Canine distemper virus: in vivo virulence of in vitro-passaged persistent virus strains

Groups of ferrets were inoculated intraperitoneally with cell lysates or equivalent doses of whole cells from 9 different cell lines persistently infected with canine distemper virus. Viral persistence in these cell lines was characterized by noncytolytic infection and restricted release of cell-free infectious virus. In vivo replication competency of the various viruses in ferrets ranged from nil to virulent and did not correlate with in vitro titers of inocula. Ferret virulence (cell lysates only) for one cell line (CCL64-RCDV) was associated with morphologic absence of virion assembly, failure to interfere with lytic virus replication after superinfection, and in vitro infectivity restricted to canine macrophage-like tumor cells. Virion protein production in the CCL64-RCDV virulent inoculum and in the CCL64-Ly avirulent inoculum was evaluated by use of the immunoblot technique. All major virion proteins were produced by infected cells. Virulence was not associated with obvious changes in electrophoretic mobility of virion proteins when profiles of ferret-virulent CCL64-RCDV were compared with those of avirulent CCL64-Ly.