Infectious anaemia in Atlantic salmon (Salmo salar) was studied by recording gross and light microscopic changes, and the development of lesions was studied in relation to haematocrit values. Gross lesions were characterised by ascites formation, congestion and enlargement of liver and spleen, congestion of the foregut and petechiae in the peritoneum. Histologically, lesions were demonstrated in the liver, being characterised by congestion in early stages (that is, haematocrit values around 25), dilatation of the sinusoids, and in later stages (haematocrit values 25 to 15) formation of blood-filled spaces bearing morphological resemblance to peliosis hepatis. At low haematocrit values (around 10), these changes comprised large areas of the liver parenchyma, that is, blood-filled areas coalesced, presenting islets of degenerate and necrotic hepatocytes. At this stage, haemorrhagic necroses were found. Spleen and kidney lesions were characterised by congestion. In the foregut, congestion and bleeding in lamina propria were observed. Liver lesions became more disseminated and severe with decreasing haematocrit values. Hypoxia due to anaemia alone cannot fully explain the development of the liver lesions.
