This is the first description of heart and skeletal muscle inflammation (HSMI), a novel disease affecting farmed Atlantic salmon Salmo salar in Norway. HSMI was first diagnosed in 1999, and there has since been a yearly increase in the number of recorded outbreaks. Atlantic salmon are commonly affected 5 to 9 mo after transfer to sea, but outbreaks have been recorded as early as 14 d following seawater transfer. Affected fish are anorexic and display abnormal swimming behaviour. Autopsy findings typically include a pale heart, yellow liver, ascites, swollen spleen and petechiae in the perivisceral fat. While mortality is variable (up to 20%), morbidity may be very high in affected cages. Until more accurate tests are available, HSMI is diagnosed on the basis of histopathology. The major pathological changes occur in the myocardium and red skeletal muscle, where extensive inflammation and multifocal necrosis of myocytes are evident. HSMI is transmissible and, although most likely caused by a virus, the causal agent has not yet been isolated. This paper describes clinical signs and pathology of HSMI from 3 field outbreaks in Norway. Microscopic lesions are compared and discussed in relation to published descriptions of pancreas disease (PD) and cardiomyopathy syndrome (CMS). It is concluded that HSMI is histopathologically distinguishable from PD and CMS.KEY WORDS: Heart and skeletal muscle inflammation · HSMI · Atlantic salmon · Myocarditis · Myositis Resale or republication not permitted without written consent of the publisherDis Aquat Org 59: [217][218][219][220][221][222][223][224] 2004 CMS was first described in Norway (Ferguson et al. 1990), and subsequently in the Faeroe Islands (Bruno & Poppe 1996) and Scotland (Rodger & Turnbull 2000). The causality of CMS is unclear, but Grotmol et al. (1997) described nodavirus-like particles in association with heart lesions consistent with CMS in Norway.As the causal agent has not yet been found, it is important to establish a protocol for distinguishing HSMI histologically from other muscular disorders of Atlantic salmon. In this paper we describe the clinical signs, gross pathology and major histopathological findings from 3 field outbreaks of HSMI in Atlantic salmon during 1999 and 2003, and compare and discuss these features in relation to previously published descriptions of PD and CMS. MATERIALS AND METHODSField samples. Atlantic salmon were sampled during disease outbreaks in a seawater farm on the Norwegian west coast in July 1999 and on 2 different locations in western Norway in May and June 2003. Clinically diseased and dead fish were sampled nonrandomly, whereas asymptomatic fish were collected randomly from affected cages. Fish sampled from the 1999 outbreak were killed by a blow to the head before blood samples were collected from the caudal vein, and macroscopic changes were recorded before tissues were sampled for histology. Fish sampled from the 2003 outbreaks were euthanised at the fish farm and sent on ice to the National Veterinary Ins...

Heart and skeletal muscle inflammation (HSMI) is a disease syndrome of unknown aetiology first observed in farmed Atlantic salmon, Salmo salar, in 1999. In the present study we have demonstrated for the first time that HSMI is an infectious disease. It was induced in Atlantic salmon post-smolts after injection with tissue homogenate from farmed Atlantic salmon previously diagnosed with HSMI. The lesions were also induced in cohabitating salmon given a corresponding injection without tissue homogenate. Six weeks post-challenge the fish that had been injected with tissue homogenate developed a serious epicarditis and myocarditis with mononuclear cell infiltrations in compact and spongy layers of the heart. Similar lesions were found in cohabitants after 10 weeks. The lesions were consistent with samples from field outbreaks of HSMI. No lesions were found in control fish. A viral aetiology is strongly suggested, as no difference in disease induction between an inoculum containing antibiotics and a non-treated inoculum was found. Further investigations are required in order to make conclusions regarding the cause and pathogenesis of HSMI.

