Aujeszky's disease virus (ADV) in mammalian wildlife on swine farms in Illinois (USA): Potential for transmission to non-infected herds

Abstract - After eradication of ADV from swine herds, non-porcine reservoirs can be potential sources for reintroduction of ADV. Vaccination of swine with modified live, gene-deleted vaccines, particularly intranasally (which promotes aerosol transmission), has the potential to immunize wildlife populations. Immunized wildlife can survive exposure to virulent wild-type ADV. Mammalian wildlife traveling between swine herds can also serve as vectors for between herd spread. The objective of this study was to determine the prevalence of infection of mammalian wildlife with wild-type and vaccine ADV on swine farms quarantined for ADV where vaccine was used, and to determine the potential for between herd spread of ADV by wildlife. Six swine farms in Illinois (USA) quarantined for infection with ADV were studied. All were in close proximity (maximum 30 km distance). Five farms studied in both 1996 and 1997 used modified live ADV vaccines with deletions for glycoprotein E (gE ^-), one dose administered intranasally at 3-4 weeks of age, and the other intramuscularly at 9-10 weeks. The sixth farm, studied only in 1997, vaccinated pigs intramuscularly at 9 weeks of age with a gE ^-gG ^- vaccine. Each farm was visited 6 times per year, between April and September. Overnight live trapping of wildlife (rodents and medium sized mammals) was conducted. Blood samples were obtained from all captured animals. Detection of anti-ADV antibodies was accomplished by using a serum neutralization (SN) test for sera from medium sized mammals and rats and a latex agglutination test (LAT) for sera from small rodents. All rodents were euthanized and their brains were harvested. Upon first capture, each medium sized mammal trapped was tattooed for identification and released. Upon recapture, if a medium sized mammal had been identified as seropositive for ADV on a previous trip, it was euthanized and its brain, trigeminal ganglia, and tonsils were removed. For all ADV seropositive animals, a polymerase chain reaction (PCR) for viral detection was performed on tissue samples (brains for rodents; brains, tonsils, and trigeminal ganglia for cats), using primers for glycoproteins C (gC) and E (gE). For each seropositive rodent, 4 matched (by trapping trip on the same farm) seronegative rodents were also tested using PCR. As another control, 6 rodents were selected from each of 2 trips on the only seronegative farm and tested using PCR. There were seropositive cats on 2 farms, house mice [Mus] on 3 farms, and field mice [Peromyscus] , rats , and rabbits on 1 farm each; 5 of the 6 farms had seropositive wild mammals. There were no seropositive samples for raccoons , opossums , or skunks . There were 3 seropositive cats that were recaptured and euthanized. Two were PCR ⁺ for gC and gE, indicating infection with wild-type ADV. One was PCR ⁺ only for gC, suggesting infection with gE ^- vaccine. Among the 64 individual cats, 8 (13%) were trapped on 2 farms, of which 2 were seropositive. The only seropositive rat was gC ⁺ and gE ^- by PCR, indicating infection with gE ^- ADV vaccine. Among the 11 seropositive mice, 9 (82%) were gC ⁺gE ⁺ (indicating infection with wild-type ADV) and 2 were gC ⁺gE ^- (indicating infection with gE ^- ADV vaccine). However, for the 40 matched seronegative mice on farms seropositive for wildlife, 38 (95%) were gC PCR ⁺, with 29 (73%) gE ⁺ (indicating wild-type ADV) and 8 (20%) gE ^- (indicating infection with gE ^- ADV vaccine). For the one farm that was entirely seronegative for wildlife, the 12 rodents tested were PCR ^-. All PCR ⁺ results were confirmed by Southern blot. It was not possible to isolate ADV from any of the animals that were tested by PCR. Cats and rodents living on swine farms may be infected with ADV, either with wild-type or vaccine genotypes. Thus, these species may serve as a reservoir for possible reintroduction of ADV to swine. Seropositive cats can travel between farms, indicating that cats may serve as a vector for the transmission of ADV between swine farms. A comparison of LAT and PCR results for rodents indicates that the prevalence of ADV infection in rodents may be much higher than estimated by serological testing. ADV eradication programs must consider the role of rodents as a reservoir for reintroduction of ADV into swine herds, and cats as a possible vector for transmission of ADV between herds.