Comparative susceptibility of Deschutes River, Oregon, Tubifex tubifex populations to Myxobolus cerebralis.

Dams along the Deschutes River (DR) in central Oregon have blocked fish migration for over 40 years. Reestablishment of anadromous fish runs above the dams as part of a fish passage plan may introduce fish pathogens, such as Myxobolus cerebralis, the myxozoan parasite that causes salmonid whirling disease. This parasite is carried by adult salmon that stray into the DR system during their return to enzootic areas of the upper Columbia River basin, and it is now known to be established in at least one lower DR tributary. The life cycle of M. cerebralis involves two obligate hosts: a salmonid and the oligochaete worm Tubifex tubifex. To determine the likelihood of parasite establishment above the DR dams, we conducted benthic sediment surveys between 1999 and 2007 and found that T. tubifex had a patchy distribution and low relative abundance. Mitochondrial 16S ribosomal DNA gene analysis indicated that two lineages of T. tubifex (III and VI) were present both above and below the dams. Laboratory susceptibility studies to characterize differences in infection prevalence and parasite production between nine T. tubifex populations revealed that production varied considerably among exposed groups and was proportional to the number of lineage III worms present. Our results suggest that M. cerebralis could become established above the dams if infected fish are allowed passage into the upper DR system, but not all areas of the DR basin can be classified as having the same likelihood for parasite establishment, and the potential impact will be location dependent.

Distribution and abundance of the salmonid parasite Parvicapsula minibicornis (Myxozoa) in the Klamath River basin (Oregon-California, U.S.A.).

The distribution and abundance of the myxosporean parasite Parvicapsula minibicornis in the Klamath River mirrored that of Ceratomyxa shasta, with which it shares both its vertebrate and invertebrate host. Assay of fish held at sentinel sites and water samples collected from those sites showed that parasite prevalence was highest below Iron Gate dam, which is the barrier to anadromous salmon passage. Above this barrier parasite levels fluctuated, with the parasite detected in the free-flowing river reaches between reservoirs. This was consistent with infection prevalence in the polychaete host, Manayunkia speciosa, which was greater than 1% only in populations tested below Iron Gate dam. Although a low prevalence of infection was detected in juvenile out-migrant fish in the Trinity River, the tributaries tested did not appear to be a significant source of the parasite to the mainstem despite the presence of large numbers of infected adult salmon that migrate and spawn there. Rainbow trout became infected during sentinel exposure, which expands the host range for P. minibicornis and suggests that wild rainbow trout populations are a reservoir for infection, especially above Iron Gate dam. High parasite prevalence in the lower Klamath River is likely a combined effect of high spore input from heavily infected, spawned adult salmon and the proximity to dense populations of polychaetes.