Molecular characterization and phylogenetic analysis of the meq gene of Marek's Disease Virus strains from Vietnamese commercial poultry flocks

Marek's disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by Marek's disease virus serotype 1 (MDV-1) and remains a major concern for the global poultry industry despite widespread vaccination. Although current vaccines effectively prevent clinical disease, the emergence of more virulent strains has been recorded. Molecular surveillance of field strains is essential, particularly in regions where data remain limited. This study aimed to molecularly characterize MDV-1 isolates from recent cases in Vietnamese commercial poultry flocks, focusing on the meq gene, a key virulence-associated marker, and to compare variants with vaccine strains CVI988/Rispens. A total of 12 MDV-1 positive samples from multiple regions of Vietnam were analyzed using Sanger sequencing of the full-length meq gene. Phylogenetic analysis grouped the Vietnamese isolates into three genetic clusters (G1–G3). The attenuated-like G1 isolates showed high similarity to CVI988/Rispens clones and contained four to seven proline-rich repeat (PPPP) motifs. In contrast, G2 and G3 isolates were classified as field virulent genotypes and contained two to three PPPP motifs. Comparative sequence analysis revealed multiple amino-acid substitutions in the Meq protein relative to CVI988/Rispens, including conserved mutations characteristic of Asian field strains. Vietnamese isolates clustered primarily with contemporary Asian lineages from China and India. The results demonstrate the presence of both vaccine-derived and virulent MDV-1 strains in this Vietnamese sample set and highlight the importance of continued molecular surveillance to monitor viral evolution and support vaccination strategies.