Serum antibody response of rainbow trout, Oncorhynchus mykiss to two species of pathogenic bacteria; Streptococcus iniae and Lactococcus garviae

One of the most important bacterial diseases in aquaculture is Streptococosis/loctococosis. Meanwhile this disease has been known as a zoonosis disease, and then fish vaccination against this disease is important. In this study the specific immunological responses of rainbow trout (Oncorhynchus mykiss) to vaccination with bivalent lactococcal/streptococcal vaccine were investigated. Two hundred and fourty rainbow trout fingerlings (30±1.7g) were randomly divided into two equal groups in triplicates. Fish in vaccinated treatment intraperitonealy injected with 100 microliter vaccine on the first day and booster vaccine were directed by immersion route on day 30th of study. The control group was injected with sterile PBS, identical to vaccinated group. Blood samples were collected from vaccinated and non-vaccinated groups at days 0, 14. 30, 45 and 60 of study and antibody titer were subsequently performed. Sixty days after initial immunization all groups were challenged by lethal dose of Streptococcus iniae and Lactococcus garviae, mortality rate was calculated and compared between the groups. Survival rates of vaccinated fish after challenge with S. iniae and L. garviae were 62±7.22% and 68±.8.67% respectively. The results demonstrated that specific plasma IgM antibody titer induced by commercial vaccine were significantly higher compared to the level in the control group (p<0.05). Our results demonstrated that not only these bivalent vaccines can effectively protect rainbow trout against L. garvieae and S. iniae, but also can increase antibody titer in rainbow trout, then it could offer an appropriate strategy to prevent these infections in rainbow trout farms in Iran.