Cardiomyopathy Syndrome (CMS)

Overview

What is CMS?

Cardiomyopathy Syndrome (CMS) is a severe viral cardiac disease affecting farmed Atlantic salmon (Salmo salar). It is commonly known as “hjertesprekk” (“heart rupture”) in Norwegian because cardiac rupture is a characteristic finding in advanced cases. It is caused by the piscine myocarditis virus (PMCV), leading to inflammation and necrosis of the heart muscle. First identified in Norway in the late 1980s, CMS has since been reported in other major salmon farming regions, including Scotland, Ireland, the Faroe Islands, and Canada, causing significant economic losses 1 2 3. Although CMS-like cardiac lesions have also been reported in wild Atlantic salmon and PMCV has been detected in wild individuals, available investigations do not indicate that wild salmon constitute a significant reservoir of the virus 11.

Clinical Signs of Cardiomyopathy Syndrome

Common Signs

Physical Signs:
- Cardiac lesion and rupture: Inflammation and necrosis of the heart muscle detected microscopically. Enlargement of the heart atrium. In advanced cases, rupture of the atrial wall causes cardiac arrest and results in the accumulation of blood and blood clots in the pericardial cavity 11.
- Liver changes: Darkened, severely discolored, or mottled liver, frequently covered by a fibrin layer due to prolonged circulatory dysfunction 11.
- Ascites: Accumulation of fluid in the abdominal cavity is a classical sign of circulatory failure. It is sometimes accompanied by abdominal distension 11.
- Exophthalmia: Bulging eyes can be observed in affected fish 11
- Skin haemorrhages and oedema: In terminal stages, small pinpoint haemorrhages, particularly on the underside of the abdomen, and fluid accumulation in the scale pockets, which may cause raised scales 11.
- Absence of External Signs and Good Body Condition: Affected fish are frequently the largest, healthiest-looking individuals in the sea cage, showing no outward symptoms prior to sudden death 12
Behavioural Changes: Fish affected by CMS may show few or no obvious behavioural changes before sudden death. In cases with a more prolonged clinical course, changes may become apparent, including reduced appetite and sluggish swimming.
- Loss of Appetite: Affected fish may reduce or stop feeding, particularly during a more prolonged disease course. In acute cases, fish may continue feeding until sudden death 3 12
- Reduced activity: Some affected fish may become lethargic or swim sluggishly 3 4 12

Progression of Symptoms

Early Stages: Cardiac lesions may develop without obvious clinical signs. PMCV can be detected before clinical disease develops 11 3.
Advanced Stages:
- Acute (Sudden Collapse):
  - Sudden cardiac rupture or acute heart failure occurs, most often triggered during routine stress handling events, such as delousing operations, leading to a sudden high increase in mortalities 11 12.
  - Autopsies frequently reveal fresh, intact feed in the stomach and intestine, confirming abrupt mortality 12.
- Protracted form:
  - The heart gradually weakens, and affected fish may develop reduced appetite, sluggish swimming, protruding eyes and haemorrhages on the underside of the abdomen 12 3.
  - Mortality may remain slightly or moderately elevated over an extended period, resulting in substantial cumulative losses 11.
  - Internal findings may include substantial fluid accumulation in the abdominal cavity and a markedly discoloured liver, consistent with circulatory dysfunction 12.
Impact on Fish Health: CMS severely compromises heart function and reduces the fish’s ability to tolerate stress and physical exertion. Because cardiac tissue is structurally weakened, routine handling operations—such as delousing, crowding, or sorting—frequently trigger acute heart collapse or rupture. Consequently, CMS stands as one of the primary infectious drivers of pre-harvest mortality and economic loss in Norwegian marine salmon farming 1 4 11 12 13.

