Saprolegniosis is a skin infection caused by a microorganism that affects fish whose protective mucus layer has been altered or that are already compromised by disease or stress. Once the epidermis is contaminated, this microorganism, which looks like a fungus, destroys the superficial and deep layers of the skin.

Wildlife at risk

All freshwater fish can be affected by saprolegniosis. The salmonids family (salmon and trout) appear to be particularly vulnerable to this type of infection. All life stages of fish, including eggs, can be affected. Saprolegniosis can also be found in crustaceans, amphibians and molluscs.

Signs of the disease

In water, fish have slightly elevated lesions that take on the appearance of cotton wool, but have a gelatinous consistency adhering to the skin. Out of the water, the lesions resemble a thin mat of gelatinous consistency. Initially white, the lesion then turns brownish with a red border. The gills can also be affected.

Lesions can quickly spread to the surface of the skin. The disease can cause fish death.

Transmission and persistence in the environment

The microorganisms of the genus Saprolegnia, which cause the disease, are ubiquitous in the environment. They are found in moist soils and in water. They usually feed from decaying organic matter in water (saprophyte). Some species are parasitic.

By reproducing asexually, microorganisms of the genus Saprolegnia produce and release mobile spores. These spores infect fish. These microorganisms are common in freshwater. They rarely affect healthy fish. Instead, they attack fish whose protective mucus layer has been altered (mechanical friction, skin wound) or whose immunity is already compromised by another disease (bacteria, virus) or stressor (irritants, chemicals, toxic algae). The spawning season of fish is particularly conducive to fungal infections, due in part to the risk of injury.

Protection and prevention

Risk for wildlife health

Saprolegniosis can result in the death of fish, and this condition is generally secondary to another underlying health condition.

Risk for domestic animal health

Fish reared in fish farms and aquarium fish can also suffer from saprolegniosis.

Risk for human health

Transmission to humans was never reported. While the presence of saprolegniosis reduces the interest and value of recreational and commercial fishing, fish that are minimally affected remain edible. Since the presence of the microorganism may conceal another health problem, it is recommended to manually remove all affected parts before the fish is completely cooked. It is not advisable to consume fish that are highly affected and those that appear to be diseased. Adopting some safe practices when preparing and cooking wild fish remains relevant.

Surveillance and control

It is not unusual to find a fish in the wild with saprolegniosis. Since this type of infection affects fish that are injured, sick or weakened by a stressor, it may be indicative of another much more important problem. If you see multiple fish with this disease, contact the ministère de l'Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs quickly at 1 877 346-6763.

Prevention is achieved by reducing stressors and skin injuries.

Last update: January 8, 2024


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