Suspected toxicopathic hepatic necrosis and megalocystosis in pen-reared Atlantic salmon Salmo salar in Puget Sound, Washington, USA

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Abstract
Severe liver disease was associated with mortality in Atlantic salmon Salmo salar maintained in seawater net pens in Port Townsend Bay, Washington, USA. Approximately 2 mo after seawater introduction prominent diffuse hydropic degeneration and pyknosis of hepatocytes were observed in moribund fish. As the disease progressed, the livers of affected fish exhibited multifocal areas of regenerating hepatocytes intermixed throughout a necrotic parenchyma. Nuclear pleomorphism, karyomegaly and hepatic megalocytosis were prominent in fish surviving at 6 mo post seawater introduction, and many of the enlarged hepatocyte exhibited nuclear inclusions which arose from cytoplasmic invaginations. The lesions were consistent with toxicopathic changes; bacterial and viral examinations revealed no infectious agent associated with the liver damage. The source of the suspected toxicant was most likely direct water contact or natural food rather than the commercial feed because the liver lesions were not detected in Atlantic salmon fed the same commercial diet but reared at a different seawater location (Port Angeles, Washington). Furthermore, fish at both locations originated from the same freshwater hatchery and were of the same genetic strains. Cumulative mortalities at the Port Townsend sites were over 90%, attributed primarily to the liver lesions. However, some moribund fish also had idiopathic pancreatitis; Aeromonas salmonicida, Cytophaga-Flexibacter and sea lice Lepeophtheirus salmonis infections; others were lost to seal predation.

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