of Variant Marek's Disease Virus Isolants in Vaccinated and Unvaccinated Chickens

SUMMARY Marek's disease (MD) virus isolants Md/5 and Md/11, obtained from commercial broiler flocks vaccinated with turkey herpesvirus (HVT) but having excessive condemnation losses from MD, seemed similar to prototype MD viruses in oncogenicity for susceptible chickens, immunodepressive ability, and antigenicity. Compared with prototype MD viral strains JM/102W and GA/22, however, both field isolants were classed as biological variants on the basis of: 1) higher induction of acute cytolytic infection, characterized by atrophy of bursa and thymus and early death in the absence of lymphomas; 2) higher oncogenicity in genetically resistant chickens; and 3) higher oncogenicity in chickens immunized with HVT. A standard field dose (l103 plaque-forming units) of HVT vaccine protected 92-95% of chickens challenged with JM/ 102W but only 57-72% of chickens challenged with Md/5; the calculated dose of HVT required to protect 90% of chickens was at least 1000-fold as great for challenge with Md/5 as for challenge with JM/102W. The extent to which such variant MD viruses are responsible for MD vaccine failures in commercial flocks was not determined. These studies suggest that susceptibilities of chickens to acute cytolytic infection and MD lymphomas are mediated independently, and confirm that the severity of acute cytolytic infection in MD is influenced by virus strain.