Exposure of Pigs to a Pseudorabies Virus Formed by in Vivo Recombination of Two Vaccine Strains in Sheep

Abstract

Several genetically altered pseudorabies virus (PRV) vaccines have been created in recent years. These vaccines contain deletions in the thymidine kinase (TK) virulence gene; some contain serodiagnostic glycoprotein (gX, gI, gIII) deletions as well .5,6,9,11 Recently, we demonstrated that a gXand TK-negative, beta-galactosidase (Bgal)-positive vaccine strain recombined with a gXand TK-positive, Bgal-negative vaccine strain in sheep to produce a TKand Bgal-positive, gX-negative progeny virus (Henderson LM, Katz JB, Erickson GA, Davis AJ: 1988, Abstr Conf Res Workers Anim Dis #313, p. 56). In that work, l0 plaque forming units (PFU) of both vaccine strains were mixed together in the same syringe and injected intramuscularly into 3 adult sheep. Each test animal thus received 2 x l0 PFU of the viral mixture; 4 control sheep received 2 x l0 PFU of only one or the other of the vaccine strains. All 3 test sheep exhibited classical neurological signs of PRV infection within 72-96 hours, whereas the control animals did not exhibit these signs. Brain and spinal cord suspensions were inoculated onto fetal porcine kidney cell cultures. Pseudorabies virus was identified by direct immunofluorescence during the first cell culture passage. Recovered PRV was then serially passaged 4 times in cell cultures selective for TK-positive virus recovery. Plaques expressing Bgal activity were identified by their blue color and were plaque purified twice in Bgal expression media. The resulting virus was characterized both phenotypitally and by restriction enzyme and DNA hybridization analyses as a TKand Bgal-positive, gX-negative PRV designated recombination-derived PRV (RD-PRV) (Henderson LM, et al.: 1988, Abstr Conf Res Workers Anim Dis #313). The present study was designed to examine the diagnostic serological and virological consequences of exposing young swine to RD-PRV. Twenty-four recently weaned swine were randomly assigned to 4 groups; each group contained 4 animals inoculated with RD-PRV and 2 sentinel control animals. In groups A and B, inoculated swine were injected intramuscularly with 10 and 10 PFU of RD-PRV, respectively. In groups C and D, the inoculated animals received 10 and l0 PFU, respectively, of RD-PRV intranasally.