Vaccine efficacy in spotted wolffish Anarhichas minor: relationship to molecular variation in A-layer protein of atypical Aeromonas salmonicida

Atypical Aeromonas salmonicida strains comprise a heterogeneous group in terms of molecular and phenotypic characteristics. They cause various conditions of ulcer diseases or atypical furunculosis and are being isolated in increasing number from various fish species and geographical areas. Several marine fish species susceptible to atypical A. salmonicida, including spotted wolffish Anarhichas minor O., are now being farmed and new vaccines may be needed. A commercial furunculosis vaccine for salmon is reported to protect wolffish poorly against experimental challenge with atypical A. salmonicida. The protective antigen(s) in furunculosis vaccines is still unclear, but in oil-adjuvanted vaccine for Atlantic salmon Salmo salar L., the surface A-layer was shown to be important for protection. In spotted wolffish, the efficacy of atypical furunculosis vaccines seems to vary with the atypical A. salmonicida strains used as bacterin in the vaccine. In the present study we investigated whether differences in the A-layer protein among atypical strains might be responsible for the observed variation in vaccine efficacy. Atypical A. salmonicida strains from 16 fish species in 11 countries were compared by genome polymorphism analysis using amplified fragment length polymorphism (AFLP) fingerprinting and by comparative sequencing of the vapA genes encoding the A-protein. The A-protein sequences appeared to be highly conserved except for a variable region between Residues 90 to 170. Surprisingly, the grouping of strains based on AFLP- or A-protein sequence similarities was consistent. In addition, serological differences in the A-protein among the strains were demonstrated by an A-protein-specific monoclonal antibody. Vaccines based on atypical A. salmonicida strains possessing genetically and serologically different A-layer proteins were shown to result in significantly different protection in spotted wolffish.