Genomic characteristics and distinctive genetic polymorphisms of Marek's disease virus strains circulating in Japan.

Marek's disease virus (MDV) is an oncogenic herpesvirus that causes malignant lymphomas in chickens (Marek's disease, MD). Although MD is currently well controlled by vaccination, field strains of MDV tend to increase in virulence, and sporadic outbreaks have been reported worldwide, including in Japan. Previously, we determined the whole-genome sequence of a Japanese MDV strain, Kgs-c1, isolated in 2014, and analyzed its genetic characteristics. However, to date, Kgs-c1 is the only MDV strain that has been subjected to whole-genome sequencing, leaving the possibility that the key genetic features of the Japanese strains remain undiscovered. Therefore, we conducted whole-genome sequencing of two MDV strains, Kgs-c2 and Sg-c1, isolated in 2016 and 2019, respectively, and analyzed their genetic characteristics. Consistent with our previous findings, the genomic sequences of the Japanese strains were classified into the Eurasian cluster, suggesting that the Japanese strains are genetically closer to the European and Chinese strains than they are to the US strains. In contrast, the Japanese strains exhibited genetic features different from those of pathogenic European and Chinese strains. Furthermore, polymorphisms in the meq and infected cell protein 4 (ICP4) genes, which are candidates involved in the enhanced virulence of MDV, were observed among the newly isolated strains and Kgs-c1. Thus, three MDV strains isolated in Japan had different genetic features from those in other countries. These findings emphasize the necessity of periodic monitoring of MDV field strains in order to determine shifts in their characteristics and effectively control MD.