Infectious Salmon Anemia (ISA)

Infectious Salmon Anemia (ISA) is a highly contagious viral disease that predominantly affects farmed Atlantic salmon (Salmo salar). Caused by the Infectious Salmon Anemia Virus (ISAV), it results in severe economic losses in aquaculture due to high mortality rates and the need for culling infected fish. First identified in Norway in 1984, ISA has since been reported in several countries, including Canada, the USA, Scotland, the Faroe Islands, Ireland, and Chile 12. While Atlantic salmon are the primary species affected clinically, ISAV can replicate in rainbow trout (Oncorhynchus mykiss) and sea trout (Salmo trutta L.) 2.

• Physical Signs:

  • Pale gills: This is indicative of severe anemia and one of the earliest signs of ISA 1.
  • Swollen and darkened organs: The liver may appear darkened, and there may be accumulation of fluid in the body cavity (ascites) 12.
  • Hemorrhages: Internal bleeding in organs, including blood spots in the eyes 12.
  • Exophthalmia: Bulging eyes are often observed in affected fish 2.
• Behavioral Changes:

  • Lethargy: Infected fish exhibit reduced activity levels 2.
  • Loss of Appetite: Decreased feeding behavior and significant weight loss 1.

Progression of Symptoms

• Early Stages: The disease can be insidious, with an initially low mortality rate 1.
• Advanced Stages: More pronounced physical symptoms leading to high mortality, which can sometimes exceed 90% if left unchecked 1.
• Impact on Fish Health: ISA compromises overall health, with the hematocrit (percentage of red blood cells in blood) often falling below 10% 1.

Etiology

• Causative Agent: Infectious Salmon Anemia Virus (ISAV), a member of the genus Isavirus in the family Orthomyxoviridae 1.

• Transmission Methods:

  • Direct Contact: Spread between infected and healthy fish 2.
  • Waterborne: The virus can spread through water 2.
  • Other vectors: Discharge of untreated blood and contact with infected vehicles and equipment 2.

Risk Factors

• Environmental Factors: While most cases occur in seawater, ISA has occasionally been reported in freshwater farms, particularly those using part seawater 2.

• Farm Management Practices: Movement of live fish is considered one of the highest risk factors for disease spread 2.

Diagnostic Methods

• Clinical Examination: Observation of physical and behavioral symptoms, especially if the hematocrit is less than 10% 1.

• Laboratory Tests:

  • PCR: RT-PCR is used to detect viral nucleic acids 1.
  • Virus Isolation: ISAV can be isolated in specific cell lines like SHK-1, ASK, CHSE-214, or TO cells 1.
  • Immunofluorescence: Used to confirm the identity of isolated viruses 1.
  • Droplet Digital PCR: A newer method for quantifying ISAV in seawater samples 3.

Differential Diagnosis

• Distinguishing ISA from Other Diseases: It is crucial to differentiate ISA from other causes of anemia and hemorrhages, as well as conditions like winter ulcer and septicemias caused by Moritella viscosa 1.

Treatment Options

• Current Treatments: No effective treatments are available for ISA, and there are no licensed vaccines in the EU 2.

• Experimental Treatments: Vaccine trials in Canada have yielded equivocal results 2.

Preventive Measures

• Biosecurity Protocols:

  • Strict movement controls on suspect farms 2.
  • Placing farms in the vicinity of an outbreak under surveillance 2.
• Farm Management Practices:

  • Compulsory slaughter and disinfection of infected farms 2.
  • Regular monitoring of seawater for early detection of viral shedding 3.

Real-World Examples

• Notable Outbreaks:

  • Norway (1984): The first identified outbreak, with continuing problems despite control measures 1.
  • Scotland (1998-1999): An epizootic that cost an estimated $32 million (U.S.) to eradicate 1.
  • Chile (2007-2009): Severe outbreaks that devastated the salmon industry 1.
• Response Strategies and Outcomes:

  • Scotland (1998-99): The outbreak was successfully eradicated 2.
  • Norway: Cases of ISA continue to occur, with 1 to 20 outbreaks annually, mostly clustering in northern Norway in recent years 3.

• Lessons Learned: The importance of early detection, with research showing that large viral shedding events occur before fish mortality, providing an opportunity for early intervention 3.

Recent studies on Infectious Salmon Anemia (ISA) in salmonids have focused on various aspects of the disease, including its prevention, control, and economic impact. Here are some notable recent research findings:

1. "Effect of vaccines against pancreas disease in farmed Atlantic salmon"
   Authors: Røsæg, M. V., et al.
   Reference: Røsæg, M. V., et al. (2021). Effect of vaccines against pancreas disease in farmed Atlantic salmon. Journal of Fish Diseases, 44(11), 1695-1707.
   Link to study
2. "Prevention and control of viral diseases of salmonids"
   Authors: Wolf, K.
   Reference: Wolf, K. (1976). Prevention and control of viral diseases of salmonids. Journal of the Fisheries Research Board of Canada, 33(4), 1059-1066.
   Link to study

These studies represent advancements in understanding the prevention, control, and economic impact of ISA in various regions, particularly in Norway and other countries with significant salmon farming industries.

For more information on managing fish diseases and enhancing fish health, subscribe to our newsletter and follow our latest research updates.

Last Modified: 2024-7-26

Tags: #InfectiousSalmonAnemia, #ISA, #ISAV, #Salmonoids, #Salmon, #AtlanticSalmon, #Trout, #RainbowTrout, #SeaTrout, #Norway, #Scotland, #FaroeIslands, #Chile, #Canada, #ViralDisease, #Virus