United States Office of Water September 2000
Environmental Protection Agency 4304
What is Giardia?
Giardia (je-ar’de-ah) are protozoan parasites which occur in a trophozoite and an oval-shaped cyst form. Cysts excreted in the feces of an infected host move passively through the environment. If cysts are ingested, infection may be transmitted to another vertebrate host. The trophozoite causes infection. Excystation to the trophozoite form is initiated in the stomach and completed in the small intestine. The
trophozoites divide, attach to the small intestine, and then detach for unknown reasons. During the encystment process, they become rounded and elaborate a cyst wall that protects the cyst as it is excreted and carried through water and other media.