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Title: PASSIVE IMMUNIZATION OF NILE TILAPIA (OREOCHROMIS NILOTICUS) PROVIDES SIGNIFICANT PROTECTION AGAINST STREPTOCOCCUS AGALACTIAE

Author
item Pasnik, David
item Evans, Joyce
item Klesius, Phillip

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2006
Publication Date: 10/1/2006
Citation: Pasnik, D.J., Evans, J.J., Klesius, P.H. 2006. Passive immunization of nile tilapia (oreochromis niloticus) provides significant protection against streptococcus agalactiae. Fish and Shellfish Immunology. Vol 21 Issue 4 pages 365-371.

Interpretive Summary: The emerging fish pathogen, group B Streptococcus agalactiae, has been shown to cause significant morbidity and mortality among a variety of freshwater and saltwater fish species throughout the world. Because of its potential economic and ecological impacts to aquaculture and fisheries, a vaccine composed of concentrated extracellular products and formalin-killed S. agalactiae whole cells was constructed and determined to be significantly efficacious. Further studies initially suggested that specific antibodies were involved in protective immunity against S. agalactiae, but a passive immunization study would more definitively determine the role of antibodies. Therefore, the study presented here was designed to assess the role of specific antibodies in protection against S. agalactiae, determine whether complement is involved in protection, and further resolve whether specific antibodies play an important role in immunity against Streptococcus spp. Adult Nile tilapia (Oreochromis niloticus) were injected with tryptic soy broth as control or with S. agalactiae vaccine. Ninety days later, fish were injection challenged with live S. agalactiae. Blood was drawn from all fish 90 d after vaccination and 25 d after challenge, and the acquired serum was injected into fingerling Nile tilapia. These passively immunized fish were subsequently challenged 72 h later with live S. agalactiae, and significantly less mortalities were noted among fish administered serum containing specific anti-S. agalactiae antibodies (0.0-10.0% mortalities) than in control groups injected with tryptic soy broth (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 or vaccinated-challenged fish. Pre-challenge serum samples indicate that the passively immunized fish had significantly increased specific antibody levels over control fish. A highly significant correlation between increased pre-challenge specific serum antibody 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.

Technical Abstract: A series of studies were 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 x 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 x 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. Keywords: Antibody; Oreochromis niloticus; Passive immunization; Streptococcus agalactiae; Tilapia