Immunoresponse of Eel against Edwardsiella tarda Antigens

From Edwardsiella tarda EF-1 strain, three antigens were obtained: lipopolysaccharide (LPS), culture filtrate and formalin killed whole cells. Eel were injected intramuscularly with the antigens twice at an interval of one week. The agglutination and passive hemagglutination titers in serum were determined three weeks after the second immunization. In the another experiment, eel were challenged by intramuscular injection with live bacteria three weeks after the second immunization and the agglutination titer was determined in the serum of the surviving eels. Antigenic relationships between the LPS and the culture filtrate were examined by OUCHTERLONY method and the immunoelectrophoresis. All the immunized eel showed an increase in titer to the three antigens, but a large increase in the titer was not observed. The highest survival rate was recorded in challenged eel immunized with LPS, which indicated the LPS role may correlate with the protective effect of the vaccine. Agglutination titer increased significantly in the serum of the immunized eel, but no correlation between survival rate and agglutination titer was observed among the immunized groups. The results suggest that the agglutination titer using whole cell antigen does not correlate with the level of the protection observed in eels. The increase in the titer with whole cell antigen was considered to be a function of an antigen detected in the LPS and the culture filtrate.