Molecular Epidemiologic Survey On Marek's Disease Virus In Some Districts Of Sichuan Province

Marek's disease (MD) is a common lymphoproliferative disease of chickens, usually characterized by lymphoid neoplasms form in all kinds of organs and tissues, and is also an economically important viral disease of chickens, and its pathogen is Marek's disease virus (MDV), belong to Mardivirus,α-Herpesvirinae. With the use of vaccines, the disease was well controlled, but field MDVs have shown continuous increasing in virulence and acquired the ability to overcome the immune response induced by vaccines. Nowadays, MD continues to be a serious threat to poultry industry, isolation and characterization of MDVs are essential for monitoring changes of viruses and evaluating the effectiveness of existing vaccines.Between 2008 and 2010,563 blood samples of 23 chicken farms were detected by PCR and 145 blood samples were positive, the average infection rate of field MDVs in chickens is 25.7%. The infection rate in HVT vaccinated broiler chickens is about 20%-60%, in HVT vaccinated breeder chickens is about 12.5%-42.5%, but almost no positive chickens were detected in CVI988 vaccinated breeder chickens. From 23 different chicken farms,18 field MDV strains were isolated from vaccinated chicken flocks in Sichuan province, China. Three oncogenic genes including Meq, pp38 and vIL-8 genes of the 18 isolates were amplified and sequenced successfully. Through alignment analysis of the oncogenic genes of the 18 isolates and the other reference strains published in GenBank utilizing DNAstar software, the homologies of the nucleotides of the three genes were 97.7-99.6%,99.4-100%and 99.0-99.5%; and the homologies of the amino acids of the three genes were 95.0-98.8%,99.3 -100%and 97.0-98.5%, respectively. In addition, Alignment analysis of the amino acid sequences of the three oncogenic genes showed that four amino acid mutations in Meq gene (position 80,115,139 andl76) and two amino acid mutations of in vlL-8 gene (position 4 and 31) were unique in MDVs circulating in some regions of Sichuan, which could be considered as unique features of field MDVs prevalent in some regions of Sichuan. Phylogenetic analysis based on Meq and vIL-8 amino acid sequences revealed that the 18 isolates in Sichuan, together with field MDVs in other regions of China generated an independently evolutionary direction. Viral virulence study and vaccine protective test showed that the 18 isolates in Sichuan belonged to highly virulent strains and the vaccine CV1988 can induce effective immunity in chickens.In conclusion, the study and other published data in the GenBank have demonstrated the main characteristics of Meq, pp38 and vIL-8 genes of MDVs circulating in recent years in Sichuan, China. In the three genes, mutations, deletion or insertion were observed, even some mutations could be considered as the molecular marks of the MDVs circulating presently in some regions of Sichuan. The paper supplies some valuable information not only for the moleculer features of MDV but also for control and prevention of MD in Sichuan.