Isolation and pathogenic analysis of virulent Marek's disease virus field strain in China

Abstract

Marek's disease (MD) has become increasingly common in China, resulting in considerable economic loss. The etiological agent is unclear. In this study, we isolated a field MD virus (MDV) strain, designated SX1301, from CVI988/Rispens-vaccinated chickens with tumors. Co-infection of avian leukosis virus, reticuloendotheliosis virus, and chicken infectious anemia virus was excluded by polymerase chain reaction, enzyme-linked immunosorbant assay, DNA blotting hybridization, and indirect immunofluorescence assay. As with most strains isolated in China, SX1301 had the same amino acid mutation of meq protein at positions 77(E), 80(Y), and 115(A) Animal experimental results showed development of lethal MD in 57% and MD tumor in 23% of the specific pathogen-free chickens inoculated with SX1301, with tumors mainly distributed in spleen, liver, and kidney. CVI988/Rispens protected 83% of chickens upon challenge with SX1301, with a mortality rate and tumor incidence of 10% and 7%, respectively. These results implicated SX1301 as a virulent MDV strain, with commercial MDV vaccine CVI988/Rispens unable to confer adequate protection against SX1301. There have been no reports of very virulent (vv) plus MDV in China, but frequently occurring virulent MDV may account for the repeated outbreaks of MD. Vaccines with greater efficacy are needed to protect against MDV.

Revisiting cellular immune response to oncogenic Marek’s disease virus: the rising of avian T-cell immunity

“…Therefore, it is believed that the wide use of MDV vaccines is driving the evolution of MDV field viruses toward greater virulence [1,[11][12][13][14]. Indeed, virulent or very virulent plus MDV (vv+ MDV) field strains were documented to break through the protection conferred by CVI988 vaccine clinically or experimentally [14][15][16][17][18], highlighting a worry of the lack of a better alternative to CVI988 to combat increasingly virulent MDV strains in the future. Unfortunately, progress in developing such vaccines has been slow, since most MDV vaccines under development are no more efficacious or safer than CVI988 [19,20].…”

Increased Horizontal Transmission of Recombinant Marek’s Disease Virus Due to Reticuloendotheliosis Virus Long Terminal Repeat Is the Major Competitive Advantage of the Virus Being a Prevalent Strain

“…Marek's disease has been prevalent among Chinese chicken flocks over the last 20 years (Cui et al, 2016;He et al, 2018). The incidence of MDV was highly heterogeneous in different regions and flocks because of the various geographical environments, different feeding patterns, and varieties of chicken breeds in China (Zhang et al, 2017).…”

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

“…However, vaccination failure of MD has often occurred in China in recent years [14,23,24,25,26]. Here, a field MDV strain BS/15 was isolated from a Chinese chicken flock in 2015 that had been vaccinated with the commercial vaccine CVI988 and had developed severe MD.…”

“…SD2012-1 has a long latent period and causes subsequent death with a mortality rate of 70.3% in unvaccinated SPF chickens; however, SD2012-1 can break through the protection provided by vaccine HVT (FC126 strain) or bivalent vaccine HVT plus SB-1, causing mortality rates of over 60% in vaccinated SPF chickens [23]. In another study, the CVI988 vaccine could only protect 83.0% of SPF chickens from SX1301 infection, despite the low mortality rate of 57.0% caused by SX1301 in unvaccinated SPF chickens [24]. In our previous study, the field MDV strain LTS induced a very low mortality (23.1% by 60 dpc) in unvaccinated chickens, but the vaccine CVI988 could not provide efficient protection (PI: 66.7) against LTS [14].…”