Pathogenicity of an unusual highly virulent Marek's disease virus isolated in the United Kingdom

A highly virulent strain of Marek's disease virus, designated C12/130, was isolated in 1991 from a field outbreak in HVT vaccinated broiler breeders in the UK. The pathogenicity of the disease induced by this virus was studied in genetically susceptible and resistant specific-pathogen-free chickens. The virus induced severe early cytolytic disease causing mortality up to 25% in the first 2 weeks with marked lasting atrophic changes of the thymus and bursa of Fabricius. Severity of the disease varied between different lines and expression of the meg gene was detected by in situ hybridization in the infected tissues.