March 19, 2002 The New York Times by Sandra BlakesleeResearchers studying mice have made a discovery that is certain to upset anyone who loves the rump end of beef but worries about mad cow disease. In mice, anyway, the aberrant proteins that cause the condition seem to accumulate in muscle tissue, especially the hindquarters.
Until now, the harmful proteins had been seen only in brain or nervous system tissue. The researchers stressed that the finding had been seen only in mice and may not be applicable to animals that people eat. But, they said, it is alarming enough to warrant urgent testing of obviously infected livestock to see if their meat harbors the aberrant proteins, called prions.
The prions cause spongiform encephalopathy, a lethal neurological condition called mad cow disease when it occurs in cattle, chronic wasting disease in elk and deer, and scrapie in sheep. As far as anyone knows, only the cattle disease has infected people, who are believed to have contracted it by eating meat from diseased animals.
The United States Department of Agriculture is now killing all deer and elk herds that have been exposed to chronic wasting disease, said Dr. Linda Detwiler, a veterinarian who leads the mad cow disease working group at the department. The U.S.D.A. will look into the new finding and do cooperative studies with scientific laboratories around the world, she said.
As of March 4, 116 people, mostly from Britain, had died or were dying from the human prion disease, called variant Creutzfeldt-Jakob disease.
Until now, scientists believed that harmful prions were found exclusively in brain, spinal cord and lymph tissues of infected animals and that meat became contaminated with them in the rendering and slaughtering process. To prevent prion diseases from spreading, many governments ban nervous tissue from entering human and certain animal food supplies.
"The message from this work is that we should not take anything for granted," said Dr. Giuseppe Legname, an adjunct assistant professor of neurology, who helped carry out the study at the University of California at San Francisco. It should be possible, he said, to test 50 infected animals from various kinds and breeds of livestock to determine quickly if deadly prions are found in any of their muscle meats.
Cattle continue to suffer mad cow disease in Britain, where the epidemic began in the early 1980's, although the number of newly diagnosed cases in animals is falling each year. Meanwhile, the disease is increasing on mainland Europe, where many cattle ate infected meat and bone meal exported from Britain in the 1990's. An epidemic of chronic wasting disease in deer and elk is haunting game farms and hunting grounds in the United States and Canada.
The mouse experiment, being reported today in The Proceedings of the National Academy of Sciences, was begun more than five years ago in the U.C.S.F. laboratory of Dr. Stanley Prusiner, who was the first person to describe prion diseases.
Prions are normal proteins found on the surface of cells throughout the body. Their function is not known. Infectious prions are produced when normal prions, for unknown reasons, assume a flattened shape that prevents them from being broken down by the body. Instead, the abnormal prions accumulate into toxic clumps that somehow produce lethal holes in brain tissue. Infectious prions are able to convert healthy prions into abnormal ones, relentlessly propagating the disease.
"Prions we knew could be found in tissues outside the central nervous system, but it was not clear how they got there," said Dr. Patrick J. Bosque, an assistant professor of neurology at the University of Colorado Health Sciences Center.
It was widely believed that prions were not found in skeletal muscle, Dr. Bosque said. People looked in the early years of the mad cow epidemic, but the tests available then were extremely time-consuming and not very sensitive. Moreover, researchers may have looked at the wrong muscles, he said.
To find out if prions could replicate in muscle tissue independently, the scientists created mice that "expressed" or made normal prions in muscle but nowhere else in the body. Whatever prions do, Dr. Legname said, other proteins must be able to compensate in other tissues since the animals appear healthy. Scientists also know that normal prions must be present for the disease to take hold, which in this transgenic mouse could happen only in muscle.
When the mice were injected with abnormal mouse prions, the infection spread inside muscle, Dr. Legname said, but not just any muscle. Infectious prions seemed to favor the mouse's hind leg and were found there in very high levels. "We don't know how to explain it," he said.
But transgenic mice are not normal creatures, and the ones used in this experiment were especially sensitive to prions. They were injected with, rather than fed, the disease-causing agent. So it is too soon to say whether the findings will be widely applicable. But Dr. Legname said it would be prudent to test livestock known to carry prion diseases. He said the tests used in the laboratory mice could be adapted for cattle, deer and other animals.
While such tests are not commercially available, InPro Biotechnology in South San Francisco, a company that is developing findings from the U.C.S.F. laboratory, said it could do the muscle tests in larger animals in a matter of months.
Even if muscle meat of deer or elk is found to carry prions, Dr. Detwiler said, the tissue will have to be injected into other animals to see if the infection crosses between species, a process that may take years.