Low Risk of Establishing Salmonid Whirling Disease in the Deschutes River Basin
Whirling Disease Mini-Workshop – Central Oregon Environmental Center
Don Ratliff, Portland General Electric
November 17, 2009
After more than 10 years of monitoring and study, final conclusions indicate that under current conditions salmonid whirling disease should not be a problem causing disease in native salmon, steelhead, or trout. The parasite, Myxobolus cerebralis, could become established at low levels both downstream and upstream of the Pelton Round Butte Project (PRB). In anticipation of having fish passage at the Pelton Round Butte Project, the Licensees have funded continuing studies of M. cerebralis, the aquatic worm Tubifex tubifex (alternative host for M. cerebralis), and its affect on Deschutes salmonids. Final recommendations are for continued vigilance and monitoring as T. tubifex. This aquatic worm does best in areas of high sediment and high organic nutrient enrichment. For instance, high populations of T. tubifex were found in the settling pond at Wizard Falls Fish Hatchery. In my presentation I will highlight the major findings by OSU microbiologists and ODFW Fish Heath Specialists that should limit disease expression. Study findings include:
1. Myxospores of M. cerebralis, the causative pathogen, have been deposited by stray steelhead and Chinook annually into the lower Deschutes River basin since 1987.
2. Sentinel live-box studies using very susceptible fry have only detected the triactinospore (TAM) stage, the stage that infects salmonids, in very low numbers some years in lower Trout Creek, Mud Springs, near Oak Springs, and Buck Hollow Creek. Most diagnoses were by PCR, a very sensitive genetic test. Characteristic myxospores were only observed from a small percentage of sentinel fry exposed to lower Trout Creek in 1997. Disease was not observed in these sentinel fry. Myxospores have not been found in juvenile or adult wild steelhead from Trout Creek.
3. No Deschutes Basin resident or anadromous salmonids have ever been found to have myxospores after more than 10 years of looking.
4. Studies of water temperatures, and the alternative host, T. tubifex, show why whirling disease has not become a problem over the 20+ years of introduction of M. cerebralis into the lower Deschutes Basin by stray adult steelhead and salmon from Snake River watersheds. These studies also indicate that if the parasite is introduced with adult salmon or steelhead passed above PRB with fish passage, it should not result in whirling disease within the middle Deschutes Basin because of the mitigating factors of water temperatures, and the alternate host not aligned for parasite proliferation.
Study results that determined these factors include:
Water temperatures in most of the Deschutes Basin are either too cold or too warm for high production of TAM stage of M. cerebralis when salmonids are in the susceptible fry stage. In T. tubifex, water temperatures below 10oC (50oF) significantly delay the development of the TAM stage, and those above 20oC (68oF) disrupt the parasite’s development.
T. tubifex are not abundant in the Deschutes Basin. Where found they normally make up a very small percentage of the aquatic oligochaete (worm) population. In other words, of the worms in the sediment, most all of them are not T. tubifex. An exception is lower Trout Creek where T. tubifex is more abundant.
In many of the T. tubifex populations, the M. cerebralis-resistant, genetic lineage VI worms will actually reduce the numbers of TAMs produced.
Even in the susceptible lineage III populations, there is low potential production of TAMs from most populations as demonstrated in laboratory studies at the OSU Salmon Disease Laboratory. In these studies different T. tubifex populations were fed myxospores, and the TAMs produced enumerated. The exception is the T. tubifex population from lower Trout Creek that produced relatively large numbers of TAMs. This may be why lower Trout Creek is the only place where some transmission has been documented by actually observing myxospores in a small percentage of susceptible fry exposed during sentinel live-box studies.