Malformed Proteins Found in Sheep Muscle
May 24, 2004
By DONALD G. McNEIL Jr.
Prions, the misfolded proteins that are widely believed to
cause brain-wasting diseases, have been found in sheep
muscle, scientists announced yesterday - the first time
they have been discovered in animal flesh that many humans
But the scientists emphasized that the finding did not mean
that lamb or mutton posed a danger to humans.
"The risk of transmission from sheep to humans is very,
very low," said Olivier AndrÈoletti, a prion specialist at
the National Veterinary School in Toulouse, France, and
lead author of the study, which was published yesterday in
The prions were found at one five-thousandth the
concentrations that are found in sheep brains, and
therefore likely to be much less infectious.
Also, the animals were infected with scrapie, a prion
disease that is not the same as bovine spongiform
encephalopathy, or mad cow disease. Although the symptoms
of scrapie have been described in sheep for centuries, and
scientists believe that mutated scrapie prions may have
caused the British epidemic of mad cow disease of the
1980's, no case of scrapie transmitted to humans has ever
Moreover, according to Dr. Paul Brown, a prion expert at
the National Institutes of Health, in nearly 40 years of
trying, no researcher has ever infected a healthy animal
with a prion disease by injecting it with liquefied muscle
from a sick one - not even when the injection was directly
into the brain.
It is even less likely that a human could be infected by
lamb or mutton that has passed through the acidic process
of digestion, scientists said.
The study team, based at three French research institutes,
found prions in the leg muscles of sheep that were
naturally infected with scrapie and in sheep deliberately
infected with it. In one naturally infected sheep, they
found scrapie eight months before the animal showed any
signs of the disease, which include itching that makes
animals scrape themselves against trees and fences (hence
the name), tremors, stumbling gaits and, eventually,
lethargy and death.
Although he agreed that prion levels in the meat were low,
Dr. Giuseppe Legname, a prion expert at the University of
California at San Francisco, called the finding "a
Two years ago, in collaboration with Dr. Stanley Prusiner,
who won a 1997 Nobel Prize in Medicine for his work in the
field, Dr. Legname found prions in the muscles of mice and
showed that they could replicate there. Since then, Swiss
researchers have found prions in the muscles of humans with
sporadic Creutzfeldt-Jakob disease, a degenerative brain
disease that is thought to arise spontaneously in one in a
That prions exist in the edible parts of livestock, Dr.
Legname said, suggests that the United States should screen
livestock to minimize the chances that Americans will be
exposed to infected animals. The United States Department
of Agriculture, which tested about 20,000 cattle a year for
mad cow disease before finding one positive for it in
December, has announced it will test more than 200,000
animals starting this summer, but that is still only a
small fraction of the number of animals that Europe tests.
Dr. Brown, who spent decades on prion research, said he
was not surprised that they had been found in sheep muscle.
"In the last few years, the sensitivity of immunoblot tests
has been ramped up so much that people are beginning to
find the protein all over the place," he said.
He ventured a prediction: "Within the next year, somebody
will make a big splash by finding it in the muscles of
cattle," he said, "and the beef industry will go crazy."
Nonetheless, he said it was still his instinct that
beefsteak had not been the culprit in transmitting mad cow
disease from cattle to humans in Europe.
"Mechanically recovered" meat, which is squeezed off
chopped-up bones under pressure, a process that in many
cases mixes in spinal cord and nerve tissue, was a "much
better vehicle," he said. Late last year, spinal cord and
nerve tissue from cows over 30 months old was banned from