Submitted to: American Association of Veterinary Laboratory Diagnosticians
Publication Type: Abstract Only
Publication Acceptance Date: 11/5/2005
Publication Date: 11/5/2005
Citation: Greenlee, J.J., Kunkle, R.A., Hamir, A.N. 2005. Experimental intracerebral and oral inoculation of scrapie to swine: preliminary report [abstract]. Proceedings of the American Association of Veterinary Laboratory Diagnosticians 48th Annual Conference. p. 38.
Technical Abstract: Transmissible spongiform encephalopathies (TSEs, prion diseases) are chronic neurodegenerative diseases that occur in humans, cattle, sheep, goats, cervids, and a number of laboratory animal models. In a laboratory setting, the host range of a given TSE can be tested by inoculating animals with brain tissue from affected animals through various routes including oral and intracranial. There is no evidence of the natural occurrence of any form of TSE in the pig, but pigs have been shown to be susceptible to bovine spongiform encephalopathy (BSE) infection by multiple-route parenteral challenge. However, pigs orally exposed at eight weeks of age to large amounts of brain from cattle clinically affected with BSE did not support infection after seven years of observation. In the United States, feeding of ruminant by-products to ruminants is prohibited, but feeding of ruminant materials to swine and poultry still occurs. The potential for swine to have access to scrapie-contaminated feedstuffs exists, but the potential for swine to serve as a host for replication/accumulation of the agent of scrapie is unknown. The purpose of this study was to perform oral and intracerebral inoculation of the U.S. scrapie agent to determine the potential of swine as a host for the scrapie agent and their clinical susceptibility. This study utilized 26 swine randomly divided into three groups: controls (n=6), oral inoculates (n=8), and intracranial inoculates (n=12). Brain homogenate (10%) derived from scrapie-affected sheep was given by a single intracranial injection of 0.75 ml or by oral inoculation of 15 ml on four consecutive days. Scrapie inoculum was derived from clinically ill sheep inoculated with material derived from 13 sheep in seven source flocks. A sample of this material was also inoculated back into sheep to assure infectivity. Necropsies were planned for six months post inoculation, at approximately the time the pigs were expected to reach market weight. Samples collected were examined microscopically after routine staining (hematoxylin and eosin) and staining by standard immunohistochemical methods for prion protein (PrP**Sc). After approximately six months incubation time, no histologic lesions suggestive of spongiform encephalopathy or immunohistochemical evidence of prion infection were obtained. No evidence of scrapie infection was demonstrated in this short-term study, but positive results after an incubation period of only six months would be uncharacteristic. The only TSE with an incubation of six months or less known at this time is transmissible mink encephalopathy in mink, skunk, or raccoon hosts. However, scrapie in the raccoon model has a two-year incubation period. A replicate of littermate pigs has been inoculated and will be studied after long-term (3-7 years) incubation, and a similar study is underway with pigs inoculated with material derived from elk, mule deer, and whitetail deer affected by chronic wasting disease (CWD).