Giardia is probably the most common intestinal parasite in the United States. It inhabits the duodenum and upper jejunum, where the alkaline pH creates a favorable milieu. Cysts are ingested in fecally contaminated water or food or are passed by hand-to-mouth transmission. Waterborne outbreaks have become more common in the United States because the cysts are resistant to chlorination. Day-care centers have been increasingly implicated in promoting giardiasis.
Symptoms depend on the duration of infection at the time of presentation. Patients may complain of explosive, watery, or foul-smelling diarrhea, flatus, abdominal distention, fatigue, and fever or chronic diarrhea with weight loss or general debilitation. Stools should be examined for cysts and trophozoites with routine and concentration techniques. Giardia antigen can be detected in the stool with immunofluorescence or ELISA technique. Occasionally, duodenal aspiration, the string test (Entero-test), or small-bowel biopsy is necessary to make the diagnosis. The drug of choice for treatment is metronidazole.
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