A severe anemia occurring in sea-cultured coho salmon (Oncorhynchus kisutch) has been associated with serious losses in the northern coastal water of Miyagi Prefecture. The death due to the disease has been recorded from February to May, since 1985.
Clinical signs of the disease were pale gills, yellowish liver, blood retain in the atrium and watery blood.
The hematological parameter of the diseased fish exhibited lower hematocrits, erythrocyte counts, hemoglobin concentrations, mean corpuscular hemoglobin concentration and greater numbers of immature erythrocytes in comparison to healthy fish.
Blood smears showed a few bodies of inclusion like structure ranging in size from 1 to 2 μm and a large number of rods in the cytoplasm. Electron microscopy revealed virus particles scattered randomly in the cytoplasm, and marked swelling in mitochondria with occasional loss of mitochondrial cristae. It was thought that bodies of inclusion like structure were formed by deposited electron dense materials in the swollen mitochondria.
Histopathological examination showed necrosis of muscle fibers in the ventricle and atrium, and deposits of hemosiderin and ceroid like materials were also found within macrophages in the spleen, kindney and liver.
As a result, the cause of the disease was thought to be as follows : The virus first infected erythrocytes to make hemolysis, leading to hypoxemia, and then produced phospholipid peroxidation; finally heart lesions and hemolysis were progressed by hypoxemia and phospholipid peroxidation.
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