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

Abstract

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 × 10 4 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 × 10 4 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 ( r 2 = 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.