Causes of Cardiomyopathy Syndrome

Etiology

Causative Agent: Piscine myocarditis virus (PMCV).
Transmission Methods:
- Horizontal transmission (from fish to fish): Waterborne transmission of PMCV has been demonstrated in cohabitation experiments. Field observations suggest that the virus may also spread between cages within a farm. However, how PMCV is introduced to farms and transmitted under field conditions, including between farms, remains incompletely understood 1 2 11
- Vertical transmission (parent to offspring) of PMCV has been suspected and is being investigated 1

Risk Factors

Environmental Factors: Poor water quality, high stocking densities, and stressful conditions can increase susceptibility to CMS 11. Mortality spikes are usually triggered by physical exertion and handling stress—particularly mechanical or thermal delousing, crowding, sorting, or pumping 11.
Farm Management Practices: Ineffective biosecurity measures and lack of routine health monitoring can facilitate the spread of PMCV 1 3.
Fish size: The risk of a CMS outbreak increases directly with the duration of the marine growth phase. CMS primarily strikes large, high-value salmon during their second year at sea, though outbreaks in smaller post-smolts (100–300g) have also been recorded 11 12.
Previous Infections: Salmon cohorts that have previously suffered from Pancreas Disease (PD) or Heart and Skeletal Muscle Inflammation (HSMB) carry a significantly higher risk of developing CMS 11.
Hatchery Supplier Effect: Field data shows a strong correlation between CMS outbreaks and specific freshwater hatcheries, with fish from the same smolt suppliers repeatedly showing higher disease incidence year after year at sea 11.
Stocking Season (Sea Transfer Window): Fish transferred to sea cages in the autumn carry a higher risk of developing CMS compared to spring-stocked smolts 11.

Diagnosis

Diagnostic Methods Diagnosis of CMS is based on clinical history, necropsy findings and characteristic histopathological changes in the heart, supported by detection of PMCV RNA.

Clinical Examination: Observation of physical and behavioural symptoms indicative of CMS. However, affected fish may show few obvious signs, so clinical examination alone cannot confirm the diagnosis 11.
Laboratory Tests:
- Histopathology: Microscopic tissue examination serves as the primary definitive diagnostic tool for CMS. It identifies characteristic severe inflammation and cellular necrosis localized primarily within the inner spongy myocardium of the atrium and ventricle, while leaving the compact outer heart layer relatively intact 11.
- RT-qPCR (Real-time quantitative PCR): Detects PMCV genetic material in fish tissues. It is essential for routine population screening, and monitoring viral load before clinical outbreaks occur 11.
- Immunohistochemistry: Detects viral antigens in tissue samples 5.
- In Situ Hybridization (ISH / RNAscope): Advanced staining techniques visualize PMCV RNA directly inside active heart tissue lesions 11.

Differential Diagnosis

Distinguishing CMS from Other Diseases: It is crucial to differentiate CMS from other diseases that cause cardiac lesions, such as Heart and Skeletal Muscle Inflammation (HSMI) and Pancreas Disease (PD) 4. Unlike PD, CMS produces no tissue necrosis in the exocrine pancreas, and unlike HSMI, CMS does not affect skeletal muscle tissue. These distinct differences in lesion distribution allow pathologists to definitively differentiate the conditions during histopathological examination 11.

Treatment and Prevention

Treatment Options

Current Treatments: There are no effective antiviral treatments available for CMS. Management focuses on limiting mortality and protecting fish welfare by minimising handling, physical exertion and other stressful events, and mitigating the impact through biosecurity measures and supportive care.
Experimental Treatments: Ongoing research into potential antiviral agents and other therapeutic approaches 2 3.

Preventive Measures

Biosecurity Protocols:
- Implementing strict quarantine measures for new stock to prevent the introduction of PMCV.
- 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. RT-qPCR can be used to monitor PMCV infection in fish groups 11.
- Implementing all-in/all-out stocking practices to reduce the risk of spreading infections 1 2 3.
- Handling and other stressful procedures should be minimised following a CMS diagnosis. Grading, transfer and delousing can increase mortality because affected fish tolerate stress and physical exertion poorly. Management should aim to minimise these pressures until the fish can be harvested 11.
Genomic Selection: Breeding companies offer salmon eggs selected for increased resistance to CMS using quantitative trait loci (QTL) markers and genomic selection. In experimental challenge tests, fish carrying favourable QTL variants showed lower PMCV loads and less cardiac damage than fish carrying unfavourable variants. However, this should not be presented as complete protection against infection or outbreaks 2 11.
Functional Feeds: Specialized anti-inflammatory cardiac feeds ("functional feeds") can be administered during risk periods to reduce myocardial tissue damage and possibly lessen mortality during outbreaks 11.
Vaccination Status: There is currently no commercial vaccine available against PMCV, although vaccine development remains an active area of research 11.

