Marek’s disease virus (MDV) also known as Gallid herpes virus 2 (GaHV-2), was an oncogenic poultry herpes virus, causing lymph proliferative and demyelinating disorder in infected chickens. In the 1970s, herpes virus of turkey (HVT) vaccine, which belonged to serotype 3 vaccines, was mainly used to control the disease. In the mid-1980s, a serotype 2 vaccine, such as SB-1 strain was used in combination with HVT against the enhanced virulence strains. With further increase in virulence of field viruses, CVI988 vaccine, which belonged to serotype 1 vaccine, was introduced for widespread use in the 1990s. Recently, the failures of CVI988 vaccination have been reported, when CVI988 vaccine is used either alone or in combination with serotype 2 and/or serotype 3 vaccines, suggesting the emergence of high virulent strains.
At present, the widespread use of vaccines against Marek’s Disease (MD) was suggested to have led to the evolution of field viruses with greater virulence, and a number of patho types classified as vMDV, vvMDV, and vv+MDV have been isolated, and more virulent strains could overwhelm the protection conferred by currently available vaccines. In recent years, MDV isolates in China have been reported in breeder or layer flocks which have been vaccinated with HVT or CVI988. The virulent Marek’s Disease virus strains (MDVs) circulating in China seemed to constitute a separate genotype different from exotic MDV reference strains, and these isolates were high pathogenic for chickens. In this study, detection of MDV very virulent strain and analysis of its Meq gene were carried out for a clinical case in a chicken farm infected with Marek’s disease.