Salmon furunculosis
Salmon furunculosis is a serious bacterial disease that affects salmon and other salmonids farmed or living in freshwater and sea.\nIt is caused by a bacteria called Aeromonas salmonicida.\nThis disease is so named because it can cause boil...
What is it?
Salmon furunculosis is a serious bacterial disease that affects salmon and other salmonids farmed or living in freshwater and sea.\nIt is caused by a bacteria called Aeromonas salmonicida.\nThis disease is so named because it can cause boils, i.e. abscesses under the skin of the fish.\nIt affects both farmed and wild fish, and can cause significant economic losses in fisheries. aquaculture farms.
Key point
Disease that can lead to high and rapid mortality in affected farms if not managed.
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:
Affected fish often become lethargic, swim near the surface or abnormally, and lose their appetite.\nSwellings or bumps may be seen under the skin, which may develop into full-blown boils or abscesses containing pus.\nWhen these lesions break open, they leave open sores and ulcers that do not heal easily.\nHemorrhages (small bleeds) may appear at the base of the fins, around the the mouth or on the stomach.\nIn very acute forms, fish can die quickly without showing obvious external signs, which makes the disease sometimes difficult to spot in time.\nAbnormal coloring, pale gills and darkening of the skin are also possible.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by the bacterium Aeromonas salmonicida, which exists in several subspecies.\nThis bacteria can survive in water, sediment and culture equipment, making it easier for it to persist in a fish farming site once introduced.
Stress plays a major role in triggering the disease: water that is too hot, poor water quality, too high a density of fish in tanks or cages, transport or rough handling can weaken the fish's natural defenses.\nSkin injuries, caused for example by parasites or handling, facilitate the entry of the bacteria.\nFarms where several generations of fish follow one another without complete disinfection of the site are particularly at risk.
Transmission
The bacteria is transmitted mainly through contaminated water, by direct contact between sick and healthy fish, and by contaminated equipment or equipment (nets, tanks, tools).\nCarrier fish without visible signs can also spread the bacteria, particularly during periods of stress.\nTransmission can also occur through eggs in certain cases, which complicates disease control between breeding sites.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian or fish health specialist makes the diagnosis by observing clinical signs and then confirming with laboratory tests.\nThis usually includes taking a sample of tissue or lesions to isolate the bacteria in culture, as well as more extensive testing such as PCR (a technique that detects the DNA of the bacteria) to confirm the identification.\nSwabs from several fish from the same tank or cage are often necessary to assess the extent of the problem.
Treatment
Treatment is based on the use of antibiotics prescribed and supervised by a veterinarian, adapted after a sensitivity test of the bacteria if possible.\nSupportive measures, such as improving water quality and reducing stress, often accompany the treatment.\nHealth management of the site (isolation of sick fish, disinfection) is an integral part of the overall care.
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 farming practices: maintaining optimal water quality, avoiding overpopulation in ponds or cages, limiting stress during handling and transport.\nRegular disinfection of equipment and installations helps to reduce the presence of the bacteria on the site.\nVaccination of salmon is widely used in many producing countries and constitutes an important prevention tool.\nQuarantine of new fish introduced to a site is also recommended before mixing them with the rest of the stock.
Vaccination
Vaccines against Aeromonas salmonicida exist and are used in several salmon-producing countries, particularly in the form of an injection administered to young fish before their transfer to sea.\nVaccination is one of the main prevention strategies in modern farms.
Possible complications
Without treatment, furunculosis can lead to significant mortality within a breeding tank or cage.\nSecondary infections can attach to open skin lesions.\nThe bacteria can also settle permanently on a breeding site, making recurring episodes more difficult to completely eliminate.
When to see a veterinarian
Fish that are lethargic or swim unusually close to the surface.\nAppearance of bumps, swellings or sores under the skin.\nAbnormal or sudden mortalities in a tank or cage.\nWidespread loss of appetite within a group of fish.
Prognosis
The prognosis depends on the speed of treatment and the form of the disease.\nIn peracute forms, mortality can occur very quickly.\nWith early treatment and good breeding conditions, many fish can survive, but the bacteria remains difficult to completely eradicate from a site once established.
Frequently asked questions
Is salmon furunculosis dangerous for humans?
No, this disease only affects fish. It is not known to be transmitted to humans or to make fish unsafe to eat once cooked.
Can you treat a fish that already has visible boils?
A specialist veterinarian can offer antibiotic treatment, but the lesions already present do not always disappear completely. Early treatment gives better chances of recovery.
How do I know if my breeding is at risk?
Sites with poor water quality, high fish density or a history of disease are at greater risk. Regular monitoring with an aquaculture health professional helps limit risks.
Does vaccination provide 100% protection?
No, vaccination greatly reduces the risk and severity of the disease but does not guarantee total protection. It must be associated with good breeding practices.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 25 September 2026
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