http://www.nytimes.com/2004/05/24/science/24sheep.html 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 normally eat. 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 AndrÃ©oletti, a prion specialist at the National Veterinary School in Toulouse, France, and lead author of the study, which was published yesterday in Nature Medicine. 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 been found. 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 warning." 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 million humans. 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 human food.