Both Balantidium coli trophozoites and cysts are found in stool. Trophozoites are characterized by: their large size (40 µm to 200 µm), the presence of cilia on the cell surface, a cytostome, and a bean shaped macronucleus which is often visible and a smaller, less conspicuous micronucleus. Cysts are seen less frequently and range in size from 50 µm to 70 µm. B. coli can also invade tissue.
A, B, C: B. coli cysts in a wet mount, unstained.
A: B. coli trophozoite in a wet mount, 500× magnification. Note the visible cilia on the cell surface.
B: B. coli trophozoite under differential interference contract (DIC) microscopy, 500× magnification
C: B. coli trophozoite in a wet mount, 1000× magnification. Note the visible cilia on the cell surface. Image contributed by the Oregon Public Health Laboratory.
D: B. coli trophozoite in a Mann’s hematoxylin stained smear, 500× magnification. Note the cytosome (black arrow) and the bean shaped macronucleus
A, B: Balantidium coli trophozoites in colon tissue stained with hematoxylin and eosin (H&E). Image A taken at 200× magnification and Image B at 400×.
C: Balantidium coli trophozoites in tissue stained with H&E
Cysts are the parasite stage responsible for transmission of balantidiasis . The host most often acquires the cyst through ingestion of contaminated food or water . Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine . The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by binary fission, during which conjugation may occur . Trophozoites undergo encystation to produce infective cysts . Some trophozoites invade the wall of the colon and multiply. Some return to the lumen and disintegrate. Mature cysts are passed with feces .