WASHINGTON — Mad cow disease is one of a family of fatal diseases in humans and other mammals caused by bizarre infectious agents unlike any others known to biology.
Neither viruses nor bacteria nor single-celled protozoans, these infectious elements, known as prions (PREE-ons), consist of a mere strand of protein and carry no DNA or other genetic material — a fact that for years left most scientists skeptical about their ability to cause disease.
How, scientists wondered, could something multiply within an animal’s body and even spread to other animals if it could not make copies of itself? And how could something make copies of itself without some kind of hereditary material such as DNA?
For answering those two questions, California scientist Stanley Prusiner won a Nobel Prize in 1997.
Prusiner found that prions ("proteinaceous infectious particles"), are tiny pieces of protein, even smaller than viruses, which can exist in two forms — a "normal" form and an infectious, disease-causing form. The two are virtually identical, except one is folded and tangled into a slightly different shape than the other.
Researchers still are not sure what normal prions do, though there is some evidence they play a role in transporting copper within the body.
But Prusiner found that once an aberrantly shaped prion finds its way into an animal (by hitchhiking within contaminated meat products, for example), it can attach itself to a normal prion in the body and force that prion to adopt its "bad" conformation. So unlike standard infectious agents that make more of themselves through replication — in effect, having babies — "bad" prions make more of themselves by recruiting good prions to their cause. In the end, the effect is the same: An expanding wave of conversions results in more and more "bad" prions accumulating in the body. They travel through the animal’s nervous system and begin to take a toll on the brain.
Over time they cause pockets of nerve damage, giving the brain a spongy appearance and inexorably killing the animal.
Prion diseases, known collectively as "transmittable spongiform encephalopathies" or TSEs, vary somewhat depending on which animal each occurs in. In cows there is mad cow disease. In sheep it is called scrapie. In humans, prions cause Creutzfeld-Jacob disease, Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia and kuru (a disease that for decades affected a tribe in Papua New Guinea that had a tradition of eating the brains of their dead relatives).
In the mid-1990s scientists recognized a new version of human prion disease, called variant CJD, caused by eating beef from prion-infected cows. Like other prion diseases, the incubation period between infection and symptoms is long — as long as four decades — making it very difficult to track the source of infection and also difficult to stop an epidemic early, since it is invisible for many years.
There is no treatment for prion diseases, and prions are famed for being virtually indestructible. The proteins are resistant to standard means of sterilization, including heat, chemicals and radiation — which is why the United States and other countries have since 1997 banned the use of even processed beef parts as meal for other cows.
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