Efficacy of single and combined Streptococcus iniae isolate vaccine administered by intraperitoneal and intramuscular routes in tilapia (Oreochromis niloticus)

We evaluated the effectiveness of Streptococcus iniae vaccines prepared fro m formalin-killed cells and concentrated extracellular products (greater th an 2 kDa) of a single isolate of S. iniae (ARS-10) and a combination of ARS -10 + ARS-60 S. iniae isolates in tilapia, Oreochromis niloticus, for the p revention of streptococcal disease. Two routes of administration, intraperi toneal (i.p.) and intramuscular (i.m.) injection, were evaluated for vaccin e efficacy. Tilapia were vaccinated by i.p. or i.m. injection of the vaccin e or tryptic soy broth (control) and challenged by i.p. injection with 1x10 (8) colony-forming units of S. iniae 30 days post-immunization. Tilapia i.p . immunized with ARS-10 vaccine and challenged with the homologous isolate (ARS-10) had a relative percent survival (RPS) of 45.6%. Tilapia i.p. immun ized with ARS-10 vaccine and challenged with a heterologous isolate (ARS-60 ) had an RPS of 93.7%. In contrast, we found that the RPS was 17.7% in tila pia i.m. immunized with the ARS-10 vaccine and challenged with the same iso late, ARS-10. Heterologous (ARS-60) isolate challenge resulted in an RPS of 59.5%. However, the i.m. administration of a vaccine combining the ARS-10 + ARS-60 isolates provided an RPS of 63.1 against ARS-10 isolate and RPS of 87.3% against ARS-60 isolate in comparison to RPS of 17.7 and 59.5 against ARS-10 and -60 isolates provided by the single isolate ARS-10 vaccine. Vac cination significantly reduced abnormal behavior and morphology. We found a highly significant (P < 0.01) negative correlation between the behavioral and morphological score and RPS. Serologic evaluation revealed that both i. p. and i.m. immunizations stimulated significant (P < 0.05) antibody titers in comparison to the non-immunized tilapia. Significantly (P < 0.05) incre ased antibody titers were also produced by i.m. immunization of the combine d ARS-10 + ARS-60 vaccine. Administration of the single isolate ARS-10 vacc ine by i.m. injection was marginally protective in preventing streptococcal disease caused by homologous and heterologous isolates of S. iniae. The va riability seen in the protective ability of a single S. iniae isolate vacci ne indicates that antigenic heterogeneity exist in S. iniae. Recognition of this antigenic variability is very important to development of efficacious streptococcal vaccines. Thus, this research suggests that efficacious vacc ines, that may be administered by either i.p. or i.m. injection, are depend ent on the antigenic composition of the S. iniae. Published by Elsevier Sci ence B.V.