Comparison of canine distemper viral strains: an electron microscopic study.

A canine distemper (CD) viral strain, designated R252, originally obtained from a dog with demyelinating encephalomyelitis has been shown to reproduce this disease in gnotobiotic dogs in a high incidence in contradistinction to other CD viral strains which produce an acute fatal disease. Because comparision of R252 strain with the Snyder Hill (SH) and Onderstepoort (Ond) strains revealed differences in in vitro behavior, the 3 viruses were ultrastructurally investigated. The results revealed differences among the 3 viruses: Cytoplasmic nucleocapsid aggregates were characteristic of R252, diffuse aggregates of nucleocapsids were characteristic of SH, and although budding viral particles were recognized in Ond-infected cells, few nucleocapsids were seen in the cytoplasm. Cytoplasmic fibrillar structures were characteristic of R252- and Ond-infected cells. The budding viral particles observed with R252 and SH were similar, whereas Ond budding particles seemed to contain fewer nucleocapsids. Intranuclear inclusions recognized in R252- and SH-infected cultures appeared as areas of clearing of nucleoplasm along with disruption of the nucleolus. The presence of nucleocapsid-like structures was variable. Ultrastructurally, the cytoplasmic inclusions in cells infected with all 3 viruses progressed from focal aggregates of nucleocapsids to electron-dense bodies.