CWDs: the impossible diseases
The brain-eating "transmissible spongiform encephalopathies," or TSEs, were first seen in sheep in the 1700s. They were strange. Uniquely, the infectious agent lacked DNA and RNA, and survived high temperatures and massive radiation.

Judd Aiken is trying to identify the prion that caused the Wisconsin outbreak of chronic wasting disease.

In 1982, Stanley Prusiner, a biochemist at the University of California at San Francisco, blamed scrapie on a normal protein that was, literally bent out of shape. He thought these so-called "prions" started a chain-reaction that destroyed more prions and eventually bored holes in brains - producing the Swiss-cheese effect.

Normal prions, we now know, are found on the surface of some cells. They probably play a role in communicating across the cell membrane.

Prions are proteins, and proteins are the structural element of biology. In innumerable variety, proteins compose tissues and enzymes. Built inside cells from chains of amino acids, proteins can only work if they fold into the right shape. Because up to one-third of proteins fold wrong, the body has mechanisms to recycle rejects. If this fails, junk protein can build up like rubbish in a July garbage strike. Eventually, just as sky-high trash heaps render a city unlivable, mutant prions can destroy cells.

From heresy to conventional wisdom
Considered bunk at first, Prusiner's "prion hypothesis" gradually gained currency. In 1997, it gained Prusiner the Nobel Prize for Physiology or Medicine. Today, most scientists accept that prions cause the TSE diseases, including CWD and mad cow.

(We don't have time to elaborate, but prions may play a role in Alzheimer's, Parkinson's, and other diseases.)

Although prions lack genes, genes play a role in TSEs. Prion researcher Judd Aiken observes that human victims of new-variant CJD (caused by eating mad-cow products), share a gene that causes or allows incorrect prion folding.

Leapin' prions
A key question of prion diseases is the species barrier. Typically, pathogens infect only a limited range of species. Monkeys, for example, aren't infected by HIV, the AIDS virus.

The first TSE, scrapie, appeared more than 200 years ago in sheep. Scrapie causes sheep to incessantly rub ("scrape") themselves to assuage an itch caused by Swiss-cheese brains. Then it kills.

Scrapie was scary, but did not seem to affect other ruminants -- or mutton-eaters either. How do prion diseases get established in a new species? That was a major question as British cattle began staggering and drooling in the 1980s.

This Suffolk ewe sheep is infected with scrapie, the first-known prion disease. Courtesy Jack Dykinga, USDA.

The answer appeared in a little-known practice of the agricultural industry - recycling the by-products of "forcibly retired" animals into feed for living animals. The mad cow epidemic probably started when British cows ate food containing the brains of diseased sheep. This industrial recycling of animal parts is now banned in the United Kingdom, the United States, and many other countries.

British cattle seemed healthy for many years, but as healthy cows ate the by-products of mildly diseased cows, the mutant prion apparently adapted and multiplied.

By 1986, the mad-cow epidemic was galloping around British barnyards. Mutant prions, fortified by repeated cycling through other cows, were causing the ominous neurological signs of mad cow.

As Aiken explains, prion diseases take a long while to build up and gain potency after jumping species.

Dear deer
A silent epidemic probably preceded the appearance of chronic wasting disease in elk, mule deer and white-tail deer in Colorado and Wyoming about 40 years ago.

Sheep are again the likely source of the prions, says Aiken. "If sheep scrapie [the original TSE] was transmitted into deer, it could be at a low level. Initially, the deer would never show symptoms, and it could be transmitted to other deer."

A new test can detect scrapie prions in lymph tissue from a sheep's eyelid. Live-animal tests for scrapie -- and chronic wasting disease -- could help eradicate the stubborn disease from sheep and deer. Jack Dykinga, USDA.

Eventually, the prions apparently moved from deer to deer, concentrating and adapting much as they had in British bovines, Aiken suggests. "Because of this passaging between deer that clearly occurs, there was potential for interspecies transmission from sheep to deer."

In other words, just infecting a deer is not enough to cause obvious disease, because mutant prions start slowly. Only after "cycling" through many generations of deer can mutant prions build up their concentration and infectious potential.

That's informed speculation, as nobody fully understands how prion diseases. Let's keep speculating.

How did CWD reach Wisconsin?

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