Presence of wild type Aujeszky's disease virus in swine identified as subclinical low-prevalent serological test reactors within qualified virus-negative herds.

Subclinical low-prevalent Aujeszky's disease (AD) serological test reactors are defined as those few swine within a qualified AD virus (ADV)-negative herd that have antibodies to wild type virus. However, clinical signs of the associated disease are not observed in these putatively infected swine or elsewhere in the herd. Twelve such animals, including 7 previously vaccinated with a genetically modified ADV, were identified in Illinois (USA) during a 2.5 year period. The humoral immune responses of the 5 nonvaccinated swine were assessed by an enhanced virus neutralization test and a radioimmunoassay. Anti-ADV antibodies were determined to be present in the serum from 4 of these swine. Attempts to isolate ADV by in vitro and in vivo inoculations of cell cultures and weanling mice, respectively, of tonsillar and trigeminal nerve ganglionic tissue preparations from each animal (vaccinated and nonvaccinated) were unsuccessful. Tonsillar and trigeminal nerve ganglionic tissues of each animal were screened for the presence of wild type and/or vaccine viral genomes by a soluble polymerase chain reaction (PCR) coupled with Southern hybridization. Unique PCR primers were used to distinguish between wild type and vaccine viral DNAs. An additional PCR procedure, which amplifies a portion of the essential and highly conserved viral gp50 gene, also was employed in an effort to detect viral genomes. Wild type viral DNA was found in the tissues from at least 5 of the vaccinated and 3 of the nonvaccinated swine. These results indicate that such animals should be considered as being infected with ADV. Further, these findings emphasize the need to develop highly specific and sensitive antemortem testing methods for accurate assessment of ADV infection in herds containing such subclinical, yet serologically positive, swine.