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[Giardia intestinalis (syn. Giardia lamblia)]

Cysts are resistant forms and are responsible for transmission of giardiasis.  Both cysts and trophozoites can be found in the feces (diagnostic stages) .  The cysts are hardy and can survive several months in cold water.  Infection occurs by the ingestion of cysts in contaminated water, food, or by the fecal-oral route (hands or fomites) .  In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites) .  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 .  Encystation occurs as the parasites transit toward the colon.  The cyst is the stage found most commonly in nondiarrheal feces .  Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible.  While animals are infected with Giardia, their importance as a reservoir is unclear.