Balantidium coli, trophozoites, tissue

Balantidium coli can cause invasive intestinal disease in a similar fashion to Entamoeba histolytica: the trophozoites invade through the mucosa and then laterally into the underlying submucosa, resulting in production of flask-shaped ulcers. The two parasites can be differentiated by identifying the trophozoites in tissue; the trophozoites of Balantidium coli are very large, measuring 40 to 200 micrometers in maximum dimension, and are covered by a circumferential layer of cilia which can be appreciated even in tissue sections. Depending on the plane of section, the large "kidney-bean" shaped macronucleus can also often be seen. In comparison, the trophozoites of E. histolytica are much smaller (10 to 60 micrometers), are not ciliated, and have a very different appearing nucleus.