The Science of Whirling Disease

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Understanding the Challenge of Whirling Disease

Myxobolus cerebralis (Mc) is a metazoan parasite that penetrates the head and spinal cartilage of fingerling trout where it multiplies very rapidly, putting pressure on the organ of equilibrium.

Triactinomycon ("TAM") spores infect juvenile fish. This causes the fish to swim erratically in a whirling pattern. This handicap makes feeding and avoiding predators much more difficult. In severe infections, the disease causes high rates of mortality in young fish. Those that do survive until their cartilage hardens to bone can live a normal life span, but are marred by skeletal deformities. This reduces reproduction, although fish can reproduce without passing on the parasite to their offspring A Mc spore can live up to 30 years in harsh nvironments.

The tiny parasite Myobolus cerebralis is native to Eurasia. It was introduced into North American waters in the late 1950s. So far, it has been most detrimental to wild rainbow trout populations, although many other salmonid species can become infected. Some carry the parasite without showing signs of the disease.For example, brown trout (also a European import) become infected by M. cerebralis but rarely suffer from the clinical disease. Regardless of the presence of disease symptoms, when an infected fish dies, many thousands to millions of the parasite spores are released to the water, making control very difficultThe M. cerebralis spore is virtually indestructible -- it can live in a stream (or former stream) for 20 - 30 years through freezing temperatures and drought. Eventually it will be ingested by its alternate host, a tiny, common aquatic worm known as Tubifex tubifex. In the worm, the spores become the Myxobolus cerebralisthat can infect trout fry, called Triactinomycon, or "tams."

Because of its life cycle and subsequent hardiness, the parasite is easily spread to uncontaminated drainages since it is so easily and unknowingly transported by animals, birds and humans.

Whirling Disease Management
Whirling disease is not the only important fish disease that affects salmon and trout in the western states. However, because it was not naturally occurring in this country, its spread has had unique consequences compared with other salmonid pathogens. Because the disease was introduced from Europe, species of trout and salmon native to North America have not evolved to be resistant to whirling disease as they have to other pathogens that naturally occur in the West. Mc has been reported in 23 states and whirling disease has caused severe population declines in many of the intermountain West’s finest trout streams.

WD poses a grave threat to already threatened native trout populations. In Yellowstone Park, substantial declines in populations of native Yellowstone cutthroat trout have been linked to whirling disease infections in several Yellowstone Lake spawning tributaries. In Yellowstone's Pelican Creek, a spawning population of over 10,000 Yellowstone cutthroat trout has been eliminated by a severe whirling disease infection. The disease is also a grave threat to native trout recovery programs. In Montana, recovery plans for east slope cutthroat trout are being thwarted by presence of the disease. Gila and Apache trout recovery programs in Arizona and New Mexico are also at risk due to whirling disease. While not yet present in Alaska, the risk to Alaskan sport and commercial salmon, trout, and steelhead fisheries is being carefully evaluated by Alaska’s fish and game department.

States have had to develop specific policies on the consequences of detecting Mc in private, state and federal facilities as well as in wild fish populations. State fish and game agencies and private aquaculture in the West have spent considerable fund in attempts to clean up infected hatcheries and protect hatchery water sources. In Colorado alone, where 11 of 16 Colorado Division of Wildlife trout production facilities have tested positive for the parasite, over $10 million has been spent on hatchery clean up. Still, unmanageable Mc infections have caused the decommissioning of many public and private aquaculture facilities.

Although there have been vast improvements in detecting infected fish and regulations put in place to stop the spread of whirling disease, it continues to be a problem. Wild fisheries that suffered dramatic declines have not yet recovered and there is still concern. the impact on state and local economies is estimated to be millions of dollars each year due to the decline or loss of recreational fishing opportunities.

References:

• Bartholomew, J. L&C.J. Wilson. 2002. Whirling Disease Reviews and Current Topics, American Fisheries Society Symposium 29.
• Bartholomew, J. L., B. L. Kerans, R. P. Hedrick, S. C. MacDiarmid and J. R. Winton. 2005. A risk assessment based approach for the management of whirling disease. Reviews In Fisheries Science 13:205-230.
• Nehring, Barry R., February 2006. Colorado’s Cold Water Fisheries: Whirling Disease Case Histories and Insights for Risk Management.