Life cycle of Giardia intestinalis
Giardia cysts are resistant forms and are responsible for transmission of giardiasis. Both cysts and trophozoites (the active stage) can be found in the feces (diagnostic stages) (1). The cysts are hardy and can survive several months in cold water. Infection occurs by the ingestion of cysts in contaminated water or food or by the fecal-oral route (hands or fomites, i.e., inanimate objects or substances capable of transferring pathogens) (2). In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites) (3). Trophozoites multiply by longitudinal binary fission, remaining in the lumen of the proximal small bowel, where they can be free or attached to the mucosa by a ventral sucking disk (4). Encystation occurs as the parasites move toward the colon.The cyst is the stage found most commonly in nondiarrheal feces (5). Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible. Non-human animals are infected with Giardia, but their importance as a reservoir for human infection is unclear.
From Centers for Disease Control Parasites and Health website