Protection of Japanese flounder Paralichthys olivaceus against experimental edwardsiellosis by formalin-killed Edwardsiella tarda in combination with oral administration of immunostimulants

We studied the effects of oral administration of immunostimulants (curdlan and quillaja saponin) and formalin-killed Edwardsiella tarda cells (FKC) on the protection against edwardsiellosis in Japanese flounder Paralichthys olivaceus. Fish were fed experimental diets containing immunostimulants and FKC singly or in combination for three weeks. The survival rates were observed for 19 days after challenge with E. tarda, and the agglutination titers against the bacterium were measured before and after the challenge. Fish fed the immunostimulants showed higher survival rates than the control group, with the agglutination titers being as low as those of the control group. On the other hand, fish fed on FKC-containing diets exhibited significantly (p < 0.05) higher survival rates and titers than the control group. The immunostimulants were suggested to activate immune systems in the vaccinated fish, which led to enhancement of resistance against edwardsiellosis.