Studies on humoral immune response of yellowtail [Seriola quinqueradiata] against pseudotuberculosis

"Pseudotuberculosis caused by Pasteurella piscicida is one of the most important diseases of cultured yellowtail, Seriola quinqueradiata, Since 1969, many yellowtail farms on the west coast of Japan had been damaged by the epizootics of this disease every year. Recently, antibacterial agents have been used for control of the disease. However, the occurrence of multiple drug resistant bacteria has made drug selection difficult, and chemotherapy in fish culture brought out subjects on public health. Prevention of this disease by vaccination would avoid the use of drugs. In the present report, the author have investigated the humoral aspects of immune response against pseudotuberculosis in yellowtail, as a basic study on the development of the techniques based on the immunocompetence of the host to prevent and control this disease. The antigenic structure of P. piscicida was studied with the cross agglutination and the immunodiffusion tests, the results suggested that all strains isolated from yellowtail and other marine fishes in Japan were serologically homogeneous. The humoral aspects of post - infectious immunity of yellowtail was examined on the production of agglutinating antibody against the infectious agent after the outbreak of pseudotuberculosis in 1979. Peaks of agglutinating antibody activity against P. piscicida in the sera of cultured 0-year-old yellowtail were observed in the middle of July and the end of October. Rising and falling periods of water temperature ranging about 23-25degC corresponds to these peaks. It would be inferred from these that there was a suitable environment for growth of the pathogen in fall, but the recurrences of this disease were prevented, by secondary immune response in yellowtail. The production of agglutinating antibody in yellowtail immunized by intraperitoneal injection with formalin-killed P. piscicida cells in Freund's complete adjuvant was examined. Agglutinating antibody titers in the immunized fish sera began to increase in 1-2 weeks after immunization, and maximum titers were obtained from 3 weeks after immunization. Both post-infectous and artificially acquired antibody produced in yellowtail sera were macroglobulin and their activities were inactivated by reduction with 2-mercaptoethanol. Yellowtail immunoglobulin(Ig) was purified from the sera by the procedures which involved gelfiltration on Sepharose 6B and ion exchange chromatography on DEAE-Sephadex A-25. Yellowtail Ig thus purified was found to be a 15.6S macroglobulin having the electro-phoretic mobility of beta-globulin in agar gel immunoelectrophoresis. Yellowtail Ig showed a higher agglutinating antibody activity at 20-25degC than at 37degC. The immunoglobulin was dis-"