Conference 25 - 2016 Case: 4 20170503

Signalment:

Post-smolt, unknown gender, Atlantic salmon (Salmo salar).This case was submitted as part of a sample of five fish from a seawater tank in which fish were presenting with loss of appetite and lethargy.

Gross Description:

There was no digesta/ingesta in the gastrointestinal tract with small amounts of soft yellow material (fecal casts) in the distal intestine.

Histopathologic Description:

Pyloric caeca/pancreas: one section is examined consisting of multiple cross sections pyloric caeca and adjacent adipose tissue. There is extensive loss of exocrine pancreatic tissue. Multifocally, there are small to moderate numbers of mononuclear inflammatory cells (lymphocytes and macrophages). Remaining exocrine epithelial cells have reduced numbers of zymogen granules, and the islets of Langerhans remain intact.

Heart: There are degenerative changes of the cardiomyocytes in the spongy and compact layers of the ventricle, characterized by marked cytoplasmic hypereosinophilia, with variable vacuolation and occasional shrunken nuclei. In these areas, there are low to moderate numbers of lymphocytes and macrophages. Multifocally, there is increased cellularity, prominence and occasional karyomegaly of endocardial endothelium (hyperplasia). Multifocally, the epicardium is expanded by mild to moderate numbers of lymphocytes and macrophages.

Skeletal muscle: Multifocally, there is marked loss of myofibres in the red muscle. The majority of the remaining myofibres are shrunken with densely eosinophilic sarcoplasm, surrounded by moderate numbers of mononuclear inflammatory cells.ï¿½ In the white muscle, there are scattered necrotic and degenerating myofibres, with swollen fragmented, eosinophilic sarcoplasm, central migration of nuclei, and sarcoplasmic infiltration by macrophages. Affected myofibres are surrounded by low numbers of lymphocytes and macrophages.

Morphologic Diagnosis:

Exocrine pancreas: Severe, diffuse, pancreatic degeneration and loss. Heart, myocardium: Marked, multifocal to coalescing, cardiomyocyte degeneration and necrosis, with endocardial hyperplasia and hypertrophy, and mild to moderate, multifocal, lymphohistiocytic epicarditis. Skeletal muscle (red myofibres): Moderate to marked, chronic, multifocal to coalescing, necrotizing myositis. Skeletal muscle (white myofibres): Mild to moderate, chronic, multifocal, necrotizing myositis.

Lab Results:

Positive for salmonid alphavirus (SAV) by RT-PCR

Condition:

Salmonid alphavirus

Contributor Comment:

The microscopic features of this case are consistent with previously published findings for pancreas disease (PD) in farmed Atlantic salmon, and the histopathological diagnosis was confirmed by RT-PCR. PD is caused by salmonid alphavirus (SAV), which was first described in farmed Atlantic salmon from Scotland in the mid-seventies.¹⁶It has subsequently been responsible for major economic losses to the Atlantic salmon farming industries in Scotland, Ireland, and Norway.

At present, six closely related subtypes have been identified for SAV^4,5, which differ in host specificity, geographical location^7,10 and aquatic environments (Table 1). SAV-2 is the only subtype commonly detected in freshwater systems, causing sleeping disease in freshwater trout. Current research also suggests there may be differences between strains in the infection dynamics⁷ as well as minor differences in prevalence and severity of the tissue damage⁸. Naturally occurring outbreaks of PD in farmed Atlantic salmon have only been reported in the seawater phase of production.¹⁵ To date, there has been no evidence of vertical transmission of the disease¹¹ with horizontal transmission being by far the most important means of spreading the virus^1,19, with shedding of mucus and feces described as transmission routes for SAV.

Table 1. Summary of Salmonid alphavirus subtypes, their geographical distribution and species susceptibility

SAV Subtype	Species	Production Phase	Country
SAV-1	Atlantic salmon (Salmo salar)	Seawater	Ireland and Scotland
SAV-2	Rainbow trout (Oncorhynchus mykiss)	Freshwater	France, England, Scotland, Spain, Croatia and Germany
SAV-2 (Marine)	Atlantic salmon (Salmo salar)	Seawater	Scotland and Norway
SAV-3	Atlantic salmon (Salmo salar) Rainbow trout (Oncorhynchus mykiss)	Seawater	Norway
SAV-4	Atlantic salmon (Salmo salar)	Seawater	Ireland and Scotland
SAV-5	Atlantic salmon (Salmo salar)	Seawater	Scotland
SAV-6	Atlantic salmon (Salmo salar)	Seawater	Ireland

JPC Diagnosis:

1. Pancreas: Loss, diffuse, severe with mild lymphocytic pancreatitis, Atlantic salmon, Salmo salar.

2. Heart, ventricle: Epicarditis and myocarditis, lymphocytic, diffuse, moderate with multifocal mild myocardiocyte necrosis.

3. Skeletal muscle, red: Degeneration and necrosis, multifocal to coalescing, severe with histiocytic myositis.

4. ï¿½Skeletal muscle, white: Degeneration and necrosis, multifocal to coalescing, mild to moderate with histiocytic myositis.

Conference Comment:

This excellent case demonstrates the prototypical constellation of lesions associated with chronic salmonid alphavirus (SAV) infection. Histological examination of infected fish typically reveals a near complete loss of exocrine pancreatic tissue, epicarditis and myocarditis centered on the ventricle, and skeletal muscle degeneration and necrosis of the white and red muscle.^9,15 Each subtype of SAV listed in Table 1 produces similar morphologic changes and are unable to be distinguished histologically. One of the most difficult tasks for the novice histopathologist is to recognize the complete absence of a normal structure in a tissue section, especially when there is little to no inflammation associated with that loss. Within the multiple cross sections of the pyloric cecae, in this case, conference participants astutely noted a near diffuse loss of exocrine pancreas with only small clusters of remaining exocrine pancreatic cells containing brightly eosinophilic zymogen granules surrounded by mild lymphocytic inflammation that extends into the peripancreatic fat. The islets of Langerhans are mostly unaffected. This diffuse loss of exocrine pancreas with relatively mild inflammation is typical for the chronic phase of this disease. As mentioned by the contributor, the acute phase is associated with a short-lived with rapid necrosis of the exocrine pancreatic tissue and an inflammatory response ranging from no inflammation to moderate mononuclear cell infiltration. The contributor provides an outstanding and thorough review to SAV and includes helpful tables to rule out potential differential diagnoses based on the lesions present in this case.

Both pancreas disease (PD) of salmon, seen in this case, and sleeping disease (SD) of rainbow trout is caused by related SAV infection. This virus is in the genus Alphavirus and family Togaviridae, a group of important enveloped ssRNA viruses.^9,15 Other members of the Togaviridae family include eastern, western, and Venezuelan equine encephalitis viruses. Generally, this family of viruses is associated with transmission by insects, usually mosquitoes. An aquatic arthropod vector has not yet been identified for SAV; however, direct horizontal transmission has been well documented.^9,15 The virus can survive for extended periods of time in the water outside of the host with a marked increase in survival in sea water compared to fresh water. The virus can also survive for long periods within the fat of dead fish and leaked fat droplets floating on the surface may contribute to long distance spread of the virus. Vertical transmission has not been shown to be a significant route of infection.^9,15

References:

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