lyansmissible Spongiform Encephalopathies 



General Introduction 



Transmissible spongiform encephalopathies (TSEs) are rare forms of progressive 

 neurodegenerative disorders that affect both humans and animals and are caused 

 by similar uncharacterized agents that generally produce spongiform changes 

 in the brain. Specific examples of TSEs include scrapie, which affects sheep and 

 goats; bovine spongiform encephalopathy, which affects cattle; transmissible mink 

 encephalopathy; feline spongiform encephalopathy; chronic wasting disease of mule 

 deer, white-tailed deer, black-tailed deer, and elk; and, in humans, a variant form of 

 Creutzfeldt-lakob disease. TSEs have also been repented in Europe in captive wild 

 ruminants, cats, and monkeys. 



TSEs are insidious degenerative diseases of the central nervous system. Historically, 

 the diagnosis of TSEs has been based on the occurrence of clinical signs of the 

 disease, which was confirmed only by postmortem examination of brain tissue. More 

 recently, identifying the presence of abnormal prion protein by various techniques 

 has improved the ability to make a disease diagnosis. 



A characteristic feature of all TSEs is the lack of a measurable host immune response 

 to the agent, meaning that no antibodies are produced. No conventional serologic 

 test can be used to identify infected animals. Scientists usually diagnose TSEs in the 

 laboratory by histopathologic examination of the brain followed by one or more 

 supplemental tests. 



Scrapie 



Scrapie is a fatal, degenerative TSE disease affecting the central nervous system of 

 sheep and goats. First recognized as a disease of sheep in Great Britain and other 

 countries of Western Europe more than 250 years ago, scrapie has been reported 

 tlnoughout the world, hi the United States, scrapie has primarily been reported in 

 the Suffolk breed. It also has been diagnosed in numerous other breeds, in several 

 crossbreeds of sheep, and in goats. The first case of scrapie in the United States was 

 diagnosed in 1947 in a Michigan flock. From this first case through lanuary 2005, 

 scrapie has been diagnosed in more than 1,295 flocks in this country. Through 

 lanuary 2005, about 2,678 cases in sheep and 14 cases in goats have been reported. 



The agent responsible for scrapie and other TSEs is smaller than the smallest known 

 virus and has not been completely characterized. Three main theories have been 

 advanced on the nature of the scrapie agent: ( 1 ) the agent is a prion, an exclusively 

 host-coded protein that is modified to a protease-resistant form after infection; 

 (2) the agent is a virus with unusual characteristics; and (3) the agent is a virino, a 

 small, noncoding regulatory nucleic acid coated with a host-derived protective protein. 



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