The Giardia life cycle begins by consuming food or water contaminated with cysts, just as in Entamoeba. A major difference is that once Giardia excysts, the trophozoite stage lives in the small intestine instead of the colon. Also, though it can attach to the mucosa, it typically does not invade the tissue or metastasize to other organs, as does Entamoeba spp. It reproduces by longitudinal binary fission. The trophozoite is often firmly attached via its ventral disk to the duodenal epithelium. If it moves, it is generally to the jejunum, where it will encyst (change into the resistant cysts form). The cyst form is passed in the stool, as is the trophozoite, but only the cyst will survive long enough to infect the next host.
Regarding Dientamoeba fragilis, much less is known about the life cycle, and some sources will not list a life cycle. It generally was considered not to have a cyst form; however, recent reports in the literature indicate that there is a cyst form, but it is rare and difficult to identify, so for our purposes, we will consider it not to have a cyst. Two ways of possible infection are the fecal-oral route (although this has never been definitively proven) or it is passed along with helminth eggs.
