Eww, gross!" thought many Americans as they read about the United Kingdom's
mad-cow epidemic-all the while contentedly chewing their own hamburgers. The gruesome,
incurable disease, which causes dementia and motor-skill loss, has led to 79 human deaths
and the preemptive slaughter of hundreds of thousands of cattle. Now a similar condition
is making an appearance among game animals in the Rocky Mountain West. Dubbed
"mad-deer disease," it may already be responsible for the deaths of three people
who are believed to have eaten infected venison.
Both mad-cow and mad-deer disease are transmissible spongiform encephalopathies, in
which brain proteins called prions deform, forcing other brain cells to degenerate along
with them. Cows get a version called bovine spongiform encephalopathy; sheep get a similar
disease called scrapie; and humans contract, very rarely and by an unknown natural
process, Creutzfeldt-Jakob disease.
Those thought to have been infected through the
consumption of tainted beef have what is called "new-variant CJD." The course of
the disease is similar in all species, starting with a little wobbliness or memory loss,
then progressing to full dementia and death, usually in the space of a few months.
Mad-deer disease, more formally known as chronic wasting disease (CWD), is found mostly
in northeast and central Colorado and southeastern Wyoming, where it infects about 5
percent of mule deer on game farms and 1 percent of free-ranging deer. Infected animals
have also been found on game farms in Nebraska, Montana, South Dakota, and Oklahoma. Game
farms sell meat and the velvet from antlers (marketed as a health supplement or
aphrodisiac), or sometimes ship live animals to other states to bulk up their supply of
huntable wildlife. Thus one farm with infected animals could potentially spread the
disease far and wide.
In Larimer County, Colorado, the seeming epicenter of the outbreak,
some reports put the infection rate of wild deer as high as 15 percent. By way of
comparison, says John Stauber, coauthor of Mad Cow U.S.A., "At the height of mad-cow
disease in Britain, only one to two percent of cows on any given farm were showing
symptoms of the disease." Given the lack of a vaccine and the growing interstate
commerce in farmed deer and elk, he says, "the eventual spread of chronic wasting
disease across North America seems inevitable."
Mad-deer disease is apparently passed from doe to fawn, as well as through shared
feeding facilities, salt licks, and touching noses. It is not yet clear how-and even
whether-the disease can spread to humans. Ingestion of infected meat is suspect, as is
mere contact with infected tissue, putting both hunters and venison-eaters at risk. Deer
in advanced stages of CWD are awkward and slow, making them easier to shoot and more
likely to end up on a dinner plate. Moreover, while dressing game, hunters expose
themselves to the most infectious tissues, the brain and spinal cord.
Another nexus for transmission may be the rendering process, in which animal carcasses,
including roadkill, are ground up as feed for other animals, some of which are then eaten
by people. In the United Kingdom, feeding infected cows to other cattle amplified the
disease's transmission rate. The U.S. Department of Agriculture has only partly learned
this lesson, says Stauber: While it forbids cow cannibalism, it allows the carcasses of
deer, elk, and sheep known to be infected with CWD or scrapie to be rendered and
subsequently fed to chickens, pigs, and pets. Those animals may be rendered in turn and
fed to cattle. Last year, a coalition of public-health groups petitioned the agency to
halt the practice-so far to no avail.
Dr. Byron Caughey of the National Institutes of Health has studied the transmissibility
of spongiform encephalopathies. While his findings indicate that "there is a
substantial [interspecies] barrier" and that the efficiency of transmission between
mad deer and humans seems to be low, it may be "in the same ballpark" as that
between mad cattle and humans.
Unfortunately, the states in which mad-deer infection is highest also rely heavily on
the sale of hunting licenses, making them loath to publicize the possibility that eating
venison might turn your brain into a sponge. Currently, detection of the disease requires
waiting for an animal to drop dead under suspicious conditions and then relying on hunters
or game farmers to call in a vet for a post-mortem rather than simply shipping the carcass
to the rendering plant.
"Obviously, when the animals are dying of the disease, it's
much too late to have prevented the disease from getting from one animal to another,"
says Dr. Mary Jo Schmerr, lead scientist of prion diseases at the Agricultural Research
Service. "If you could detect it before they show these clinical signs, you could
remove them from the flock or herd."
Schmerr thinks she may soon be able to do that with a simple blood test she's
developing that can be performed on living animals. In the absence of the widespread use
of such a test, you might just want to pass on that venison haunch.