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Bovine Spongiforn Encephalopathy (BSE) is a slowly progressive
degenerative disease affecting the central nervous system
of cattle. It appears to be caused by an unconventional infectious
agent, originally described as a "slow virus", a
"self-replicating protein" and more recently as
a "prion". The prion is an aberrantly folded normal
cellular protein (PrP). This agent is extremely resistant
to heat and to normal sterilization processes, and does not
evoke a detectable immune response or inflammatory reaction
in host animals.
The Spongifonnencephalopathies receive their name from the
histologic appearance of affected brain tissue. The neuropil
of the brain become vacuolated (i.e., full of holes like a
"sponge"). Many neuronal cell bodies also become
vacuolated and eventually degenerate. Biochemically these
intraneuronalvacuoles contain aberrantly folded prion protein
in a Beta-sheet conformation (amyloid) that cannot be broken
down by normal cellular degradation processes. This accumulation
of "amyloid" protein leads to neuronal cell degeneration
and eventual cell death. In this way the slowly progressive
clinical signs develop. Their severity is related to the number
and speed with which neurons die.
There are two theories about the pathogenesis of this disease.
Research increasingly supports Prusiner's hypotheses, whereby
a prion protein molecule contacts a normal PrP molecule within
the neural tissue of the brain, and induces it to refold into
the aberrant conformation. Refolded molecules induce the same
change in other normal PrP molecules, creating additional
replicas from normal protein. The second theory suggests that
the infective form of the prion protein enables a gene to
produce additional infective damaging molecules.
BSE is considered a "common source" epidemic, meaning
that animals contract the disease from a common element in
their environment. Semen, chemicals, autosomalinheritance,
biologies and pharmaceuticals have been ruled out as this
common cause. There are different scientific hypotheses concerning
the origins of BSE. One theory is that BSE existed in undetectable
levels in the British cattle population prior to 1988. Another
theory stems from epidemiologic data suggesting that BSE in
Britain
was caused by feeding cattle rendered protein produced from
the carcasses of scrapie-infected sheep. (The practice of
using products such as meat and bone meal in cattle rations
as a source of protein has been common for several decades.)
Changes in rendering operations in the early 1980's - particularly
the discontinuation of a solvent-extraction process and the
elimination of a second steam-heat treatment - may have allowed
transmission of the infective agent to cattle resulting in
the large number of cases that developed. The feeding of animal
protein specifically derived from ruminants ceased in Britain
in July 1988.
In natural cases of BSE in cattle, the agent has been found
only in brain tissue, spinal cord and retina. It has not been
found in meat or milk. Similarly, it has not been proven experimentally
to be transmissible through meat or milk. Traces of the BSE
agent were detected in the small intestines of calves that
had been fed large doses of meal from BSE-infected animals.
Failure to identify the agent in tissues may indicate either
a true absence of the agent or simply inadequate sensitivity
of current detection methods.
BSE causes a progressive degeneration of the nervous system
in mature cattle. Affected animals may display changes in
temperament, such as nervousness or aggression, abnormal posture,
incoordination and difficulty in standing, decreased
milk production, loss of body weight (despite continued appetite)
or anorexia, an abnormal stilted gait, high stepping, heightened
sensory perception, itching, excessive licking and death.
The common name "Mad Cow Disease", is related to
abnormal motor nerve control coupled with aggressiveness.
Most cases have been reported in the Holstein-Fresian breed,
although all cattle are susceptible to the disease. Onset
of clinical signs have been observed in cattle as young as
1 year 10 months, and may be precipitated by stress, estrous
or calving. The disease course varies from less than two weeks
to 14 months usually resulting in death or humane destruction
within four months.
Confirmation of disease is only possible by post-mortem examination
of brain tissue. Because the prion protein is an incorrectly
folded normal protein, no inflammatory or immune response
is generated. This, is part of the reason no test currently
exists to detect BSE in a live animal. The diagnosis must
be made at necropsy by examining brain tissue histologically
and biochemically. Ongoing nationwide surveillance has not
detected BSE in any cattle in the United
States.
