Prions have been found in sheep muscle, scientists announced Saturday — the first time they have been discovered in animal flesh that many humans normally eat.

The animals were infected with scrapie, a prion disease that is not the same as bovine spongiform encephalopathy. Scientists believe that mutated scrapie prions may have caused the British epidemic of mad cow disease of the 1980’s, but no case of scrapie transmitted to humans has ever been found.