Whirling Disease - Fact sheet

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General information

Whirling disease is an infectious disease of finfish. It is caused by a parasite called Myxobolus cerebralis, which belongs to a group of microscopic parasites in the Class Myxosporea.

There is no treatment for whirling disease. Once introduced, the elimination of the parasite in wild finfish populations is not usually possible.

Whirling disease is not a health risk for humans or pets. The parasite that causes whirling disease does not affect humans or animals other than finfish, such as trout and salmon. There is no risk associated with swimming or eating fish caught from infected waters.

Species of finfish that can be infected by whirling disease

Each species of finfish may have several common names, but only 1 common name is listed.

Species susceptible to whirling disease that occur in the natural environment in Canada:

  • 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)

Signs of whirling disease

Whirling disease can cause death in the younger life stages of susceptible freshwater finfish. Overall deaths of infected young fish can reach 90%.

Affected finfish may exhibit any of the following signs:

  • behaviour
    • whirling swimming pattern
  • appearance
    • skeletal deformities of the body or head, for example, shortening of the mandible and indentations on the top of the head
    • tail may appear dark or even black

Diagnosing whirling disease

Diagnosing whirling disease requires laboratory testing. Not all infected finfish show signs of disease. It is also possible for finfish to have signs consistent with whirling disease that are due to other causes.

Detections of whirling disease

Whirling disease has been found in Alberta and British Columbia. Surveillance for whirling disease occurs through Parks Canada, the Canadian Food Inspection Agency (CFIA), the Department of Fisheries and Oceans, and provincial departments. Confirmed detections are posted to the CFIA website.

Increased sampling and testing will help determine the extent of disease presence and the most appropriate disease response.

Disease prevention and control

Whirling disease is not spread directly between finfish. The parasite is spread through contact between finfish and a freshwater worm (Tubifex tubifex).

People can spread whirling disease by moving any of the following:

  • infected live or dead finfish
  • infected worms
  • contaminated equipment, or
  • contaminated water or sediment

Movement controls and permits

If you are moving species susceptible to whirling disease and the movement poses a risk for disease spread, ensure you have a domestic movement control permission from the CFIA Domestic Movement Control Program (DMCP). For more information on when a permission is required, see the Domestic Movement Control Program.

When importing fish determined as a species susceptible to whirling disease, ensure you have a CFIA import permit.

  • An import permit is required from the CFIA for certain species of finfish
  • People bringing finfish into Canada should check other federal, provincial and/or territorial requirements before entering the country
  • For more information on importing aquatic animals see Aquatic animal imports

Parks Canada, the Provinces, and Department of Fisheries and Oceans may have additional movement controls in place. Please check with them prior to moving fish which may pose a risk of disease spread to ensure they do not have additional requirements.

How the public can help control the spread of whirling disease

  • If you frequently handle or work with finfish, be aware of the clinical signs of whirling disease
  • Do not import susceptible finfish into Canada without proper permits
    • An import permit is required from the CFIA for certain species of finfish
    • People bringing finfish into Canada should check other federal, provincial and/or territorial requirements before entering the country
  • Do not introduce live finfish from another country into the natural waters of Canada without prior approval
    • People releasing finfish into the natural waters or in rearing facilities within Canada should check if federal or provincial and/or territorial permits are required
  • Do not use finfish that were bought in a grocery store as bait for catching finfish or other aquatic animals
  • When cleaning and gutting finfish, dispose of all waste in your municipal garbage
  • Do not visit Canadian aquaculture sites, zoos or aquariums for 14 days if you have travelled to another country and
    • visited an aquaculture site, or
    • had contact with wild finfish
  • If you are fishing, boating or paddling, it is important to follow measures to prevent the spread of whirling disease
    This includes the measures above, as well as cleaning, draining and drying your watercraft and equipment prior to moving to another water body. It is also important to wash and disinfect the footwear you wore, and wash your clothing thoroughly and dry it at a high temperature.

Although whirling disease has been found in parts of Alberta and British Columbia, it is still very important to follow the measures above even in infected areas to prevent further spread between waterbodies. This is because not every susceptible finfish population in an infected area has whirling disease and these measures also help keep the level of the parasite low to reduce the impact on finfish.

What the CFIA is doing to protect Canadian aquatic animals from whirling disease

Whirling disease is a reportable disease in Canada. This means that anyone who owns or works with aquatic animals, who knows of or suspects whirling disease in their finfish, is required by law to notify the CFIA.

The Canadian Food Inspection Agency, Department of Fisheries and Oceans, Parks Canada, and Provinces work closely together to determine the appropriate disease control response.

More information

For more information about reportable diseases, visit the CFIA Aquatic Animal Health page or contact your local CFIA Animal Health Office.

Reference material

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