A prospective study of giardiasis and water supplies in Colorado.

A prospective study of 484 visitors to Vail and Aspen/Snowmass, Colorado, was conducted to determine the risk of acquiring giardiasis. Of the 259 visitors to Vail, no cases of giardiasis were confirmed and only one of 12 water filtrates were positive for Giardia cysts. Of 225 visitors to Aspen/Snowmass two cases of giardiasis were confirmed and 12 of 20 water filtrates were positive for Giardia cysts. The regular occurrence of Giardia cysts in Aspen and Snowmass water was associated with lower rates of giardiasis acquisition than reported during outbreaks of waterborne giardiasis.