Specific humoral immune response and protection against Vibrio parahaemolyticus in orange-spotted grouper Epinephelus coioides

This study shows the specific antibody response of orange-spotted grouper Epinephelus coioides to the formalin-killed bacteria Vibrio parahaemolyticus and the protection after experimental challenge with the bacteria. The vaccination was performed by intraperitoneal injection using bacterin combined with adjuvant. Specific antibodies were analyzed by ELISA using whole bacteria as antigen, and the immunoreactivity to the various bacteria protein components were detected by western blotting post vaccination. In addition, the protective efficacy of the bacterin was tested by challenge with V.  parahaemolyticus at 30 and 60 days post vaccination.The results showed that significantly high level of antibodies was found in vaccinated fish using a bacterin with adjuvant. At 60 days post injection, the antibody levels were lower and more similar in all groups. By western blotting, immunoreactive bands were observed in bacterin vaccinated whereas no bands were seen in the two control groups. The challenge tests showed that the cumulative mortality rate was low in the vaccinated groups (10 and 40%) after 30 days post vaccination while high mortality occurred in control groups (70% and 90%). However, the mortality rate had increased in all groups at day 60 post vaccination. The bacterin/adjuvant vaccinated groups showed lowest mortality of 40% and the other groups including controls above 60%. Vaccination using formalin killed bacteria V. parahaemolyticus mixed with adjuvant FIA provided good protection in cultured E. coioides. This was supported by findings of specific antibodies to the bacteria in sera from vaccinated fish. The results indicate that vaccination, using adjuvants can be a prophylactic measure in orange spotted grouper and further studies should focus on obtaining longer time protection and use of various adjuvants.