Following is a synopsis of the current knowledge about scrapie and resource information.
Causative agent, transmission: Scrapie is a transmissible spongiform encephalopathy widely thought to be caused by a misfolded prion protein that accumulates in the brain. However, there are also two other theories—that the agent is a virus with unusual characteristics, or a virino, a small, noncoding regulatory nucleic acid coated with a host-derived protective protein. According to Dr. Katherine O'Rourke, a USDA research microbiologist in Pullman, Wash., "Infection is believed to be caused by ingestion of that misfolded protein [through the placenta and placental fluids] by the lamb or other adults nearby. The protein bumps into the normal folded protein in the gut and is amplified—that is, the bad proteins make the good ones go bad. Then it's thought to be transported to the brain by the vagus nerve."
Destroying the infectious agent: The prion is resistant to heat and most sterilization processes. Bleach is effective in decreasing detectable amounts of the TSE agent.
Incubation period: Typically two to five years, but according to Dr. Diane Sutton, national scrapie program coordinator, six to nine years for animals exposed after weaning. For this reason, it is presumed that some flocks are infected but remain undetected.
Usually the disease is diagnosed with immunohistochemistry on brain tissue. A live animal test has been developed and is currently being used in infected and exposed flocks by staff of the Department of Agriculture's Animal and Plant Health Inspection Service. This test uses a biopsy of lymphoid tissue from the third eyelid. Of 100 sheep, Dr. O'Rourke said that 85 percent have third-eyelid tissue suitable for biopsy. If all 85 specimens were positive, 88 percent would show it on this test. She said that a sheep with a positive test result will die within 28 months.
Vaccine: There is no vaccine.
Treatment: There is no successful treatment.
Breeds involved: In the United States, scrapie has been reported primarily in the Suffolk breed, but it has also been diagnosed in Hampshire, Cheviot, Southdown, Shropshire, Rambouillet, North Country Cheviot, Dorset, Finnsheep, Corriedale, Merino, Montadale, Columbia, Cotswold, Border Leicester, Texel, and crossbreds. Although scrapie has been most common in blackfaced breeds, it is also being diagnosed in whitefaced Columbia sheep. It is believed that most breeds and breed crosses of sheep and goats are susceptible.
Goats: Only nine cases have been found in goats in the past 12 years. When cases do occur, they are usually caused by ingestion of the placenta or placental fluids of infected sheep or cutaneous contact through abraded skin. Goats are included in the eradication program so that infected ones are traceable and an indemnity can be paid to affected goat producers.
Prevalence: As of Jan. 31, 2002, there were 61 infected or source flocks in the United States. It is presumed that some flocks are infected but remain undetected because of the long incubation period and the difficulty in making a diagnosis.
Scrapie-free countries: The United States recognizes only two countries as being free of scrapie—Australia and New Zealand.
Resources: The USDA scrapie Web site is www.aphis.usda.gov/vs/scrapie.htm, or write National Scrapie Eradication Program Coordinator, USDA-APHIS, Veterinary Services, 4700 River Rd., Unit #43, Riverdale, MD 20737-1231. The Animal Agriculture Web site is www.animalagriculture.org/scrapie. Contact your local APHIS Veterinary Services area office by calling (866) USDA-TAG (873-2824).