Giardia is a single-celled protozoan parasite that causes intestinal infections in both dogs and cats. Dogs become infected when they ingest Giardia cysts that may be present in water or other areas of the environment that have been contaminated with feces. Symptoms are more visible in puppies and younger dogs than in older dogs and can be acute, transient, intermittent, or chronic in nature. In some cases, dogs will exhibit diarrhea that is soft, frothy, greasy, and with a strong odor or excessive mucus.