The newly described piscine reovirus (PRV) appears to be associated with the development of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon Salmo salar L. PRV seems to be ubiquitous among fish in Norwegian salmon farms, but high viral loads and tissue distribution support a causal relationship between virus and disease. In order to improve understanding of the distribution of PRV in the salmon production line, we quantified PRV by using realtime PCR on heart samples collected at different points in the life cycle from pre-smolts to fish ready for slaughter. PRV positive pre-smolts were found in about 36% of the freshwater cohorts and a general increase in viral load was observed after their transfer to seawater. A reduction in viral loads was recorded when fish approached slaughter (18 mo in sea cages). Sequencing of positive samples did not support the hypothesis that outbreaks are caused by the spreading of a particular (virulent) strain of PRV. KEY WORDS: Piscine reovirus · PCR · Salmon · Aquaculture · HSMI Resale or republication not permitted without written consent of the publisherDis Aquat Org 99: [7][8][9][10][11][12] 2012 After initial success with experimental transmission of the disease (Kongtorp et al. 2004a), HSMI was recently linked to a novel reovirus, piscine reovirus (PRV) (Palacios et al. 2010). Although the virus appears to be ubiquitous in Norwegian salmon farms, elevated viral loads are associated with outbreaks of HSMI (Palacios et al. 2010). A causal relationship is also supported by in situ hybridisation using formalin-fixed heart tissue where a correlation between the presence of viral nucleic acids and histopathological lesions has been found (Palacios et al. 2010). PRV has been detected in healthy, wild salmon, albeit with lower prevalence and virus load. The virus has also been found in archive samples taken from salmon in the late 1980s and also in rainbow trout Oncorhynchus mykiss Walbaum (Palacios et al. 2010). Farmed Atlantic salmon with HSMI-like lesions have also been reported from Scotland (Ferguson et al. 2005).In the present study we quantified PRV loads at various stages in the Atlantic salmon production line, from pre-smolts in fresh water to fish ready for slaughter. Samples were collected from farms where no heart lesions were observed and from farms with clinical outbreaks of HSMI. We also sequenced parts of the PRV genome detected in different samples in order to investigate the molecular epidemiology of the virus and potentially virulent PRV strains. Our results indicate that PRV is a ubiquitous virus in Norwegian fish farms and that careful virus quantification and/or study of tissue distribution of viral particles/genomes need to be complemented with his topathology for correct diagnoses. The results should be useful when interpreting PRV loads both in the context of HSMI/PRV epidemiology and diagnosis. MATERIALS AND METHODSViral loads were established for 2 sets of samples; one set of 171 field samples was collected from 23 mar...

The present paper describes, for the first time, clinical signs and pathological findings of pancreas disease (PD) in farmed Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in sea water in Norway. Similarities and differences with reports of PD from Ireland and Scotland are discussed. Samples of 68 rainbow trout from disease outbreaks on 14 farms and from 155 Atlantic salmon from outbreaks on 20 farms collected from 1996 to 2004 were included in the present study. The histopathological findings of PD in Atlantic salmon and rainbow trout in sea water were similar. Acute PD, characterized by acute necrosis of exocrine pancreatic tissues, was detected in nine Atlantic salmon and three rainbow trout. Salmonid alphavirus (SAV) was identified in acute pancreatic necroses by immunohistochemistry. Most fish showed severe loss of exocrine pancreatic tissue combined with chronic myositis. Myocarditis was often but not consistently found. Kidneys from 40% and 64% of the rainbow trout and Atlantic salmon, respectively, had cells along the sinusoids that were packed with cytoplasmic eosinophilic granules. These cells resembled hypertrophied endothelial cells or elongated mast cell analogues. Histochemical staining properties and electron microscopy of these cells are presented. SAV was identified by RT-PCR and neutralizing antibodies against SAV were detected in blood samples.

Heart and skeletal muscle inflammation (HSMI) is a transmissible disease of farmed Atlantic salmon, Salmo salar L. It is characterized by significant epi-, endo- and myocarditis, as well as myositis, particularly involving red skeletal muscle. The aetiology of HSMI is currently unresolved, though a viral cause is suspected. Since its discovery in 1999, HSMI has become an increasing problem for the Norwegian farming industry, with some farms experiencing yearly outbreaks and subsequent economic losses. In the present study an Atlantic salmon farm was studied from December 2003 to April 2005. Samples from apparently healthy as well as clinically diseased fish were collected monthly and examined histopathologically. The first fish to be diagnosed with HSMI was sampled in May, 8 months after transfer to sea. A clinical outbreak of HSMI followed in June, when all fish in the sample had lesions consistent with HSMI. Subsequent samples revealed that cardiac lesions decreased in severity 2 months after the start of the outbreak, but that multiple foci of cellular infiltration and necrosis persisted throughout the year. There appeared to be a shift in lesion location from being most severe in the compact myocardium in early stages of disease to a greater involvement of the atrium and spongy layer of the ventricle in later samples. Late samples also showed increased fibrosis of cardiac tissue. In conclusion, HSMI appears to be a severe disease with elevated mortality, morbidity close to 100% and prolonged duration.

