Imported

Belgium/Netherlands

Sheep Questions and

Answers

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

Factsheet

 

  

APHIS
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 host’s 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 animal’s 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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United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture