Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 30, 2010
Publication Date: July 18, 2010
Citation: Zanella, E.L., Lager, K.M., Miller, L.C., Swenson, S.L., Bigelow, T.T., Kehrli, Jr., M.E. 2010. Pathogenesis and Transmission of Feral Swine Pseudorabies Virus Isolates. In: Proceedings of the International Pig Veterinary Society Congress, July 18-21, 2010, Vancouver, Canada. p. 875. Technical Abstract: Introduction. Aujesky’s Disease or pseudorabies, is one of the oldest recognized swine diseases. It is caused by pseudorabies virus (PRV), an alpha-herpesvirus that can induce respiratory disease, reproductive failure, and affect the central nervous system. PRV vaccines, in conjunction with serologic tests, have been used to eradicate the virus from swine herds and even from entire countries. One weakness in PRV control and eradication programs is the indigenous PRV infection of feral swine. This wild-life reservoir is responsible for frequent infection of domestic swine (1). A better understanding of this source of infection may lead to improved PRV control programs. This report summarizes studies investigating the pathogenicity and transmissibility of feral swine PRV isolates from the USA. Materials and Methods. Seventy-three four-week-old pigs were randomly allocated to one of nine treatment groups, each housed in a separate isolation room. Pigs in each group received either a low dose (LD - about 10**3CCID50) or high dose (HD - about 10**6CCID50) intranasal challenge of virus: Group 1(n=7) LD feral swine isolate FS268; Group 2 (n=7) HD FS268; Group 3 (n=7) LD feral swine isolate C3R Ossabaw Is; Group 4 (n=7) HD C3R Ossabaw Is; Group 5 (n=7) LD domestic swine isolate ISUVDL4892; Group 6 (n=7) HD ISUVDL4892; Group 7 (n=10) LD panther isolate of a feral swine virus FP117-05; Group 8 (n=10) HD FP117-05; Group 9 (n=11) controls that received three ml of cell culture media. Nasal swabs (NS) were collected on days 0, 2, 4, 7, 9, 11, 14, 21, and at necropsy 28 days-post-inoculation (dpi) at which time blood, lung lavage and tissues were collected for virus isolation and detection of viral DNA by real-time PCR (2). On 2 dpi, ten direct contact pigs (DC) were comingled with the pigs inoculated with HD C3R Ossabaw Is isolate, and five DC pigs were comingled with the pigs inoculated with HD FP117-05. After five days of contact, each group of DC pigs were moved to a clean isolation room and maintained for about 16 weeks. Results. Clinical signs: Mean body temperature spiked 2 dpi in all PRV inoculated groups and returned to normal by 5-7 dpi. For each virus pair the HD pigs were more affected than LD pigs. By 1 dpi some pigs were lethargic and at 2 dpi respiratory distress (sneezing, dyspnea, dysphonia (altered squeal (laryngeal edema)), congested nasal passages, and dog sitting) was observed. Transient anorexia was detected in most pigs. Central nervous system disease was observed in some pigs (ataxia, opisthotonos, paddling, and seizure). Most pigs from the feral swine isolate groups recovered; however, six out of seven group 6 pigs were euthanized or died in the first 7 dpi. In group 4, two pigs and in group 1, one pig were euthanized in the first 8 dpi. Clinical signs in DC pigs exposed to C3R Ossabaw Is isolate were mild. In contrast, all DC FP117-05 exposed pigs had moderate to severe respiratory disease, but each pig did recover. Virology: Virus was isolated from NS from 2-14 dpi with most positives between 2-9 dpi, and more frequently from the HD than LD challenge groups. Virus was isolated infrequently from 28 dpi tissues. Serology: Based on a commercially available ELISA, all PRV-inoculated pigs seroconverted by 14 dpi. Likewise, all DC pigs sera-converted by 14 days post contact and were seropositive throughout the duration of the study, about four months. Discussion. Collectively, results from these studies indicate natural infection of domestic swine with feral swine PRV isolates should be detected either through detection of virus during the acute phase of the infection, or through detection of antibody in the convalescent phase of the infection. This conclusion supports many current control programs that screen sera for PRV-specific antibody, and may test tissues for virus by real-time PCR. Acknowledgements. We thank Dr. Romero from the University of Florida for the feral swine isolate FS268, Dr. Stallknecht from the Southeastern Cooperative Wildlife Disease Study for the feral swine isolate C3R Ossabaw Is and the panther isolate FP117-05, and Dr. Yoon from Iowa State University for the domestic swine isolate ISUVDL4892. Funding was provided by USDA Veterinary Services and Agricultural Research Service. References: 1. Wyckoff, AC, et al., J Wildlife Dis 2009:45;422–429. 2. Ma, W, et al., J Vet Diagn Invest 20:440-447.