Cwd research a long, uphill fight
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CWD research a long, uphill fight

March 14, 2002 Wisconsin State Journal by Ron Seely
We have grown to rely so heavily upon science and its wonders that many of us tend to assume there are explanations for every quirk of nature.

And then along comes something weird - such as chronic wasting disease, or CWD.

To study CWD, to begin reading the journal articles and to talk to the scientists who are working in their labs, is a humbling experience. There is a lot we don't know. Here's what we know for certain. The disease is part of a family of diseases called transmissible spongiform encephalopathies, or TSEs. Mad cow disease is the form of TSE that affects cattle. Other TSEs include scrapie in sheep and feline spongiform encephalopathy in cats.

There are also several forms of TSEs that affect humans. They include Creutzfeldt-Jakob disease, new variant Creutzfeldt-Jakob disease, fatal familial insomnia and kuru.

All of these diseases are neurological. They are fatal diseases of the central nervous system that affect the brain and the spinal cord.

These are things you've probably already heard in the wake of the discovery of CWD in three deer shot near Mount Horeb. And now, people are asking questions.

They're wondering whether CWD can spread to people, especially people who regularly eat venison. And many, especially wildlife biologists, are wondering how the disease spreads from animal to animal and whether the disease can leap from deer or elk to domestic livestock.

Good and important questions. Unfortunately, science cannot yet provide definitive answers. Here's why.

Research to date has shown that TSEs such as chronic wasting disease are caused not by viruses or bacteria, which we know something about and can fight, but by a protein, a deformed protein. This is an unusual thing.

Normally, proteins make our bodies work; they are chains of amino acids folded into special shapes that do everything from carrying oxygen through our blood to helping our muscles relax and contract.

The protein involved in TSEs is called a prion. Researchers don't know for sure what prions do, although according to a May 1996 article in the journal Nature, research on mice suggests that they may have something to do with protecting the brain against dementia and other degenerative problems associated with old age.

Prions are found mostly in the brain and the spinal cord although they also occur in the eyes, spleen, tonsils and lymph nodes (this explains the warning from the state Department of Natural Resources for hunters to be careful handling these parts when they clean their deer).

The same article in Nature explained that TSE is thought to be caused by a mutant or rogue prion. This rogue is misshapen in such a way that it can link with normal prions and convert them into copies of themselves.

These deformed prions build up, according to University of Wyoming researcher Elizabeth Williams, and eventually cause the brain lesions that are the primary pathological feature of TSEs.

All of this background is important because the transmission of the disease - CWD, here in Wisconsin - from species to species depends on the rogue prion being similar enough to the host prion to be able to lock into its structure and do its dirty work.

According to the Nature article, transmission works best between animals of the same species. But there is some evidence that rogue prions can jump between species, "provided that the prions of donor and host are similar enough for the conversion process to occur."

It is reassuring, then, that researchers have noted that human beings are extremely distant on the evolutionary scale from bovids such as sheep and cattle and, more important for us, from ungulates such as sheep and deer.

Prions from deer are quite different from prions found in humans or gorillas and chimpanzees.

Indeed, in her research, Williams has shown that the barrier between species is apparently very difficult to breach when it comes to CWD. In her work, she has inoculated mixed beef cattle with CWD derived from infected deer.

Thirteen animals had the disease injected directly into their brains; after two years, three of the inoculated cattle had the disease. But of the animals who were inoculated with the disease orally, none had the disease 4 years later.

Still, researchers such as Williams warn that the science to date is far from conclusive. Indeed, research at the Nagasaki University School of Medicine in Japan, published in the April 1996 issue of Nature, showed that the normal prion found in cattle shares two biochemical features with the prion found in humans and apes.

Even more worrisome is that the similarities occur in a part of the protein thought to be involved in the onset of TSEs.

Such laboratory findings are reason enough for care by those who eat venison (there are guidelines on the DNR web site at www.dnr.state.wi.us/org) and for an aggressive campaign against the disease by state agencies.

EDITOR-NOTES: Seely covers science and the environment for the State Journal. He can be reached by e-mail at rseely@madison.com or by telephone at 252-6131.

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