Imported Belgium/Netherlands Sheep Questions
and Answers
Veterinary Services
January 2002
Q: Why is the U.S. Department of Agriculture (USDA) acquiring
three flocks of sheep in Vermont?
A: The USDA's Animal and Plant Health Inspection Service (APHIS)
regulates the importation of foreign animals and works with U.S. producers
to contain and eradicate animal disease. On July 10, 2000, several sheep
in three Vermont flocks tested positive for a transmissible spongiform
encephalopathy (TSE). USDA intends to acquire these sheep to prevent
the spread of a TSE to other livestock.
Q: What threat do these sheep pose?
A: These sheep pose a threat to the entire U.S. sheep industry.
TSEs, a class of diseases, can hide in animals for a long time before
the animals show signs of illness. Thus, a TSE could spread from coast
to coast and no one would know until the infected sheep started to die.
TSEs are almost always fatal.
Also, the type of TSE present in the Vermont sheep has not been determined.
While it could be the kind that affects only sheep, they could also
be carrying the TSE that affects cattle, which could destroy the cattle
industry, too.
Q: Where did these sheep come from and where are they now?
A: Between May and December 1996, two shipments of sheep from
Belgium were imported into the United States. These shipments included
primarily East Friesian milk sheep, which originated both from Belgium
and the Netherlands. A total of 65 sheep were imported. The imported
sheep were originally consigned to two farms in Vermont. These sheep
have been in milk production flocks since their import, and at this
time the majority of the imports are between 4-5 years of age. Most
of these original sheep imports bore offspring.
Since the sheep entered the United States, USDA has tracked the movements
of the original sheep and their progeny. At this time, all of the imported
sheep and their offspring are accounted for.
Forty-eight of the original imports are still alive. Of the 17 which
have died or been euthanized, 15 were tested for TSEs with negative
results. One died shortly after import and another died later.
Q: Why did USDA allow importation of these animals?
A: In the early 1990's, there was significant interest from
various sources in obtaining both live sheep and germplasm from overseas.
This was intended either to improve the genetic base of the domestic
sheep population or to gain access to breeds not commonly found in the
United States. USDA-APHIS evaluated the situation and came to the conclusion
that sheep could be imported from certain countries with various restrictions
that would reduce the risk of disease transmission. After this conclusion,
there was a brief window of time in 1996 when imports were allowed from
certain countries. This window was closed in late 1996, after information
was published that outlined both the experimental transmission of BSE
to sheep via oral inoculation and a wider tissue distribution of the
agent than previously established.
Q: What actions have been taken since the sheep were imported
and why?
A: In 1993, the first indications about bovine spongiform encephalopathy
(BSE) infectivity in experimentally inoculated sheep were published.
In 1996, additional research demonstrating wider tissue distribution
of the agent was published. In late 1997, both Belgium and the Netherlands
reported their first cases of BSE in native cattle. Subsequent to these
findings, the European Union's Scientific Steering Committee (SSC) issued
an opinion in 1998 that stated it is highly likely that European sheep
were exposed to feed contaminated with the BSE agent.
The combination of all of these factors led USDA to the conclusion
that the sheep in Vermont could possibly have been exposed to BSE while
in Europe and therefore a decision was made by the State of Vermont
at the request of APHIS to quarantine them. Subsequent to the quarantine,
APHIS obtained information that the flocks of origin had been fed concentrates
prepared at local mills. This practice has been shown as the most likely
route of BSE exposure for the infected cattle in Belgium.
The two flocks of sheep imported in 1996 and their offspring have
been under State quarantine in Vermont since October 1998. These sheep
and their progeny are prohibited by the State quarantine from entering
either the human or animal food chains. If any of the sheep become ill
or died, tissues are collected for diagnostic purposes and the carcasses
are incinerated at APHIS' expense.
Q: What tests were done? When, how, and what were the results
that led to the conclusion that these animals have TSE?
A: Each of the sheep that recently tested positive for a TSE
underwent four separate tests. The first two tests were conducted in
June 1999. The first of these was histopathology. Histopathology examines
the brain of the subject for microscopic changes. The sheep tested did
show some changes indicative of a degenerative neurological condition,
but the changes were not diagnostic for a TSE.
The second test in June 1999 was the immunohistochemistry. This test
examines sections of the brain of the subject for the abnormal prion
proteins that are an indicator of TSE infection. No abnormal prion protean
was detected in this test.
