Research note: Molecular epidemiology of Marek's disease virus in China during 2022 and 2023.

Marek's disease (MD) is a highly contagious and oncogenic viral disease that causes immunosuppression in chickens. A molecular epidemiological investigation of Marek's disease virus (MDV) was conducted in chicken flocks in China during 2022 and 2023. Real-time PCR was used to detect MDV serotype 1 (MDV-1) in 544 clinical tissue samples. MDV-positive samples were subsequently tested for reticuloendotheliosis virus (REV), avian leukosis virus subgroups A and B (ALV-A/B), avian leukosis virus subgroup J (ALV-J), and chicken infectious anemia virus (CIAV). Full-length sequencing of the meq and pp38 genes was performed on 18 selected MDV-positive samples. The results revealed an MDV positive rate of 19.67% (107/544), with 73.83% (79/107) of these samples showing co-infection, predominantly MDV and CIAV (54.21%, 58/107). The MDV positive rates were higher in native chickens (28.43%, 29/102) and Hy-Line layers (27.62%, 29/105), whereas the co-infection rate of MDV with other avian immunosuppressive pathogens was highest in Arbor Acres broilers (91.67%, 22/24). Molecular analysis of the meq gene from circulating field MDV strains revealed typical amino acid substitutions associated with Chinese MDV isolates, including K77E, D80Y, V115A, T139A, P176R, and P217A. Notably, two additional novel substitutions, A88T and Q93R, were identified in 16 of the 18 isolates. Phylogenetic analysis indicated that the 16 isolates clustered with virulent MDV isolates from Guangdong Province (2018), whereas the remaining two grouped with virulent MDV strains circulating one decade earlier. These findings suggest that MDV infection was prevalent in chicken flocks across some regions of China during 2022 and 2023, frequently co-occurring with other avian immunosuppressive pathogens, and that virulent MDV strains with novel molecular characteristics are currently circulating in chicken populations.