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Giardia Antigen

Giardia intestinalis, also known as G. lamblia and G. duodenalis, is a flagellated protozoan. Giardiasis is the most commonly reported intestinal parasitic infection in the United States with an annual incidence of 1.2 million cases per year.

Giardia infection is transmitted by the fecal-oral route and results from ingestion of Giardia cysts in fecally contaminated food or water or through person to person or animal to person transmission. Drinking untreated water from lakes and rivers, swimming, having contact with some animal species, and sexual practices involving fecal contact might increase risk for giardiasis. Giardiasis is often detected in international travelers and among internationally adopted children. Transmission to close contacts of infected persons can also occur, especially in child-care settings. People with occupational exposure to human waste are also at risk.

Cysts are infectious immediately upon being excreted in feces. They are environmentally hardy and tolerant to chlorine. Infectious dose is very low; ingestion of 10 cysts can cause infection. Infected persons have been reported to shed more than 100 cysts in their stool per day and to excrete cysts for months.

Giardia most frequently infects children who attend daycare centers and their contacts, foreign travelers, nature lovers such as skiers and hikers exposed to untreated water, and customers of contaminated public water systems. Ingestion of as few as 10 cysts may cause infection.

Giardiasis should be included in the differential diagnosis when a patient experiences diarrhea lasting more than 3 days. Giardiasis is generally a self-limited illness characterized by diarrhea, abdominal cramps, bloating, weight loss, and malabsorption. Asymptomatic infection is also common.

While an O&P exam can detect Giardia, the sensitivity is low due to sporadic shedding of the parasite. As many as 7 or 8 specimens may be required for Giardia to be excluded by O&P exam.

Enzyme immunoassay for Giardia antigen in the stool has a sensitivity of 95% and is the test of choice for giardiasis. Specimen requirement is 10 g of a random stool sample submitted in a screw-capped, plastic container with 10 % formalin.

Giardia is also included in the FilmArray Gastrointestinal Panel that tests for 22 common pathogens. Specimen requirement is a fresh diarrheal stool sample from inpatients or a stool sample submitted in Cary-Blair transport media for outpatients. A single specimen is sufficient.

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