As reported by the San Francisco Chronicle on March 19, 2002, researchers at the University of San Francisco discovered prions in laboratory mouse muscle tissue, raising the possibility that livestock could be similalrly infected. Thus far, tests in Europe done on muscle tissue of food animals have been negative. The following is an abreviated version of the Chronicle rerport.

Prions, the elusive and mysterious protein particles that cause mad cow disease, have been detected for the first time multiplying in the muscle tissues of laboratory mice. As a result of the unexpected finding, prion researchers are urging that cattle muscle tissue -- especially the kind that winds up in cuts like rump steak -- be studied for evidence of the lethal proteins.

Prions (pronounced pree-ons) have been implicated in several extremely rare but invariably fatal human diseases, in particular a new form of Creutzfeldt-Jakob disease linked to consuming contaminated beef products in Britain and a few other countries. No cases of the new variant of CJD, nor any diseased cattle, have been reported in the United States. The new report adds another dimension to what has been a major medical mystery since the infectious proteins were discovered and linked to animal diseases 20 years ago.

The report on the laboratory findings is being published today in the Proceedings of the National Academy of Sciences by a team of scientists working in the laboratory of Dr. Stanley B. Prusiner, the Nobel laureate neurologist at the University of California in San Francisco. It was Prusiner and his colleagues who discovered that the disease-causing proteins could act exactly like infectious bacteria or viruses and multiply in animal and human bodies even though they contain no genetic material of their own. In their experiments at UCSF, Prusiner's team inoculated mice with the lethal form of the prions and found that they multiplied and accumulated in skeletal muscles -- mostly by far in their hind legs.

Although normal mice infected with the prions appear to remain healthy so far, because the incubation period of the disease is extremely long, a group of genetically engineered mice infected with the prions quickly became diseased and died, according to Dr. Giuseppe Legname, an immunologist in the Prusiner laboratory who was one of the experimenters. The novel finding that prions could multiply in the muscle tissues of an infected mammal caused the Prusiner group to urge an intensive study of cattle -- as well as sheep, deer and elk -- to see whether the muscles of those animals too might be susceptible to prion infection.

At this point, however, the researchers do not know whether the disease-causing proteins could in fact exist or multiply in the muscle tissue of any animals besides the laboratory mice. "I think it is important to undertake such studies in cattle and other animals, but I myself am still perfectly happy to eat beef or any other meat," Legname said during an interview.

The prion disease is unknown among cattle in America. Herds here have been under intensive surveillance by U.S. Agriculture Department inspectors since May 1990, according to the federal Centers for Disease Control and Prevention. Importing foreign animal products derived from brain spinal cord or lymph tissues is strictly barred. Until now, surveillance of American livestock has not included testing of muscle tissues, but Legname said the Prusiner laboratory at UCSF is well equipped for that task. It has a battery of highly sophisticated equipment, a variety of specialized antibodies and genetically engineered mice that can be used to screen thousands of tissue samples rapidly, he said.

"We are already looking into the problem of chronic wasting disease in deer and elk," Legname said. "We are setting up tests for the prions in sheep tissue, and then we will start looking at beef." "But all the tests must be very carefully controlled, and the numbers must be large enough so that the results will be statistically significant, whatever we find," he said.

Humans and most animals carry normal prions in their bodies, and they do no harm. The strange particles are composed entirely of amino acids shaped into spherical strands and contain no genes at all. But occasionally -- and for reasons wholly unknown to science so far -- their shapes can flatten into sheets that pile up on one another and multiply to become as infectious as any conventional microbe with conventional genetic machinery. The experiments in Prusiner's lab began five years ago. Lead author of the team's report is Dr. Patrick J. Bosque, who was a postdoctoral fellow at UCSF and is now a neurologist at the University of Colorado's Denver Medical Center.