The third and fourth tests were conducted in June and July of this
year. The third, the Western-blot test, is another method of detecting
the abnormal form of prion proteins in the brain tissue of a test subject.
Abnormal prion protein was detected by this method in four of the sheep.
Thus, the sheep tested positive for a TSE.
The fourth test, capillary electrophoresis, detects the presence of
abnormal prion protein in the blood of a subject. In this case, the
same four sheep that tested positive for the Western-blot test also
were positive with the capillary electrophoresis test. However, this
test is still considered to be experimental; therefore, the USDA actions
were not based on these test results.
Q: Isn't more testing needed to be sure of these results? Why
don't you do more testing?
A: The test that was done - Western-blot analysis - is an approved
test authorized by APHIS, and it was done at a USDA-cooperating laboratory.
The method used for this test has been published in literature and is
an accepted methodology. The tissue samples were from best location
in the brain to find PrPres (an indicator of TSE infection) if it is
present.
Q: What actions are being taken by the USDA and what further
actions are planned?
A: As of August 1, 2000, USDA-APHIS has purchased one flock
and is involved in legal actions with the owners of two other flocks.
The sheep in the purchased flock were euthanized, samples were obtained,
and the carcasses were incinerated.
Q: Why is USDA taking this action?
A: The test that was positive - Western-blot - cannot differentiate
between scrapie and BSE. The only known method to differentiate between
these two diseases requires a series of mouse bioassay systems, which
take at least 2-3 years for completion. These sheep could possibly have
been exposed to BSE in Europe. If they are actually infected with BSE,
this would present a significant animal and human health risk. This
possibility warrants the conservative actions USDA has taken to minimize
any potential risks.
Q: If the sheep have been in the country since 1996, why has
USDA waited this long to take the action?
A: The flocks were placed under quarantine due to the possibility
of exposure to BSE that could have occurred prior to their importation.
The quarantine allowed for increased monitoring and surveillance of
these animals. Although many animals had been examined from these flocks
prior to this date, there had not been any definitive evidence of a
TSE agent. The USDA has been trying to negotiate with the flock owners
to voluntarily sell the sheep to the USDA so that the risk could be
removed. Without this new definite evidence, the USDA did not have authority
to move forward. At this time, we now have confirmed evidence of a TSE
agent and are moving forward with more stringent actions based on this
evidence.
Q: The government must know it is BSE or why would it declare
an extraordinary emergency?
A: BSE cannot be differentiated from scrapie through routine
methods of testing and diagnosis. The only current method differentiation
uses a series of mouse bioassay systems and can take 2-3 years. The
USDA does not know if the agent affecting these flocks is BSE or scrapie.
However, we do know that the sheep were fed concentrate from local mills
prior to being imported. This type of concentrate has been determined
to be the most likely cause of BSE in cattle in those countries. Therefore,
these sheep could have been exposed to BSE in Europe.
BSE in sheep has the potential to spread from animal to animal and
contaminate the environment, as it appears to behave like scrapie. BSE
in cattle behaves very differently and is spread primarily through feed
contamination. Since BSE in sheep could be spread laterally (i.e., animal
to animal) feed bans would not effectively contain or prevent new infections
of BSE in sheep. The most prudent course of action, considering the
greater potential of spread for BSE in sheep coupled with the devastating
effect to the U.S. livestock industry should BSE become established
in the United States, and the public health consequences, dictates that
extraordinary actions be taken, even without confirmation of which TSE
agent is involved.
In addition, the sheep were imported with numerous restrictions to
prevent the introduction of a different strain of scrapie into the U.S.
This action would also prevent the introduction of a different and possibly
more virulent strain of scrapie.
Q: How will USDA dispose of the sheep?
A: USDA will euthanize the sheep in a humane manner, take samples
for further diagnostic studies, and incinerate their remains. No tissues
will enter either the human or animal food chain.
Q: What can be done with the farms?
A: USDA is evaluating various options at this time. There are
some indications in research that the agent can contaminate the environment,
so USDA is exploring options to address these concerns. The samples
collected from the animals will provide additional information that
can be used to help determine the extent of any possible contamination
of the farms. This information will assist in USDA's attempt to make
an informed decision of what precautions must be taken in regard to
the environment on these farms.
Q: What is the difference between BSE in sheep and scrapie?
