Pathology

Learn more about the general causation, symptoms, diagnosis and prevention of these pathogens.

Infectious Salmon Anemia virus (ISAv)

Long-distance transport of pathogens plays a critical role in the emergence of novel diseases. Shipping is a major contributor to such transport, and the role of ships in spreading disease has been recognized for centuries. However, statistical confirmation of pathogen spread by shipping is usually impractical. Authors present evidence of invasive spread of infectious salmon anemia in the salmon farms of Scotland and demonstrate a link between vessel visits and farm contamination. The link is associated with vessels moving fish between sites and transporting harvest, but not with vessels delivering food or involved in other work.

Infectious salmon anaemia virus (ISAV), genus Isavirus (family Orthomyxoviridae), is present in all large salmon (Salmo salar)-producing countries around the North Atlantic. The target species for this virus are members of the genus Salmo, but the virus may also replicate in other salmonids introduced to the North Atlantic (Oncorhychus spp.). Existing ISA virus isolates can be divided into two major genotypes, a North American (NA) and a European (EU) genotype, based on phylogenetic analysis of the genome. In 1999 an ISA virus belonging to the NA genotype was isolated from Coho salmon in Chile, and in 2007 the first outbreaks of ISA in farmed Atlantic salmon was observed. Several salmon farms in Chile were affected by the disease in 2007, and even more farms in 2008. In this study, ISA virus has been isolated from salmon in a marine farm suffering an outbreak of the disease in 2008 and from smolts with no signs of ISA in a fresh water lake. Sequencing of the partial genome of these ISA viruses, followed by phylogenetic analysis including genome sequences from members of the NA and EU genotypes, showed that the Chilean ISA virus belongs to the EU genotype. The Chilean ISA virus groups in a clade with exclusively Norwegian ISA viruses, where one of these isolates was obtained from a Norwegian brood stock population. All salmonid species in the southern hemisphere have been introduced from Europe and North America. The absence of natural hosts for ISA viruses in Chile excludes the possibility of natural reservoirs in this country, and the close relationship between contemporary ISA virus strains from farmed Atlantic salmon in Chile and Norway suggest a recent transmission from Norway to Chile. Norway export large amounts of Atlantic salmon embryos every year to Chile; hence, the best explanation for the Norwegian ISA virus in Chile is transmission via these embryos, i.e. vertical or transgenerational transmission. This supports other studies showing that the ISA virus can be transmitted vertically.

Heart and Skeleton Muscle Inflammation (HSMI)

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.

Atlantic salmon (Salmo salar L.) mariculture has been associated with epidemics of infectious diseases that threaten not only local production, but also wild fish coming into close proximity to marine pens and fish escaping from them. Heart and skeletal muscle inflammation (HSMI) is a frequently fatal disease of farmed Atlantic salmon. Although pathology and disease transmission studies indicated an infectious basis, efforts to identify an agent were unsuccessful. Here authors provide evidence that HSMI is associated with infection with piscine reovirus (PRV). PRV is a novel reovirus identified by unbiased high throughput DNA sequencing and a bioinformatics program focused on nucleotide frequency as well as sequence alignment and motif analyses. Measures must be taken to control PRV not only because it threatens domestic salmon production but also due to the potential for transmission to wild salmon populations.

Salmon Alphavirus (SAV)

Disease associated with salmonid Alphavirus (SAV) infection is a significant problem for farm production of salmonids in Europe. The SAV subtype 3 (SAV3) is a Norwegian subtype present exclusively in production systems for Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss in western Norway. It has been suggested that SAV3 is transmitted through smolt transport from the main area for SAV disease in western Norway to as far as northern Norway. One explanation for this type of spread is that SAV is present at freshwater production sites for Atlantic salmon smolts. The present study confirms this, showing that SAV3 is present at smolt production sites in Norway.

A series of laboratory studies were undertaken to investigate the survival of salmonid alphaviruses (SAV) under a range of conditions relevant to waste disposal, persistence and spread in the field, and to laboratory studies and testing. SAV was found to be rapidly inactivated in the presence of high levels of organic matter at 60 degrees C at pH 7.2 and at pH 4 and pH 12 at 4 degrees C, suggesting that composting, ensiling and alkaline hydrolysis would all be effective at inactivating virus in fish waste. Testing was conducted under sterile conditions at 4, 10, 15 and 20 degrees C in sea water, half-strength sea water and fresh (hard) water, both in the absence and the presence of added organic matter. Virus survival was shown to be inversely related to temperature, and to be reduced by the presence of organic matter. Calculated half lives (t(1/2)) under these conditions ranged from 61.0 to 1.5 days. Testing in non-sterile sea water resulted in reduced t(1/2) values. The half life of SAV in serum was also found to be inversely related to temperature, emphasizing the need for rapid shipment of samples at 4 degrees C to laboratories for virus isolation studies.

