Pathological assessment of farmed rainbow trout <i>Oncorhynchus mykiss</i> infected by <i>Staphylococcus warneri</i>

Spontaneous Staphylococcus warneri infection was found in rainbow trout broodstocks in raceway culture where the fish were kept at a density of 110 kg m -3 and at water temperature of 7– 9 °C. Signs of disease occurred a week after the transport and transfer of the fish and the clinical signs of moribund fish manifested themselves as anorexia, lethargy and slow swimming on the surface of the water. The external clinical signs included exophthalmos, eye haemorrhage and pale gill. Internally there were hyperaemia over the abdominal fat in pyloric region, pale liver, yellow liver, ecchymotic or petechial haemorrhages in liver and hyperaemia of the intestinal wall; on histological examination, there was eye oedema, the liver displayed moderate or severe acute haemorrhagic hepatitis, inflammatory reaction around bile ducts, moderate focal vacuolation of hepatocytes, and congested haemorrhagic spleen, with infiltration into visceral fat. Haematological examination revealed moderate to severe cytoplasmic lesions on peripheral smear of the observed erythrocytes. The case description suggests that this might represent a secondary or opportunistic infection associated with immunosuppression due to handling stress.