Emerging natural recombinant Marek's disease virus between vaccine and virulence strains and their pathogenicity

Abstract

Marek's disease virus (MDV), an oncogenic virus belonging to the subfamily Alphaherpesvirinae, causes Marek's disease (MD). Vaccines can control MD but cannot block the viral infection; they are considered imperfect vaccines, which carry the risk of recombination. In this study, six natural recombinant MDV strains were isolated from infected chickens in commercial flocks in China. We sequenced and analysed the genetic characteristics of the isolates (HC/0803, CH/10, SY/1219, DH/1307, DH/1504 and Hrb/1504). We found that the six strains resulted from recombination between the vaccine CVI988/Rispens (CVI988) strain skeleton and the virulence strain's partial unique short region. Additionally, a pathogenicity study was performed on recombinant strains (HC/0803 and DH/1307) and reference strains (CVI988 and LHC2) to evaluate their virulence. LHC2 induced 84.6% mortality in infected chickens; however, no mortality was recorded in chickens inoculated with HC/0803, DH/1307 or CVI988. However, HC/0803 and DH/1307 induced a notable spleen enlargement, and mild thymus and bursa atrophy at 11–17 days post‐challenge (dpc). The viral genome load in the HC/0803‐ and DH/1307‐challenged chickens peaked at approximately 107 viral copies per million host cells at 17 dpc and was similar to that in LHC2‐challenged chickens, but significantly higher than that of CVI988‐challenged chickens. In summary, HC/0803 and DH/1307 displayed mild virulence with temporal damage to the immune organs of chicken and a higher reproduction capability than the vaccine strain CVI988. Our study provides direct evidence of the emergence of recombinant MDV strains between vaccine and virulence strains in nature. The emergence of natural recombinant strains suggests that live vaccines can act as genetic donors for genomic recombination, and recombination may be a safety concern when administering live vaccines. These findings demonstrate that recombination promotes genetic diversity and increases the complexity of disease diagnosis, prevention and control.

Comparative Analysis of Multiple Consensus Genomes of the Same Strain of Marek’s Disease Virus Reveals Intrastrain Variation

“…Secondly, the 16 unique SNPs in US of CVI988 GenBank.BAC impacting US2, US3, SORF4 and US6 / gD have been proposed as evidence of naturally occurring recombination between CVI988 and virulent strains (63). To date, a total of seven isolates from China have been described as natural recombinants of CVI988, largely on the basis of these 16 SNPs (64)(65)(66). The two new consensus genomes of CVI988 generated as part of this study share the same nucleotides in all 16 positions with these isolates, calling into question whether they are in fact natural recombinants of CVI988.…”

“…The copyright holder for this preprint this version posted September 4, 2023. ; https://doi.org/10.1101/2023.09.04.556264 doi: bioRxiv preprint US6 / gD have been used as the main evidence of naturally occurring recombination between CVI988 and virulent strains, starting in 2018 by He et al (59). Multiple studies have followed, resulting in a total of seven isolates from China being described as natural recombinants of CVI988 on the basis of these 16 SNPs (60)(61)(62). Our two new consensus genomes of CVI988 share the same nucleotides in all 16 positions with these seven isolates, implying that they are likely to be field isolates of CVI988 and not natural recombinants as previously described.…”

Coinfection in the host can result in functional complementation between live vaccines and virulent virus

“…Recombination was not seen in these experiments, but we cannot exclude the potential for recombination in the natural setting. In fact, it was recently shown that recombination occurs in nature between MD vaccines and circulating virulent strains in China [ 2 ]. Here, we showed that both MDV and MD vaccines dually infect FFE skin cells during coinfection confirming the dual infection of cells occurs in vivo that is prerequisite for recombination or complementation to occur.…”

“…Apart from mutations incurred during replication, recombination and complementation are two mechanisms in which viruses can evolve to greater virulence, altered host adaptation, or pathogeneses. The risk of vaccines recombining in hosts to create virulent virus has long been raised and shown to occur in nature using poultry vaccines as a model [ 1 , 2 ]. Complementation – the interaction between viral gene products or gene functions during coinfection that results in increased or decreased yield of parental viruses [ 3 ] - by mixed or coinfections has been demonstrated in bacteriophage [ 4 ] and plant viruses [ 5 ] in nature, but thus far not demonstrated for animal viruses outside of cell culture [ 6 ] or non-natural experimental model systems [ 7 ].…”

Immunopotentiating effect of lentinan on chicks and its inhibitory effect on Marek's disease virus infection

“…Based on the serologic response of the virus, MDV is classified into three serotypes, MDV 1, MDV 2, and MDV 3, and 4 pathotypes of serotype 1 MDV are currently recognized: m (mild), v (virulent), vv (very virulent), and vv+ (very virulent plus) ( Lee et al, 1983 ; Witter, 1997 ; Witter et al, 2005 ). Recent epidemiological investigations have shown an increased prevalence of wild strains of MDV1 in China ( Zhang et al, 2022 ; Yu et al, 2023 ). Vaccine immunization is the most effective means of controlling MDV infection ( Nair, 2004 ).…”