A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

What's the story on prions, which supposedly cause mad cow disease?

June 15, 2001

Dear Cecil:

I know about bacteria. I know about viruses, sort of. But what are prions? I've been reading about them in connection with mad cow disease, and I gather that they're some new kind of germ, and that someone got a Nobel Prize for discovering them. But that's about it. Maybe I'm just getting old, Cecil, but what the hell — bacteria and viruses weren't good enough? We need some new kind of high-tech bug that sounds like you make it by pelting muons with bosons in the Tevatron?

Cecil replies:

You're not the only one to wonder about prions (pronounced PREE-ons). The term was coined by Dr. Stanley Prusiner, who received the Nobel Prize in 1997 for discovering the tiny pathogens. Prion is supposedly an acronym for "PROteinaceous INfectious particle." Obvious problem: this gives us proins, not prions. I'm not saying this necessarily tells you anything about Prusiner, but you can see where some would have their doubts.

(To be fair, some say Prusiner switched the letters for the sake of euphony, though in one's bitter heart one suspects he did it to produce a cool, scientific-sounding term like muon, boson, etc. Prusiner was traveling when I called, so the question remains unresolved.)

You may ask: How come I never heard of prions before now? Probably because, until the appearance of mad cow disease, you never heard of the diseases prions are believed to cause. These include Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker disease, and fatal familial insomnia, all of which are fairly rare, thank God. Symptoms of fatal familial insomnia include: four months of progressive insomnia accompanied by panic attacks and bizarre phobias; five months of hallucinations, panic, agitation, and sweating; three months of total insomnia, weight loss, and possibly incontinence; six months of dementia and total insomnia; and finally "sudden death after becoming mute." Say, you don't look at all well. Are you getting enough sleep?

Almost forgot: kuru. That's the disease in New Guinea believed to be transmitted by infected brain tissue consumed during cannibalistic funeral rites.

Back to mad cow disease, properly known as bovine spongiform encephalopathy (BSE) — literally, "disease in which cow brains deteriorate and wind up looking like a sponge." As of December 2000, approximately 180,000 cases had been confirmed in Great Britain, plus about 1,300 elsewhere, mostly on the continent but also in other parts of the world, including (eep) Canada. In every case the cattle had been imported from the UK. The number of new cases has dropped sharply since the peak in 1992-'93. "The outbreak may have resulted from the feeding of scrapie-containing sheep meat-and-bone meal to cattle," the Centers for Disease Control inform us. (Scrapie is a disease of sheep.) "There is a strong evidence and general agreement that the outbreak was amplified by feeding rendered bovine meat-and-bone meal to young calves."  You getting the message here? I don't care how hungry you are. Cannibalism is bad.

The reason people are so worried about BSE, apart from their concern over the British cattle industry's economic prospects, is that BSE has been linked to outbreaks of variant Creutzfeldt-Jakob disease (vCJD), a progressive neurological disorder in which the human brain deteriorates and winds up looking like a sponge. I'm getting so absent-minded I almost forgot to mention this. OMIGOD, AM I GETTING CREUTZFELDT-JAKOB DISEASE? THAT MEANS THE ALIEN SPACE COMMANDERS IN MY TOMATO PLANTS WILL FLY UP MY NOSTRILS AND INVADE MY BRAIN! Wait, those are the symptoms of fatal familial insomnia, not vCJD. Never mind.

Prusiner's Nobel Prize-winning breakthrough was that BSE and similar brain diseases, collectively known as transmissible spongiform encephalopathies (TSEs), are caused by the pathological transformation of a protein on the surface of certain brain cells. As one writer describes it, the protein gets folded the wrong way and becomes a prion, which then causes adjacent proteins to fold the wrong way, which messes up still other proteins — I'm envisioning Ice-9 from Vonnegut's Cat's Cradle and before you know it, your brain looks like something you could use to scrub out the septic tank.

Now to the skeptics. Although the existence of prions is generally accepted by scientists, there are a few holdouts, notably Laura Manuelidis, a professor of neuropathology at Yale. As she points out, there are dozens of strains of TSEs, so there must be something more complicated than a simple binary fold/no-fold prion-type mechanism at work. Something like a virus. So far she hasn't discovered it, but you understand her beef (so to speak). Other disease processes — bacterial and viral infections, tumors, genetic disorders — have broad applicability and manifest themselves in numerous ailments great and small. Prions are mostly involved in certain fairly rare (except for BSE) diseases of the brain. More importantly, prions reproduce without benefit of DNA or RNA, the only such instance in all of biology. It offends one's sense of aesthetics. (For a good summary of the case against prions, see http://www.slate.com/articles/news_and_politics/hey_wait_a_minute/1997/10/nobel_gas.html.) Whatever the truth of the matter, it's not just cattle breeders in Britain who hope they get it sorted out soon.

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