Attenuation of Marek's Disease Virus and Study of Its Properties in Two Different Cell Cultures 1

SUMMARY-Cytopathology of Marek's disease virus (MDV) was changed in duck embryo fibroblast (DEF) culture, and pathogenicity of the virus was lost as a result of long cell-culture passage. The virus was propa-gated in chicken embryo fibroblast (CEF) cultures by cocultivation with MDV-infected DEF, and some differences were observed between properties of the virus in these two cultures. Cell.free virus was recovered in growth medium of infected CEF, and MDV microplaques in this culture consisted of round cells. No cell-free virus was recovered in growth medium of infected DEF, and the microplaque consisted of syncytial cells. Behavior of MDV in these two cultures is discussed.- J Nat Cancer Inst 44: 1257-1267, 1970. MAREK'S DISEASE virus (MDV) was firs isolated (1-3) from HPRS-16 and JM strains of the disease. These reports suggested the implication of the virus in the etiology of the disease. A similar virus was later isolated from several other strain