Division of Environmental Health Giardiasis Giardiasis, a disease caused by the protozoan parasite Giardia lamblia , is characterized by chronic diarrhea that usually lasts one or more weeks. Stools are often malodorous and have a pale greasy appearance. Infection without symptoms is also common.
Trophozoites stay in the upper small intestinal tract where they actively feed and reproduce.
When trophozoites pass down the bowel, they change into the inactive cyst stage by developing a thick exterior wall that protects the parasite after it is passed in feces. The organism does not invade other parts of the body or cause any permanent damage. In infants and small children, however, the severe diarrhea can lead to dehydration and shock if adequate fluid intake is not maintained. In the United States, G. lamblia is the parasite most commonly identified in stool specimens submitted to state laboratories for parasitologic examination. Other surveys conducted in the United States have demonstrated G. lamblia prevalence rates ranging from 1 percent to 30 percent, depending on the location and ages studied.