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 from formalin-killed cells and concentrated extracellular products (greater than 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 prevention of streptococcal disease. Two routes of administration, intraperitoneal (i.p.) and intramuscular (i.m.) injection, were evaluated for vaccine efficacy. Tilapia were vaccinated by i.p. or i.m. injection, of the vaccine or tryptic soy broth (control) and challenged by i.p. injection with 1 X 10(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. immunized 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 tilapia i.m. immunized with the ARS-10 vaccine and challenged with the same isolate, 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. Vaccination 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) increased antibody titers were also produced by i.m. immunization of the combined ARS-10 + ARS-60 vaccine. Administration of the single isolate ARS-10 vaccine by i.m. injection was marginally protective in preventing streptococcal disease caused by homologous and heterologous isolates of S. iniae. The variability seen in the protective ability of a single S. iniae isolate vaccine 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 vaccines, that may be administered by either i.p. or i.m. injection, are dependent on the antigenic composition of the S. iniae.