Reduced Vaccinal Protection of Turkey Herpesvirus Against Field Strains of Marek's Disease Herpesvirus ,

Four isolates of Marek's disease herpesvirus (MDV) were obtained from 21-week-old broiler breeders with lesions and mortality from Marek's disease (MD). They had been properly vaccinated with turkey herpesvirus (HVT). The isolates were designated Ala-7, 8, 9, and 10. These 4 MDV strains were compared with the GA isolate of MDV in chickens vaccinated with graded doses of HVT. Challenge with the MDV strains was also with graded doses. All 5 isolates of MDV were similar in virulence. However, protection afforded against MD lesions by HVT was significantly lower in the Ala-7, 8, and 9 isolates than with the Ala-10 or GA isolates. The Ala-7, 8 and 9 isolates of MDV induced small-cell plaques in chick embryo fibroblasts, the Ala-10 isolate induced both large and small cell per plaque types, and the GA isolate produced predominantly small cell plaques with a few plaques of the large cell type.