A Highly Pathogenic Marek’s Disease Virus Isolated From a Immunized to Bivalent Vaccines Chicken Farm in China

Abstract Marek’s disease virus (MDV) is an important oncogenic poultry pathogen that can generally be controlled by vaccination. However, MDV still occasionally occurs on vaccinated farms owing to possible genetic variation among MDV strains as well as management-related issues. In this study, a novel MDV strain (designated LZ1309) was isolated from a poultry flock that had been previously vaccinated using the HVT plus CVI988 vaccine strains. Animal experiments showed that LZ1309 infection led higher morbidity (100%) and mortality (90%). Moreover, existing vaccines only provided partial protection against LZ1309, which protection indexes of HVT, CVI988, and HVT plus CVI988 were 68.4%, 85%, and 90%, respectively. In conclusion, we have shown that the more virulent of Marek’s disease virus existed in vaccinated with HVT plus CVI988 in poultry farms in China. And the emergence of LZ1309 poses a new potential threat to poultry farms. In future studies, the development of new treatment strategies should be of high priority.