G. intestinalis Lifecycle

 

1-2. Giardia cysts are the infective stage of G. intestinalis.  As few as 10 cysts can cause infection (1).  These cysts are ingested by consuming contaminated food or water, or fecal-orally.  They can survive outside the body for several months, and are also relatively resistant to chlorination, UV exposure and freezing.(19)

3. When cysts are ingested, the low pH of the stomach acid produces excystation, in which the activated flagella breaks through the cyst wall(1). This occurs in the small intestine, specifically the duodenum.(19)  Excystation releases trophozoites, with each cyst producing two trophozoites. (13)

4. Within the small intestine, the trophozoites reproduce asexually (longitudinal binary fission) and either float free or are attached to the mucosa of the lumen.(13)

5. Some trophozoites then encyst in the small intestine.(13) Encystation occurs most likely as a result of exposure to bile salts and fatty acids, and a more alkaline environment.(7)  Both cysts and trophozoites are then passed in the feces, and are infectious immediately or shortly afterward (13).  Person-to-person transmission is possible(13).  Animals can also be infected with Giardia, and beavers have been associated with giardia outbreaks, although not definitively(5).

http://www.dpd.cdc.gov/dpdx/HTML/Giardiasis.htm