A sequential study of the light and electron microscopic liver lesions of infectious anemia in Atlantic salmon (Salmo salar L.).

The present study describes light and electron microscopic changes in the liver of Atlantic salmon during the development of infectious salmon anemia (ISA). Atlantic salmon postsmolts weighing 80-100 g were infected by intraperitoneal injections, and liver samples were collected sequentially between day 0 and day 25 post infection (p.i.), with time intervals of 3-4 days. At each collection time, livers from five infected fish and two control fish were examined. Changes involving the perisinusoidal macrophages were observed by transmission electron microscopy, from day 4 p.i. Large vacuoles, containing a fine-granular material with low electron density, accumulated in the cytoplasm. These changes persisted and became more severe throughout the investigation, leading to a considerable increase in the size of the cells. At day 14 p.i., degenerative features of the sinusoidal endothelium were observed. By day 18 p.i., areas of the liver were devoid of a sinusoidal endothelial lining, bringing hepatocytes in direct contact with blood cells. At this stage, the sinusoids were moderately congested. From day 21 p.i., heavy sinusoidal congestion, peliosis hepatis, and degeneration of the hepatocytes were observed. No virus was observed in any of the inhabitant cell types of the liver. Gross and light microscopic changes were first recorded at day 18 p.i., as was a significant decrease in the hematocrit values. By day 25 p.i., characteristic multifocal, confluent, hemorrhagic necroses were present. Results of the present investigation suggest that the liver lesions observed with ISA are not the result of the development of an anemia alone or caused by direct viral damage to hepatocytes. Hepatocellular degeneration succeeded changes in the perisinusoidal macrophages and degeneration of the sinusoidal endothelium. These changes may have impeded the sinusoidal blood flow and hence caused an ischemic hepatocellular necrosis.