Table 2.

Nature and frequency¹ of significant histopathology lesions in tissues of Atlantic salmon affected with heart and skeletal muscle inflammation (HSMI) and coho salmon affected with HSMI-like disease

Tissue			Atlantic salmon			Coho salmon
			Histopathological description	Diagnosis	Frequency	Histopathological description	Diagnosis	Frequency
Heart	Epicardium		Multifocal to diffuse infiltration of mononuclear cells.	Mononuclear epicarditis, subacute, multifocal to diffuse, moderate to severe.	33/36	Multifocal to diffuse infiltration of mononuclear cells.	Mononuclear epicarditis, subacute, multifocal to diffuse, moderate to severe.	18/18
	Myocardium	Spongy layer	Multifocal to diffuse infiltration of mononuclear cells of spongy layer and compact layer. Multifocal to diffuse myocardial degeneration & necrosis.	Mononuclear myocarditis, degenerative, necrotic, subacute to chronic, multifocal to diffuse, moderate to severe.	33/36	Multifocal to diffuse infiltration of mononuclear cells of spongy layer. Some mononuclear cells are present in the compact layer. Focal to multifocal myocardial degeneration & necrosis.	Mononuclear myocarditis, degenerative, necrotic, subacute to chronic, multifocal to diffuse, moderate to severe.	15/18
		Compact layer				Not significant findings
Red muscle			Multifocal to diffuse infiltration of mononuclear cells in red muscle. Multifocal to diffuse myocytic degeneration & necrosis.	Mononuclear myocarditis, degenerative, necrotic, subacute to chronic, Multifocal to diffuse, moderate to severe.	30/36	Multifocal to diffuse infiltration of mononuclear cells in red muscle.	Mononuclear myocarditis, Multifocal to diffuse, mild to moderate.	12/18
Liver			Blood vessels surrounded by a mononuclear cell infiltrate. Degeneration, necrotic, hemorrhagic, acute to subacute, focal to multifocal, moderate to severe.	Mononuclear hepatitis, necrotic, hemorrhagic, acute to subacute, focal to multifocal, moderate to severe.	16/36	Blood vessels surrounded by a mononuclear cell infiltrate. Degeneration, necrotic, acute to subacute, focal to multifocal, mild to moderate.	Mononuclear perivascular hepatitis, necrotic, acute to subacute, focal to multifocal, mild to moderate.	14/18
Tissue	Atlantic salmon				Coho salmon
	Histopathological description		Diagnosis	Frequency	Histopathological description		Diagnosis	Frequency
Spleen	Diffuse infiltration of mononuclear cells, interstitial. Diffuse congestion with evidence of increase in circulating granulocytes & intravascular erythrophagia.		Splenitis, subacute, diffuse, moderate.	11/36	Diffuse infiltration of mononuclear cells, interstitial. Diffuse congestion with evidence of increase in circulating granulocytes & intravascular erythrophagia.		Splenitis, subacute, diffuse, moderate.	10/18
Kidney	Diffuse infiltration of mononuclear cells, interstitial.		Interstitial nephritis, subacute, diffuse, moderate.	4/36	Diffuse infiltration of mononuclear cells, interstitial.		Interstitial nephritis, subacute, diffuse, moderate.	11/18
Pyloric caeca	Extensive proliferation of fibroblasts-like cells, mononuclear, eosinophilic cells with eccentric nucleus & macrophage-like cells. Large empty vacuoles adjacent to the pancreatic area observed in some cases.		Vaccine induced peritonitis	11/36	No significant findings.		No significant findings.	18/18
Gill	Lamellar fusion across multiple filaments.		Branchiosis multifocal, moderate.	3/36	Lamellar fusion across multiple filaments. Numerous interlamellar vesicle formation. Neoparamoeba perurans showing nucleus & “parasome". Granular eosinophilic intracytoplasmic inclusions in hypertrophied epithelial cells.		Focal to multifocal bacterial & parasitic hyperplasia, subacute to chronic, moderate to severe, with Paramoeba perurans & epitheliocystis-like inclusions.	16/18
Posterior gut	Multifocal to diffuse infiltration of mononuclear cells in lamina propria & sub mucosa.		Mononuclear enteritis, multifocal to diffuse, moderate.	5/36	Multifocal to diffuse infiltration of mononuclear cells in lamina propria & sub mucosa.		Mononuclear enteritis, multifocal to diffuse, moderate.	4/18

¹Frequency corresponds to number of fish with lesion over total examined. 36 Atlantic salmon were examined from two farms corresponding to four sampling times; 18 coho salmon were examined from two farms corresponding to four sampling times