A possible association between plaque type and pathogenicity of Marek's disease herpesvirus.

Eighteen isolants of Marek's disease herpesvirus (MDHV) were examined for their plaque types in chicken embryo fibroblast cell cultures and their pathogenicity to an experimental line of White Leghorns highly susceptible to MD. Seven isolants (Id-1, GA, WSU-GF, and 4 field isolants) induced morphologically similar small-cell plaques (SC plaques) consisting of small round refractile cells, whereas 11 isolants (HN and 10 isolants from zoo birds) all induced large-cell plaques (LC plaques) containing large cells along with small refractile cells. The 7 SC plaque isolants all produced clinical MD with high incidences of gross MD tumors, while all 11 LC plaque isolants produced subclinical infection with only minimal histologic lesions. This observation strongly suggests a possible association between plaque type and pathogenicity of MDHV isolants.