“Efficacy of Commercial Vaccines Available in Japan Against Streptococcus iniae in Six Salmonid Species (6980)”

Efficacy of Commercial Vaccines Available in Japan Against Streptococcus iniae in Six Salmonid Species

Introduction: Streptococcus iniae infects a variety of freshwater and marine fish species. Formalin-inactivated vaccines are commonly used for preventive approaches against this bacterium. In Japan, several commercial formalin-inactivated vaccines are approved for use exclusively in some marine fish species, but not in freshwater fish. To clarify whether commercial vaccines designed for marine fish are effective in freshwater fish, the vaccines were evaluated for the efficacy in six salmonid species cultured in freshwater in Japan.

Methodology: Six salmonid species (rainbow trout, Oncorhynchus mykiss; masu salmon, O. masou; coho salmon, O. kisutch; Japanese char, Salvelinus leucomaenis; brown trout, Salmo trutta; whitefish, Coregonus maraena) were challenged with S. iniae TB-0107, originating from diseased rainbow trout, at different doses (10³⁻⁷ CFU/fish) by intraperitoneal inoculation. The median lethal dose (LD₅₀) was then calculated from the cumulative mortality at 21 days post-challenge, and the susceptibility to S. iniae was compared among the fish species. Subsequently, two commercially available vaccines, approved for use in marine fish, were administered intraperitoneally to six salmonid species. At 21 days post-vaccination (dpv), the fish were challenged with S. iniae at 10⁵⁻⁶ CFU/fish, and the relative percent survival (RPS) was calculated. Serum samples were also collected at 21 dpv to determine specific antibody titers against S. iniae by ELISA in all the fish species tested and by agglutination test in rainbow trout and whitefish.

Results: The LD₅₀ of S. iniae infection in rainbow trout, masu salmon, coho salmon, Japanese char, brown trout, and whitefish were 1.4×10⁴, 6.6×10⁴, 5.1×10³, 1.2×10⁴, 3.1×10⁵, and 5.3×10³ CFU/fish, respectively. The survival rates of the vaccinated groups were higher than those of the control groups across all the fish species. RPS values were above 60% in five salmonids except whitefish, for which efficacy was comparatively lower. Serum antibody titers following the vaccination were significantly increased in the five salmonids except whitefish, compared with the control groups, although no increase was observed in whitefish.

Conclusion: All the six salmonid species were susceptible to S. iniae. Commercial vaccines showed high protection in rainbow trout, masu salmon, coho salmon, and brown trout, but not in whitefish. Following vaccination, no increase in antibody titers was observed in whitefish, whereas antibody titers increased in the other five species. These results suggest that vaccine efficacy against S. iniae infection is dependent on specific antibody production against the bacteria.

Funding: This study was funded by the Ministry of Agriculture, Forestry and Fisheries of Japan.

1. MORIMOTO, Natsuki, JAPAN FISHERIES RESEARCH AND EDUCATION AGENCY, Presenter
2. UMEDA, Kousuke, JAPAN FISHERIES RESEARCH AND EDUCATION AGENCY, Author
3. MAEDA, Tomoki, JAPAN FISHERIES RESEARCH AND EDUCATION AGENCY, Author
4. NAKAGAWA, Yukihiro, JAPAN FISHERIES RESEARCH AND EDUCATION AGENCY, Author
5. TAKANO, Tomokazu, JAPAN FISHERIES RESEARCH AND EDUCATION AGENCY, Author
6. MATSUYAMA, Tomomasa, JAPAN FISHERIES RESEARCH AND EDUCATION AGENCY, Author