5. The weirdest agent

GLOSSARY:
Bovine spongiform encephalopathy (BSE): Mad cow disease.

Chronic wasting disease: A TSE in deer and elk.

Creutzfeldt-Jakob disease (CJD): Fatal, human TSE, strikes one person in a million.

Prion: A protein that, when misshapen, can cause other prions to fold wrongly. Causes all TSEs.

Scrapie: A TSE in sheep. Known since the 1700s; does not infect people.

Transmissible spongiform encephalopathy: TSE. Infectious brain diseases caused by prions.

Variant CJD (vCJD): The "human" version of mad cow. Similar to CJD; strikes younger people.

This 4,000 pound-capacity stainless steel tank is the heart of a state-of-the-art mobile tissue digester. The $900,000 digester uses heat, pressure and caustic chemicals -- sodium hydroxide or potassium hydroxide -- to reduce animal and microbial tissues to a sterile slurry that can be safely disposed of in a sanitary sewer. The digester is being used to dispose of deer carcasses tested in the context of Wisconsin's outbreak of chronic wasting disease. Photo by Jeff Miller, ©UW-Madison

What's a prion?
When scientists talk prions, they’re referring to different varieties of one protein -- PrP -- which appears widely in all mammals. In its normal form (PrPc, the c standing for cellular), the protein sits on the outside of cell membranes and is entirely benign. Presumably, it has a function, but oddly, despite a lot of work on the subject, scientists have yet to pin it down.

Illustration shows normal and misfolded PrP proteins. Both normal and diseased prions have different structures but the same amino-acid sequence. Normal PrPC is mostly alpha-helix (the tubes) and some beta-structures (the waved arrows) connected by loops (the linear sections). The yellow gadget at bottom attaches the protein to cell membranes. The misshapen protein PrPSc has more of a beta-structure, which may explain its resistance to -degrading enzymes, and thus its durability. Image: ©McCusker and Novakofski, University of Illinois

What's a prion disease?
PrP can misfold into a shape called PrPSc (named for the disease scrapie), usually through contact with the infectious agent,that bundles into clumps. The clumps cause plaques in the brain that are the trademark of the transmissible spongiform encephalopathies, or TSEs. As the name suggests, an infected brain becomes riddled with holes, until it resembles a sponge, at least under a microscope.

Microscopic image of spongy brain tissue.
Tiny holes in the brain's grey matter gives the brain of BSE-affected cows a spongelike appearance when tissue sections are examined in the lab. Photo by Dr. Al Jenny USDA.

In people, the most common prion disease is Creutzfeldt-Jakob disease, or CJD, which affects about one person in 1 million. Much rarer are Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia, kuru and Alpers Syndrome. All human varieties are similar, and with the exception of kuru (which is only known to have occurred among isolated tribes in Papua New Guinea, who once practiced cannibalism) are distinguishable mostly by age of onset.

Cows get bovine spongiform encephalopathy (BSE), AKA mad cow disease. Sheep can contract scrapie, the oldest TSE, and deer and elk can carry chronic wasting disease. Sometimes, as probably happened with mad cow disease in Europe, the disease may jump to another species unlucky enough to eat a diseased animal.

How do people contract prion diseases?

Acquired infection occurs through diet (eating infectious parts of mad cow) or medical procedures (including brain surgery with contaminated equipment), growth hormone injections (which were once extracted from human pituitary glands, which is located near the brain) or corneal transplants. As doctors have identified these routes, medical procedures have changed to eliminate them.

Hereditary transmission, caused a rare mutation in the gene that codes for the normal form of the protein, explains about 15 percent of human prion disease cases.

Spontaneous disease. The prion changes shape for no known reason.

According to the Centers for Disease Control, 213 Americans died of Creutzfeldt-Jakob disease in 2000, including inherited and spontaneous disease. None of the cases was blamed on eating meat.

Whatever their original source, prion diseases seem to undergo a chain reaction in the brain: The distorted form of the prion (PrPSc) causes native PrP to contort into the diseased form. Since prions appear largely in central nervous system tissue -- brain and spinal cord -- prion diseases can spread through eating infected tissue. Most scientists think bovine spongiform encephalopathy (BSE, or mad cow disease) causes the human disease variant Creutzfeldt-Jakob disease (vCJD).

Is there any treatment?
No treatment can cure or control any prion diseases. Although some drugs can relieve pain and muscle spasms, about 90 percent of patients die within a year.

What's the risk?
Man guides a dead cow into giant a metal vat. According to the Centers for Disease Control, the risk of contracting vCJD from BSE-infected beef remains very low in the United States.

Experts say boneless cuts of beef are the safest, since they are least likely to contain nerve tissue. Processed beef products, including hamburger, hot dogs, and sausage, may pose a higher threat since spinal tissue is more likely to enter the product. Mechanically recovered meat, which is extracted from bones at high pressure, is even more risky. Added to ground meat and other meat products as a kind of meat pulp, mechanically recovered meat is facing tighter regulation.

Organic beef, reports Consumers Union, carries the least risk, because organic beef producers do not use animal by-products.

We know they aren't alive, but can you "kill" them?
As lone proteins with no genetic material, prions aren't considered organisms, so they can't be killed. But proteins can be destroyed or, in scientific jargon, "denatured," using enzymes, heat or other means. Prions, however, are notoriously hardy, and resist normal disinfection techniques like boiling or irradiation.

How to stamp out prions? The list is short, the methods vicious:

Incineration at 1000 degrees C

Autoclaving (steam heating) at 134 degrees C for 15 minutes or longer
Exposure to highly concentrated sodium hydroxide (lye) for 15 minutes in a boiling solution.
Exposure to concentrated bleach for one hour.

Yum. Plenty of infectious protein in our bibliography.

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