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

Abstract

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.