Juranek, Dennis D. Centers for Disease Control and Prevention, Atlanta, Georgia.
Last reviewed:February 2016
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A disease caused by the protozoan parasite Giardia lamblia, characterized by chronic diarrhea that usually lasts one or more weeks. The presence of the protozoan parasite Giardia lamblia (also known as Giardia intestinalis and Giardia duodenalis) in the human small intestine is known as giardiasis. It is one of the most common types of waterborne disease. This condition also affects many mammals, birds, and reptiles, allowing for possible zoonotic transmission. The diarrhea caused by giardiasis may be accompanied by one or more of the following symptoms: abdominal cramps, bloating, flatulence, fatigue, or weight loss. The stools are malodorous and have a pale greasy appearance. Infection without symptoms is also common. As with most other protozoa inhabiting the intestinal tract, the life cycle of Giardia involves two stages: trophozoite and cyst (see illustration). Trophozoites stay in the upper small-intestinal tract, where they actively feed and reproduce. When the trophozoites pass down the bowel, they change into the inactive cyst stage by rounding up and developing a thick exterior wall, which protects the parasite after it is passed in the feces. Humans become infected either directly by hand-to-mouth transfer of cysts from feces of an infected individual (or animal) or indirectly by drinking feces-contaminated water. After the cyst is swallowed, the trophozoite is liberated through the action of digestive enzymes and stomach acids, and becomes established in the small intestine. See also: Diarrhea; Diplomonadida; Gastrointestinal tract disorders; Infant diarrhea; Protozoa; Water-borne disease; Zoonoses
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