Ohio State University Extension Factsheet

Ohio State University Fact Sheet

Veterinary Preventive Medicine

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Update on Testing for Scrapie

VME-0004-01

William P. Shulaw, D.V.M.
Extension Veterinarian, Cattle and Sheep, Ohio State University Extension

What Is Scrapie and Why Is It Important?

Scrapie is one of several diseases of humans and animals that are more generally called transmissible spongiform encephalopathies (TSEs.) These diseases are all characterized by a very long incubation period, slow onset of neurologic disease, and inevitable death. Other diseases in this group include transmissible mink encephalopathy (TME-mink), bovine spongiform encephalopathy (BSE-cattle), chronic wasting disease in deer and elk of North America (CWD), feline spongiform encephalopathy (FSE-cats), and in human beings, kuru, Creutzfeldt-Jacob disease (CJD), Gerstmann Straussler Syndrome (GSS), and fatal familial insomnia (FFI). Certain tissues, such as brain, spinal cord, placenta, and others, are considered to contain the infectious agent that causes the disease.

Scrapie has been reported since at least the early 1700s and has existed in the United States since 1947. It is the most studied of the TSEs because of the readily available animal model. In the United Kingdom, at least 20 "strains" of scrapie have been described based upon the results of mouse inoculation experiments.

Over the years, several theories have been proposed regarding the etiology of these diseases. It was first thought that the cause was a conventional virus. This has largely been discounted. The agent has remarkable resistance to physical and chemical methods of destruction including heat, formalin, ionizing radiation, and enzymes known to destroy DNA and RNA. The second hypothesis is that the agent is a virino - a very small piece of RNA (DNA is less likely) covered by a resistant protein coat. This hypothesis is still not proven, and few scientists are proponents of it.

The current belief held by many scientists (including Stanley Prusiner who won the Nobel Prize in 1998 for his work in this area) is that the agent is an abnormal form of a normally occurring host protein found on cells in the nervous system and spleen and lymph nodes. This theory holds that the abnormal protein is infectious and that it produces disease by causing a conformational change in the normal host protein. The conformational change leads to loss of normal cellular function and cell death. Prusiner has coined the term "prion" to describe this proteinaceous infectious particle. Although not all problems concerning this theory have been resolved, many scientists hold to it.

Scrapie has received a great deal of attention since the late 1980s following the appearance of BSE. Many people still believe that scrapie in sheep, combined with changes in animal feeding and rendering practices, led to the outbreak of BSE in the United Kingdom. The BSE crisis has led to tremendous financial loss and public outcry in the UK and in much of Europe. In 1996, a British advisory committee suggested that some unusual cases of CJD in humans might have been caused by the BSE agent acquired by eating beef or beef products. As of February of 2001, 88 definite and probable cases of this "variant" of CJD, as it is called, have been reported. Consensus exists in the scientific community that this variant form of CJD is caused by the BSE agent. Furthermore, more than 80 confirmed cases of FSE in cats (in England) have been reported since 1990, and a spongiform encephalopathy in various exotic species of ruminants and lions and tigers has been reported from several zoological collections. All these are thought to be caused by the feeding of BSE-contaminated feeds. Lastly, in April of 1999, a report of spongiform encephalopathy, with a pattern of lesions characteristic of BSE, in several species of primates in three French zoos suggested widespread infection of primates in zoological collections and further supports the zoonotic nature of the disease.


Figure 1. This ewe shows the weight loss and behavioral changes associated with scrapie. She is slightly uncoordinated in the rear legs when she walks.

Clinical Signs

Scrapie in sheep is characterized initially by subtle changes in behavior and gait. Animals may stand apart from the flock and stumble slightly. Later the behavior change may be more pronounced and a peculiar "bunny hopping" gait may be observed when the animal attempts to move rapidly. Signs usually progress to obvious behavior changes, and eventually the animal cannot rise from its bed. Sheep may, or may not, show the characteristic rubbing, or "scraping," for which the disease was named. Wool loss may be slight to extensive. Once, we believed that the characteristic, microscopic, spongy appearance of the brain (spongiform encephalopathy) could only be seen after a period of clinical signs. Today, occasional animals are detected histologically and immunohistochemically when few signs are observed clinically.

Evidence exists that development of clinical signs is influenced by the genetic makeup of the sheep with respect to the gene coding for production of normal prion protein. Several mutations of this gene are known in sheep, but the most important may be at codons (positions) 136, 154, and 171. In the United States, the mutation at codon 171 is thought to affect the development of the strain of scrapie most commonly seen here. This has been studied primarily in Suffolk sheep, and much less is known about the effect of genotype in other breeds. Those sheep that are homozygous for glutamine (designated QQ) at this position on the prion gene appear to be more likely to develop clinical disease. Those that are QR (glutamine/arginine) or RR (arginine/arginine) appear "resistant" to clinical scrapie. This is not necessarily true for all strains of scrapie, and it is not yet known whether sheep with the QR or RR genotype can carry the infection and transmit it without developing the clinical signs. One RR sheep and a few QR sheep from outside the United States have been shown to have developed scrapie. Many Suffolk breeders have begun selection in their flocks for QR or RR sheep.

