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The Deschutes River, located in central Oregon, is a tributary of the Columbia River. The migration of anadromous fish into the upper Deschutes River basin was blocked in the late 1960s by a series of dams constituting the Pelton Round Butte Hydroelectric Project (PRB). Plans to reintroduce anadromous fish runs above the PRB are complicated by concerns over the introduction of (the agent of whirling disease), which is carried into the lower Deschutes River by adult salmon that stray on their return to enzootic regions of the upper Columbia River basin. To develop a fish passage strategy that minimizes the risk of parasite introduction into the upper Deschutes River basin, it is important to determine whether establishment of the life cycle in waters below the PRB has occurred. Fry of susceptible rainbow trout and steelhead (anadromous rainbow trout) were exposed as sentinel fish at various locations in the Deschutes River and its tributaries between 1998 and 2007 to detect . The parasite was found periodically at low prevalence by polymerase chain reaction (PCR) or quantitative PCR assay in fish held at several locations. However, in 2007, the parasite was detected from fish exposed in Trout Creek based on visual examination (pepsin-trypsin digest preparations and histological sections) and molecular methods. This result confirms that is established in the Deschutes River basin below the PRB. Continued introduction of the parasite into the river system below the PRB via stray hatchery summer steelhead was also documented. These findings have implications for managing movement of within the watershed. [ABSTRACT FROM AUTHOR]