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 (Δalr1Δalr2 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 × 10⁶ CFU of FK V. anguillarum plus 1 × 10⁴ CFU of AL V. anguillarum (FK-10⁶ + AL-10⁴) showed complete protection (100% RPS) against V. anguillarum. In comparison, fish immunized with 1 × 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.