A: Both BSE and scrapie are TSEs. TSEs are forms of progressive
neurodegenerative disorders that affect both humans and animals and
are caused by similar uncharacterized agents that generally produce
spongiform changes in the brain. In addition to BSE and scrapie, other
examples of TSEs include: transmissible mink encephalopathy; feline
spongiform encephalopathy; chronic wasting disease of mule deer, white-tailed
deer, black-tailed deer, and elk; and in humans, kuru, Creutzfeldt-Jakob
disease, Gerstmann-Straussler syndrome, fatal familial insomnia, and
variant Creutzfeldt-Jakob disease (vCJD).
The common characteristics of the TSEs are long incubation periods
of months to years, the presence of scrapie-associated fibrils in the
brain, and the ability to transmit the disease to laboratory animals
by an injection into the brain of brain tissue from the diseased animal.
Scrapie was first recognized as a disease in sheep in Europe more
than 250 years ago. It was first diagnosed in U.S. sheep in 1947. Scrapie
is not known to be a human pathogen.
BSE was first recognized in Great Britain in 1986 and has been considered
primarily a disease of cattle. BSE has not been diagnosed in native
cattle outside of Europe. There are various scientific hypotheses concerning
the origin of BSE. BSE is thought to be the most likely cause of vCJD,
which is a fatal human disease. There have been over 70 vCJD cases in
the UK, 2 in France, and 1 in the Republic of Ireland.
BSE can be orally transmitted to sheep with as little as one-half
gram of infected brain tissue. Sheep infected with BSE showed the same
signs as scrapie and routine tests cannot differentiate between the
two.
There is one method of distinguishing between scrapie and BSE in the
same species. This method involves conducting bioassay studies via the
inoculation of infected material into mice.
These mouse bioassay studies have been done to identify both BSE in
cattle and strains of scrapie in sheep. When these studies were done
on brain material from sheep experimentally infected with BSE, the study
demonstrated that the agent was similar to the BSE agent as identified
in cattle rather than the scrapie agents identified from sheep.
BSE has not been diagnosed as a natural disease in sheep to date. However,
it must be pointed out that the studies to differentiate between scrapie
and BSE take years and not many samples have been completed.
Q: What causes BSE?
A: Little is known about the actual agent that causes BSE and
other TSEs. So far, scientists know that the TSE agent is smaller than
most viral particles. It is highly resistant to heat, ultraviolet light,
ionizing radiation, and common disinfectants that normally stop viruses
or kill bacteria. Also, the agent does not cause the hostís immune
system to create detectable antibodies. The TSE agent has not yet been
observed under a microscope.
Three main theories on the nature of the agent have been proposed:
1) An unconventional virus.
2) A prion or a partially protease-resistant protein that is rebuilt
into an abnormal prion.
3) An incomplete virus (i.e., a small piece of DNA) that protects itself
by using a host protein. This is called a virino.
Currently, the abnormal prion theory has gained acceptance among some
in the science community. This theory suggests that the TSE agent is
actually a normal host prion changed into an abnormal prion. This abnormal
prion then reproduces itself by forcing other normal prions to change.
It is theorized that the transformation of the prion protein may occur
from mutation of the prion gene or from contact with outside abnormal
prions.
BSE seems to be caused by a single strain type. This BSE strain is
different from historical or contemporary isolates from sheep or goats
with natural scrapie or cervids with chronic wasting disease, as determined
by study of incubation periods and brain "lesion profiles"
in mice.
Q: Can BSE infect sheep?
A: BSE can be transmitted experimentally to sheep through the
feeding of small amounts (.5 gr) of infected cattle brain. This indicates
a theoretical possibility that some sheep could have contracted BSE
through the consumption of contaminated feed. Investigations on the
feeding practices of sheep in Europe found that it was common practice
in some countries to feed sheep meat and bone meal. Since continental
European imported significant amounts of BSE-contaminated meat and bone
meal from the United Kingdom, sheep in the European Union were most
likely exposed to the BSE agent. Defining the natural occurrence of
BSE in native European sheep will most likely take 2-3 years.
BSE in sheep cannot be differentiated from scrapie though routine
methods of diagnosis (current differentiation uses a series of mouse
bioassay systems and takes 2-3 years). BSE in sheep appears to cause
infectivity in more tissues than BSE in cattle and may spread from one
sheep to another, unlike BSE in cattle. If BSE occurs naturally in sheep
and behaves like scrapie (i.e., transmits laterally), feed bans will
not prevent the spread of disease.
