microscope.

The interior of the Balantidium coli trophozoite has two nuclei. One small “micronucleus” that can be difficult to see either in unstained or stained specimens. The other nucleus is a large “macronucleus” that is described as kidney bean shaped. The cytoplasm of the trophozoite is “dirty” with numerous bacteria, food particles, and contractile vacuoles. The posterior end of Balantidium coli contains a cytophyge (small opening) in which vacuoles empty. At the anterior narrowed end of the organism is a curved depression.

Balantidium coli cyst are most of time spherical in shape, but can be oval as well measuring 50-70u. As in the trophozoite there is the presence of a micronucleus and macronucleus. A thick wall surrounds the cyst contents with cilia visible immediately after the cell wall. Contractile vacuoles can be seen in the cytoplasm of young cysts while in older cysts the cytoplasm appears granular.


Balantidium coli trophozoite – note the cilia surrounding the organism. Usually a micronucleus and macronucleus are visible. In this image only the micronucleus is visible.

Balantidium coli illustration showing important identifying characteristics to observe along with its unique large size – Illustration by W. Vientos