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Giardiasis is the most frequent parasitic cause of gastroenteritis in the U.S. 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. 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. Fortunately, enzyme immunoassay for Giardia antigen in the stool has a sensitivity of 95% and is the test of choice for giardiasis. If other parasites are possible as well, O&P exam should be done in addition to Giardia antigen detection. A full O&P exam and Giardia antigen detection can be performed on the same specimen of stool or duodenal aspirate. Submission of a total of 3 stool specimens collected every other day is recommended. |
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