Giardia lamblia cysts
is the protozoan that causes giardiasis, with its accompanying severe diarrhea, abdominal discomfort, malabsorption, and/or weight loss. Outbreaks occur often in the United States due to consumption of impure water containing Giardia cysts. The cysts are somewhat resistant to chlorine. Boiling of water and/or filtration are normally required to kill them. Because Giardia may be acquired by drinking water in the wilderness without purification, its common name is "backpacker's parasite". Giardia moves with flagella, placing it in the class Mastigophora.Cysts of Giardia lamblia are shown at 1000 X magnification in fecal material. Each cyst has an oval shape and usually two to four nuclei. The nuclei of Giardia may contain a central karysome that stains darkly. An axostyle, appearing like a dark line through the center of the cyst, may be visible. Parabasal bodies may also appear as comma-shaped structures. Diagnosis depends on finding cysts or trophozoites (trophs) in the stool.
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