The development of live vaccines for furunculosis lacking the A‐layer and O‐antigen of Aeromonas salmonicida

Abstract. Mutants of Aeromonas salmonicida strains lacking either the A‐protein, O‐antigen or both of these major surface antigens were tested in rainbow trout, Oncorhynchus mykiss (Walbaum), for their suitability as live vaccines (LV). All of these mutants were shown to be attenuated, as fish receiving ∼5 × 10⁷ of the respective strains showed no clinical signs of furunculosis. Immersion vaccination of fish in 5 × 10⁷ cfu ml^‐1 of these strains with an identical immersion dose 14 days later resulted in significant protection by all strains from challenge with a heterologous virulent strain of A. salmonicida 5 weeks later. The levels of protection conferred were all greater than or equal to that provided by an injected bacterin using the same vaccination schedule. With one exception, all LV strains that still possessed a functional O‐antigen provided protective indices (PI) four‐ to seven‐fold greater than the PI for the fish injected with bacterin. When antibody responses of vaccinated fish were compared, it was found that only vaccination by bacterin gave rise to a measurable agglutinating litre. Western immunoblots using the immune fish sera failed to reveal any major differences in antigen recognition in fish that received any of the vaccines tested. These data suggest that the immune response generated by the use of live vaccine strains is different from that generated by a bacterin, and that these useful mutations may be incorporated into existing furunculosis LVs for further attenuation.