Giardia intestinalis, the flagellated protozoan originally named G. lamblia, is the most commonly identified parasitic pathogen in the U.S. Children in day care and their close contacts are at greatest risk, as are backpackers and campers (by drinking unfiltered, untreated water), persons drinking from shallow wells, travelers to disease-endemic areas, and men who have sex with men. Giardia cysts can be excreted in the stool intermittently for weeks or months, resulting in a protracted period of communicability. Transmission occurs when cysts (as few as 10) are ingested through person-to-person or animal-to-person contact, or by ingestion of fecally contaminated water or food. The majority of Giardia infections occur without symptoms. When symptomatic, patients report a variety of gastrointestinal complaints including chronic diarrhea, steatorrhea, abdominal cramps, bloating, frequent loose and pale greasy stools, fatigue and weight loss.


In 2003, the reported incidence of giardiasis in Oregon was nearly twice that of the rest of the U.S., with 11.5 cases per 100,000 population. All 2003 cases were reported as sporadic or household-associated disease; no outbreaks were detected. Children <5 years of age continue to have the highest incidence, with 33 cases/100,000.

Prevention depends upon good personal hygiene (hand washing!), and avoiding consumption of fecally contaminated water. Travel warnings on water quality should be heeded.