The discovery of a cow suffering from bovine spongiform encephalopathy (BSE) in Washington State at the end of last year brought fears of mad cow disease to the U.S. New findings provide further insight into how prions, the misshapen proteins behind the transmissible spongiform encephalophathy (TSE) diseases, behave. Prions are most often associated with organ tissue, but recent findings have indicated that they can accumulate in the muscles of rodents and humans who have succumbed to TSEs. According to a report published online today by the journal Nature Biotechnology, prions can be detected in the muscles of sheep infected with the TSE known as scrapie several months before the disease can be clinically diagnosed.

Olivier Andreoletti of National Veterinary School in Toulouse, France, and his colleagues infected six sheep with scrapie. After the animals died, the researchers analyzed their muscle tissue and found "small but consistent amounts" of the infectious prion in the creatures¿ forelimbs and hindlimbs. The scientists also examined a flock of sheep that had naturally succumbed to scrapie and detected prions in the muscles of two animals out of a dozen that were tested. One of these, which was 13.5 months old when it died, tested positive even though animals typically display clinical signs of scrapie only after 22 months of age.

Fewer of the naturally infected animals tested positive for prions in their muscles, suggesting that the infectious proteins spreads to muscles less efficiently in the wild than in the laboratory. In addition, the prions in the muscle tissues were 5,000 times less infective than were those recovered from brain tissue. Although this is the first observation of prions accumulating in the muscles of animals that enter the human food chain, the authors caution that the results cannot be extrapolated to BSE in cattle. In addition they note that "dietary exposure to scrapie is currently considered nonhazardous to humans."