ARS | Publication request: Identification of atypical scrapie in Canadian sheep

Submitted to: Canadian Veterinary Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 2009
Publication Date: May 1, 2010
Citation: Mitchell, G.B., Orourke, K.I., Harrington, N.P., Soutyrine, A., Simmons, M.M., Dudas, S., Zhuang, D., Laude, H., Balachandran, A. 2010. Identification of atypical scrapie in Canadian sheep. Canadian Veterinary Journal. 22(3):402-408.

Interpretive Summary: Scrapie is a fatal brain disease of sheep, occurring in most sheep producing areas of the world. The disease occurs as a classical form, infectious within a flock and largely associated with particular forms of the prion gene, and an atypical form, usually found in older animals and often with no clinical signs of disease. There is as yet no evidence for transmission of atypical scrapie Intense eradication efforts in Europe and North America include random testing of clinically normal animals collected at slaughter facilities and testing of sheep dying of unknown causes on farms. The Canadian National Scrapie Surveillance program, through the national and international (Office of International Epizootics) reference laboratory in Ottawa, has identified three cases of atypical scrapie in Canadian sheep. These sheep had genetic, biochemical and histologic profiles similar to those observed in the United States, European, and British cases of Nor98. The finding of atypical scrapie in Canadian sheep was not unexpected and demonstrates the value of the current surveillance program in identifying both classical and atypical scrapie. Coordination of the testing programs in Canada and the U.S. allows both governments to gather comparable data on the nature and extent of Nor98 in sheep flocks in North America and develop harmonized control programs.

Technical Abstract: Scrapie, a transmissible spongiform encephalopathy of sheep and goats, exists in most small ruminant producing countries of the world. An atypical form of this disease, originally termed Nor98, was discovered in large abattoir surveillance of clinically normal, predominantly older sheep and rarely in clinical suspects. Nor98 is presumptively diagnosed by the unconventional findings in enzyme linked immunosorbent and immunohistochemistry assays of the abnormal prion protein in the brain and lymph nodes. Nor98 is usually differentiated from classical scrapie by Western blot analysis and the unique biochemical profile following passage in transgenic mice carrying the ovine prion gene. The Canadian National Scrapie Surveillance Program was initiated in 2005 and involves the active testing of sheep over 12 months of age which are slaughtered at federal and provincial abattoirs, or are identified as fallen stock from rendering companies, sales barns, cull ewe feedlots or farms. This report describes the first three cases of atypical scrapie detected in Canadian sheep. Two of the animals were older than 5 years of age and apparently clinically normal prior to sampling. Two of the sheep carried a polymorphism at codon 141 frequently associated with atypical scrapie and one sheep had a genotype relatively resistant to classical scrapie. Enhanced scrapie surveillance efforts have identified three cases of atypical scrapie in Canada. The classical form of scrapie has been extensively studied, with surveillance and breeding programs currently aimed at reducing disease prevalence. The existence of atypical scrapie in the Canadian sheep flock is not unexpected and additional cases will undoubtedly arise during continued scrapie surveillance efforts. Rapid identification and differentiation of these cases is necessary to understand the national prevalence of atypical scrapie and complement the Canadian initiative to control classical scrapie.