Biological diversity among serotype 2 Marek's disease viruses

Selected biological characteristics were determined for 14 low-passage serotype 2 Marek's disease virus (MDV) isolates. Four of these isolates were also tested after extensive serial passage in chicken embryo fibroblast cultures. Observations were made on replication in vitro and in vivo, pathogenicity by in ovo inoculation, antigenicity, and protection against virulent MDV challenge. Among the low-passage isolates, there were some differences in pathogenicity after in ovo inoculation but relatively little difference in other characteristics, with the exception of the HN-1 strain, which replicated more rapidly in cell culture but produced generally lower in vivo responses than other isolates. After extended in vitro passage, isolates replicated much more readily in cell culture and produced lower pathologic responses in vivo than low-passage isolates, as has been reported for serotype 1 isolates. No antigenic differences among isolates were detected, but high-passage isolates induced lower levels of precipitating antibodies than low-passage isolates, indicating a possible reduction in A antigen production. The observed diversity associated with strain and passage level may be of value in the selection of optimum vaccine strains.