en fish disease fish disease پژوهشي Orginal research papers The main objective of this study was to evaluate the pathogenicity of isolated &nbsp;&nbsp;<em>Yersinia ruckeri</em> &nbsp;from Caspian trout. <em>Y. ruckeri </em>specimens were isolated from infected rainbow trout farms located in north of Iran. The identification was confirmed by biochemical tests and 16s rRNA gene sequencing. The pathogenicity test was carried out to determine the virulence of the <em>Y. ruckeri </em>by IP injection, and histopathological and hemato-biochemical changes were evaluated pre and post challenge. Based on the results, LD₅₀ were calculated as 1&times;10⁵ CFU ml^-1, while 1&times;10⁷ CFU ml^-1 caused 100% mortality after 10 days in the experimental groups. The main histopathological changes were seen in liver, kidney, spleen and intestine, including erosion and necrosis, infiltration of inflammatory cell, hyperplasia and <em>catarrhal enteritis</em> in the infected organs. Also, a significant decrease in glucose, RBC counts, Hb values and HCT percentage and significant increase in the WBC counts, neutrophils percentage, AST, ALP and LDH values were observed in infected fish after challenging. Finally, Caspian trout is susceptible to Yersiniosis and can play important role in transmission of <em>Y. ruckeri </em>to cultured and wild fish.&nbsp; Caspian trout, Yersinia ruckeri, Pathogenicity, Hemato-biochemical changes, Histopathological changes 2421 2410 http://jifro.ir/browse.php?a_code=A-10-335-9&slc_lang=en&sid=1 M. Adel miladadel85@yahoo.com 100319475328460036382 100319475328460036382 Yes Department of Food Hygiene and Public Health, School of Veterinary Medicine, Shahrekord University, Shahrekod, Iran. M.Y. Ina-Salwany 100319475328460036383 100319475328460036383 No Department of Aquaculture, Faculty of Agriculture, Universiti Putra Malaysia, , Selangor, Malaysia. S. Yeganeh skyeganeh@gmail.com 100319475328460036384 100319475328460036384 No Department of Fisheries, Faculty of Animal Sciences and Fisheries, Sari Agricultural Sciences and Natural Resources University (SANRU), Sari, Iran I. Karimi 100319475328460036385 100319475328460036385 No Department of Pathobiology, Faculty of Veterinary Medicine, Shahrekord University, Shahrekod, Iran. A. Zafariian 100319475328460036386 100319475328460036386 No Department of Fisheries, Faculty of Animal Sciences and Fisheries, Sari Agricultural Sciences and Natural Resources University (SANRU), Sari, Iran