Serological relationships among the strains of pathogenic Vibrio isolated from cultured eels and between the eel isolates and the species of the genus Vibrio were studied by means of Ouchterlony immunodiffusion techniques and agglutinin adsorption tests with rabbit antisera prepared against representative strains of the present isolates. The present eel isolates were antigenetically very homologous : All representative five isolates produced four lines of identity in Ouchterlony plates, and possessed agglutinins (flagellar and at least one heat-stable antigens) in common. The species of the genus Vibrio exhibited slight relationships to the eel isolate : V. anguillarum strains, V. parahaemolyticus strain, and V. alginolyticus strain shared one precipitin band with the present eel isolate, and V. anguillarum strains, V. fischeri strains, V. parahaemolyticus strain, and V. alginolyticus strain had agglutinins, most likely flagella antigen, common to the present eel isolate. These results well agree with those of the previous DNA studies, and present another piece of evidence supporting that the present eel isolate is a new member of the genus Vibrio.
A search was made for the diagnostic tests for the disease of cultured eels due to the present bacterium. It is considered impossible to differentiate the disease of eels caused by the present bacterium, by means of external signs of diseased fish, from that caused by other eel pathogens such as Aeromonas hydrophila, Edwardsiella tarda, Vibrio anguillarum, and Pseudomonas anguilliseptica. This necessitates the isolation and identification of the infectious agent for diagnosis. Positive proof was obtained that the diluted rabbit antisera prepared against representative strains of the present bacterium can be used for diagnostic purpose. In addition, a simplified identification method by the use of API 20E was shown. It was proposed that the disease of cultured eels caused by the present vibrio be named as vibriosis of eels Type B ; that due to V. anguillarum as Type A.