In vitro and in vivo properties of the virus causing natural canine distemper encephalitis

A group of dogs with naturally occurring canine distemper developed prodromal systemic symptoms followed by neurological disorders. The post-infection courses of these diseases lasted approximately 2 months. A varying degree of demyelination and inclusion body formation was found mostly in the cerebella of virologically confirmed cases with little or no inflammatory response. The distribution of canine distemper virus antigen coincided with the histopathological lesions. The animals had moderate to high neutralizing titres to the virus in their sera and a low level of interferon-like activity in their cerebrospinal fluids. Isolation of viruses was most successful by the cocultivation method for brain specimens, but was possible by the direct method using lung homogenates. In infected Vero cells, the isolates derived from brain caused the formation of distinct plaques consisting of multinucleate giant cells, but the isolates from lung induced a cytopathic effect mainly consisting of cell rounding which eventually spread throughout the culture. The former infection produced less extracellular virus than the latter. The synthesis of the viral surface proteins H and F, and of M, was markedly reduced compared with that of the internal viral proteins such as NP, P and L. The SDS-PAGE migration pattern of the P protein varied from case to case, but was similar when isolates from different tissues of the same case were compared. In the affected tissues, the amount of viral polypeptides decreased markedly relative to that of the NP and there was also an absolute decrease compared to their abundance in Vero cells. This decrease was more obvious in the brain than in the lung. The relevance of these results is discussed.