The emergence of a highly pathogenic Marek's disease virus featuring newly identified mutations in Meq oncogene in Southern China

Background Marek's disease virus (MDV) is a significant threat to global poultry health, causing high mortality, tumor formation, and severe immunosuppression, leading to substantial economic losses. Continuous monitoring and research on MDV field strains are essential for developing effective vaccination strategies and biosafety measures. Methods In this study, peripheral blood was collected from a chicken farm in southern China where an outbreak of MDV occurred despite vaccination with the CVI988 vaccine. A virulent field strain of MDV was isolated and identified through plaque purification experiments, PCR, and indirect immunofluorescence assays. The molecular characteristics of the Meq gene were analyzed by cloning and sequencing. Finally, animal experiments were conducted to evaluate the pathogenicity, oncogenicity, and vaccine protective efficacy of the isolated strain. Results A highly pathogenic MDVstrain, designated as SS1901, was isolated and purified from a chicken farm vaccinated with CVI988. Sequencing of the oncogenic protein Meq revealed mutations at K77E, D80Y, V115A, T139A, P176R, and P217A, con-sistent with mutations found in other prevalent Chinese strains. Additional mutations at A88T and Q93R were identified, which have been observed in MDV strains from southern China, India, and Italy, indicating the circulation of such strains. Challenge protection experiments demonstrated that SS1901 caused 100% mortality and 80% tu-mor incidence in SPF chickens, with tumors primarily in the heart, spleen, and liver. HVT, 814 and CVI988 vaccines provided 46.7%, 80% and 73.3% protection, respectively. Combination of CVI988 and HVT rendered the highest protection at 86.7%. Conclusions This study reports new mutation sites in the Meq gene of a prevalent Chinese MDV strains and confirms the isolate SS1901 could overcome existing vaccine-induced immunity, providing valuable new information for monitoring new MDV field strains and developing novel control strategies.