Direct isolation and identification of recombinant pseudorabies virus strains from tissues of experimentally co-infected swine.

Tissue homogenates were obtained from swine co-infected with 2 vaccine strains of pseudorabies virus (PRV). Viral isolates derived by serial plaque purification directly from tissue homogenates, without an intervening step of isolation and amplification on cell cultures, were characterized as recombinant and parental PRV genotypes on the basis of thymidine kinase and glycoprotein X gene combinations. Use of limiting dilutions and recovery of virus isolates as individual plaques minimized the likelihood of in vitro recombination serving as a confounding source of recombinant PRV. The thymidine kinase and glycoprotein X gene sequences were classified as wild-type or deleted, using a battery of polymerase chain reaction assays. Results substantiate the observation that PRV vaccine strains can form genetic recombinants in vivo after experimentally induced co-infection.