The genome evolution of Marek's disease viruses in chickens and turkeys in China.

The virus that causes Marek's disease (MD) is globally ubiquitous in chickens, continuously evolving, and poses a significant threat to the poultry industry. Although vaccines are extensively used, MD still occurs frequently and the virus has evolved increased virulence in China. Here, we report an outbreak of MD in vaccinated chickens and unvaccinated turkeys in a backyard farm in Guangdong province, China, in 2018. Phylogenetic analysis revealed two lineages of MDVs at this farm, with one lineage, containing isolates from two turkeys and five chickens, clustering with virulent Chinese strains and displays a relatively high genetic divergence from the vaccine strains. These new isolates appear to have broken through vaccine immunity, yielding this outbreak of MD in chickens and turkeys. The second lineage included four chicken isolates that clustered with the CVI988 and 814 vaccine strains. The large diversity of MDVs in this single outbreak reveals a complex circulation of MDVs in China. Poor breeding conditions and the weak application of disease prevention and control measures make backyard farms a hotbed for the evolution of viruses that cause infectious diseases. This is especially important in MDV as the MD vaccines do not provide sterilizing immunity, which allows the replication and shedding of virulent field viruses by vaccinated individuals and supporting the continuous evolution of MDVs. Hence, constant monitoring of the evolution of MDVs is necessary to understand the evolution of these field viruses and potential expansions of their host range.