Characterization of two avian reoviruses that exhibit strain-specific quantitative differences in their syncytium-inducing and pathogenic capabilities

We previously proposed that the conservation of the nonessential syncytium-inducing phenotype among all reported avian reovirus (ARV) isolates may reflect a mechanism for enhanced virus dissemination in vivo, which in turn could contribute to the natural pathogenicity of ARV. Direct testing of this hypothesis has been hampered by the lack of available virus strains with defined differences in their fusion-inducing capability. We now report on the characterization of two ARV strains, ARV-176 and ARV-138, that exhibited strain-specific differences in their fusogenic properties, which correlated with their pathogenic potential in embryonated eggs. Moreover, both virus strains possessed similar replicative abilities in cell culture, suggesting that the weakly fusogenic ARV-138 virus is specifically inhibited in its syncytium-inducing ability. To test the use of these viruses for reassortant studies aimed at assessing the role of cell fusion in viral pathogenesis, a preliminary genetic analysis was undertaken using a monoreassortant that contained nine genome segments from the parental ARV-138 virus and the S1 genome segment from the highly fusogenic and pathogenic ARV-176 parental virus. The monoreassortant possessed the full fusogenic potential of the ARV- 176 parental virus and displayed enhanced embryo pathogenicity, providing the first genetic evidence implicating the ARV S1 genome segment in both syncytium formation and viral pathogenesis.