Genomic analysis of a Chinese MDV strain derived from vaccine strain CVI988 through recombination

Disease caused by Marek's disease virus (MDV), a highly oncogenic alpha-herpesvirus, is controlled mainly by vaccination. Since 1990s, CVI988 has been widely used as vaccine strain. However, as an attenuated live vaccine, CVI988 has the potential of virulence enhancement and the risk of recombination that should be considered. In this study, we sequenced the whole genome of a Chinese strain HNLC503 and found the close relationship between HNLC503 and CVI988. Further study indicated that HNLC503 had undergone recombination in US region, the same position as that previously occurred in Eurasian strains isolated from 2010 to 2014. By comparing ORFs, it was found that non-synonymous mutations were introduced in US2, US3, SORF4 and gD genes by recombination, while natural mutations occurred in RLORF1, vIL-8, UL36, VP22 and gE, in HNLC503. In summary, our study revealed the phenomenon of MDV vaccine strain recombination, warning that vaccine strains have the potential to enhance virulence through recombination.