Prevalence of Myxobolus cerebralis at Juvenile Salmonid Acclimation Sites in Northeastern Oregon

—Fry of rainbow trout Oncorhynchus mykiss held as sentinel fish became infected with Myxobolus cerebralis when held at all juvenile acclimation sites for spring Chinook salmon O. tshawytscha and steelhead O. mykiss (anadromous rainbow trout) and thus expand the known range of M. cerebralis in northeastern Oregon. In spring 2001, replicate cages of rainbow trout fry were placed at the intake site of each facility and sampled at approximately 14, 28, and 50 d. Infection prevalence, as determined by the presence of M. cerebralis DNA following polymerase chain reaction (PCR) amplification, was relatively low (,17.5%) at most acclimation sites. However, a significantly higher prevalence of infection in sentinel fry held at the Wallowa facility (65%) identified this site as presenting a higher risk for juvenile steelhead to M. cerebralis exposure than other sites. Further, PCR analysis of ossified cranial elements from steelhead juveniles held at the Wallowa facility for 6 weeks demonstrated a comparable prevalence of infection, 52%. Detection of similar infection prevalence in fry and juveniles at the same site demonstrates the usefulness of sentinel exposures as a surrogate for lethally sampling valuable salmonid juveniles. In addition, this study suggests that infection of anadromous salmonids may complicate management efforts to contain the parasite within northeastern Oregon. Myxobolus cerebralis, the etiologic agent of whirling disease in salmonids, was first detected in the Pacific Northwest in 1986 during an inspection of a private trout hatchery on the Lostine River, in northeastern Oregon (Lorz et al. 1989). Since the first observation of the parasite in the region, M. cerebralis has been detected in wild and feral fish collected from several tributaries of the Grande Ronde River (Catherine Creek, Lostine River, and Wallowa River) and in one tributary, Little Sheep Creek, of the Imnaha River (Oregon Department of Fish and Wildlife, unpublished data). Myxobolus cerebralis is presently considered enzootic in both river basins (Sandell et al. 2001); however, because fish collected in surveys may have migrated from the original site of infection, the extent of parasite establishment in the region is not known. Susceptibility to M. cerebralis infection and subsequent development of whirling disease is influenced by several factors, the most important being fish age (Markiw 1991, 1992; MacConnell and Vincent 2002). For rainbow trout Oncorhynchus mykiss, clinical disease is significantly reduced when initial exposure to the parasite is delayed until 9 weeks posthatch (Eileen K. N. Ryce, personal communication), whereas Chinook salmon * Corresponding author: [email protected] Received October 24, 2002; accepted April 30, 2003 O. tshawytscha appear to develop resistance to clinical disease as early as 3 weeks posthatch (Sollid et al. 2003). Although fish are less likely to suffer clinical disease when exposed at a later age, they may become infected and become lifelong carriers of M. cerebralis. Water temperatures also influence both the seasonal occurrence of the parasite and the severity of the infection in a fish. Studies conducted in Montana demonstrated that peak infection rates occur during the spring and fall when temperatures are between 128C and 158C (MacConnell and Vincent 2003). This was supported by laboratory studies demonstrating peak release of the infectious triactinomyxons stage of the parasite at temperatures between 108C and 158C (El-Matbouli et al. 1999). However, infections may occur at lower temperatures, as evidenced by infection of sentinel fish in the Lostine River in November, when water temperatures were as low as 58C (Sandell et al.