Introduction:
Scrapie is an infectious disease of sheep and goats. It is part of a group of diseases known as transmissible spongiform encephalopathies (TSE). Other TSE diseases include bovine spongiform encephalopathy (BSE), chronic wasting disease (in deer and elk), transmissible mink encephalopathy, Kuru (in humans), and Creutzfeldt-Jakob Disease (in humans). In the United States, there have been and continue to be cases of scrapie, transmissible mink encephalopathy, and chronic wasting disease.Over 200 years ago, scrapie was first recognized as a disease of sheep in Great Britain and other countries of western Europe. 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. Scrapie has now been diagnosed in over 1,000 flocks throughout the United States.
There is no epidemiologic evidence that scrapie in sheep and goats is transmitted to humans. Kuru and Creutzfeldt-Jakob disease in humans have similar clinical and pathological features but are different diseases. Transmission from sheep to other animal species is possible under laboratory conditions.
Causative Agent:
Scrapie is caused by a transmissible agent, probably an abnormal form of the prion protein called PrPsc. (A prion is a poorly understood infectious agent that is thought to be composed of abnormal protein that can cause damage to cells.) While a scrapie-specific prion has been identified as the possible cause, there appears to be a genetic component to the disease. A gene has been identified that may control how susceptible an animal is to the disease or how long the animal will be infected before showing any clinical signs. However, this does not mean that the disease is spread genetically; instead, it indicates that genetics may play a role after the animal is exposed to scrapie. The exact nature of the scrapie agent is still under investigation.Clinical Signs:
Scrapie is a slowly progressing disease that causes deterioration of the central nervous system in sheep and goats. This causes behavioral changes, tremors (especially of head and neck), and uncoordinated movements. Abnormalities in movement may include a high-stepping gait of the forelimbs and a "bunny hop" movement of the back legs that is exaggerated when the animal is forced to run. Sudden noise, excessive movement, or the stress of handling may trigger tremors and convulsions in the animal. Head pressing and "star gazing" may occur in some sheep. Sheep, but not goats, tend to lose weight. Itching and rubbing (hence the name scrapie) can occur, and some animals will also bite at their legs and pull hair/wool from their sides. Affected goats are less likely to rub against fixed objects, but will scratch vigorously with their hind feet and horns. The disease will progress to the point where the animal is unable to get up and eventually dies.Disease Transmission: The disease can spread from animal to animal through direct contact. This is referred to as horizontal transmission. Purely vertical transmission to the fetus while it is in the uterus is unlikely, but has been documented. The scrapie agent is thought to be spread most commonly from the mother to her offspring and to other newborns during the first few months of life. This may occur because of direct contact with the placenta and placental fluids. The scrapie agent is found in a wide variety of tissues and body fluids, particularly the central nervous system and cerebrospinal fluid. It does not appear to be transmitted through the semen. There is less risk of an animal contracting scrapie as it gets older. The role of premise contamination through feed, water, bedding, pasture, and vectors (insects) is unknown.
The signs or effects of the disease usually appear 2 to 5 years after the animal is infected. Because there is no treatment, death usually occurs within 1-6 months after clinical signs begin.
Diagnosis: 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; however, there is a test using a sample of tissue taken from the third-eyelid (nictitating membrane) that is currently undergoing evaluation by the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) for scrapie detection in live animals.
Prevention and Control: Because there are no treatment options, prevention is the only solution to this problem. Prevention is based on maintaining a closed flock/herd. New animals (especially females) introduced into the flock/herd must come from reliable sources known to be free of scrapie.
There is a federal scrapie control program called the Voluntary Scrapie Flock Certification Program (VSFCP) managed by the USDA/APHIS. Its main objective is to measure the prevalence of scrapie over time, while simultaneously incorporating specific flock management plans, trace-back procedures, postmortem examinations, and permanent identification. The program is based on the following key concepts:
Operating an effective program to deal with this disease requires cooperation among producer organizations, allied industries, and governmental agencies. Past nation-wide eradication procedures have failed, due in part to producer noncompliance and inadequate identification systems. Individual state regulations vary, making it necessary to contact state and federal veterinarians to determine the best procedures.
For more information contact:
USDA/APHIS/VS
National Animal Health Program
4700 River Road, Unit 43,
Riverdale, MD 20737-1231.
TEL: 301-734-4913