NEW YORK (Reuters Health) – The abnormal proteins linked to ”mad cow” disease and other brain-wasting illnesses can form in muscle much more easily than previously thought, new research suggests.
The accumulation of these proteins, known as prions, has been thought to be concentrated mainly in the central nervous system and lymphatic tissues, but California researchers have demonstrated that high levels of prions can form in muscles throughout the body.
The report suggests that current strategies aimed at protecting the food supply, which include taking precautions to ensure that meat does not contain any neurological or lymphatic tissues, may not be adequate for reducing the risk of prion diseases. However, the study’s authors caution that the results are preliminary, and it is too soon to know whether prions naturally form in the muscle of animals infected with brain-wasting diseases.
Mad cow disease, or bovine spongiform encephalopathy (BSE), is one of several fatal brain-wasting diseases thought to be caused by the build-up of prions in the brain. In 1996, a new form of Creutzfeldt-Jakob disease (CJD), a human brain-wasting disease, was reported in young adults in the UK. There is strong evidence that this new type of CJD arose when people ate meat from cows infected with BSE.
Since the discovery of the apparent link between mad cow and the new type of CJD, there has been a push to exclude meat from prion-infected animals from the food supply. Besides slaughtering herds of infected cattle, this strategy has included banning brains, spinal cord and certain other tissues from meat products.
Now, Dr. Stanley B. Prusiner and colleagues from the University of California at San Francisco report that, in mice at least, high levels of prions can accumulate in muscles, too.
Prusiner and his colleagues injected brain tissue from prion-infected mice and hamsters into the muscles of uninfected mice. The researchers found that these injected prions were able to reproduce extensively in muscle. This accumulation of the abnormal proteins varied widely from muscle to muscle, with muscles in the hind legs having the highest levels.
A report on the findings appears in the March 19th issue of the journal Proceedings of the National Academy of Sciences.
The research demonstrates that skeletal muscle in mice is ”intrinsically capable” of forming high levels of the abnormal proteins, Prusiner and his colleagues conclude. The fact that high levels of prions may accumulate in muscle even when neural and lymphatic tissue is excluded “raises the concern that humans consuming meat from prion-infected animals are at risk for acquiring infection,” according to the authors.
But the risk of contracting a prion disease by eating meat is still uncertain, Prusiner and his colleagues note. They point out that the accumulation of prions may vary from species to species and from disease to disease. Also, compared to being injected with infected tissue, it is much more difficult to become infected with prions by eating infected tissue, they note. The researchers also point out that not all types of prions can move from species to species, so the risk to people is uncertain.
The study challenges the conventional thinking on where prions accumulate in the body, Dr. Giuseppe Legname, who is a co-author of the study, told Reuters Health in an interview. ”We thought that they were only accumulating in the nervous system and lymphatic system,” he said.
The formation of prions in muscle could have larger implications, according to Legname. He stressed, however, that the study was conducted in laboratory mice, so it does not prove that prions can form in the muscles of other animals. To find this out, researchers will have to perform similar experiments in other species, he said.
People should not panic about the results, Legname explained. There is “no reason to worry yet,” he said. Still, he added, “We need to be aware that there might be a possibility” that prions could be present in the muscles of other animals.
The findings may have a silver lining. If significant numbers of prions do accumulate in muscle, than it would be easier to diagnose prion diseases by analyzing a small sample of muscle tissue, the investigators note.
SOURCE: Proceedings of the National Academy of Sciences 2002;99:3812-