Imported Belgium/Netherlands Sheep Test Results
Veterinary Services
April 2002
The U.S. Department of Agriculture's Animal and Plant Health Inspection
Service (APHIS) announced that 2 sheep from a flock of 125 which were
confiscated in March of 2001 from a farm in Vermont have tested positive
for an atypical undifferentiated transmissible spongiform encephalopathy
(TSE).
TSEs are a class of degenerative neurological diseases that are characterized
by a very long incubation period and a 100-percent mortality rate. Two
of the better known varieties of TSEs are bovine spongiform encephalopathy
(BSE) in cattle and scrapie in sheep.
Additional tests will be conducted to determine exactly what TSE the
animals have-BSE or scrapie. These tests involve the use of bioassays
that consist of injecting mice with tissue from the infected animals
and waiting for them to develop disease. This testing may take at least
2 to 3 years to complete.
This flock of 125 sheep was confiscated in March 2001 after 4 animals
from an associated flock tested positive for an atypical TSE of foreign
origin. In all, APHIS acquired 380 sheep from a total of three flocks.
All of the animals were humanely euthanized, sampled, and disposed.
The animals did not enter the animal or human food supply.
The decision to confiscate the sheep was made after four sheep from
one the of the flocks tested positive for an atypical undifferentiated
TSE of foreign origin in July 2000. On July 14, 2000, USDA issued a
declaration of emergency and extraordinary emergency to acquire the
sheep.
The first flock of 21 sheep were voluntarily sold to USDA on July 14,
2000. Those animals tested negative for a TSE.
The owners of the other two flocks contested USDA's action. A federal
district court judge ruled in favor of USDA based on the merits of the
case. The flock owners appealed to the Second Circuit Court requesting
a stay, which was denied. After the request for a stay was denied, the
flocks were confiscated by USDA in March 2001.
The second flock of 234 animals was confiscated on March 21, 2001.
Testing for TSEs is ongoing for this flock. The third flock of 125 sheep
was confiscated on March 23, 2001.
Background
APHIS' mission is to "Protect American Agriculture." As part
of that mission APHIS regulates the importation of animals and works
with U.S. producers to eradicate animal diseases. To improve the genetic
base of the domestic sheep population or to gain access to breeds not
commonly found in the United States, there was a brief window of time
in 1996 when USDA allowed the importation of sheep from certain countries.
In August and November 1996, two importations of sheep from Belgium
occurred. The sheep were primarily East Friesian milk sheep which originated
from both Belgium and the Netherlands. A total of 65 sheep were imported.
These sheep were used for milk production; milk from these sheep was
used to produce cheese that was sold locally and nationally.
This brief import window was shut in late 1996 after published research
indicated that sheep that were orally infected with BSE had a wider
tissue distribution of the agent than cattle with BSE. This raised the
possibility that if sheep were naturally infected with BSE the disease
may spread from one sheep to another.
In late 1997, both Belgium and the Netherlands reported their first
cases of BSE in native cattle. In 1998, the European Union's Scientific
Steering Committee issued an opinion that stated it is highly likely
that European sheep were exposed to feed contaminated with the BSE agent.
Based on these reports, the state of Vermont, in consultation with APHIS,
imposed a quarantine on these sheep in October 1998. The two flocks
of sheep and their progeny were prohibited, by the State quarantine,
from entering either the human food or animal feed chains or being sold
for breeding purposes. 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.
If any of the quarantined sheep were to be culled or died, tissues
from animals greater than 6 months of age were collected for diagnostic
purposes and the carcasses were incinerated at APHIS' expense. Based
on four animals from one flock testing positive for an atypical undifferentiated
TSE, USDA determined that an emergency and extraordinary emergency existed
in Vermont. These declarations, which were effective July 14, 2000,
provided funds and the authority to seize and dispose of these sheep.
The declaration of emergency provides authority for USDA to seize and
destroy the sheep and authorizes payment of fair market value for the
sheep. On October, 28, 2000, Congress provided the USDA with additional
authority and funding ($2.4 million) to compensate the owners for economic
losses incurred due to seizure and destruction of the Vermont sheep.
This was in addition to fair market value funds that would have been
provided for the sheep. This additional funding, which was available
only if the sheep were destroyed on or before November 17, 2000, was
declined by the owners. The owners of the other two flocks contested
USDA's action. A federal district court judge ruled in favor of USDA
based on the merits of the case. The flock owners appealed to the Second
Circuit Court requesting a stay, which was denied.
TSE Testing
Tissues from the sheep have been subjected to three tests; histopathology,
immuno-histochemistry, and Western-blot.
Histopathology examines brain tissue for microscopic changes indicative
of a TSE. Immuno-histochemistry examines the brain tissue for the abnormal
prion protein, which is a marker for TSE disease. The Western-blot test
also detects the abnormal form of prion protein in the brain tissue.
All of these are recognized as official tests by APHIS.
The abnormal prion protein was detected by the Western-blot test in
all of the sheep that have tested positive for a TSE in these groups
of animals. The method used for this test has been published in literature
and is an accepted methodology. The tissue samples were from the obex,
which is the best location in the brain to find the abnormal form of
the prion protein (an indicator of TSE infection) if it is present.
The Western-blot test however cannot differentiate between scrapie
and BSE. The only known validated method to differentiate between these
two diseases requires a series of mouse bioassay systems, which take
at least 2-3 years for completion.
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