Characterization of two recent Japanese field isolates of canine distemper virus and examination of the avirulent strain utility as an attenuated vaccine

•We characterized two recent Japanese isolate CDV strains.•The Yanaka strain was avirulent, while the Bunkyo-K strain was virulent in dogs.•We tested the ability of the Yanaka strain for vaccination.•The Yanaka strain efficiently induced protective immunity against virulent CDVs.•The Yanaka strain is a potential candidate for a novel live attenuated CDV vaccine. Recently, several new strains of canine distemper virus (CDV) have been isolated in Japan. To investigate their pathogenesis in dogs, the Yanaka and Bunkyo-K strains were investigated by infecting dogs and determining clinical signs, amount of virus, and antibody responses. The Yanaka strain is avirulent and induced an antibody response. The Bunkyo-K strain induced typical CDV clinical signs in infected dogs and virulence was enhanced by brain passage. Molecular and phylogenetic analyses of H genes demonstrated the Bunkyo-K strains were of a different lineage from Asia-1 group including the Yanaka strain and Asia-2 group that contain recent Japanese isolates, which were recently identified as major prevalent strains worldwide but distinct from old vaccine strains. Based on these data, we tested the ability of the Yanaka strain for vaccination. Inoculation with the Yanaka strain efficiently induced CDV neutralizing antibodies with no clinical signs, and the protection effects against challenge with either old virulent strain or Bunkyo-K strain were equal or greater when compared with vaccination by an original vaccine strain. Thus, the Yanaka strain is a potential vaccine candidate against recent prevalent CDV strains.