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| SCRAPIE:
GET THE FACTS
KEY FACTS 1. Scrapie is an infectious disease of sheep and goats. 2. Scrapie is not transmitted to humans. 3. Kuru and Creutzfeldt-Jakob disease are human diseases similar
to scrapie but not related. 4. Bovine Spongiform Encephalopathy (BSE) is a cattle disease that
is similar to scrapie in sheep. 5. Scrapie
is caused by a transmissible agent, probably an abnormal form of the prion
protein called PrPSc. The scrapie transmissible agent is highly
resistant to heat and common disinfectants. 6. While a scrapie-specific protein, the (protease resistant)
PrPSc, has been identified as the causative agent, there appears to be a
genetic interaction mechanism with the presence of the prion. 7. Scrapie does not stimulate the immune system: therefore,
there is no current blood test for preclinical diagnosis. 8. Vertical transmission (to the lamb while it is in the uterus)
is unlikely but not yet proven. 9. The highest known prevalence of scrapie transmission is from
the ewe to the lamb (lateral transmission) during the first six to nine
months of life. 10. Scrapie is not a genetically transmitted disease but rather
an infectious disease with genetically influenced susceptibility. 11. A gene has been identified which may control the length of
the incubation period or susceptibility or code for the scrapie protein (PrPSc). WHAT IS SCRAPIE? Scrapie
is an infectious disease of sheep and goats
which causes a degeneration of the central nervous system resulting
in a variety of behavioral, locomotive changes and death. Scrapie is a
member of a family of diseases called Transmissible Spongiform
Encephalopathies (TSEs). How DID IT START? Scrapie was first recognized as a disease of sheep in Great Britain and other countries of Western Europe over 200 years ago. Although the worldwide distribution is unknown, Australia and New Zealand are the only major sheep-producing countries recognized as being scrapie-free. In 1947, the first case of scrapie was diagnosed in the United States in sheep originating from Britain via Canada. From this initial case until September 30, 1996, scrapie has been diagnosed in 1281 sheep in 850 flocks throughout the U.S. ARE THERE OTHER
RELATED DISEASES? There is no evidence of spread to humans or other species. Kuru and Creutzfeldt-Jakob disease in humans have similar clinical and pathological features but are different diseases. Transmission from sheep to other species is possible under laboratory conditions; but when this is done, the resulting disease condition is still scrapie, not another animal TSE. WHAT CAUSES SCRAPIE? Over the years there has been a great deal of controversy regarding the nature of the scrapie agent or prion protein. It is believed that infectivity resides in a scrapie-specific protein, also called the prion protein. The scrapie agent is highly resistant to environmental conditions and substantially resistant to procedures such as cooking or rendering, and many disinfectants. It appears the prion gene is important in controlling host susceptibility, length of the incubation period, and production of the prion protein itself. HOW DO SHEEP GET THE DISEASE? The disease can spread from sheep to sheep through direct contact. This is referred to as lateral transmission. Purely vertical transmission (to the lamb while it is in the uterus) is unlikely, yet not definitively proven one way or another. The scrapie agent is found in a wide variety of sheep tissues and body fluids, particularly the central nervous system and cerebrospinal fluid. It has not been transmitted from infected rams via the semen. The exact mode of escape of the scrapie agent from the body is not clear, although nasal secretions and placenta have been shown to contain the agent in some studies. The highest incidence of transmission is from dams to their own or other offspring during the first few months of life (birth to nine months). Therefore, there is less risk of a sheep contracting scrapie as it gets older. It is unlikely that spread occurs from the sire to offspring. The role of premise contamination (through feed, water, bedding, pasture, and vectors (insects) etc.) is unknown. WHAT BREEDS GET THIS DISEASE? It appears that scrapie is not a genetically transmitted disease but rather a disease caused by an infectious agent with genetically influenced suceptibility. Studies indicate that certain genes influence the incubation period. Sheep with short incubation genes develop scrapie in about two to four years. Sheep with long incubation genes are either resistant to infection or die from other causes before the clinical disease develops. Preliminary results indicate that testing for the gene and selective breeding for freedom from clinical disease may be practical. Over 90 percent of confirmed scrapie cases in the U.S. have been in “black-faced” sheep, mainly Suffolks. It is not known whether this is due to genetics or management practices. In some other countries, scrapie is known to occur mostly in white-faced breeds. WHAT SYMPTOMS
SHOULD
I LOOK FOR? Scrapie is a slowly progressive disease causing degeneration of the central nervous system of sheep and goats characterized by behavioral changes, tremor (especially of head and neck) and uncoordinated movements which progress to prostration and death. There is no associated fever. Abnormality in movement may include a high-stepping gait of the forelimbs and a “bunny hop” movement of the back legs which is exaggerated when the animal is forced to run. Sudden noise, excessive movement or the stress of handling may send the animal into tremors and convulsions. Head pressing and “star gazing” may occur in some sheep. Sheep, but not goats, tend to lose weight despite retaining a normal appetite. The itching and rubbing, for which scrapie got its name, occur to varying degrees from subtle and unnoticed to severe, with the wool rubbed from the sides and back. Some animals also will bite at their legs and pull wool from their sides. Affected goats are less likely to rub against fixed objects, but will scratch vigorously with their hind feet and horns. HOW IS IT DIAGNOSED? As there is no treatment, death is inevitable within one to six months after the onset of clinical signs. External parasites, listeriosis, pregnancy toxemia, plant poisoning, OPP, lead poisoning, rabies, pseudorabies and brain abscesses may all display similar clinical signs. Therefore, a diagnosis of scrapie based on clinical signs must be confirmed by microscopic examination of brain tissue and/or by immunohistochemistry which tests for the presence of the prion protein. At present there are no blood tests, cell cultures or other practical, live-animal diagnostic procedures. CAN
I PROTECT MY FLOCK FROM SCRAPIE? Prevention is based on maintaining a closed flock. New animals (especially females) introduced into the flock must come from reliable sources known to be free of scrapie. Past eradications procedures have failed, due in part to producer noncompliance and inadequate identification systems with federal regulations requiring destruction of infected and exposed animals. Individual state regulations vary, making it necessary to contact your state and federal veterinarian to determine the best procedures. There is a federal scrapie control program called the Voluntary Scrapie Flock Certification Program (VSFCP) managed by USDAIAPHIS. Its principle is to measure the absence of scrapie over time while simultaneously incorporating specific flock management plans, trace-back procedures, postmortem examinations, and permanent identification. For more information contact: USDAIAPHIS/VS, National Animal Health Program, 4700 River Road, Unit 43, Riverdale, MD 20737-1231. TEL: 301-734-4913 Primary Author Cleon
Kimberling, Extension Veterinarian, Colorado State University Contributors Cindy
Wolf, Veterinarian, University of Minnesota Paul Rodgers American Sheep Industry Association |
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Hawks Mountain Ranch
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