|WEST GREENLEE, M - Iowa State University|
Submitted to: Food Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2016
Publication Date: 12/22/2016
Citation: Greenlee, J.J., Kunkle, R.A., Smith, J.D., West Greenlee, M.H. 2016. Scrapie in swine: a diagnostic challenge. Food Safety. 4(4):110-114.
Interpretive Summary: Scrapie is a fatal disease of sheep and goats that causes damaging changes in the brain. The infectious agent is an abnormal protein called a prion that has misfolded from its normal state. Whether scrapie can transmit to other livestock such as swine is unknown. This study evaluated the potential of pigs to develop scrapie after either intracranial or oral inoculation. Our data indicates that swine do accumulate the abnormal prion protein associated with scrapie if allowed to incubate for long periods of time (51 months) after intracranial inoculation. While pigs inoculated orally did not have positive proof of scrapie infection when brain tissues were subjected to traditional diagnostic methods, bioassay of brain material in transgenic mice expressing the porcine prion protein were positive. This finding demonstrates that there is scrapie-associated infectivity in the brains of pigs that were exposed orally to the scrapie agent. Currently, swine rations in the U.S. could contain animal derived components including materials from sheep and goats. This information is useful to individuals in the sheep and goat and swine industries. These findings could impact future regulations for the disposal of offal from sheep and goats and U.S. regulators should carefully consider the new information from this study before relaxing feed ban standards designed to control potentially feed borne prion diseases.
Technical Abstract: A naturally occurring prion disease has not been recognized in swine, but the agent of bovine spongiform encephalopathy does transmit to swine by experimental routes. Swine are thought to have a robust species barrier when exposed to the naturally occurring prion diseases of other species, but the susceptibility of swine to the agent of sheep scrapie has not been thoroughly tested. We conducted this experiment to test the susceptibility of swine to U.S. scrapie isolates by intracranial and oral inoculation. Scrapie inoculum was a pooled 10% (w/v) homogenate derived from the brains of clinically ill sheep from the 4th passage of a serial passage study of the U.S scrapie agent (No. 13-7) through susceptible sheep (homozygous ARQ at prion protein residues 136, 154, and 171, respectively). Pigs were inoculated intracranially (n=19) with a single 0.75 ml dose or orally (n=24) with 15 ml repeated on 4 consecutive days. Necropsies were done on a subset of animals at approximately six months post inoculation (PI): the time the pigs were expected to reach market weight. Remaining pigs were maintained and monitored for clinical signs of TSE until study termination at 80 months PI or when removed due to intercurrent disease (primarily lameness). Brain samples were examined by immunohistochemistry (IHC), western blot (WB), enzyme immunoassay (EIA), and for a subset of pigs in each inoculation group, bioassay in mice expressing porcine prion protein. At six-months PI, no evidence of scrapie infection was noted by any diagnostic method. However, at 51 months of incubation or greater, 5 animals were positive by one or more methods: IHC (n=4), WB (n=3), or EIA (n=4). Furthermore, positive bioassay results were obtained from all inoculated groups (oral and intracranial; market weight and end of study) suggesting that swine are potential hosts for the agent of scrapie.