Balantidium coli has distinctive microscopic features and a typical reservoir. What are they?

• A. A flagellated parasite; reservoir is dogs
• B. A large ciliated protozoan; reservoir is pigs
• C. An intracellular parasite; reservoir is cattle
• D. A sporozoan; reservoir is birds

Answer: (B)

Balantidium coli is a large ciliated protozoan that colonizes the human large intestine. Its distinctive microscopic features include cilia covering the cell and a macronucleus (often kidney-shaped) along with a smaller micronucleus, with cysts forming the infectious stage. Humans typically acquire infection by ingesting cysts from water or food contaminated with pig feces, making pigs the usual reservoir. This combination—a large ciliate and pigs as the reservoir—best fits the organism’s biology and transmission. It is not a flagellated parasite, not intracellular, and not a sporozoan, and other reservoirs like dogs, cattle, or birds do not apply.