Giardiasis is caused by infection with the protozoan parasite Giardia lamblia, also called Giardia intestinalis in Europe.
Giardia infections are ubiquitous, and outbreaks occur in developed and underdeveloped nations throughout the world. Infection results from ingestion of cysts, usually contained in water or food, on hands, or on fomites contaminated with feces. The parasite is found in about 4% of stool specimens submitted to laboratories in the United States and is the most common parasite isolated. The exact prevalence of the infection in the United States is not known because it is not reportable in all states and may be difficult to isolate in the laboratory. Epidemic giardiasis in day care centers was first reported in 1977, with infection rates varying from 0 to 25%. Most children have symptoms. Chronic passage of cysts by some preschool children in day care facilities is found 5 to 6 months after the initial diagnosis, either because of continued transmission or chronic infection. Prevalence rates decline when children are toilet-trained. Sexual transmission may occur in heterosexual or homosexual contacts.
Campers and hikers are at risk because of vertical transmission from animals, and waterborne outbreaks in national parks have been reported. In addition, many outbreaks have been attributed to municipal water supplies that have not been treated with flocculation or filtration.