A Chinese Variant Marek’s Disease Virus Strain with Divergence between Virulence and Vaccine Resistance

Abstract

Marek’s disease (MD) virus (MDV) has been evolving continuously, leading to increasing vaccination failure. Here, the MDV field strain BS/15 was isolated from a severely diseased Chinese chicken flock previously vaccinated with CVI988. To explore the causes of vaccination failure, specific-pathogen free (SPF) chickens vaccinated with CVI988 or 814 and unvaccinated controls were challenged with either BS/15 or the reference strain Md5. Both strains induced MD lesions in unvaccinated chickens with similar mortality rates of 85.7% and 80.0% during the experimental period, respectively. However, unvaccinated chickens inoculated with BS/15 exhibited a higher tumor development rate (64.3% vs. 40.0%), but prolonged survival and diminished immune defects compared to Md5-challenged counterparts. These results suggest that BS/15 and Md5 show a similar virulence but manifest with different pathogenic characteristics. Moreover, the protective indices of CVI988 and 814 were 33.3 and 66.7 for BS/15, and 92.9 and 100 for Md5, respectively, indicating that neither vaccine could provide efficient protection against BS/15. Taken together, these data suggest that MD vaccination failure is probably due to the existence of variant MDV strains with known virulence and unexpected vaccine resistance. Our findings should be helpful for understanding the pathogenicity and evolution of MDV strains prevalent in China.

Marek's disease viruses circulating in commercial poultry in Italy in the years 2015–2018 are closely related by their meq gene phylogeny

“…There is a growing need for new MD vaccines that are efficacious against currently circulating viruses, due to the occurrence of breaks in vaccine immunity. Since vaccine failures are occurring worldwide (Abdallah et al, ; Abd‐Ellatieff et al, ; López‐Osorio et al, ; Sun et al, ), more studies must be conducted to evaluate the protection conferred by Rispens‐type vaccines against more recent, and not yet pathotyped, field strains with a history of high virulence. Naturally circulating low virulence strains could represent a solution if they offer improved protection over CVI988/Rispens vaccine.…”

Newly detected mutations in the Meq oncogene and molecular pathotyping of very virulent Marek’s disease herpesvirus in Tunisia

“…Due to mutations in the viral genome, the severity of disease varies among GaHV-2 isolates, leading to everchanging pathotypes that overwhelm vaccine protection, and efforts have been made to identify new emerging pathotypes in order to ensure vaccine protection [15]. Sporadic epidemics of MD have been reported worldwide, even in vaccinated flocks [16], indicating that the virulence of MDV has increased in recent decades, and some of the more recent isolates are more pathogenic to chickens than older isolates [17,18]. Thus, the increased use of vaccines has led the virus to evolve toward higher virulence, which can overcome the protection conferred by currently available vaccines [19].…”

Co-Infection with Marek’s Disease Virus and Reticuloendotheliosis Virus Increases Illness Severity and Reduces Marek’s Disease Vaccine Efficacy

“…Dramatic success has been achieved through the extensive use of MD vaccines, greatly reducing economic losses in the poultry industry [ 18 , 19 ]. Nevertheless, MD outbreaks have continued around the world in recent years, which is likely due to MDV evolution and co-infection with other viruses [ 20 , 21 , 22 , 23 , 24 , 25 ]. At present, the attenuated MDV strains CVI988 and 814 are considered the best vaccines against MD [ 26 , 27 , 28 ], and commercial CVI988 and 814 vaccines are widely used in China.…”