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Whirling Disease: An Overview and Prevention Guidelines

Whirling disease affects juvenile salmonid fish, including trout and whitefish. It is caused by the microscopic parasite Myxobolus cerebralis, which infects fish through their skin. While there is no risk to human health, the parasite can be lethal to rainbow (including steelhead) and cutthroat trout under four months of age.

Prevention Measures

To prevent the spread of whirling disease, follow the “Clean, Drain, Dry” steps for all boats, equipment, and materials.

Recent Detection in Canada

In December 2023, the Canadian Food Inspection Agency (CFIA) confirmed the presence of whirling disease in Yoho National Park (Columbia River Watershed), British Columbia. For more information on confirmed detections and updated zoning, visit the CFIA website.

The Province of BC is collaborating with Parks Canada and the CFIA to identify high-priority areas for surveillance and testing for the upcoming field season. Additional updates will be provided as they become available.

Historical Context

Whirling disease was first confirmed in Canada in 2016 in Alberta. It has since been detected in four major watersheds across central and southern Alberta. British Columbia monitors for whirling disease in priority water bodies in the East Kootenay region, near the Alberta border, due to its proximity to infected areas. Whirling disease is a federally reportable disease through the CFIA.

Impact on Fish Populations

In infected waters, whirling disease can cause high mortality in fish populations, particularly among juvenile salmonids. The severity of outbreaks varies based on habitat and environmental conditions, but extreme cases have resulted in localized population collapses of over 90% in some streams in the Western United States.

There are no health concerns for humans or other mammals swimming in or drinking water containing whirling disease, and eating infected fish is not known to cause harmful effects.

Disease Characteristics

Whirling disease is caused by Myxobolus cerebralis, which has a complex lifecycle involving both juvenile fish and bottom-dwelling aquatic worms. The parasite invades the head and spinal cartilage, causing the characteristic “whirling” swimming pattern. Infected fish release spores into the water, continuing the cycle.

Signs of infection include blackened or deformed tails and skull deformities, though these are not exclusive to whirling disease. Parasites may be present even without these external signs.

Susceptible Species

Wild fish species susceptible to whirling disease include:

  • Oncorhynchus clarkii (cutthroat trout)
  • Oncorhynchus kisutch (coho salmon)
  • Oncorhynchus mykiss (rainbow trout)
  • Oncorhynchus nerka (sockeye salmon)
  • Oncorhynchus tshawytscha (chinook salmon)
  • Prosopium williamsoni (mountain whitefish)
  • Salmo salar (Atlantic salmon)
  • Salmo trutta (brown trout)
  • Salvelinus confluentus (bull trout)
  • Salvelinus fontinalis (brook trout)

Monitoring and Prevention

Given the parasite’s small size and complex lifecycle, containment or eradication from waterbodies is challenging. Monitoring and preventing the spread via boats, fishing gear, and transported fish are crucial.

The province has established a monitoring program for waterbodies in southern B.C., especially along the Alberta border. Effective prevention requires the active participation of boaters, anglers, and others who enjoy B.C.’s streams and lakes.

Best Practices for Prevention

To prevent the spread of whirling disease, follow these best practices:

  • Never move fish or fish parts from one waterbody to another.
  • Use fish cleaning stations or dispose of fish parts in the local solid waste system.
  • Clean, drain, and dry boats and equipment before moving between waterbodies.


  • Inspect and clean watercraft, trailers, and equipment that contact water or fish.
  • Remove mud, sand, and plant material before leaving the shore.
  • Rinse or pressure wash your boat away from storm drains, ditches, or waterways.
  • Bathe pets before allowing them to enter another waterbody.


  • Drain water from watercraft and equipment onto dry land before leaving the shoreline.


  • Dry watercraft and equipment completely between trips, allowing at least 24 hours of drying time.
  • Leave compartments open on boats and equipment and sponge out standing water.

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