Histopathological studies of yolk-sac fry of Baltic salmon (Salmo salar) with the M74 syndrome

The purpose of this study was to describe the histopathological findings during the progression of the M74 syndrome in the Atlantic salmon from the Baltic Sea. We also wanted to evaluate whether there were differences in the histopathological picture between yolk-sac fry from different categories of M74, i.e. family groups with early, intermediate, or late development of disease. In the preclinical stage of M74, no lesions were detected in the yolk-sac fry. In the clinical stage, the dominant findings were necrotic lesions in the brain and a depletion of glycogen in liver and skeletal muscle and proximal tubules of the excretory kidney. In the terminal stage, the brain lesions had become more severe and, furthermore, yolk-sac fry with early development of M74 also showed focal hepatocellular necrosis and sometimes hyaline degeneration of skeletal muscle. The findings in yolk-sac fry with intermediate development of disease were generally the same as in yolk-sac fry with early development, even though the lesions were of lesser magnitude. The only major histopathological findings in yolk-sac fry with late development of disease were brain lesions and, in the terminal stage, a dilatation of saccus vasculosis.