In Europe, 2 closely related alphaviruses (Togaviridae) are regarded as the causative agents of sleeping disease (SD) and salmon pancreas disease (SPD): SD virus (SDV) has been isolated from rainbow trout Oncorhynchus mykiss in France and the UK, while SPD virus (SPDV) has been isolated from salmon Salmo salar in Ireland and the UK. Farmed salmonids in western Norway also suffer from a disease called pancreas disease (PD), and this disease is also believed to be caused by an alphavirus. However, this virus has not yet been characterised at the molecular level. We have cultured a Norwegian salmonid alphavirus from moribund fishes diagnosed with cardiac myopathy syndrome (CMS) and fishes diagnosed with PD. The virus has also been found in salmon suffering from haemorrhagic smolt syndrome in the fresh water phase. Based on the pathological changes, host species and the nucleotide sequence, we suggest naming this virus Norwegian salmonid alphavirus (NSAV). Together with SPDV and SDV it constitutes a third subtype of salmonid alphavirus (SAV) species within the genus Alphavirus, family Togaviridae.

The first alphavirus to be isolated from fish was recorded in 1995 with the isolation of salmon pancreas disease virus from Atlantic salmon, Salmo salar L., in Ireland. More recently Norwegian salmonid alphavirus (SAV) has been isolated from marine phase production of Atlantic salmon and rainbow trout in Norway. These viruses are closely related and are now considered to represent subtypes of SAV, a new member of the genus Alphavirus within the family Togaviridae. SAVs are recognized as serious pathogens of farmed Atlantic salmon and rainbow trout in Europe. This paper aims to draw together both historical and current knowledge of the diseases caused by SAVs, the viruses, their diagnosis and control, and to discuss the differential diagnosis of similar pathologies seen in cardiomyopathy syndrome and heart and skeletal muscle inflammation of Atlantic salmon.

Atlantic salmon Salmo salar pre-smolt, smolt and post-smolt, with clinical signs of haemorrhagic smolt syndrome (HSS) have been found in several locations along the Norwegian coast (Rogaland to Troms). Affected fish had pale gills and bleeding at the fin bases, but seemed to be in good physical condition with no obvious weight loss. The internal organs and body cavity showed distinct bleedings. Petechiae were found on the gastrointestinal tract, swim bladder and peritoneum, visceral adipose tissue, heart and somatic musculature. The liver was bright yellow and sometimes mottled with petechiae and ecchymoses. Acitic fluid was found in the visceral cavity and fluid was also present in the pericardial cavity. Histological examination revealed haemorrhage in most organs. The glomeruli were degenerated and the renal tubules were filled with erythrocytes. The aims of this study were to describe the pathology and discover, if possible, the aetiology of the HSS.

Salmon Leukemia virus (SLv)

A plasmacytoid leukemia (PL) has caused mortalities in chinook salmon (Oncorhynchus tshawytscha) reared in seawater netpens in western British Columbia, Canada, since 1988. Kidney or eye tissues from 11 of 13 fish from netpens with clinical PL had reverse transcriptase (RT) activity. This RT activity was associated with virus particles of retrovirus morphology and buoyant density. Many infectious neoplasms of animals, including fishes, are caused by retroviruses. The evidence in this study shows the presence of a retrovirus in chinook salmon with PL and further suggests a retroviral etiology of the disease. We are tentatively calling this virus salmon leukemia virus.

Plasmacytoid leukemia (PL) is an important disease of seawater pen-reared chinook salmon in BC. The disease is caused by an infectious agent, and the microsporidium Enterocytozoon salmonis, Renimbacterium salmoninarum (the cause of Bacterial Kidney Disease) and a retrovirus have been suggested as possible causes. Experimental transmission studies were conducted to elucidate the etiology of PL and they stronlgy indicate that a virus is the cause of the disease.

Viral Hemorrhagic Septicemia (VHS)

Viral hemorrhagic septicemia (VHS), a systemic infection of various salmonid and a few nonsalmonid fishes, is caused by a rhabdovirus designated as the viral hemorrhagic septicemia virus. The virus infection occurs in salmonids and certain other fishes of any age and may result in significant cumulative mortality. Fish that survive may become carriers.