en aquaculture aquaculture پژوهشي Orginal research papers <span style="font-size:11pt"><span style="line-height:110%"><span style="font-family:Calibri,sans-serif"><span lang="TR" style="font-size:12.0pt"><span style="line-height:110%"><span new="" roman="" style="font-family:" times=""><span style="color:black">Yersiniosis is the second important bacterial infections in coldwater fish culture with significant mortalities and economical losses in the Iranian fish farms. In the present study, the effect of extracellular proteins (ECP) on protective immunity<b> </b>of <i>Y. ruckeri</i> vaccine was evaluated in juvenile rainbow trout (7&plusmn;1.2 g). For this purpose, 540 specimens of juvenile rainbow trout were randomly divided into 6 groups each in triplicates. Group 1 (G₁) and Group 2 (G₂) were orally administrated with formalin killed cells (FKC) and FKC+ECP, respectively. Groups 3 to 5 received ECP, FKC, ECP+FKC via intraperitoneal route, respectively. Group 6 received phosphate buffer saline as the control group. The humoral antibody responses to bacterial antigens were monitored by ELISA. LD₅₀ of <i>Y. ruckeri</i> was determined used probit method sixty days after vaccination. Then, fish in each treatment were challenged intraperitoneally (I.P) with LD₅₀ of <i>Y. ruckeri</i> virulent registered strain in Iran (KCW 291153). The ELISA results indicated that ECP could increase the serum ELISA antibody titer as the humoral immune response, but ECP with the FKC could increase antibody levels significantly in serum and intestine mucus. Also survival rates in G₁to G₆ were 25, 31.25, 37.5, 56.25, 87.5 and 15 percent, respectively. Among I.P immunized fish the survival rate in G₅ was significantly higher than the other groups. Although in orally vaccinated fish with FKC (G1) survival rate did not show significant difference with the control group, the FKC /ECP group (G₂) showed a significant increase compared to control group (<i>p</i>=0.039)</span></span></span></span><span dir="RTL" lang="AR-SA" style="font-size:12.0pt"><span style="line-height:110%"><span new="" roman="" style="font-family:" times=""><span style="color:black">.</span></span></span></span><span lang="TR" style="font-size:12.0pt"><span style="line-height:110%"><span new="" roman="" style="font-family:" times=""><span style="color:black">&nbsp; It was concluded that supplementing FKC with ECP increased protective immunity of Yersiniosis vaccine principally in I.P. route. Given the great benefits of the oral vaccine, whole cell/ECP can be considered as a protective antigen to design potential vaccines against this pathogenic micro-organism. </span></span></span></span></span></span></span> Yersinia ruckeri, Immunity, Rainbow trout, Vaccination 1166 1179 http://jifro.ir/browse.php?a_code=A-10-2288-2&slc_lang=en&sid=1 Z. Tulaby Dezfuly z.tulaby@gmail.com 100319475328460046517 100319475328460046517 Yes Ph.D. student of Aquatic Science, Faculty of Veterinary Medicine, ShahidChamran University of Ahvaz, Ahvaz, Iran M. Alishahi 100319475328460046518 100319475328460046518 No Professor of Department of Clinical Sciences M. Ghorbanpoor 100319475328460046519 100319475328460046519 No Professor of Department of microbiology M.R. Tabandeh 100319475328460046520 100319475328460046520 No Associate professor of Biochemistry and Molecular Department M. Mesbah 100319475328460046521 100319475328460046521 No Professor of Department of Clinical Sciences