Piscine Reovirus (PRV)

• Physical Signs:
  • Heart and Skeletal Muscle Inflammation (HSMI):
    • Inflammation: Swelling and inflammation of heart and skeletal muscles.
    • Anorexia: Loss of appetite.
    • Lethargy: Reduced activity.
    • Ascites: Accumulation of fluid in the body cavity 1.
  • Jaundice Syndrome:
    • Yellowing: Jaundice observed in skin, gills, and eyes.
    • Anemia: Pale gills and organs.
    • Liver and Kidney Degeneration: Necrotic lesions in liver and kidney 2.

Progression of Symptoms

• Early Stages: Mild signs such as reduced feed intake and slight lethargy.

• Advanced Stages: More pronounced physical symptoms like inflammation and internal lesions.

• Impact on Fish Health: PRV infections can compromise immune function and overall vitality, potentially increasing susceptibility to secondary infections 3.

Etiology

• Causative Agent: Piscine Orthoreovirus (PRV), a double-stranded RNA virus with three known genotypes: PRV-1, PRV-2, and PRV-3 3.

• Transmission Methods:

  • Horizontal Transmission: Spread between infected and healthy fish through water or direct contact.
  • Vertical Transmission: Possible transmission from parent to offspring through eggs 3.

Risk Factors

• Environmental Factors: Stress conditions may precipitate clinical disease 1.

• Farm Management Practices: High-density aquaculture settings may facilitate virus spread.

Diagnostic Methods

• Clinical Examination: Observation of physical and behavioral symptoms indicative of PRV infection.

• Laboratory Tests:

  • RT-qPCR: Detects PRV genetic material in fish tissues.
  • Histopathology: Microscopic examination of heart and muscle tissues to identify characteristic lesions of HSMI.
  • Virus Sequencing: Molecular detection and sequencing for confirmation of virus strain13.

Differential Diagnosis

• Distinguishing PRV from Other Diseases: It is crucial to differentiate PRV-associated conditions from diseases with similar symptoms, such as Erythrocytic Inclusion Body Syndrome (EIBS) 1.

Treatment Options

• Current Treatments: There are no specific antiviral treatments for PRV. Management focuses on preventing the spread and mitigating the impact through biosecurity measures and supportive care.

Preventive Measures

• Biosecurity Protocols:
  • Implementing strict quarantine measures for new stock to prevent the introduction of PRV.
  • Regular health monitoring and early detection programs to identify and manage outbreaks promptly.

Real-World Examples

• Notable Findings:

  • Pacific Northwest: PRV has been detected in both wild and farmed salmonids since at least 1987, possibly as early as 1977 1.
  • Canada: PRV was found in 95% or more of escaped farmed Atlantic salmon in Washington State and British Columbia 5.
  • Japan: PRV-2 has been associated with EIBS and jaundice/anemia in farmed coho salmon 1.
• Research Outcomes:

  • Studies suggest that PRV-1 may be of low risk to wild Pacific salmon species, as it has been present in these populations for decades without clear association with disease 1.
  • Experimental infections have shown that PRV can be infectious for Chinook and sockeye salmon but may not cause mortality or apparent disease 1.

These findings highlight the complex nature of PRV infections and the need for continued research to fully understand its impact on different salmonid species and populations.

Piscine Reovirus (PRV) remains a significant challenge for aquaculture, necessitating diligent management and preventive measures. By implementing robust biosecurity protocols, investing in ongoing research, and fostering industry-wide cooperation, fish health managers and veterinarians can mitigate the impact of PRV and ensure the sustainability of aquaculture operations. This comprehensive guide aims to provide the necessary information and resources to effectively manage PRV and support the health and welfare of farmed fish.