Cardiomyopathy syndrome (CMS) is a disease of unknown aetiology, having significant economic impact as it primarily affects large, farmed Atlantic salmon Salmo salar L. in seawater, close to harvest. In the present study, we have demonstrated that CMS is a transmissible disease under experimental conditions. Histopathological lesions consistent with CMS were induced in Atlantic salmon post-smolts after injection of tissue homogenate from farmed fish diagnosed with CMS. Six weeks post-injection (p.i.), experimental fish started developing focal to multi-focal lesions in the atrial endo-and myocardium, with subsequent progression to the ventricle. This proceeded into severe endocarditis and subsequent myocarditis with mononuclear cell infiltration of the atrium and, to a lesser degree, the spongy layer of the ventricle. These lesions were consistent with histopathological findings in field outbreaks of CMS. From Week 33 p.i., lesions also appeared in the compact myocardium, with focal epicarditis adjacent to focal myocardial lesions. In conclusion, these results indicate that CMS has an infectious aetiology and should be treated as a potentially contagious disease.

A cohort study was initiated in the spring of 2006 to investigate epidemiological aspects and pathogenesis of salmonid alphavirus (SAV) subtype 3 infections and pancreas disease (PD). The aims were to assess involvement of the freshwater production phase, the extent and frequency of subclinical infections and to follow PD-affected populations throughout the entire seawater production cycle, as well as investigate possible risk factors for PD outbreaks. Fish groups from 46 different Atlantic salmon freshwater sites in six counties were sampled once prior to seawater transfer and followed onto their seawater sites. A total of 51 Atlantic salmon seawater sites were included, and fish groups were sampled three times during the seawater production phase. SAV subtype 3 was not identified by real-time RT-PCR from samples collected in the freshwater phase, nor were any SAV-neutralizing antibodies or histopathological changes consistent with PD. In the seawater phase, SAV was detected in samples from 23 of 36 (63.9%) studied sites located within the endemic region. No SAV subtype 3 was detected in samples from seawater sites located outside the endemic region. The cumulative incidence of PD during the production cycle amongst sites with SAV detected was 87% (20 of 23 sites). Average fish weight at time of PD diagnosis ranged from 461 to 5978 g, because of a wide variation in the timing of disease occurrence throughout the production cycle. Mortality levels following a PD diagnosis varied greatly between populations. The mean percentage mortality was 6.9% (+/-7.06) (range 0.7-26.9), while the mean duration of increased mortality following PD diagnosis was 2.8 months (+/-1.11) (range 1-6).

A new disease in farmed rainbow trout (Onchorhyncus mykiss) was described in Norway in 2013. The disease mainly affected the heart and resembled heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar L.). HSMI is associated with Piscine orthoreovirus (PRV), and a search for a similar virus in the diseased rainbow trout led to detection of a sequence with 85% similarity to PRV. This finding called for a targeted effort to assess the risk the new PRV-variant pose on farmed rainbow trout and Atlantic salmon by studying infection and disease pathogenesis, aiming to provide more diagnostic knowledge. Based on the genetic relationship to PRV, the novel virus is referred to as PRV-Oncorhynchus mykiss (PRV-Om) in contrast to PRV-Salmo salar (PRV-Ss). In experimental trials, intraperitoneally injected PRV-Om was shown to replicate in blood in both salmonid species, but more effectively in rainbow trout. In rainbow trout, the virus levels peaked in blood and heart of cohabitants 6 weeks post challenge, along with increased expression of antiviral genes (Mx and viperin) in the spleen, with 80–100% of the cohabitants infected. Heart inflammation was diagnosed in all cohabitants examined 8 weeks post challenge. In contrast, less than 50% of the Atlantic salmon cohabitants were infected between 8 and 16 weeks post challenge and the antiviral response in these fish was very low. From 12 weeks post challenge and onwards, mild focal myocarditis was demonstrated in a few virus-positive salmon. In conclusion, PRV-Om infects both salmonid species, but faster transmission, more notable antiviral response and more prominent heart pathology were observed in rainbow trout.