Salmon viral hemorrhagic septicemia
Viral hemorrhagic septicemia (VHS) is a serious viral disease that affects many species of freshwater and marine fish, including Atlantic salmon (Salmo salar). It is caused by a virus from the Rhabdoviridae family, which is very resistant t...
What is it?
Viral hemorrhagic septicemia (VHS) is a serious viral disease that affects many species of freshwater and marine fish, including Atlantic salmon (Salmo salar).
It is caused by a virus from the Rhabdoviridae family, which is very resistant to cold water.
This disease is one of the diseases requiring notification to the World Organization for Animal Health (WHOA/WOAH), due to its major economic impact on aquaculture.
Key point
Mandatory notifiable disease according to international aquaculture health regulations (OMSA/WOAH), due to its strong economic impact and its capacity for rapid spread between farms.
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:
The clinical signs of VHS are often nonspecific at first.
We observe a general depression, the fish swim abnormally, sometimes in a spiral or on the surface near the banks.
The skin and fins may show hemorrhages (small punctual bleeding).
The eyes may become bulging (out of their sockets) and red, sometimes with swelling of the abdomen related to fluid accumulation.
The gills may be pale, a sign of anemia.
Mortality can be sudden and affect a large number of fish in a short time, especially among young individuals.
Some fish show no visible signs before suddenly dying.
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 an RNA virus, viral hemorrhagic septicemia virus (VHSV), belonging to the genus Novirhabdovirus.
There are several strains (genotypes) of this virus, with varying levels of virulence depending on fish species and geographic areas.
The virus multiplies mainly in cold waters, between approximately 4°C and 14°C, and its activity decreases significantly above 15°C.
Young fish (fry, juveniles) are generally more sensitive than adults.
Stress linked to high stocking density, poor water quality or frequent handling increases susceptibility to infection.
Cold water temperatures, particularly during seasonal transitions, favor the clinical expression of the disease.
The introduction of fish or material from infected areas is a major risk factor for a farm.
Transmission
The virus is transmitted mainly through water, via the excretions (urine, feces, mucus) of infected fish.
Direct contact between sick and healthy fish promotes the spread, as does contaminated breeding equipment (nets, bins, boots) that are not disinfected.
The movement of live fish between breeding sites constitutes a major factor in geographical spread.
Some wild fish can be carriers without showing symptoms, acting as a reservoir for the virus.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The diagnosis of suspicion is based on the observation of clinical signs and abnormal mortality rates by the aquaculture veterinarian or the competent health service.
Confirmation requires specialized laboratory analyses: isolation of the virus in cell culture and molecular biology tests (RT-PCR) allowing the virus to be identified with certainty.
Organ samples (kidney, spleen, heart) are taken from sick or recently dead fish.
Due to the regulated nature of this disease, any suspicion should be reported to the competent veterinary authorities.
Treatment
There is no specific antiviral treatment for VHS.
Treatment is based on general support measures: improvement of water quality, reduction of stress and handling, isolation of sick fish.
Management of the disease mainly involves strict health measures: stamping out infected populations, disinfection of facilities, and movement restrictions, according to the directives of the veterinary authorities.
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 biosecurity: strict control of fish movements, quarantine of new arrivals, rigorous disinfection of equipment and installations.
It is essential to source from certified disease-free sites.
Regular health monitoring of farms, with screening tests, allows early detection.
In some countries and for certain strains, vaccines exist and can be used as part of official control programs.
Stress management (density, water quality, gentle handling) also helps to reduce the sensitivity of fish.
Vaccination
Vaccines against SHV exist in some countries, mainly used as part of official control programs in risk areas.
Their availability and use depend on local regulations and the health status of the breeding region.
Possible complications
The disease can lead to severe anemia, damage to internal organs (kidney, liver, spleen), and blood clotting disorders.
Massive mortality can occur within a few days on affected farms, with significant economic consequences.
Surviving fish may remain carriers of the virus and contribute to its persistence in the environment.
When to see a veterinarian
Observation of abnormal or sudden mortality in a pond or enclosure.
Fish swimming abnormally, spiraling or lethargic near the surface.
Presence of visible hemorrhages on the skin, eyes or fins.
Suspicion of recent introduction of fish or material from a risk area.
Prognosis
The prognosis is gloomy in the event of an active epizootic, with mortality rates which can be very high in young fish.
Surviving adult fish may develop some resistance but remain asymptomatic carriers.
Early detection and rapid health management make it possible to limit the extent of losses on the farm.
Frequently asked questions
Is salmon viral hemorrhagic septicemia transmissible to humans?
No, this disease only affects fish and is not transmissible to humans. The consumption of fish is not affected by a health risk linked to this virus.
Can we treat a salmon suffering from this disease?
There is no specific antiviral treatment. Management is based on collective health measures and general support of the fish, under veterinary supervision.
How can a farm protect itself from this disease?
Biosecurity is essential: control of fish movements, disinfection of equipment, quarantine of new arrivals and regular health monitoring.
Is there a vaccine against this disease?
Yes, vaccines exist in some countries and are used in official control programs, depending on local regulations.
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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