Heart and Muscle Inflammation (HSMI)

Overview

What is HSMI?

Heart and Skeletal Muscle Inflammation (HSMI) is a significant viral disease affecting farmed Atlantic salmon (Salmo salar). It is caused by the piscine orthoreovirus (PRV) and is characterised by inflammation and damage to the heart muscle, sometimes accompanied by inflammation and damage to red skeletal muscle. First detected in Norway in 1999, HSMI has since been reported in other major salmon farming regions, including Scotland, Ireland, Chile and Canada, causing substantial economic losses. It occurs most commonly during the first year at sea, although cases also occur in freshwater production 1 2 6. HSMI has so far been documented only in farmed Atlantic salmon and is caused by the genotype PRV-1. However, related strains of PRV cause similar, but distinct viral conditions across other salmonid species. Depending on the region, host species and PRV strain, these related conditions are HSMI-like disease in rainbow trout (Oncorhynchus mykiss), Erythrocytic Inclusion Body Syndrome (EIBS) in coho salmon (Oncorhynchus kisutch), and jaundice and anemia (or jaundice syndrome) in Chinook salmon (Oncorhynchus tshawytscha). This disease page focuses exclusively on HSMI in Atlantic salmon 6 7

Clinical Signs of Heart and Skeletal Muscle Inflammation

Common Signs

Physical Signs:
- Cardiac Damage: Inflammation and damage to the heart muscle, affecting its function. The heart may appear pale, and fluid or blood may accumulate in the pericardial cavity surrounding the heart 6.
- Ascites: Fluid may accumulate in the abdominal cavity 6.
- Liver discolouration: The liver may appear pale or yellowish and may show congestion associated with circulatory failure (see a reddened margin). A thin layer of fibrin may sometimes cover the liver surface 6 8.
- Enlarged spleen: Swelling of the spleen may be observed 6.
- Petechial haemorrhages: Small petechial haemorrhages may occur in the fat surrounding the internal organs 6.
Behavioural Changes:
- Abnormal Swimming And Lethargy: Fish may display weakened swimming due to muscle damage, where they may gather near cage walls and water surface 1 2.
- Loss of Appetite: Decreased feeding behaviour and subsequent weight loss after an outbreak 8.

Progression of Symptoms

Early/Subclinical phase: PRV primarily targets and replicates inside red blood cells (erythrocytes) early in the infection cycle. During peak disease, the virus spreads from blood cells to infect heart cardiomyocytes and red skeletal muscle cells 2 6. Fish may develop substantial heart inflammation and damage without obvious external signs or increased mortality. Cardiac lesions can persist before and after a clinical outbreak 6.
Clinical disease: Affected fish may show reduced feed intake, lethargy and abnormal swimming behaviour. During outbreaks with increased mortality, heart inflammation is usually extensive, and inflammation and damage to red skeletal muscle may also occur. Morbidity can be high, while mortality varies from negligible losses to approximately 20% in affected cages 1 2 6.
Recovery: Fish can recover from HSMI, although PRV-1 infection may persist long after clinical recovery. PRV-1 is detected in nearly 90% of farmed Atlantic salmon during the seawater phase in Norway, and viral RNA often remains detectable until slaughter 6.
Impact on Fish Health: HSMI severely affects cardiac function and, in severe cases, cause circulatory failure. Affected fish may tolerate stress poorly, and handling, crowding or low oxygen conditions may increase losses 6.

Causes of Heart and Skeletal Muscle Inflammation

Etiology

Causative Agent: Piscine orthoreovirus (PRV) 2 3.
Transmission Methods:
- Horizontal Waterborne Transmission: Spread between infected and healthy fish through waterborne exposure 6 1 3.
- The virus infects susceptible fish through the intestinal wall (gut enterocytes) 9

Risk Factors

Environmental Factors:
- Low oxygen levels can reduce fish tolerance of the disease 6.
- Fish with HSMI-related heart damage are vulnerable to stress. Crowding, wellboat transport, transfers and sea-lice treatments may worsen outcomes; minimising stressful handling is therefore important when cardiac function is compromised 6.
Farm Management Practices:
- Ineffective biosecurity measures and lack of routine health monitoring can facilitate the spread of PRV 1 3.
- Utilizing unsterilized or inadequately disinfected seawater intake in land-based facilities likely increases the risk of early PRV-1 introduction 10.

Diagnosis

Diagnostic Methods

Clinical Examination: Observation of physical and behavioural signs compatible with HSMI may raise suspicion of the disease. However, these findings are not specific, and clinical examination alone cannot confirm HSMI; laboratory examination is therefore required 6.
Laboratory Tests:
- PCR (Polymerase Chain Reaction): Detects PRV genetic material in fish tissues. A positive result alone does not confirm HSMI, because PRV-1 can be present in fish without characteristic lesions or clinical disease. Results should therefore be interpreted alongside histopathology and clinical history 3 6.
- Histopathology: Definitive diagnosis is based on characteristic inflammation in the heart, often involving both the compact and spongy layers of the ventricular myocardium. Inflammation and damage to red skeletal muscle provide additional support, although these lesions may be absent and are most commonly observed around the peak of a clinical outbreak 3 6.
- Immunohistochemistry & In Situ Hybridization:** These techniques can localise PRV antigens or viral RNA, respectively, within tissue sections. They can help demonstrate the relationship between viral presence and tissue lesions 1 3 6.

