New Live—Animal Test for Scrapie
Suffolk ewe infected with scrapie.
The first practical test for diagnosing the sheep disease scrapie in live
animals may help the livestock industry gain the upper hand on and eventually
eradicate one of its worst problems. Scientists with the
Agricultural Research Service teamed
with Washington State University veterinarians to invent the test.
Scrapie was inadvertently introduced from Europe in 1947. The U.S.
Department of Agriculture has been trying to eradicate it for decades, but the
disease has been intractable. Not only has it been impossible to detect scrapie
in live animals, they can harbor it for up to 8 years before showing disease
And scientists haven't fully understood how it is transmitted. Now they
believe infectious agents called prions are involved in causing scrapie, but
they are still investigating other theories also.
To glimpse what a quick, easy, inexpensive scrapie test might mean to the
sheep industry, you first have to know what sheep producers endure without such
Tom and Gail Sloan, sheep breeders in Lawrence, Kansas, specialize in
producing breeding stock from purebred Suffolk and Columbia flocks. In 1987,
they purchased a yearling ram from a large Suffolk breeder to upgrade their
family's flock of 25 Suffolk ewes.
At first the ram appeared healthy, but over the next several months he began
losing wool and seemed to have a skin problem. The ram was ultimately found to
have scrapie, a brain disease that causes sheep to behave erratically, lose
weight, and eventually die.
Sheep in this flock being observed by ARS veterinary medical officer Don
Knowles are in the early stages of scrapie infection. A new monoclonal antibody
test can detect telltale prions in lymphoid tissue taken from a sheeps
Per the federal regulations at the time, the Sloans killed the ram and all
of his progeny, carried out a rigorous cleaning program, and instituted
long-term monitoring to eliminate all traces of the disease.
The Sloans bought more ewes but again learned too late they had bought more
scrapie. After repeating the steps for ridding the flock of known and potential
scrapie carriers, they spent 6 months searching for a closed flock with a good
health program. Once again, they inadvertently purchased scrapie.
This time, the Sloans destroyed all their Suffolk sheep. They have purchased
a limited number of Suffolks, but they keep them at another farm.
"I'm afraid to let Suffolk sheep back on our home farm because of our
experience with scrapie," Gail says. "I'm not sure our family could
survive going through this again."
Effects Can Be Subtle
The Sloans' story gives only a hint of the impact scrapie has on the U.S.
The incidence of scrapie seems low only a few dozen cases are reported
annually. Black-faced sheep, including Suffolks, have had the majority of the
cases detected nationwide.
"Predation, foot rot disease, and parasitism have a much higher
monetary impact on individual flocks," says Paul Rodgers. At the American
Sheep Industry Association (ASI) in Christiansburg, Virginia, Rodgers is
Director of Animal Health, Product Safety, and Technical Services.
The disease's effect on the global marketplace, though, is far greater.
"Countries that don't have scrapie have shied away from purchasing our
sheep," says Ron Young, an Ohio sheep producer who serves on the board of
directors for the Suffolk Association. "American sheep are big, meaty, and
fast-growing, so other countries are hungry for our genetics. But from a health
protocol standpoint, we're not able to export animals to those countries."
Other livestock exports are also affected.
Scrapie is known as a transmissible spongiform encephalopathy (TSE). Related
diseases affect cattle, elk, deer, mink, cats, and humans. Bovine spongiform
encephalopathy (BSE), the TSE of cattle also known as mad cow disease has been
a problem in some other countries.
Microbiologist Katherine ORourke prepares a sample of sheep eyelid tissue
for the scrapie test.
BSE has never been detected in American cattle, and a strict surveillance
and prevention program is in place to prohibit its entry.
"Scrapie is a big stigma worldwide, " says Linda Detwiler, senior
staff veterinarian with the USDA's Animal and Plant Health Inspection Service
(APHIS). "The fact that we have scrapie affects our BSE status that is,
other countries have not declared us totally free of BSE for trading purposes,
even though we don't have that disease because any TSE is considered a risk
A Simple, Safe Test
The problem is that until now, the only way to confirm any TSE has been to
examine the brain of a dead animal. Now that could change.
A research team headed by ARS microbiologist Katherine I. O'Rourke
discovered that prions collect in pockets of lymphoid tissue in a sheep's
nictitating membrane, or third eyelid. A veterinarian can take a sample of the
tissue with only a local anesthetic.
O'Rourke works in the ARS Animal Disease Research Unit at Pullman,
Washington. Others on the team include Donald P. Knowles, who leads the Pullman
lab, Timothy V. Baszler and Steven M. Parish with Washington State University
in Pullman, and Janice M. Miller at the ARS National Animal Disease Center in
This test offers several advantages over other options currently being
pursued. While abnormal prions are known to collect in tonsils and other
lymphoid tissues, tests using those tissues require a general anesthesia. The
ARS test is safer and easier and will be much less expensive than more invasive
Another advantage is that the new test relies on a laboratory-built molecule
known as a monoclonal antibody. A monoclonal antibody is a specific series of
amino acids that recognize and bind to a specific protein, in this case prions.
The antibody can be standardized and replicated indefinitely in the laboratory.
Other existing tests use rabbit antibodies. Because one rabbit can provide
only a limited amount of serum, it isn't possible for all copies of the test
worldwide to be identical. Normal biological variation in antibodies from
multiple rabbits adds variability to a test.
In the new test, a red color indicates that scrapie prions are present in
the sample. The color comes from an enzyme on a reporter molecule that detects
the monoclonal antibody bound to prions. ARS has applied for a patent on the
monoclonal antibody and testing for prions in ruminant eyelids (Patent
Application No. 08/950,271).
Before the test can be made available, it must be validated and receive
Washington State University veterinarian Steven Parish (left) and ARS Don
Knowles apply topical anesthetic to a Suffolk ewes eye before taking an
"We need to know whether and how often the test produces false
positives or false negatives. So far, the results look promising," says
APHIS is helping to provide the ARS lab with scrapie-positive samples.
O'Rourke also plans to get samples from New Zealand, a scrapie-free country, as
scrapie-negative samples. The validation could be completed within 2 years.
Even though the test is not yet available, sheep producers are excited.
"A preclinical, live-animal diagnostic test will allow us to eventually
eradicate this disease in the United States and allow us new export
opportunities," says ASI's Rodgers.
The test will have two other important uses: confirming scrapie in dead
animals and verifying recently identified genetics that may indicate
susceptibility to scrapie.
"Sheep with certain genetics seem more likely to show clinical signs of
the disease after exposure, but we don't know if animals without the sequences
still carry and pass on the prions," says O'Rourke. "Our new test
will help us determine the relationship between genetics and prion
accumulation, which is crucial to eradicating the disease," she says.
"Once we can definitely say that an animal does not have scrapie, we'll
be able to open many doors for purebred livestock breeders," says breeder
Young. "In the past, we've just been throwing darts in the dark."
Another piece of the scientific puzzle: At what age do prions become
detectable in lambs? The new test has identified prions in 1-year-old sheep.
The lab's next step is to follow, from birth, lambs born to ewes known to have
scrapie.--By Kathryn Barry
Stelljes, Agricultural Research Service Information Staff.
Katherine I. O'Rourke and
Donald P. Knowles are in the
USDA-ARS Animal Disease
Research Unit, 337 Bustad Hall, Washington State University, Pullman, WA
99164; phone (509) 335-6020, fax (509) 335-8328.
"New Live-Animal Test for Scrapie" was published in the
November 1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.