The biological characterization of field isolates of canine distemper virus from Japan

Eight isolates of canine distemper virus (CDV) were obtained from seven dogs suffering from distemper by co-cultivation of their mononuclear cells with a marmoset B lymphoblastoid cell line, B95a. Six of the seven dogs had received one or more vaccinations. All of the isolates readily proliferated in B95a cells, but were not completely neutralized by anti-CDV canine plasma, which had high neutralizing activity against the Onderstepoort laboratory strain of CDV. Furthermore, different reactivities of monoclonal antibodies (MAbs) against CDV were observed between the field isolates and laboratory or vaccine strains of CDV in immunofluorescence studies. Immunoprecipitation analysis using MAbs detected the haemagglutinin protein of each new field isolate as 69K, 75K and 155K forms, and the fusion protein as 64K and 65K forms; the corresponding proteins of the Onderstepoort strain were detected as 75K and 61K proteins respectively. It is apparent from these results that the new field isolates of CDV have very different antigenic properties from the Onderstepoort vaccine strain.