Passive immunization of Nile tilapia (Oreochromis niloticus) provides significant protection against Streptococcus agalactiae

A study was conducted to determine the role of specific antibodies in immunity to Streptococcus agalactiae. Adult Nile tilapia (Oreochromis niloticus) were injected i.p. with tryptic soy broth as control or with S. agalactiae vaccine. Ninety days later, fish were challenged with 1.5 × 104 CFU S. agalactiae fish−1. Blood was drawn from all fish 90 d after vaccination and 25 d after challenge, and the acquired serum was injected i.p. in fingerling Nile tilapia. These passively immunized fish were subsequently challenged 72 h later with 1.5 × 104 CFU S. agalactiae fish−1, and significantly less (P < 0.0001) mortalities were noted among fish administered serum containing specific anti-S. agalactiae antibodies (0.0–10.0% mortalities) than in control groups (63.3–72.7% mortalities). Heat-inactivation of serum produced no significant differences in mortalities than non-heat-treated serum in groups administered serum containing specific antibodies from vaccinated fish (P < 0.9455) or vaccinated–challenged fish (P < 0.0781). Pre-challenge serum samples indicate that the passively immunized fish had significantly increased (P < 0.0001) specific antibody levels over control fish. A highly significant (r2 = 0.5892; P < 0.0001) correlation between increased pre-challenge specific serum antibody OD levels and survival after challenge was demonstrated when analyzing the control and passive immunization groups. The results of this study indicate that specific anti-S. agalactiae antibodies play a primary role in immunity to S. agalactiae in fish.