Disease Information (13) (Provided by Prion Molecular Biology Lab, Foreign Animal Diseases Division) ( 07/05/11 ) | |||||
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Part | Charger | leeyh | date | 07/05/11 | |
Scrapie
1. Definition
Scrapie is a transmissible spongiform encephalopathy of sheep and goats, characterized by a long incubation period followed by progressive degenerative disease of the nervous system and death. The disease was first recognized in 1730 and became an endemic problem in several countries.
Scrapie is caused by an unconventional agent, that consists of an arrangements of an altered host protein, prion, which can cause modification of the same protein when it is produced by the host thus increasing the amount of the pathogenic protein present in the host cells.
2. Host range/Transmission
Sheep and goats are susceptible to scrapie. However, while breeds of sheep vary significantly in their susceptibility to the disease, goat breeds appear to be universally susceptible. While spongiform encephalopathies occur naturally in humans, there is no evidence that the scrapie agent is transmissible to humans.
3. Geographical distribution
Scrapie is present in several European countries, especially the United Kingdom; in Canada and the United States; and in Iceland, India, Japan and Brazil. There have been isolated reports of scrapie from a number of countries, including Australia (1952), New Zealand (1954) and the Republic of South Africa(1972). In these instances the disease was confined to imported sheep and was eradicated by destruction of the affected group.
4. Clinical signs/Incubation period
The earliest signs of disease are reduced exercise tolerance, followed by the development of an unsteady gait. Animals go to water frequently but drink little and begin to rub, especially the poll, the buttocks and the rump. After about two months, animals start to lose condition, are ataxic and become rapidly fatigued. They are excitable and signs of localised rubbing are obvious from loss of wool or hair. A nibbling response can be elicited by rubbing alongside the spine over the rump. Often a popular rash appears on haired parts of the skin. By three or four months after the first signs, animals are severely affected showing marked muscle wastage, and are confused and agitated. Finally, during the next two to four weeks they become unable to stand and die.
5. Gross lesions / Diagnosis
There are no characteristic gross pathological changes but histopathology reveals spongiform changes in the brain and characteristic structures called scrapie-associated fibrils(SAFs) can be identified by electron microscopy. Currently laboratory diagnosis is based on histological changes in the brain. Laboratory examination of tissues collected at postmortem examination is essential to confirm a diagnosis. However, affected animals have characteristic clinical signs. The long incubation period results in clinical signs usually appearing when animals are between two and five years of age.
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