ISAv

Infectious salmon anemia (ISA) is a virus of farmed Atlantic salmon (Salmo salar), inducing a systemic and lethal condition comprised of severe anemia and variable hemorrhages and necrosis in several organs. It causes lethargy, poor health and can result in significant mortality (up to 90%). In the fall of 2011, ISAv was identified in several species of Pacific salmon in British Columbia.

The virus belongs to the family Orthomyxoviridae and has recently been classified as the type species of the new genus Isavirus. ISAv is related to influenza viruses, but has a number of unique characteristics that distinguish it from other known orthomyxoviruses from warm-blooded animals. The origin of ISAv is not known, but it likely an existing virus that adapted to new hosts. The virus was first detected in Norway in the early 1980s. The disease has subsequently been detected farmed and wild salmonid populations in Canada, Scotland, Faroe Islands, USA and Chile. Outbreaks in salmon farm populations can induce significant mortality. ISAv is a disease listed in the OIE (World Organization for Animal Health) Aquatic Animal Health Code (2011) and countries are obligated to report incidences of the disease to the OIE according to Chapter 1.1 of the Code.

Transmission: ISAv is transmitted horizontally by contact with infected live salmon or biological materials containing the virus, such as animal wastes or discharges from normal culture operations, and slaughter facilities. Spread of the disease is related to the movement of fish, boats or waste from fish processing plants. Research has shown that boats servicing the aquaculture industry have spread the disease between aquaculture sites. Infected fish may transmit the disease weeks before they show apparent signs of infection, with the virus found in blood, gut contents, urine, and mucus of infected salmon. The transfer of smolts (young fish) from one site to another has therefore led to the transfer of disease. Moreover, fish that survive outbreaks may continue to shed viral particles for more than a month into the surrounding water. Sea lice, a common parasite of farmed salmon have also been shown capable of spreading the disease. The disease has also been shown to transmit vertically through the eggs, which has major implications for egg importation in the farm industry.

Under experimental conditions the incubation period is usually 10-20 days. In infected populations under field conditions the infection can remain hidden for months before the disease breaks out. A disease outbreak can have either an acute course with high mortality or be more long-drawn with increase in daily mortality, which may continue for several months.

The virus can survive in seawater but is killed by ultraviolet light and by disinfectants.

Symptoms and pathology: Sick fish are lethargic, consume less feed, and may sink vertically or if in a farm, keep close to the side of the pen.

Typical findings of affected fish are pale gills, bulging or blood-spotted eyes and distended belly. As the disease progresses, bleeding in the skin, loss of scales and bloody, swollen anus can be observed. When opening the fish typical observations are dark liver, pale heart and pale gills. The heart will appear pale because of anaemia. The spleen and liver are often strongly swollen, dark and with petechial haemorrhages.

By histopathology haemorrhagic necrosis in the liver can be observed. The macroscopic changes are darkening, bleeding, especially at the abdomen and anaemic gills.

Diagnosis: Diagnosis is based on a combination of clinical, pathological and clinical chemical findings. ISA virus is isolated from organ material from infected fish by cultivation in specific cell cultures and identified by RT-PCR or immunofluorescence. There are 4 levels of detection of ISAv, including (from lowest to highest confidence): histology, PRC test, sequencing and culture. Canada only recognizes culture, citing PCRs as too sensitive and the potential for false positives too damaging to the aquaculture industry.

Prevention: There is no effective treatment upon outbreak of ISAv. Vaccinations have been used on North American populations but these do not currently seem to offer protection and immunized fish may become virus carriers. In the case of an outbreak on a farm, control programs must be applied including biosecurity, sanitary waste management, disinfection of effluent from processing plants, year class separation, culling of clinically affected populations and leaving pens fallow after disinfection to reduce the viral load.

Surveillance according to Chapter 1.4 of the OIE Aquatic Animal Health Code, (2011) is necessary to demonstrate freedom of the disease is a zone or compartment or even in a country.

References:

  1. OIE Aquatic Animal Health Code, (2011) Chapter 10.5
  2. OIE Manual of Diagnostic Tests for Aquatic Animals (2011) Chapter 2.3.5
  3. OIE Guide for Aquatic Animal Health Surveillance (2009) F. Corsin, M. Georgiadis, K. Larry Hammell, B. Hill.
  4. Alexander G. Murray, Ronald J. Smith and Ronald M. Stagg. 2002. Shipping and the Spread of Infectious Salmon Anemia in Scottish Aquaculture. Emerging Infectious Disease Vol 8 No 1.
  5. European Union Reference Laboratory for Fish Diseases
  6. Vike, S, Nylund S and Nylund A. 2009. ISA virus in Chile: evidence of vertical transmission. Arch Virol 154(1): Epub 2008 Nov 26.