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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, 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 non-diarrheal 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.
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