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. Infants and toddlers in daycare centers are more commonly infected than older children who have been toilet trained.
Infections also occur among backpackers and campers who drink untreated stream water.
Less commonly, community epidemics caused by contaminated drinking water occur. In such outbreaks, approximately 11 percent of the residents have become infected. Giardia species in dogs and possibly other animals are also considered infectious for humans. Individual immunity undoubtedly plays a role, but the exact immune mechanisms involved have not been identified. The ingestion of 10 to 25 cysts may result in giardiasis, with an incubation period from one to four weeks.