The scrapie agent is extremely resistant to heat and to normal sterilization processes. 

 It does not evoke any detectable immune response or inflammatory reaction in host 

 m animals. The scrapie agent is thought to be spread most commonly from the ewe to 

 her offspring and to other lambs in contemporary lambing groups through contact 

 with the placenta and placental fluids. Signs or effects of the disease usually appear 

 2 to 5 years after the animal is infected but may take longer to appear. Sheep usually 

 live 1 to 6 months after the onset of clinical signs and in some cases longer, but death 

 is inevitable. 



On the farm, veterinarians diagnose scrapie based on the appearance of its signs 

 combined with knowledge of the animal's history. Signs of scrapie vary widely 

 among individual animals and develop very slowly. As the result of nerve cell damage, 

 affected animals usually show behavioral changes, tremor (especially of the head and 

 neck), pruritus, and locomotor incoordination, which progresses to recumbency and 

 death. Early signs include subtle changes in behavior or temperament. These changes 

 may be followed by scratching and rubbing against fixed objects, apparently to relieve 

 itching. Other signs are loss of coordination, weight loss despite retention of appetite, 

 biting of feet and limbs, lip smacking, and gait abnormalities, including high-stepping 

 of the forelegs, hopping like a rabbit, and swaying of the back end. 



An infected animal may appear normal if left undisturbed at rest. However, when 

 stimulated by a sudden noise, excessive movement, or the stress of handling, the 

 W animal may tremble or fall down in a convulsivelike state. 



Several other problems can cause clinical signs similar to scrapie in sheep, including 

 the diseases ovine progressive pneumonia, listeriosis, and rabies; the presence of 

 external parasites (lice and mites); pregnancy toxemia; and toxins. 



Testing — The only official tests currently used for scrapie in the United States 

 are histopathologic examination of brain tissue at necropsy or detection of the 

 abnormal prion protein by immunohistochemistry (IHC) on brain or third-eyelid 

 lymphoid tissue or Western blotting. Pathological changes of scrapie are confined 

 to the central nervous system. The lesions are characteristically found in the grey 

 matter of the brain stem. They include neuronal vacuolation, other forms of 

 neuronal degeneration, astrocytosis, and a vacuolar or spongy alteration called status 

 spongiosis. The third-eyelid test is the only APHIS-approved test for scrapie detection 

 in live animals. The test uses a biopsy of lymphoid tissue from the third eyelid 

 and IHC. 



Identifying Affected Animals — Animals that are incubating the disease and may 

 be shedding the agent are rarely identified until the onset of clinical signs. The 

 only absolute way to prevent an introduction of scrapie into a flock is to prohibit 

 ^ all movements of female sheep and goats into a flock. Until a highly sensitive, 



cost-effective live-animal test is available, the risk can be substantially reduced by 



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