Efficacy of four kinds of oral vaccine for control of vibriosis in cultured ayu (Plecoglossus altivelis) was investigated. Nutrient broth inoculated with Vibrio anguillarum, the etiological agent of vibriosis in ayu, was incubated with shaking. Cells were harvested and inactivated with formalin. The bacterine thus prepared was ultrasonicated, heated or mixed with Freund incomplete adjuvant. These four kinds of vaccine were administered at a level of 0.4g wet weight of cells per kg body-weight per day. The efficacy of vaccination was tested by exposing fish to the bacteria discharged from artificially infected ayu.
The formalin-killed bacterine and formalin-killed and ultrasonicated becterine were highly effective, but production of antibody in the blood was not observed. The heated bacterinealso was effective when administered at a level of 4.0g. Administration of formalin-killed bacterine at a level of 4.0g for more than 2 weeks resulted in low mortality of about 10% in contrast to about 90% for the controls. This efficacy of vaccination was maintained for four weeks, and when booster was administered it was prolonged further. Though many antigens were contained in the bacterine, heat resistant antigens were considered to play an important role in protection against vibrio infetion.