Q: Can the difference between BSE and scrapie in sheep be determined?
A: BSE in sheep cannot be differentiated from scrapie clinically
or by routine methods of diagnosis. Current methods of differentiating
use a series of mouse bioassay systems that take 2-3 years to complete.
Q: Have natural cases of BSE in sheep been detected in any
country?
A: Currently, there have not been any naturally occurring cases
of BSE in sheep reported in any country. However, testing to differentiate
between scrapie and BSE in sheep has not been done routinely. Due to
the length of time required for the differentiation studies, only a
small number of samples have been completed. Some work has been started
in Europe to determine if any of the recent cases diagnosed as scrapie
could actually be BSE, but this will take some time. The definitive
test to differentiate is a mouse bioassay system that takes several
years to complete.
Q: Do the sheep imported in 1996 have scrapie or BSE?
A: There is no simple laboratory test that can definitively
distinguish between BSE and scrapie in animals. Mouse inoculation studies,
which take 2 or more years for completion, are necessary to define the
disease agent. USDA will request that such studies be initiated on samples
from these sheep.
Q: If the diseases look the same, how do we know if the U.S.
sheep population has scrapie or BSE?
A: We have no evidence of BSE in either our sheep or cattle
populations. BSE would have to have been introduced into our sheep population
through imports. Import restrictions based on both scrapie and BSE have
limited the possibility of such exposure. The importation of sheep from
countries affected with scrapie have been prohibited since a scrapie
eradication program was started in the United States in 1952. The only
exceptions to this policy have been for Canada, a country with a similar
animal health status as our own, and for the brief period of time in
1996, when the policy was changed. Import restrictions due to BSE took
effect in 1989, and BSE was not known to exist prior to 1986. Ten years
of active surveillance of U.S. cattle has shown no evidence of BSE.
The cattle population can be used as the best indicator of the possible
presence of BSE in the United States, as this is the same species in
which the disease naturally occurs. Our surveillance system in cattle
includes a system of reporting from diagnostic laboratories, field investigations
of central nervous system (CNS) disorders, testing of rabies-negative
animals from public health laboratories, testing of CNS-condemned cases
reported by the Food Safety and Inspection Service (FSIS), and testing
of "downer" cattle.
Q: If there is a possibility that the imported sheep have scrapie,
why will the sheep be destroyed?
A: Prior to the importation of these animals, efforts were
made to determine that they were free of scrapie. In addition, enrollment
in the Scrapie Flock Certification Program was required to ensure that
monitoring continued for an extended period of time. None of this surveillance
confirmed scrapie. We cannot completely rule out BSE as the possible
infectious agent causing the recent evidence of a TSE infection. If
these sheep were actually infected with BSE, this would present a significant
animal and human health risk, as would a more virulent or different
strain of scrapie.
Q: What assurances do we have for the American public to protect
their health?
A: The USDA policy in regards to BSE has been proactive and
preventative. APHIS has taken measures in surveillance, prevention,
education, and response to protect animal and public health. For example,
import restrictions have been in place since 1989 and active surveillance
efforts began in 1990. APHIS actively works with State and Federal agencies,
including USDA's Food Safety and Inspection Service (FSIS), the Centers
for Disease Control (CDC), the Food and Drug Administration (FDA), and
the National Institutes of Health (NIH), and stakeholders to assure
we are taking the proper actions in response to changing knowledge and
information concerning BSE.
Additional information on public health issues can be provided by
the following agencies: USDA-FSIS, FDA, CDC. The CDC can provide details
about surveillance efforts for vCJD.
Q: Is BSE in sheep a risk to humans?
A: The research on BSE in sheep is too new to provide an answer
to this question.
Q: Were meat products from these animals sold for human or
animal consumption?
A: No meat from the four sheep that tested positive in 2000
for abnormal prions was ever sold. Approximately 28 lambs went to slaughter
in 1997 and another 28 lambs went to slaughter in 1998. The meat from
these lambs, approximately 2,000 lbs., was sold to consumers in the
Greensboro, Vt., area. All the lamb meat was sold in the state of Vermont.
No lamb product was ever exported or sold by mail.
Q: Can this disease spread to the wildlife population?
A: There is no evidence to date that BSE or scrapie can affect
wildlife populations. Scrapie can be spread from animal to animal and
can contaminate the environment, especially at lambing. Experiments
have shown that BSE in sheep could act like scrapie, so it could also
contaminate the environment and wildlife could possibly be exposed.
