Molecular characterization of Marek's disease virus revealing the predominance of high-virulence MDV in Vietnam

Marek's disease (MD) is a potentially fatal viral disease caused by the Gallid alphaherpesvirus 2 (GaHV-2). The outbreaks are common in both unprotected and vaccinated flocks. The carcinogenic serotype MDV-1 was found using a PCR method in five northern provinces and two central provinces of Vietnam from 2019 to 2022. The Meq gene from 13 MDV-1 strains was sequenced in its entirety, containing 1020 nucleotides encoding 339 amino acids. The examination of these genomes revealed that all strains discovered in Vietnam correspond to the pathotypes known as highly virulent (vv). This classification was based on the presence of low proline concentration (from 20.59-21.18%) in the Meq which consisted of three PPPP motifs and three interrupted motifs. The interrupted motifs are characterized by mutations at the second position of the proline-rich region where the original PPPP sequence is altered to PPPP>P (Q / A / R) PP. Furthermore, the Vietnamese GaHV-2 strains in this study had the most similarity to high-virulence Chinese strains but had the lowest similarity to the currently used CVI988 vaccination. It is worth noting that no strains from the moderate pathogenicity (mv) or very virulence plus (vv+) groups were found among the Vietnamese isolates. The findings shed light on previously unknown molecular features of Marek's disease virus strains in Vietnam's northern provinces. The study's findings add to a better understanding of MDV in Vietnam, filling an important information gap. This knowledge can help guide efforts to reduce the burden of Marek's disease on the chicken business, improve disease prevention and control strategies and select the best vaccine for chickens in the field.