Heterosporis sp. is a parasitic fungus that produces spores affecting fish muscle cells. There are several species of Heterosporis sp., one of which is found in North America.

Animals at risk

This parasite was first identified in yellow perch. However, it can infect several other fish species, such as:

  • walleye;
  • northern pike;
  • trout-perch;
  • burbot;
  • pumpkinseed;
  • rock bass;
  • slimy sculpin.

It has also been found in aquarium fish, such as angel fish, cichlids and Betta, as well as in Japanese eel farms.

Signs of the parasite’s presence

The flesh of infected fish becomes white and opaque. It has a grainy consistency and the appearance of freezer-burned meat. Up to 90% of the muscle mass can be filled with spores and turn to a milky white colour.

The presence of the parasite is confirmed by observation of the flesh under a microscope or by laboratory analysis.

Life cycle and persistence in the environment

Fish contaminate themselves orally by ingesting loose spores into the water or flesh of infected fish. Once ingested, the parasite migrates to muscle fibres to proliferate. Spores will return to the environment when fish die and decompose in the water.

Spores remain contagious for at least two months in ambient water (21°C) and up to one year in cold water (4°C). However, they are not resistant to drying (24 hours) or freezing (-20°C for 24 hours for spores and 72 hours for fish fillets containing spores).

Protection and prevention

Risk for wildlife health 

The impact of the introduction of this parasite on wildlife health is unknown.

Risk for human health

There is no evidence that Heterosporis sp. is transmissible to humans. However, its presence significantly alters the quality and appearance of the flesh, often making it inedible. This parasite could have significant economic consequences if it spreads to fish populations valued by recreational and commercial fishers.

Adopting some safe practices when preparing and cooking wild fish remains relevant. 

Surveillance and control

The parasite Heterosporis sp. has never been identified in Québec. However, it has been recorded in the United States (Wisconsin, Michigan and Minnesota) and Canada (Lake Ontario - Bay of Quinte). Therefore, fish showing signs of its presence should be reported to the ministère de l'Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs by calling 1-877-346-6763. You have to freeze the fish while you’re waiting for it to be collected.

In order to reduce the risk of introducing this parasite into Québec, you must follow these guidelines:

  • properly dispose of suspect fish and their waste by throwing them in the garbage or burning them. Do not release them into the water;
  • do not transfer dead or live fish from one location to another;
  • clean nautical equipment with a minimum 6% bleach solution (2,200 mg/L sodium hypochlorite), let stand for 5 minutes, and rinse with clean water. Note that the parasite can survive at a lower concentration (220 mg/L or less). Allow equipment to dry completely after use for a minimum of 24 hours.

Last update: January 8, 2024


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