Lymphocystosis is a disease caused by a virus that attacks cells in the deep skin layer (dermis) and sometimes internal organs in fish.

Wildlife at risk

Lymphocystosis has been reported in over 125 fish species and is frequently observed in walleye. However, it does not appear to affect salmonids (e.g., salmon and trout), catfish and cyprinids (e.g., carp). The disease mainly affects adult fish.

Signs of the disease's presence

Infected fish have small, pinkish-grey, slightly translucent nodules of about 1 mm in diameter. Nodules can be spread on the surface of the body or on fins. In the case of a mild infection, the nodules look like sand on the body. When they are clustered together, they take on the appearance of a raspberry. Nodules can also be found in the mouth, on the gills, more rarely on the abdominal wall and on the surface of the viscera. The disease progresses slowly over several weeks. It is rarely fatal and fish can recover spontaneously.

Two other diseases that can cause growths in fish are the walleye dermal sarcoma and lymphosarcoma.

Transmission and persistence in the environment

This contagious disease affects fish that are injured or have lost scales. It is transmitted through direct contact between fish, particularly during spawning gatherings. At this point, the infectious cells break and release the virus particles that contaminate fish.

Some fish may carry the virus, which can then develop as a result of stress. High population density or handling of fish are factors that contribute to the development of the disease. The lymphocystosis virus can survive in water for one week.

Protection and prevention

Risk for wildlife health

Lymphocystosis does not appear to have a significant effect on wild fish populations.

Risk for domestic animal health

Lymphocystosis can affect ornamental fish. No treatment is needed as the disease spontaneously regresses. In the event of contamination, it is preferable to quarantine the animal so that it does not transmit the virus to others.

Risk for human health

The disease is not known to be transmissible to humans. Fish remain edible. It is still recommended that all visible growths be manually removed. Adopting some safe practices when preparing and cooking wild fish remains relevant.

Surveillance and control

Lymphocystosis is known to be present in Québec. You do not have to report suspicious fish. To stop transmission, do not discard any unused fish parts in the water. Instead, put them in the garbage or burn them.

You can disinfect with bleach (200 mg/L or more) any piece of equipment that has been in contact with affected fish.

Last update: January 8, 2024


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