Molecular and biological characterization of a Marek's disease virus field strain with reticuloendotheliosis virus LTR insert

A Marek's disease virus (MDV) field strain designated GX0101 was isolated from a layer flock and confirmed to be a recombinant virus with an insert of a long terminal repeat (LTR) from the reticuloendotheliosis virus (REV). A chimeric molecule containing an REV-LTR insert of 539 bp and its flanking sequences from MDV was amplified and sequenced. An REV-LTR downstream from the Internal Repeat Short (IRS) region has 77.4-98.6% homology to seven REV field strains isolated from different avian species in different parts of the world. The insertion site is located downstream of SORF 1 and upstream of SORF2 in the IRS region near the junction with the Unique Short (US) region in the MDV serotype 1 genome. Chicken experiments were conducted to determine the oncogenicity of the recombinant GX0101 virus and its transmissibility to contact chickens. Dot blot hybridization was used to detect the presence of the pp38 gene in feather tips from GX0101 or Md5 infected and contact birds. The pp38 was detected in GX0101 contact birds about 1-2 weeks earlier than in Md5 birds when both groups were vaccinated with HVT vaccine. Long term pathogenicity tests in specific pathogen free (SPF) chickens reveal that the recombinant GX0101 has a higher virulence than GA, but less virulence than Md5, the very virulent pathotype of MDV. This is the first report on an oncogenic serotype 1 MDV field strain with LTR insert and its pathogenicity.