A disease caused by the protozoan parasite Giardia lamblia, characterized by chronic diarrhea that usually lasts 1 or more weeks. The diarrhea may be accompanied by one or more of the following: abdominal cramps, bloating, flatulence, fatigue, or weight loss. The stools are malodorous and have a pale greasy appearance. Infection without symptoms is also common.

Giardiasis occurs worldwide. In community epidemics caused by contaminated drinking water, as many as 50 to 70% of the residents have become infected. Outbreaks also occur among backpackers and campers who drink untreated stream water. Both human and animal (beaver) fecal contamination of stream water has been implicated as the source of Giardia cysts in waterborne outbreaks. Giardia species in dogs and possibly other animals are also considered infectious for humans. Epidemics resulting from person-to-person transmission occur in day-care centers for preschool-age children and institutions for the mentally retarded. Infants and toddlers in day-care centers are more commonly infected than older children who have been toilet-trained. Epidemiology