Pathological findings experimentally infected with Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss Wabaum, 1792)

38- SUMMARY In this study, a total of 42 ( 10 control and 32 experimental) rainbow trout ( Oncorhynchus mykiss Walbaum,1792 ) (weight 85 - 95 + 10 g ) were used. Yersinia ruckeri at a concentration of 3x106 cfu/ml ( 0.1ml per fish ) was intraperitoneally injected to experimental group, whlist control group was subjected to the same volume of sterile saline. After injection, between 4 and 21 days, dead or killed fish were necropsied, microscopical and macroscopical findings were investigated. At the macroscopical examination, fish in the acut form of the disease showed hyperemia and haemorrage, at the base of fins, eyes, around the rectum, liver, swim- bladder, pyloric caeca, intestine and mesenterium. Liver, spleen and kidney were enlarged. Microscopically, diffuse oedema and exfoliated epithelial cells as well as haemorrhage were described at the secondary lamellae. Activation of the endothelial macrophages was present in the heart. In addition, a reduction in the numbers of lymphoid cells of kidney and focal necrosis of spleen were observed. In the chronic form, macroscopically, darkening or depigmentation in the skin, paled in the gills, yellow-grayish discolaration in the liver were seen. At the microscopical examination, there was hyperplasia of the interlameller cells in the gills. In liver, fat droplets in the hepatocytes and perivascular mononuclear cells infiltration of the vena centralis were detected. In the number of mononuclear cells in the haemopoietic tissue of the kidney were recognized. It was reported that previous studies were generally based on bacteriological investigation of Yersinia ruckeri. However, in this study was firstly examined with extensive pathological findings in Turkey.