Division II.4 Histology

Paraffin-embedded sections shall be cut at 5 μm and stained using haematoxylin and eosin.

Histological changes in clinically diseased Atlantic salmon are variable, but may include the following:

1. (a)

   numerous erythrocytes in the central venous sinus and lamellar capillaries of the gills, where erythrocyte thrombi also may be formed;

2. (b)

   multifocal to confluent petechiae or hepatocyte necrosis or both at some distance from larger vessels in the liver; multifocal accumulation of erythrocytes in dilated hepatic sinusoids;

3. (c)

   accumulation of erythrocytes in blood vessels of the intestinal lamina propria and eventually haemorrhage into the lamina propria;

4. (d)

   spleen stroma distended by erythrocyte accumulation;

5. (e)

   slight multifocal to extensive diffuse interstitial haemorrhage with tubular necrosis in the haemorrhagic areas, erythrocyte accumulation in the glomeruli in the kidney;

6. (f)

   erythrophagocytosis in the spleen and secondary haemorrhages in liver and kidney.