fungal disease Salmon

Saprolegniosis (mycosis of salmon eggs and larvae)

Find a veterinarian All salmon diseases

Definition Signs Causes Diagnosis Treatment Prevention Complications When to see a vet Prognosis FAQ Sources Affected species

What is it?

Saprolegniosis is a very common fungal infection in freshwater fish farming. It mainly affects salmon eggs and young fry, but can also affect weakened or injured adult fish. The fungus in question, of the genus Saprolegnia, is naturally present in fresh water. It takes advantage of a dead or damaged egg or a wound on the skin to settle in. Once developed, it forms a whitish or grayish cotton-like down, visible to the naked eye on the eggs or on the body of the fish.

Commonly observed signs

These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.

Related signs — each one links to its page:

Abnormal swimming Weakened colony

The most characteristic sign is the appearance of a white to grayish, cottony felt on the surface of the eggs or skin. On the eggs, this down gradually covers the entire surface and prevents the exchange of oxygen, which leads to the death of the embryo.\n\nIn fry and young fish, cottony patches are observed on the skin, fins or gills. Affected fish swim abnormally, stay near the surface or banks, lose their usual activity and may show a general slowdown. Affected areas may become reddish or irritated under the fungal growth.\n\nIn breeding settings, the disease is often noticed by abnormal egg mortality in incubators, with rapid spread from a dead infected egg to neighboring healthy eggs.

When to consult without delay

This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.

Causes and risk factors

Saprolegniosis is caused by filamentous aquatic fungi of the genus Saprolegnia, sometimes called “water molds”. These organisms are ubiquitous in fresh water, including clean water. They normally behave as decomposers of dead organic matter.\n\nThe problem arises when these opportunistic fungi colonize weakened living tissue: an unfertilized or dead egg, a skin wound, an area where the scales have been damaged by handling, or the skin of a fish weakened by stress, other illness or poor breeding conditions.

Several factors favor the appearance and spread of saprolegniosis:\n\nWater loaded with organic matter or poorly renewed, a cool to moderate water temperature, a high density of eggs or fish in incubators or ponds, the presence of dead or unfertilized eggs not removed quickly, rough handling causing injuries, stress linked to transport or sudden changes in water quality, and an immune system weakened by another infection.

Diagnosis and care

Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.

Diagnosis is firstly based on visual observation of the characteristic cottony fuzz on fish eggs or skin. A specialist in fish pathology or a veterinarian can confirm the identification of the fungus by a microscopic examination of a sample, showing the typical filaments of Saprolegnia.\n\nIn more complex cases, in particular to distinguish primary saprolegniosis from an infection secondary to another disease, additional laboratory analyzes may be necessary.

Treatment

The treatment uses antifungal products that can be used in a bath, suitable for eggs or live fish according to the recommendations in force in aquaculture. Mechanical removal of dead or heavily infected eggs remains an essential measure in addition to chemical treatment.\n\nImproving rearing conditions, particularly water quality and reducing density, is an integral part of treatment. The choice of product and the methods of application must be defined with a professional, because certain treatments are regulated in fish farming.

Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.

Prevention

Prevention is based above all on good breeding hygiene. It is essential to remove dead or white eggs from incubators daily, as they are the starting point for contamination.\n\nProper water renewal, effective filtration and maintaining clean water limit the amount of fungi available. Avoiding over-density of eggs and fish also reduces the risk of spread.\n\nPreventive treatments based on antifungal products suitable for aquaculture can be applied in a bath to incubating eggs, according to practices recommended by a veterinarian or fish farming specialist. Limiting stress and injuries when handling fish also helps reduce risks.

Possible complications

Without intervention, saprolegniosis can lead to a significant loss of eggs in an incubator, with the fungus spreading rapidly from a dead egg to neighboring healthy eggs by direct contact.\n\nIn live fish, an extensive infection of the skin or gills can disrupt breathing and exchanges with the environment, worsen the general condition and promote secondary bacterial infections in injured areas.

When to see a veterinarian

Appearance of a whitish or cottony fuzz on incubating eggs\nAbnormal or increasing mortality in a batch of eggs or fry\nCottony patches visible on the skin, fins or gills of a fish\nAbnormal swimming behavior or lethargy in fry or juveniles\nSkin wounds that do not heal and appear to be covered with down

Prognosis

The prognosis depends on the precocity of treatment. On eggs, rapid intervention with removal of dead eggs and antifungal treatment generally significantly limits losses. An untreated infection, on the other hand, can destroy a significant part of an incubation batch.\n\nIn live fish, a localized infection treated early has a good prognosis. Extensive damage, particularly of the gills, or associated with an already weakened general condition, darkens the prognosis.

Frequently asked questions

Can saprolegniosis be transmitted to humans?

No, this fungus is specific to the aquatic environment and is not known to infect humans.

Why are salmon eggs so susceptible to this fungus?

The fungus is a natural decomposer of dead organic matter. An unfertilized or dead egg provides it with an ideal support, from which it can then invade neighboring healthy eggs by contact.

Is a very clean aquarium or pond safe from saprolegniosis?

No, the Saprolegnia fungus is naturally present in almost all fresh waters. The risk depends mainly on the presence of dead eggs, wounds or weakened fish, not just on the visible cleanliness of the water.

How to recognize an affected salmon egg among healthy eggs?

An infected egg gradually becomes covered in white to grayish cotton-like fuzz, while a healthy egg remains translucent and smooth.