Effects On Replication,Shedding,and Meq Protein Expression In Wild Virus Of SC9-1 And CVI988 Vaccines Of Chicken Marek’s Disease

Marek’s disease(MD)is a highly contagious disease caused by Marek’s disease virus(MDV),which is characterized by the proliferation of lymphoid tissue proliferation in chickens.The clinical symptoms typically include neurological signs and visceral tumors,causing sugnificant economic losses to the poultry industry.Vaccination is a key measure to prevent and control MD.Over the past few decades,CVI988 has been widely used as vaccine strain,however,with increasing virulence of pathogenic MDV strains,there have been a few cases of virus breakout following vaccination,the continuous evolution of MDV has forced the development of new vaccines to control new and more virulent strains.SC9-1 is a recombinant vaccine prepared by our laboratory based on MDV GX0101 strain lacking the Meq oncogene and the kanamycin resistance gene.Previous studies have shown that SC9-1 provides 100%protection against vv MDV Md5 strain in SPF chickens,while CVI988 provides only 89%protection.SC9-1 provides 100% protection against vv MDV GX0101 in SPF chickens and Hailan Brown chickens,while CVI988 only provides 86.7% and 93% protection.To compare the protective effects of SC9-1 and CVI988 vaccines on Hailan Brown chickens,in this study,one-day old Hailan Brown chickens were vaccinated with SC9-1 and CVI988,respectively,and challenged with v MDV SDAU1501,to detect the effects of two vaccines on the replication,shedding with virus,and Meq gene of MDV,and the protective effects of two vaccines on chickens infected with SDAU1501 strain were compared systematically.Hailan Brown chickens of one-day-old were randomly divided into 8 groups(15in each group)and sampling,weighing,and necropsy were performed at each time point.The results of mortality and tumor incidence showed that both vaccines provide 100% protection against SDAU1501 infection in chickens.The body weight results showed that the chickens in the challenge group grew slowly and developed unevenly.Both vaccines can reduce the inhibitory effect,and chickens in the challenge group after vaccination with SC9-1 have significantly higher body weight than those in the challenge group after vaccination with CVI988 at 35 days post infection,the results indicated that SC9-1 provide better protection from the growth inhibition effect of MDV than CVI988.The liver and spleen in the challenge group were severely enlarged,and the organ index was significantly higher than other groups.The atrophy of the thymus and bursa of Fabricius was significantly different from other groups.Both vaccines can lessen the effect of the MDV on organ index,moreover,the degree of the liver and spleen enlargement,and the degree of thymus and bursa atrophy from the chicken vaccinated with CVI988 and challenged with MDV had higher than chickens vaccinated with SC9-1.The detection results of MDV load in lymphocytes showed that the virus load in the chickens from challenge group was significantly higher than other groups,and both vaccines could inhibit the replication of MDV in lymphocytes.In addition,the number of copies of MDV in chickens vaccinated with SC9-1 was significantly lower than that in chickens vaccinated with CVI988,indicated that SC9-1 can better inhibit the replication of MDV in vivo.In the detection of feather follicle shedding with virus,the number of virus copies in the challenge group after vaccinated with SC9-1 was significantly lower than in the challenge group after vaccinated with CVI988 at different time points.SC9-1 can better reduce the replication and shedding of SDAU1501 with the feather follicle than CVI988.To further compare the effects of two vaccines on the expression of the MDV oncogene Meq and Meq protein,the expression of Meq gene in lymphocytes at different time was detected by real-time quantitative PCR,the expression of Meq protein in infected chickens by Western-Blot assay.The detection results of oncogene Meq and Meq protein showed that the expression levels of Meq gene and protein in the challenge group after vaccination with SC9-1 were lower than chickens vaccinated with CVI988 and challenge with MDV.The results of this study indicate that compared to CVI988 vaccine,SC9-1 vaccine can significantly reduce the growth and development inhibition and immunosuppressive effects caused by MDV infection,while effectively inhibiting the replication of MDV in vivo and virus shedding in feather follicle,and significantly reducing the expression of Meq genes and Meq proteins.