Abstract
Recently, chickens vaccinated with the CVI988/Rispens vaccine showed increased tumor incidence. Moreover, many strains of Marek’s disease virus (MDV) that were naturally integrated with the long terminal repeat (LTR) of the avian reticuloendotheliosis virus (REV) have been isolated, which means it is necessary to develop a new vaccine. In this study, two LTR sequences were inserted into Rispens to construct a recombinant MDV (rMDV). Then, the safety and efficacy of rMDV were evaluated separately in chickens. The growth rate curves showed that the insertion of REV-LTR into MDV enabled a faster replication in vitro than Rispens. Chickens immunized with high or repeated dose rMDV had no MD clinical signs. Further, no tumor, tissue lesions, or evident pathological changes were observed in the chicken organs. Polymerase chain reaction (PCR) and virus isolation revealed that rMDV had the ability to spread horizontally to non-immunized chickens and had no impact on the environment. After five passages in chickens, there were no obvious lesions, and the LTR insertion was stable. There were also no deletions or mutations, which indicates that rMDV is safe in chickens. In addition, rMDV has an advantage over Rispens against vvMDV Md5 at low doses. All results demonstrate that the transgenic strain of rMDV with REV-LTR can be used as a live attenuated vaccine candidate.
Highlights
Marek’s disease (MD), which is caused by the MD virus (MDV), causes major economic losses to the poultry industry [1]
CVI988/Rispens was preserved in a laboratory, and the vvMDV Md5 strain was provided by Professor Cui of Shandong Agriculture University
Marek’s disease virus (MDV) strain containing long terminal repeat (LTR) was isolated in the chicken feather follicles and named the RM1 virus [20]
Summary
Marek’s disease (MD), which is caused by the MD virus (MDV), causes major economic losses to the poultry industry [1]. The feather follicle epithelium of sick and infected chickens contains large quantities of MDV virions, which could cause environmental pollution [4,5]. MDV-infected chickens continuously release infectious virions that, when mixed with dust, spread through the air, increasing the risk of spreading MDV through flocks [5]. There are three types of variations in a virus: one caused by mutation of the nucleic acid sequence in the virus replication; one caused by an exchange of the genome segments between different viruses; and one involving gene recombination between viruses in different genera [6]. In the 1990s, Isfort et al discovered that MDV and reticuloendotheliosis virus (REV) can facilitate natural gene recombination when
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