Improved Protective Efficacy in Rainbow Trout (Oncorhynchus mykiss) Against Vibrio anguillarum Through Immunization with a Combination of Formalin-Killed and Auxotrophic-Live V. anguillarum

Vibriosis caused by Vibrio anguillarum has been an important bacterial disease in cultured rainbow trout (Oncorhynchus mykiss). In the present study, we evaluated the protective efficacy of a vaccine that consists of formalin-killed (FK) V. anguillarum and the alr genes knockout auxotrophic-live (AL) V. anguillarum (Delta alr1 Delta alr2 V. anguillarum). Fish were immunized with a high dose of the FK V. anguillarum vaccine or four different combinations of FK and AL V. anguillarum. In the challenge test, fish immunized with 1 x 106 CFU of FK V. anguillarum plus 1 x 104 CFU of AL V. anguillarum (FK-106 + AL-104) showed complete protection (100% RPS) against V. anguillarum. In comparison, fish immunized with 1 x 107 CFU of FK V. anguillarum (FK-107) showed much lower survival rates. In the result of ELISA, the antibody titer of fish immunized with FK-106 + AL-104 was significantly higher than that of the PBS group, but the titer was not higher than FK-107, suggesting that the higher protection by the FK + AL combination vaccine might be mediated by not only humoral immunity but also other protective factors conferred by live bacteria in the combination vaccine. In conclusion, the present FK + AL combination vaccine efficiently protected rainbow trout with approximately 10 times lower doses of bacteria than FK-107, which could lessen the safety problem caused by a high-tittered live bacteria vaccine and the lower immunogenicity problem of killed bacteria vaccine.