Current Testing Options

Histopathology and Immunohistochemistry

Until recently, definitive diagnosis of scrapie could only be made following necropsy and examination of the brain. Characteristic vacuoles, or holes, and degenerative changes in nerve cells are typically observed microscopically following clinical scrapie. The development of polyclonal and monoclonal antibodies that allow the immunohistochemical detection of the abnormal prion protein in histologic sections of the brain has allowed us to confirm scrapie in those animals that did not have all the classic microscopic changes and in those specimens damaged by collection techniques or postmortem degeneration. This is a significant advancement as it will allow slaughter surveillance to be used to determine the national prevalence of scrapie, and it allows for the detection of the prion protein in other tissues, such as spleen and lymphoid tissue, as well as nervous tissue. Now we can study the pathogenesis of the disease and trace the infection through multiple tissues. Experimental evidence suggests that this monoclonal antibody will also detect abnormal prion in CWD of deer and elk. Other research has led to the development of other monoclonal antibodies and techniques that may be useful in diagnosis of other TSEs.

Genetic Testing

Commercial testing to determine the genetic makeup of sheep, with respect to the mutation of the prion gene, at codons 136 and 171, is available in the United States. Because only one strain of scrapie appears to be predominant in this country, most producers are only testing for amino acid coding at codon 171. Research is in progress at the Agricultural Research Service (ARS) laboratories in Washington to determine the susceptibility of QR and RR sheep to scrapie. Whether QR and RR sheep can silently carry the infection and transmit it is not yet known and is the subject of another project.

Genetic testing for susceptibility (to clinical scrapie) and using the result for animal selection is currently being practiced by many breeders. If it can be shown that the so-called "resistant" sheep do not carry the scrapie agent, significant progress can be made toward eradicating scrapie. However, most of the sheep (Suffolk) tested thus far are QQ, and we do not have a good explanation about why this genotype is over represented. If the QR or RR sheep have some production or longevity problem, it will be some time before it is evident. In addition, there is reason to believe that the mutation at codon 171 may only affect scrapie susceptibility for some strains of scrapie. If other types of scrapie unaffected by mutations at codon 171 exist in the United States, they could eventually become predominant.


Figure 2. Good restraint and local anesthesia are needed to collect a sample of tissue from the third eyelid.

Figure 3. A small piece of lymphoid tissue is removed from the inside of the third eyelid.

Figure 4. Capillary immunoelectrophoresis testing can be done on a peripheral blood sample.

Third Eyelid Testing

Dr. Katherine O'Rourke and colleagues at the ARS laboratory in Washington have developed a method of biopsy of the lymphoid tissue of the third eyelid with subsequent immunohistochemical testing using a specific monoclonal antibody. This methodology identifies the abnormal prion protein in this tissue and has correctly identified scrapie-infected animals as young as 12-14 months of age. The collection of samples requires that the animal be adequately restrained and a local anesthetic used. The collection instruments must be destroyed to prevent possible transmission to other animals.

This test has the potential to allow detection of scrapie-infected animals months to years before development of clinical signs. Testing and culling positive animals could assist producers in eradicating the disease from their flocks. This test looks promising but awaits further validation. Some laboratories in the United States now offer this testing service.

Commercial availability raises some ethical and legal issues. In many states, scrapie is a reportable disease, and its diagnosis, or suspected presence, is reportable to the Animal Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA). Although the third eyelid test is not yet an "official" test as far as APHIS is concerned, it may be considered a presumptive test by many states. In our state, a positive test on the third eyelid would require the reporting of the flock and the test result to regulatory veterinarians. This puts the veterinary practitioner in a difficult situation if producers ask for the test and receive positive results. In Ohio, any lay person is expected to report the existence of suspected scrapie, so even producers would be defying the law if they have test-positive sheep and do not report the results.

Capillary Electrophoresis

Dr. Mary Jo Schmerr and colleagues at the ARS laboratory in Ames, Iowa, are working on another test that uses monoclonal antibodies to detect scrapie-infected sheep. The significant difference is that this test is performed on a blood sample. This would obviously make testing sheep technically easier for the producer and the practitioner. This test also awaits complete validation, but preliminary work appears very promising. It is not commercially available at this time.

For many years, producers and practitioners have wanted a preclinical test for scrapie. It appears that we are much closer now than ever. In addition, confirmation of a scrapied animal is much more certain today. It will be necessary for producers, veterinarians, and regulatory officials to work together to use these new tests to the best advantage in developing new strategies for controlling or eradicating scrapie.

Testing for Scrapie Genotype

Gene Check, Inc.
1629 Blue Spruce Dr., Suite 106
Fort Collins, CO
800-822-6740
970-472-9951
Fax: 970-472-9956

Colorado State University Veterinary Diagnostic Laboratory
300 West Drake
Fort Collins, CO 80523
Phone: 970-491-1281
Fax: 970-491-0320

Infigen, Inc.
1825 Infinity Dr.
DeForest, WI 53532
608-846-0555

Third eyelid testing (Unapproved)
Gene Check, Inc.
1629 Blue Spruce Dr., Suite 106
Fort Collins, CO
800-822-6740
970-472-9951
Fax: 970-472-9956

Colorado State University Veterinary Diagnostic Laboratory
300 West Drake
Fort Collins, CO 80523
Phone: 970-491-1281
Fax: 970-491-0320

Reviewers

Diane L. Sutton, DVM
Senior Staff Veterinarian
Animal and Plant Health Inspection Service
Veterinary Services
National Animal Health Programs
United States Department of Agriculture
4700 River Road
Unit 43, 3A39
Riverdale, MD 20737

Susan Skorupski, MS, DVM
Area Epidemiologist
Animal and Plant Health Inspection Service
Veterinary Services
United States Department of Agriculture
12927 Stonecreek Drive
Pickerington, OH 43147


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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181



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