Increase in immunobiological activities of yellowtail [Seriola quinqueradiata] immunized with formalin-killed Enterococcus seriolicida

In the present study, immunobiological activities in the serum and phagocytes of yellowtail Seriola quinqueradiata immunized with formalin-killed E. seriolicida (FKC) by intraperitoneal injection were investigated to elucidate the induced protective mechanisms. Immunized fish showed 66.7% survival, while no survivors were observed in the control group 4 weeks after the challenge. Agglutinating antibody levels, alternative complement pathway activity, antibacterial activity, opsonic activity of serum were also higher in immunized than in non-immunized fish. Phagocytic activity and intracellular bactericidal activity of macrophages did not show differences between the immunized and non-immunized fish. The results suggest that non-cellular defense activities could be important mechanisms of yellowtail in protection to E. seriolicida infection.