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Giardiasis is caused by the protozoan parasite Giardia duodenalis (formerly called G. lamblia or G. intestinalis). This anaerobic flagellate protozoan is found on surfaces or in soil, food, or water that has been contaminated by feces from humans or animals hosting the parasite. The disease has substantial global burden; for 2010, the World Health Organization estimated that there were 183.8 million cases worldwide (95% uncertainty interval, 130.0–262.8 million).1 Annually, more than 1 million cases occur in the United States.2


Giardia has two stages in its life cycle—cyst and trophozoite—and is transmitted by the fecal–oral route. While both cysts and trophozoites can be found in the feces, only cysts cause disease transmission, partly because their outer shell allows them to survive for weeks to several months in the environment and makes them moderately chlorine tolerant.3 These properties, combined with a low infectious dose, make Giardia ideally suited for transmission through ingestion of contaminated water.4,5 However, transmission also occurs through contact with feces (e.g., through caring for an infected person), eating contaminated food, or contact with contaminated surfaces.4–9

Infection typically begins with cyst ingestion from contaminated water, food, or fomites (e.g., hands). In the small intestine, excystation releases trophozoites, with each cyst producing two trophozoites.4–6 Trophozoites multiply by longitudinal binary fission, remaining in the lumen of the proximal small intestine 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 nondiarrheal feces.


While Giardia infections can be asymptomatic,10 patients with giardiasis classically present with gradual onset of two to five loose stools per day and gradually increasing fatigue.11,12 Stools are often described as foul-smelling, greasy, and tend to float. Other symptoms include nausea, anorexia, abdominal cramps, bloating, flatulence, and dehydration, whereas fever, vomiting, pruritus, hives, and swelling of the eye and joints are more uncommon.11–14 Weight loss may occur over time, and giardiasis is also associated with fat, lactose, vitamin A, and vitamin B12 malabsorption.11,12,15,16

Giardiasis has an incubation period of 1–2 weeks and generally resolves within 2–4 weeks.5,12 However, chronic sequelae have also been recognized, including reactive arthritis17,18 and irritable bowel syndrome6,19; in children, in particular, severe giardiasis may cause growth stunting, developmental delay, failure to thrive, and malnutrition.20–24


Giardia is the most common intestinal parasite of humans identified in the United States,25,26 and a common cause of outbreaks associated with untreated surface and groundwater.27–29 Annually, an estimated 1.2 million cases of giardiasis occur in the United States,2 costing U.S.$34 million in hospitalizations.30

Transmission occurs indirectly through the ...

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