Abstract:
Myxobolus cerebralis, a myxozoan parasite that infects almost all species of wild
and cultured salmonids, was first identified in northeastern Oregon in the Lostine River in
1986. Fish that are heavily infected with M. cerebralis develop whirling disease, which
was determined to be the cause of catastrophic declines in rainbow trout populations in
Montana and Colorado in 1996. There have been no reports of severe salmonid
population declines attributed to whirling disease in Oregon, although many populations
are at levels that warrant protective listing under the Endangered Species Act. This study
was undertaken to I) establish the distribution and seasonal prevalence of M. cerebralis
infection in the Lostine River, 2) investigate the epizootiological factors influencing the
occurrence of whirling disease in this river, 3) determine the potential impact of the
parasite on chinook salmon, and 4) evaluate the suitability of a non-lethal tissue sampling
protocol for the detection of M. cerebralis in juvenile and adult salmonids for use in
monitoring the effects and distribution of the parasite in protected salmonid populations.
Results of this study indicate that the parasite is present throughout the main stem of the
Lostine River during all seasons, and that rainbow and anadromous steelhead trout fry
emerging in the lower river are most likely to become infected and develop whirling
disease. Chinook salmon were shown to be relatively resistant to M. cerebralis at low
levels of exposure, but this effect was less evident when the parasite exposure increased.
The non-lethal sampling protocol was successful in detecting M. cerebralis in juvenile
salmonids but not in adults, where it is most needed. The information gathered during
these studies was then correlated with the life histories of resident and anadromous salmonids in the Lostine River to determine the potential impact of M. cerebralis On
salmonid populations in this region.
