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Potential Dispersal of the Non-Native Parasite Myxobolus cerebralis in the Willamette River Basin, Oregon: A Qualitative Analysis of Risk

Myxobolus cerebralis, the myxosporean parasite responsible for whirling disease in salmonids, was first detected in the United States in 1958. It has since spread across the country, causing severe declines in wild trout populations in the intermountain west. This study qualitatively assesses the risk of introduction and establishment of the pathogen into the Willamette River basin, Oregon, by examining potential routes of dissemination and relationships among obligate hosts, the parasite, and the environment. The approach is a synthesis of historical data, literature, and original research. The risk of M. cerebralis introduction in the Willamette River basin is addressed as a function of three main elements of dispersal: (1) movement of infected fish by humans, (2) natural dispersal (via migratory birds and stray anadromous salmonids), and (3) recreational activities. Establishment of the parasite is dependent upon several environmental and biological factors, including water temperatures, density, and spatial/temporal overlap of hosts, and the distribution and genetic composition of the oligochaete host, Tubifex tubifex. This study finds the probability of introduction of the parasite to vary throughout the Willamette River basin. Areas with greater probability have been identified as the Clackamas and Santiam River subbasins. If the pathogen were introduced, probability of establishment is high in certain areas of the basin as conditions are appropriate for propagation of the parasite lifecycle.