Red vent syndrome is an inflammation of the fish’s anal orifice (or anal vent). The inflammation of the anus region is associated with the presence, and especially an unusually high concentration, of the parasite Anisakis simplex in the tissues surrounding the anus.

Wildlife at risk

Red vent syndrome is only observed in Atlantic salmon This hyperlink will open in a new window. (in French). However, the parasitic worm Anisakis simplex, which is responsible for the disease, is also present in over 200 marine fish species, such as cod, mackerel and herring. In these species, the parasite does not appear to cause inflammation in the tissues surrounding the anus.

Signs of the disease

The anus region of the infected fish will be red due to the inflammation. It is associated with the presence, and especially an unusually high concentration, of the parasite Anisakis simplex in the tissues surrounding the anus.

Life cycle and persistence in the environment

To develop, the parasite Anisakis simplex needs to transit through several living organisms called hosts. The adult parasite develops in the stomach of a marine mammal, such as a whale or seal. As an adult, it lays eggs. The latter are then released into the water through mammal feces. The eggs hatch in the water and the larvae are eaten by a crustacean, which can then be eaten by fish. However, fish are optional hosts since the parasite does not have to pass through them to complete its development. The parasite’s life cycle is completed when a marine mammal eats an infected crustacean or fish. Humans can become an accidental host of the parasite when eating raw or undercooked fish or squid.


No treatment is available for salmon that have developed signs of the disease. However, the level of infection decreases when salmon spend time in fresh water after returning from the sea. In some cases, complete healing of the skin around the anus is even observed after a few months in fresh water.

Protection and prevention

Risk for salmon health

Current scientific knowledge indicates that red vent disease does not lead to decreased survival or reproductive success in salmon.

Risk for human health

Ingestion of live Anisakis simplex larvae by humans can lead to health problems. Symptoms may include gastrointestinal pain, nausea or vomiting. Eating raw fish (meat, eggs, liver or other viscera) or improperly prepared fish (insufficient cooking or salting) puts humans at risk of infection.

Anisakis simplex larvae are found primarily in the viscera of infected fish, but may also be present in their flesh. Rapid evisceration of fish or cold storage will limit the migration of the parasite into the flesh that will be consumed. To avoid contamination, learn about the precautions to take when preparing and cooking wild fish.

It should be noted that the ingestion of Anisakis simplex larvae can sometimes lead to allergic disorders of varying severity, ranging from hives to anaphylactic shock. According to some studies, neither cooking nor freezing fish would completely eliminate the risk of allergic reactions.

Surveillance and control

In Québec, the first Atlantic salmon with symptoms of red vent syndrome were observed in 2008 in different regions. To learn more, a research project was conducted in 2009 on 10 rivers. The results are available in the Stratégie québécoise sur la santé des animaux sauvages - Rapport des activités de surveillance-2009 This hyperlink will open in a new window. (in French only).

In recent years, observations of salmon with symptoms associated with red vent syndrome are less significant. It is not required to report suspect salmon to the ministère de l'Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs.

Last update: January 8, 2024


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