Whirling disease

Description

Whirling disease is a fatal infectious disease that affects juvenile finfish in the salmonid family (salmon and trout). It is caused by a freshwater parasite, Myxobolus cerebralis. This parasite needs two hosts to complete its life cycle. The definitive (i.e. final) host is a freshwater Tubifex tubifex or sludge worm. The intermediate host must be a fish from the Salmonidae family.

Wild animals at risk

In Québec, the species vulnerable to this disease are:

Cutthroat trout
Coho salmon
Rainbow trout
Sockeye salmon
Chinook salmon
Atlantic salmon
Brown trout
Brook trout

Signs of the disease

Fish that have been infected with whirling disease may show symptoms such as:

Spinal deformity
Skull deformity (shortening of mandible, indentations on top of head)
Dark or black colour in the tail
Swimming in circles (whirling)

Some infected fish, however, may not exhibit any signs of the disease.

Young salmonids with black tails — Black tails in young salmonids. © S. Hallet/OSU

Three young salmonids with Skeletal deformities — Skeletal deformities in young salmonids. © S. Hallet/OSU

Life cycle and persistence in the environment

Whirling disease is not transmitted directly from one fish to another. The parasite moves around freely and infects the fish. The disease can also be transmitted when a salmonid fish eats an infected tubifex worm. The parasite’s spores ^{Read the content of the note 1} are highly resistant and can survive for several years in damp conditions. They can withstand freezing at –20°C and can remain viable in mud at 13°C for several months. Spores can also survive in the digestive systems of various animals. They can even attach themselves to equipment used for swimming, fishing and water sports.

Humans may spread whirling disease by transporting:

infected fish, whether living or dead
infected worms
contaminated equipment
contaminated water
contaminated sediments (earth or mud)
contaminated aquatic plants

The parasite can also be spread naturally through the movement of fish and water into new environments. Animals that consume contaminated fish or water may transport the parasite into another environment by displacing the fish or releasing spores in their feces.

Life cycle of the parasite responsible for whirling disease. Triactinomyxon type 1 spores (TAM) from the Myxobulus cerebralis multiply in the cartilage of the fish’s spine and skull, where they next transform into type 2 (myxosporea). These type 2 spores are released into the water when the fish dies. Red T. tubifex worms consume the type 2 spores, which transform in their intestine into type 1 spores. Type 1 spores are then released into the water. These type 1 spores enter the fish and the cycle begins again. — Life cycle of the parasite that causes whirling disease @ Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs

Protection and prevention

Québec’s waterways are ideal breeding grounds for the parasite responsible for whirling disease. Here are some steps for preventing the introduction and spread of this disease:

Do not move living or dead fish from one body of water to another.
Dispose of fish carcasses and unused baitfish in the garbage.
Clean, empty and dry all boats and equipment that have been in contact with waterways or their banks.

To facilitate cleaning and drying, we recommend avoiding the use of equipment that does not dry easily, such as wooden oars and fishing waders.

Risk to the health of wild animals

The disease only affects salmonids, mainly fish less than a year old, with mortality rates of up to 90% in some species. The parasite can also infect adult fish throughout their lives and cause their death. The disease can severely deplete wild salmonid populations, including the endangered cutthroat trout. The risk that a fish may die from whirling disease depends on several factors, such as the presence of hosts, the intensity of infection, and environmental conditions favourable to parasite development.

Health risk for pets

Whirling disease poses no danger to domestic animals.

Risk to human health

Whirling disease does not pose a danger for humans. There is no risk in swimming or eating fish caught in infected waters. It is always important that we take the usual precautions before consuming wild fish .

Treatment

There is no cure for this disease.

Monitoring and control

To date, no cases of whirling disease have been detected in Québec. The disease was confirmed to be present in Alberta in 2016 and in British Columbia in 2024.

In Canada, whirling disease is an officially reportable disease. Any person who suspects or detects this infection is required by law to notify the Canadian Food Inspection Agency (CFIA), the Ministère de l'Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs (MELCCFP) and the Ministère de l'Agriculture, des Pêcheries et de l'Alimentation (MAPAQ). Refer to the Reportable animal diseases page to find out who to contact.

In your email report to the MELCCFP, you must include photos from multiple angles and the following information:

date, time and place of observation;
fish species;
number of fish suspected of being infected.

It is important not to kill the suspected fish.