viral disease Trout

Viral Haemorrhagic Septicaemia in Trout

Viral haemorrhagic septicaemia (VHS) is a serious viral disease affecting many species of freshwater and saltwater fish, including trout. It is caused by a rhabdovirus and produces internal and external haemorrhages that are often fatal. Th...

Find a veterinarian All trout diseases

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

What is it?

Key point

Notifiable disease in many countries because of its major economic impact on aquaculture; any suspected case must be reported to the competent veterinary authorities.

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:

Fatigue Reduction Abnormal swimming Edema Pale mucous membranes Swelling Jaundice Difficulty breathing Circling

Infected fish become lethargic and swim abnormally, sometimes in a spiral or circular pattern. Abdominal swelling (ascites) is often seen, along with protruding eyes (exophthalmia) and pale gills indicating anaemia. Pinpoint haemorrhages may appear on the skin, at the base of fins, in the eyes, and in internal organs. The skin may darken. Mortality can be sudden and severe, particularly in young fish, sometimes with few visible signs before death.

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 a rhabdovirus, the viral haemorrhagic septicaemia virus (VHSV). This virus infects cells in many organs, particularly blood vessels, which explains the characteristic haemorrhages. Several viral strains exist with varying degrees of virulence depending on fish populations and geographical regions.

Cold to moderate water temperatures (typically between 4 and 14°C) encourage the disease to develop. Stress from high stocking density, poor water quality, transport, or handling of fish increases susceptibility to infection. Young fish are often more vulnerable than adults.

Transmission

The virus spreads mainly through water via excretions and secretions from infected fish (urine, mucus, reproductive fluid). Contamination can also occur through direct contact between fish, contaminated farm equipment, or transfer of infected fish or eggs between sites. Some wild fish may carry the virus without showing symptoms and act as a reservoir.

Diagnosis and care

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

Diagnosis is based on observing clinical signs associated with abnormal mortality, combined with specialized laboratory analysis: viral isolation in cell cultures, molecular tests (PCR), or immunological tests. These analyses are performed in official veterinary laboratories, as the disease is notifiable by law.

Treatment

There is no specific antiviral treatment for this disease in fish. Farm management relies on sanitary measures: isolation of affected animals, improvement of farm conditions to reduce stress, and in most cases, sanitization through culling of affected stock to prevent spread, under supervision of the competent authorities.

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

Prevention

Prevention relies mainly on strict biosecurity measures: health screening of fish and eggs before introduction to a farm, disinfection of equipment, quarantine of new arrivals, and limiting exchanges with contaminated areas or farms. Maintaining good water quality and reducing stress also limit the risk of clinical disease. In recognized disease-free zones, regular monitoring is carried out by veterinary authorities.

Possible complications

The disease can cause very high mortality in a farm, especially in young fish. Survivors may remain carriers of the virus and transmit it. Economically, contamination of a fish farm can have serious consequences, ranging from temporary closure of the site to trade restrictions.

When to see a veterinarian

Unusual or sudden mortality in a trout pond or farm.

Fish showing abnormal swimming, circling, or disorientation.

Visible hemorrhages observed on the skin, eyes, or at the base of fins.

Abdominal swelling or exophthalmia (bulging eyes) in several fish.

Suspicion of a notifiable disease: contact the official veterinary services immediately.

Prognosis

The prognosis is guarded to grave when clinical signs appear, with mortality rates that can be very high, particularly in fry and juveniles. Some adult fish may survive the infection but remain carriers of the virus. Early management and prompt notification to the health authorities are essential to limit the impact on the farm.

Frequently asked questions

Can viral hemorrhagic septicemia infect humans?

No, this virus only infects fish and poses no risk to humans, including through fish consumption.

Can you treat a fish that has contracted this disease?

There is no specific antiviral treatment. Management relies on isolating sick animals and implementing strict sanitary measures decided in coordination with the competent authorities.

How can a trout farm protect itself from this disease?

By implementing strict biosecurity measures: health screening of introduced fish, equipment disinfection, quarantine, and limiting exchanges with high-risk areas.

Do all infected fish die?

No, some fish, particularly adults, can survive the infection but remain carriers of the virus and contribute to its spread.