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Geographic Distribution

The reported incidence of infection with Giardia varies widely from country to country. The parasite has been noted in inhabitants and travelers from all over the world. In recent decades it has become an important etiology of endemic and epidemic diarrhea, contributing to chronic, often debilitating diarrheal illness in native populations and tourists alike. In general, it has been found in 4-16% of otherwise normal people in the tropics and in 3-20% of children in parts of the southern United States. However, in some developing countries, studies have indicated that almost all children have been infected with G. lamblia by 3 years of age. A parasitology survey in a poor district of Cali, Colombia from 1956-1960 found a 47% incidence of Giardia infection, with a peak in the first 9 years. The organism is found most often in children, especially those in large families and in institutions such as schools, orphanages, and day care centers. The reported incidence in institutionalized people ranges from 2-50%.

In the United States giardiasis is the most common protozoan disease and Giardia lamblia is the most frequent enteric parasite and the leading infectious organism associated with waterborne outbreaks of diarrhea., being responsible for an estimated minimum of 4000 hospital admissions per year (WHO, 1992). From 1965 to 1984, G. lamblia accounted for 90 outbreaks in the U.S. that affected over 23,500 people.

Epidemics have occurred in cities such as Aspen, Colorado, where leakage of sewage into part of the town's water supply was responsible for the outbreak. From December 1974 to April 1975 physicians at Griffiss Air Force Base in Rome, New York treated 61 patients with giardiasis confirmed by positive stool exams for cysts. In Harrisburg and other towns in Pennsylvania, the disease has been transmitted by beavers contaminating reservoirs; in Wilkes- Barre at least 296 people fell ill with debilitating diarrhea and intestinal gas and cramps from infection with G. lamblia from October 1983 to March 1984. There, as well as in the Rocky Mountains of North America, it is known as "beaver fever". An August 1982 report in Tropical Medicine and Hygiene News noted 140 confirmed cases of giardiasis in the tourist town of Banff in Alberta, Canada, where beavers infected with G. lamblia were discovered in the reservoir. Highly endemic regions for giardiasis elsewhere in the world are Russia and the former Soviet Union, especially St. Petersburg, and India and Central America. In 1992 the WHO estimated that there were 500,000 new cases of giardiasis worldwide.

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