The flocks in Vermont predominantly lambed inside, which reduces the
possible risk to wildlife.
Since there is a theoretical risk that wildlife could be exposed to
this agent, USDA plans to monitor this situation. Specifically, the
free-ranging deer population provides a good monitor for any such exposure.
In cooperation with State authorities, USDA is planning to collect samples
from hunter harvest surveys to monitor whether any such exposure has
occurred.
Q: Why can't you save the sheep and study them?
A: Retaining these sheep would require sufficient biocontainment
holding facilities to handle the BSE agent until it can be determined
which TSE agent is affecting this flock. These facilities are limited.
In addition, BSE is not known to exist in the United States and because
there is a potential risk that the agent affecting these flocks could
be BSE, it is most desirable to remove all possible sources of this
agent.
As TSEs are neurological diseases, it is necessary to study the brains
of the animals. This can best be done after the animals have been euthanized.
Q: Do genetics influence or contribute to the transmission
of TSEs?
A: Research has demonstrated links between genetic variations
in sheep and the development of scrapie. Genetic variations among different
breeds of sheep may play a role in whether sheep will become infected
with scrapie and how quickly clinical signs may appear. At this time,
it is not know whether genetics contributes to the development of BSE
in cattle. Research into all of these subjects is ongoing.
Q: What are the testing methods for TSEs?
A: Histopathology: Bilaterally symmetrical degenerative changes
are usually seen in the gray matter of the brain stem when a TSE is
present. These changes are characterized by vacuolation or microcavitation
of nerve cells in the brain stem nuclei. The neural perikarya and axons
of certain brain stem nuclei contain intracytoplasmic vacuoles of various
sizes, giving the impression of a spongy brain. Hypertrophy of astrocytes
(astrocytosis) often
accompanies the vacuolation.
Electron Microscopy: A TSE diagnosis may also be made
when scrapie-associated fibrils (SAF) are detected using negative stain
electron microscopy.
Supplemental tests: Supplemental tests are available
to enhance the diagnostic capabilities for TSEs. Research shows the
partially protease-resistant form of the prion protein (PrPres) is found
in the brain of TSE-infected animals. Two tests that have been used
routinely to detect PrPres in animals showing clinical signs of a TSE
are immunohistochemistry and a Western-blot technique. In the past,
if the brain tissue was not harvested shortly after the animalís
death, autolysis might make it very difficult to confirm a diagnosis
by histopathology, but these tests permit a diagnosis of a TSE based
on finding PrPres even if the brain has been frozen or if autolysis
has occurred.
Last year, the European Commission published a preliminary report
on the evaluation of four companies' tests for the diagnosis of TSE
in cattle brain samples. These included a modified Western-blot test
developed by Prionics A.G. of Switzerland; a chemiluminescent ELISA
test using a polyclonal antiPrP antibody for detection developed by
Enfer Technology, Ltd., of Ireland; a sandwich immunoassy for PrPres
developed by Commissariat a l'Energie Atomique (CEA) of France; and
a two-site noncompetitive immunometric procedure using monoclonal antibodies
and DELFIA technology to generate a signal developed by E. G. &
G. Wallace, Ltd., of the United Kingdom. The Prionics test is currently
being used in Switzerland to test "fallen stock." Other countries,
such as Germany and France, are going to start using the Prionics test
or one of the other three tests to increase surveillance for BSE in
cattle.
A number of tests have been proposed and are in the initial process
of being validated for the preclinical diagnosis of TSEs in sheep. These
include 1) immunohistochemistry testing of eyelid associated lymphoid
tissue and tonsil biopsies, 2) use of capillary electrophoresis and
fluorescent labeled peptides to detect PrPres in the blood of animals
infected with a TSE, and 3) improved Western-blotting techniques with
very good sensitivity to detect PrPres in blood, cerebrospinal fluid,
or small pieces of biopsied tissues.
Agent Isolation: As the agents that cause TSEs have
not been fully characterized or isolated, one method used to detect
infectivity in an animal is to inoculate laboratory animals with brain
material from the affected animal and monitor them for evidence of disease.
This method may take more than 2 years to produce results; hence, it
is not practical for routine testing. The most common animal used for
this type of bioassay is the mouse. Another problem with the mouse bioassay
method when testing cattle or sheep samples is that the species barrier
may prevent detection of low levels of infectivity.
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