Research Article: Effect of extracellular proteins (ECP) on protective efficacy of Yersiniosis vaccine in juvenile rainbow trout (Oncorhynchus mykiss)

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 of Y. ruckeri vaccine was evaluated in juvenile rainbow trout (۷±۱.۲ g). For this purpose, ۵۴۰ specimens of juvenile rainbow trout were randomly divided into ۶ groups each in triplicates. Group ۱ (G۱) and Group ۲ (G۲) were orally administrated with formalin killed cells (FKC) and FKC+ECP, respectively. Groups ۳ to ۵ received ECP, FKC, ECP+FKC via intraperitoneal route, respectively. Group ۶ received phosphate buffer saline as the control group. The humoral antibody responses to bacterial antigens were monitored by ELISA. LD۵۰ of Y. ruckeri was determined used probit method sixty days after vaccination. Then, fish in each treatment were challenged intraperitoneally (I.P) with LD۵۰ of Y. ruckeri virulent registered strain in Iran (KCW ۲۹۱۱۵۳). 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 G۱ to G۶ were ۲۵, ۳۱.۲۵, ۳۷.۵, ۵۶.۲۵, ۸۷.۵ and ۱۵ percent, respectively. Among I.P immunized fish the survival rate in G۵ was significantly higher than the other groups. Although in orally vaccinated fish with FKC (G۱) survival rate did not show significant difference with the control group, the FKC /ECP group (G۲) showed a significant increase compared to control group (p=۰.۰۳۹).  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.