Differential Diagnosis

Distinguishing HSMI from Other Diseases: It is crucial to differentiate HSMI from other diseases which can also cause cardiac lesions and circulatory disturbances, such as Pancreas Disease (PD) and Cardiomyopathy Syndrome (CMS). Differentiation relies primarily on histopathological assessment of the nature and distribution of lesions within and between organs, supported by pathogen-specific testing 1 2.

Disease	Primary Pathogen	Heart Affected?	Red Muscle Affected?	Pancreas Affected?
HSMI	Piscine orthoreovirus (PRV-1)	Yes	Yes (red muscle fibres only)	No
Pancreas Disease (PD)	Salmonid alphavirus (SAV)	Yes	Yes	Yes
Cardiomyopathy Syndrome (CMS)	Piscine myocarditis virus (PMCV)	Yes	No	No

Treatment and Prevention

Treatment Options

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 reducing losses through careful handling and minimising stress 6.

Preventive Measures

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. Experimental studies have shown that iodine, Virocid and extreme acidic or alkaline conditions can inactivate PRV when sufficient concentrations and exposure times are used. PRV is relatively resistant to heat and UV 11.
Farm Management Practices:
- Avoid unnecessary handling and crowding when fish have HSMI-related cardiac damage. Sea-lice treatments, transfers and other stressful operations should be carefully assessed because affected fish may tolerate them poorly 6. If handling or sea-lice treatment cannot be postponed during an HSMI outbreak, sedation may be considered to reduce handling-related stress, following assessment by the responsible fish-health professional 8.
- 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 1 3 4.
Functional & Anti-Inflammatory Feeds: Utilizing specialized functional feeds is reported to have some effect on how serious heart inflammation the fish gets 6
Selective breeding: Have been used to improve resistance or reduce disease severity. The Norwegian Veterinary Institute notes that these approaches (functional feeds and selective breeding) have not demonstrably reduced the number of HSMI cases in Norway 6.
Vaccine Status & Research:
- No commercial HSMI vaccine is currently available on the market. Commercial production has been blocked primarily because PRV cannot be cultured in standard continuous cell lines.
- Experimental vaccines have shown partial protection by reducing heart lesions and, in some trials, delaying infection, but they do not consistently prevent infection 6 12 13

Case Studies

Real-World Examples

Notable Outbreaks:
- Norway (1999): The first identified outbreaks led to significant economic losses and increased regulatory scrutiny 6.

Data Insights

Disease Impact by Country

Norway

Incidence in Norway:
- HSMI was first diagnosed in Norway in 1999 1
- HSMI is the most frequently diagnosed viral disease in sea- farmed Atlantic salmon, with approximately 150–188 site diagnoses reported annually across combined public and private diagnostic laboratories 14.
- PRV-1 is detected in approximately 90% of sea-farmed salmon in Norway 6.
Geographical Spread:
- The disease appears to have spread along the Norwegian coastline since its initial detection.
- While cases occur across all marine production areas, the highest concentration of clinical outbreaks occurs in Central and Northern Norway 14.
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, with cumulative mortatilty reaching up to 20% in some cases 5.
  - Reduced growth and feed conversion in affected fish 8.
Treatment & Management:
- No specific treatment exists for HSMI 2.
- An important management priority is to minimise stress and handle affected fish carefully to reduce losses, particularly when cardiac function is compromised 6.
- Improved biosecurity measures and monitoring of fish health are key strategies.

Research and References

Latest Research Findings

"Piscine orthoreovirus (PRV) replicates in Atlantic salmon (Salmo salar L.) erythrocytes ex vivo"
Authors: Wessel, Ø., et al.
Reference: Wessel, Ø., et al. (2015). Veterinary Research, 46(1), 26.
Link to study
"Experimental transmission of heart and skeletal muscle inflammation (HSMI) in Atlantic salmon Salmo salar"
Authors: Kongtorp, R. T., et al.
Reference: Kongtorp, R. T., et al. (2004). Diseases of Aquatic Organisms, 59(2), 105-112.
Link to study
"Heart and skeletal muscle inflammation (HSMI) disease diagnosed on a British Columbia salmon farm through a longitudinal farm study"
Authors: Di Cicco, E., et al.
Reference: Di Cicco, E., et al. (2017). PLOS ONE, 12(2), e0171471.
Link to study

These studies represent significant advancements in understanding HSMI, its association with PRV, and its geographic distribution. They highlight the ongoing research efforts to better characterize and manage this disease in various salmon-producing regions.

Conclusion

Heart and Skeletal Muscle Inflammation (HSMI) remains a significant threat to salmon aquaculture, necessitating diligent management and preventive measures. By implementing robust biosecurity protocols, focusing on reducing losses through careful handling and minimising stress, investing in ongoing research, and fostering industry-wide cooperation, fish health managers and veterinarians can mitigate the impact of HSMI and ensure the sustainability of salmon farming operations. This comprehensive guide aims to provide the necessary information and resources to effectively manage HSMI and support the health and welfare of farmed salmon.

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Title: Heart and Skeletal Muscle Inflammation Date: 2026-22-10 Tags: #HeartandSkeletalMuscleInflammation, #HSMI, #Salmonids #Salmon, #AtlanticSalmon, #CohoSalmon #PiscineOrthoreovirus, #PRV, #Trout, #RainbowTrout, #ViralDisease, #Virus