- by David Diaz,ECVFG
- edited by Lydia
Andrews-Jones, DVM
Additional information added
5/21/2003
NCBA (National Cattleman's Beef Association) Advisory
May 20, 2003
Subject: BSE situation update, talking points
The U.S. has suspended beef and cattle trade with Canada.
During the press conference with Canadian government officials,
it was disclosed that the 8-year-old cow diagnosed with BSE
was slaughtered on January 31, but did not show clinical symptoms
of BSE, so it was a lower priority for testing. The animal
did have pneumonia, so the carcass was condemned and did not
go into human food supply. Canada conducted tests on the head
on Friday, and final results with the BSE diagnosis from UK
tests were received this morning. You can link to the USDA
statement at <http://www.usda.gov/news/releases/2003/05/0166.htm>
The United States Department of Agriculture has taken swift
action to stop trade with Canada until further investigation,
complying with existing BSE regulations. This action was taken
as the result of one cow being detected with BSE, Bovine Spongiform
Encephalopathy.
The Canadian Minister of Agriculture announced today that
they have a confirmed case of BSE in one cow from one herd
in Alberta, Canada. This cow was part of a 150 animal herd.
The remaining 149 animals have been quarantined.
The United States has an aggressive triple firewall system
that has prevented the introduction and spread of BSE in the
United States.
Import ban: Live animal and beef ban from any country
known with BSE beginning in 1989
Surveillance: In 1990, the United States began a surveillance
program focusing on animals with the highest risk of neurological
disease. The United States was the first country to institute
a surveillance program without having the disease within
its borders.
In fiscal year 2002, the USDA tested 19,990 cattle for
BSE using a targeted surveillance approach designed to
test the highest risk animals, including downer animals
(animals that are non-ambulatory at slaughter), animals
that die on the farm, older animals and animals exhibiting
signs of neurological distress.
Samples are shipped to a federal lab overnight, and
the average turn-around time for the test is 8 days.
This process has not yielded any positive samples in
the United States.
Feed ban: In 1997, the Food and Drug Administration banned
the use of feed supplements containing by products, such
as meat and bone meal from cattle and other animals.
A multi-year risk analysis conducted by Harvard University
in 2001 concluded: "Our analysis finds that the U.S.
is highly resistant to any introduction of BSE or a similar
disease. BSE is extremely unlikely to become established in
the U.S."
BSE is not contagious. It does not spread animal to animal.
Rather, it is believed to spread through contaminated meat
and bone meal. Meat and bone meal feeding was banned in the
United States in 1997.
BSE affects cattle over 36 months of age. The Canadian cow
that was identified was 8 years old. This cow did not go into
the food supply.
BSE is found in central nervous system tissue such as the
spinal cord and brain, and is not found in the meat.
All cattle that have entered the United States from Canada
are identified. The USDA is determining appropriate actions.
Here are some data regarding U.S.-Canadian trade:
In 2002 imports of Canadian beef to the U.S. were 1.090
billion pounds, which is 4 percent of U.S. beef production.
In the first quarter of 2003, 271 million pounds of Canadian
beef was imported into the U.S.
In 2002 1.075 million head of Canadian Slaughter Steers,
Heifers, Cows and Bulls were imported into the U.S. This
is 3 percent of total U.S. cattle slaughter. From January
1 to May 3, 2003, 337,583 Canadian Slaughter Steers, Heifers,
Cows and Bulls were imported into the U.S., also constituting
3 percent of total US slaughter.
Of the Canadian slaughter cattle imported into the U.S.
in 2002, 319,372 were beef cows, dairy cows and bulls (majority
were cows). This is 5 percent of U.S. beef cow, dairy cow
and bull slaughter. From January 1 to May 3, 2003, 94,337
slaughter beef cows, dairy cows and bulls were imported
into the U.S. from Canada.
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