Case Studies

**Real-World Examples

Notable Outbreaks:
- Norway (1980s): The first identified outbreaks led to significant economic losses and increased regulatory scrutiny.
- Scotland: Ongoing challenges with managing CMS in salmon farming operations.
- Canada: Reports of CMS have prompted industry-wide efforts to improve biosecurity and disease management 1 2 3 4.

Data Insights

Disease Impact by Country

Norway

CMS Incidence in Norway:
- CMS is now considered one of the most important problems for the Norwegian aquaculture industry 10.
- In 2025, cardiomyopathy syndrome (CMS) was among the five most important factors contributing to mortality and reduced welfare in farmed Atlantic salmon during seawater production. 9
- Official diagnostic surveillance recorded a sharp increase in CMS cases, rising to 114 diagnosed sites across Norway (up from 78 sites the previous year) 13.
- CMS is not categorized as a reportable disease in Norway, meaning there is no mandatory state-enforced eradication or stamping-out protocol.
Geographical Spread:
- CMS affects salmon farms along the entire Norwegian coast 10
- In recent years, CMS cases have declined in northern Norway, while most cases have occurred in central and southern Norway 13.
Economic Impact:
- CMS most often affects large salmon in their last year at sea, leading to major financial consequences 10.
- In 2023, 62.8 million salmon (16.7%) died in the sea phase of production, the highest recorded mortality rate so far 6 9.

Scotland

CMS Incidence:
- CMS is a growing problem for salmon farmers in Scotland 10.

Research and References

Latest Research Findings

"Epidemiologisk studie av Kardiomyopatisyndrom (CMS): Spredning, risikofaktorer og sykdomsforløp i norsk lakseoppdrett (CMS-Epi)"
Authors: Camilla Fritsvold, et al.
Reference: Norwegian University of Life Sciences (NMBU) study, (2022).
Link to article
"Cardiomyopathy syndrome in Atlantic salmon Salmo salar L.: A review of the current state of knowledge"
Authors: Å H Garseth, C Fritsvold, J C Svendsen, B Bang Jensen, A B Mikalsen Reference: Å H Garseth, et. al. J Fish Dis. 2018. 41(1),11-26. Link to article
"Persistent immune responses in the heart determine the outcome of cardiomyopathy syndrome in Atlantic salmon (Salmo salar)"
Authors: Baojian Sun, Marco A. Vindas, Simona Kavaliaskiene, Håvard Bjørgen, Erling Olaf Koppang, Helene Wisløff, Michael Frisk, Hege Lund, Ida B. Johansen Reference: Baojian Sun, et. al. Science Direct. 2024. Volume 147. Link to article

Recent Research Findings

Research has confirmed that CMS is a contagious disease that can affect salmon at various life stages, including pre-smolts, post-smolts, and early sea phases 10.
CMS-sick salmon do not seem able to fight the PMCV infection or get rid of the virus, and heart inflammation does not appear to heal over time 10.
Heart samples are the best for detecting PMCV-specific RNA in fish with clear CMS changes 10.
Early detection may be possible through plasma from blood samples and tissue samples from the central kidney 10.

These studies represent advancements in understanding the prevalence, transmission, and impact of Cardiomyopathy Syndrome (CMS) in salmon farming, particularly in Norway and other European countries with significant salmon farming industries.

Conclusion

Cardiomyopathy Syndrome (CMS) remains a significant threat to salmon 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 CMS and ensure the sustainability of salmon farming operations. This comprehensive guide aims to provide the necessary information and resources to effectively manage CMS and support the health and welfare of farmed salmon.

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Last Modified: 2026-10-13

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