Heart and Muscle Inflammation (HSMI)

Physical Signs:

• Muscle Lesions: Inflammation and degradation of skeletal muscles, often visible as pale or white streaks.
• Cardiac Damage: Inflammation and damage to the heart muscle, affecting its function.
• Anemia: Pale gills and organs due to reduced red blood cells.
• Pericardial Effusion: Accumulation of fluid around the heart.
• Other Signs: Pale heart, yellow liver, ascites, swollen spleen, and petechiae in the perivisceral fat 12.

Behavioral Changes:

• Lethargy: Reduced activity and swimming stamina.
• Loss of Appetite: Decreased feeding behavior and subsequent weight loss.
• Abnormal Swimming Patterns: Fish may display erratic or weakened swimming due to muscle damage 13.

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

Advanced Stages: More pronounced physical symptoms, including muscle lesions and cardiac damage, leading to increased mortality if not managed. Morbidity may be very high, while mortalities are variable and can reach up to 20% in affected cages 123.

Impact on Fish Health: HSMI severely compromises heart and muscle function, making fish more susceptible to secondary infections and reducing overall vitality.

Causative Agent: Piscine orthoreovirus (PRV) 34.

Transmission Methods:

• Direct Contact: Spread between infected and healthy fish through close proximity.
• Waterborne: The virus can be transmitted through water, potentially spreading from one farm to another 14.

Environmental Factors: Poor water quality, high stocking densities, and stressful conditions can increase susceptibility to HSMI.

Farm Management Practices: Ineffective biosecurity measures and lack of routine health monitoring can facilitate the spread of PRV 14.

Clinical Examination: Observation of physical and behavioral symptoms indicative of HSMI.

Laboratory Tests:

• PCR (Polymerase Chain Reaction): Detects PRV genetic material in fish tissues.
• Histopathology: Microscopic examination of heart and muscle tissues to identify characteristic lesions of HSMI.
• Immunohistochemistry: Detects viral antigens in tissue samples 145.

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

Experimental Treatments: Ongoing research into potential antiviral agents and other therapeutic approaches 134.

Biosecurity Protocols:

• Implementing strict quarantine measures for new stock to prevent the introduction of PRV.
• Regular disinfection of equipment and facilities to minimize the risk of contamination.

Farm Management Practices:

• Reducing stocking densities to decrease the risk of virus transmission.
• Regular health monitoring and early detection programs to identify and manage outbreaks promptly.
• Implementing all-in/all-out stocking practices to reduce the risk of spreading infections 145.

• Norway (1999): The first identified outbreaks led to significant economic losses and increased regulatory scrutiny.

Incidence in Norway:

• HSMI was first diagnosed in Norway in 1999 1.
• There has been a yearly increase in the number of recorded outbreaks since its initial detection1.
• By 2004, HSMI had been reported in 417 farms in Norway 6.

Geographical Spread:

• HSMI has been observed throughout Norway's salmon farming regions 3.
• The disease appears to have spread along the Norwegian coastline since its initial detection.

Economic Impact:

• While specific economic data for HSMI in Norway is limited, the disease can cause significant losses due to:
  • High morbidity rates in affected cages 6.
  • Variable mortality rates, reaching up to 20% in some cases 6.
  • Reduced growth and feed conversion in affected fish.

Treatment & Management:

• No specific treatment exists for HSMI 3.
• Management focuses on reducing environmental stress during outbreaks 3.
• Improved biosecurity measures and monitoring of fish health are key strategies.