Efficacy of Streptococcus agalactiae (group B) vaccine in tilapia (Oreochromis niloticus) by intraperitoneal and bath immersion administration.

We evaluated the effectiveness of a Streptococcus agalactiae vaccine in tilapia (Oreochromis niloticus) for prevention of streptococcal disease. The vaccine was prepared from formalin-killed cells and concentrated extracellular products (greater than 3 kDa) of a single isolate of S. agalactiae (ARS-KU-MU-11B). Intraperitoneal (IP) and bath immersion (BI) vaccine trials were conducted at two temperatures, 32 and 26 degrees C, and mean fish weights, 5 and 30 g. Control tilapia were injected with tryptic soy broth. Thirty gram tilapia vaccinated and challenged by IP injection with 1.5 x 10(4) colony-forming units (CFU)/fish of Streptococcus agalactiae at 30 days post-immunization had a relative percent survival (RPS) of 80. Smaller tilapia vaccinated and challenged under similar conditions had an RPS of 25. An RPS of zero was noted in 30 g fish IP vaccinated with Streptococcus iniae and IP challenged with S. agalactiae. The 5 and 30 g tilapia bath immunized with S. agalactiae and IP challenged with 3.6 x 10(5) and 1.7 x 10(6) CFU/fish of S. agalactiae had RPS values of 34. Intraperitoneal administration of the vaccine provided efficacious protection only in the 30 g tilapia regardless of whether the fish were immunized and challenged at 26 or 32 degrees C. Bath immunization of both 5 and 30 g tilapia resulted in RPS values that were two times lower than those achieved with IP vaccination. The results of this study suggest that there is a lack of cross-protection of S. iniae bacterins against S. agalactiae challenge. Protection against S. agalactiae infection is, however, provided through vaccination with a S. agalactiae modified bacterin vaccine.