The disease known as scrapie has been recognized for more than 250 years. The unusual name was coined from sheep trying to relieve the intense itching which results in "scraping" off the wool. In 1947, scrapie was introduced into a Michigan flock through sheep imported from Britain. Scrapie has spread throughout the U.S. since that time.
Scrapie is a member of a family of diseases known as Transmissible Spongiform Encephalopathys (TSEs). TSEs are caused by an infectious protein called a prion. After prions are ingested, they enter the lymphatic system and travel to lymph nodes. After many months, the prions are found in the brain where they cause "holes" in the brain tissue giving it a sponge-like appearance. Other TSE-type diseases are Bovine Spongiform Encephalopathy in cattle, Chronic Wasting Disease in deer, and Creutzfeldt-Jakob disease in humans.
Sheep (and goats) are infected with scrapie at a very young age, but may not show symptoms of the disease until two to six years of age. Goats are susceptible to scrapie when raised together with sheep, but goats do not appear to spread the disease. Scrapie symptoms develop slowly and may go unrecognized at first. Symptoms may include:
Since scrapie affects the central nervous system, it can be confused with other diseases. Remember, symptoms of scrapie may take months to develop. Scrapie is always fatal.
A positive diagnosis of scrapie in a flock is based on symptoms, duration of illness, and submission of brain tissues from an affected animal. The presence of prions in a microscopic section of brain tissue is the only method to be certain that sheep are infected with scrapie. A test of lymph tissue contained in the rectum of sheep can be performed by a regulatory veterinarian in some instances, but this test is not used for routine scrapie diagnosis. If you suspect that one of your sheep may be infected with scrapie, you should contact your local veterinarian for a diagnosis.
Scrapie and Genetics
Research has shown that certain genes in the DNA of sheep play a role in the development of scrapie. The testing for the resistance or susceptibility of sheep to scrapie can be done with a blood sample drawn from the sheep in question. An animal’s genotype never changes; therefore, it can be tested at any age. One pair of genes which affect scrapie susceptibility and the disease’s incubation time has been identified by scientists. This gene is known as PRNP, the PRioN Protein gene.
Each sheep has two copies of the PRNP gene; one derived from each parent. An approved laboratory can determine the resistance or susceptibility to scrapie by examining the DNA at two codons which are particularly important for scrapie susceptibility: Codon 136 and Codon 171 of the genetic make-up. Letter designations are reported for each strand of the DNA. An "A" (Alanine) at Codon 136 indicates resistance to scrapie, whereas, the presence of "V" (Valine) indicates susceptibility. An "R" (Arginine) at Codon 171 indicates resistance to scrapie, whereas, a "Q" (Glutamine) or "H" (Histidine) indicates susceptibility. For most scrapie strains, genetic susceptibility has been determined by Codon 171:
Since, sheep have two copies of the PRNP gene (one from each parent), they can produce one of four possible combinations of amino acids at codon locations 136 and 171 as shown below:
|Amino Acid Combinations Indicating Susceptibility/Resistance
*Histidine at Codon 171 is considered to have the same susceptibility as Q (Glutamine).
By knowing the genetics of breeding animals, producers can actually breed more resistance to scrapie into their flock. Producers who retain their own replacement ewe lambs can begin influencing their flock resistance to scrapie by selecting rams that have been DNA tested and certified by an approved lab. When both copies of the PRNP gene are considered, a sheep can have one of six genotypes. The following genotypes at codons 136 and 171 are used to determine the scrapie susceptibility of sheep as indicated below:
Genotype Susceptibility/Resistance Combinations:
- AA RR – Sheep are resistant
- AA QR – Sheep are rarely susceptible
- AV QR – Sheep are susceptible to some scrapie strains* (believed to occur with low frequency in the U.S.)
- AA QQ – Sheep are highly susceptible
- AA QQ – Sheep are highly susceptible
- VV QQ – Sheep are highly susceptible
At this time, no resistant genotypes have been identified in goats; therefore, all goats are assumed to be susceptible.
Approved Laboratories for Sheep Scrapie Genetic Susceptibility (Genotype) Testing
Colorado Department of Agriculture
Rocky Mountain Regional Animal Health Laboratory
2331 West 31st Avenue Denver, CO 80211
Gene Check, Inc.
1175 58th Ave. Suite 100
Greeley, CO 80634
4711 Innovation Dr.
Lincoln, NE 68521
USDA's Animal Plant Health Inspection Service (APHIS) launched a new and enhanced Scrapie Eradication Program in November of 2002. All states agreed to be consistent with the program so producers could transport sheep between states. Scrapie costs American sheep producers an estimated $25 million annually. Trade barriers exist with scrapie-free countries, such as Australia and New Zealand. Rendering companies will not pick up dead sheep because of scrapie concerns.
The Scrapie Eradication Program utilizes genetics in a flock based clean-up plan. Slaughter surveillance of cull ewes will identify infected flocks. Infected flocks will be DNA tested and the scrapie infected and susceptible sheep removed.
USDA has funds available for the clean-up of infected flocks, including the costs associated with DNA testing and the purchase of infected and susceptible sheep, at fair market value. Other aspects of the eradication program include identification of sheep and goats with official USDA scrapie tags in:
Producers must also keep good records including names and addresses of purchases and sales of sheep from the flock.
Voluntary Scrapie Flock Certification Program
With the implementation of the Voluntary Scrapie Flock Certification Program in 1992 and modified in 1997, USDA provided producers with the opportunity to protect their sheep from scrapie and to enhance the marketability of their animals through certifying their origin in scrapie-free flocks. The intent of the program is to monitor flocks over a period of five years or more to identify flocks that are free of scrapie. Because there is no live animal test for this disease and scrapie has a long incubation period, a flock is considered free of disease if no sheep have been diagnosed with scrapie over a period of time. The longer a flock is enrolled and following the requirements of the program, the more likely the sheep in the flock are free of scrapie.
The economic value of animals in enrolled flocks increases the longer they are in the program, especially once the flock is certified. Animals from certified flocks are a valuable source for Scrapient breeding animals, especially when the genetics of the Scrapients are known.
When participating in the program, flock owners must: