Pathology of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon Salmo salar.

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.

Platyspondyly and shortness of vertebral column in farmed Atlantic salmon Salmo salar in Norway--description and interpretation of pathologic changes.

Body malformation due to shortness of the vertebral column, in most cases of unknown cause, has been observed in fish for more than 100 yr. It periodically occurs with high prevalence in farmed Atlantic salmon Salmo salar in Norway, and this paper describes the results of macroscopic, radiographic and histologic examination of parr and seawater-transferred fish. The vertebral bodies in both age groups did not acquire the length that they normally should due to a growth disturbance leading to the condition of platyspondyly and shortness in the column. The pathologic changes became visible at different ages in both groups and the process apparently starts in intervertebral tissues. There was proliferation of connective tissue and blood vessels, and sometimes infiltration with inflammatory cells, around affected vertebrae, especially in seawater-transferred fish. This is the first description of inflammation in abnormally short-spined fish, and it may indicate an infectious etiology, at least in farmed seawater-transferred salmon.