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Tissue homogenates were obtained from swine experimentally co-infected with two vaccine strains of Aujeszky's disease virus (ADV). Viral isolates were derived by serial plaque purification directly from tissue homogenates, without an intervening step of isolation and amplification on cell cultures. 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 ADV. The tyhmidine kinase and glycoprotein X gene sequences were classified as wildtype or deleted, using a battery of polymerase chain reaction assays. On the basis of pairwise combinations of the allelic forms of the thymidine kinase and glycoprotein X genes, the isolates were characterized as recombinant and parental genotypes. The results substantiate the observation that ADV vaccine strains can form genetic recombinants in vivo after experimentally induced co-infection. A full description of this study is available (Am. J. Vet. Res. 54 (4) 540-545, 1993).

The natural development of Aujeszky's disease virus (ADV) vaccine-derived recombinants has been proposed as a conceivable, although unproven, outcome of the practice of using modified-live vaccines on infected herds. Herpesviral recombination has been studied in vitro using high multiplicity of infection (m.o.i.) to maintain synchronous co-infection of target cells and one-step multiplication cycles. However, natural in vivo recombination is unlikely to occur at high m.o.i. with synchronous infection of all available target cells by both parental virus strains. Using a battery of gene-specific polymerase chain reaction assays, an analysis of recombination frequency at comparatively low m.o.i. was performed, which indicates that at low m.o.i. the rate of in vitro genetic recombination can be expressed as a function of the number of virus-cell interactions. This finding indicates that the rate of generation of recombinant ADV genotypes in swine infected by multiple ADV strains would be modulated by factors that affect the distribution of different virus strains to common target cells, thereby limiting or enhancing the likelihood of cellular co-infection. A full description of this study is available (J. gen. Virol., in press).

The eradication of Aujeszky's disease virus (ADV) in Pennsylvania has suffered because the modes of viral transmission between herds are unknown. Our objective was to identify the major risk factors involved in viral transmission by comparing proportions of operation type, density, quarantine level, and vaccination status of neighboring herds for infected case and uninfected control herds. Multivariate analysis demonstrated an elevated risk of infection with local increases in the proportion of finishing floors. A local increase in the proportion of ADV-quarantined herds was shown to be a significant risk factor, as was an increase in the proportion of vaccinating herds. A reduced risk of infection was found with local increases in the number of feeder pig producer operations.