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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Developing enough capacity in Wisconsin laboratories to test for CWD is part of the strategy to track the disease. © Robert Queen
Developing enough capacity in Wisconsin laboratories to test for CWD is part of the strategy to track the disease.

© Robert Queen

October 2002

Reducing the CWD risk

Understanding the risks of CWD exposure and the tactics we are taking to stem the disease.

Robert Manwell


The CWD testing strategy | Judging risks to people
Quicker tests, more testing capacity and the importance of the hunt

What's in a number? What is significant about 1.6 million, 40,000, five hundred or 24? The answer for conservation-minded citizens and hunters is plenty. One million six hundred thousand is the estimated white-tailed deer population in Wisconsin this fall; 40,000 is the number of deer Wisconsin intends to test for chronic wasting disease (CWD) this fall; 500 is the minimum number of deer that will be tested in almost every county in the state; and 24 is the total number of CWD-infected deer found to date.

Since the discovery of chronic wasting disease in Wisconsin in February 2002, wildlife biologists, veterinarians and epidemiologists have raced to define the extent of contamination and to form plans to eradicate this fatal disease of deer and elk. Although there is no existing evidence that the disease can be transmitted to humans through consuming venison, or to domestic livestock, there is also no iron-clad proof that it can't. Wisconsin citizens seem to be caught in one of life's sticky "what if" questions, without as much information as we'd like to make an informed decision.

Chronic wasting disease is one of a family of diseases known as transmissible spongiform encephalopathies or TSEs. Caused by an aberrant protein, called a prion, the ultimate result of the disease is deterioration of the infected deer or elk's brain and death. Some forms of the disease have been known for centuries in agriculture, such as scrapie, which is found in sheep. Other forms, such as CWD, have been documented for as little as 30 years or so. A human form exists too in the form of Creutzfeldt-Jakob disease (CJD) and new variant Creutzfeldt-Jakob disease (nvCJD).

There is much to be learned about chronic wasting disease. Many of the questions will need to be answered by human medical researchers and epidemiologists. Other questions will be investigated by wildlife and animal health specialists. In addition to mapping out where CWD exists in Wisconsin's deer herd, we hope to learn a great deal more about the dynamics of the disease; knowledge that could prove invaluable in future years.

Whether you hunt or not, all Wisconsin residents and visitors have a stake in solving this complex CWD riddle. The stakeholders most closely tied to whitetails – hunters, tourism, agriculture, business, as well as others who eat venison – are hoping for fast answers. One of the most widely held hopes is that every hunter wishing to have his or her deer tested for the disease could do so. This just isn't possible at this time, according to wildlife health officials at the federal and state levels.

But an ambitious plan to test 40,000 to 50,000 hunter-harvested deer for chronic wasting disease this fall will more completely answer where the disease exists in Wisconsin. And more importantly, where it doesn't. The expansive sampling plan will determine with a high degree of assurance the presence or absence of the disease for every county in the state.

"The fact is, on-demand testing simply won't be available for hunters this fall," says Tom Hauge, chief of DNR's Bureau of Wildlife Management. "But this plan will tell hunters and citizens with great certainty that the disease does or doesn't exist in the deer population in a given county. We hope that this level of assurance will reduce some of the anxiety people may have regarding venison and the health of the deer herd in Wisconsin."

The CWD testing strategy

Testing will take a tiered approach where more deer will be sampled the closer one gets to places where CWD has been detected. The most intensive testing will take place within the CWD intensive harvest zone. There every deer harvested during the special summer landowner shooting periods and during the October through January gun hunt will be sampled. This sampling will yield information critical to understanding the dynamics of the disease. Those results will help guide future CWD management actions and will help answer many questions including: distribution of the disease; age/sex relationships of deer and the disease; how and when CWD is transmitted within and among social groups of deer; vulnerability to harvest of CWD-infected deer and more.

A second tier will test approximately 500 deer over one year of age from each deer management unit in the larger CWD management zone, which extends about 40 miles out what we believe is the current center of the area infected with CWD. Sampling in this zone may begin with the early archery season, but will be heaviest in October and November. The objective with this sampling effort is to establish a baseline for later comparison to see if the infection is spreading outward.

The location of each deer shot in the Eradication Zone is plotted on aerial photographs so researchers can pinpoint where the disease may be spreading, contained or absent. © Greg Matthews
The location of each deer shot in the Eradication Zone is plotted on aerial photographs so researchers can pinpoint where the disease may be spreading, contained or absent.

© Greg Matthews

In the third tier, approximately 500 deer will be tested for CWD from each of the remaining counties in the state. Some counties, where deer harvests are traditionally light, will be lumped together with sampling from nearby counties to form a 500-deer sample. At selected registration stations, we will be asking hunters if we can remove deer heads. These will be subsequently sampled for tissues that harbor signs of this disease. Some stations will be sampled during the October 24-27 Zone T hunt (where applicable) and some will be staffed during the November opening weekend.

"The 500-deer-per-county on Deer Management Unit number was not pulled out of the air," said Hauge. "This sampling level will give very high probability that we will detect CWD in an area even if only one percent of the deer in that area are infected."

Make no mistake about it. This high degree of assurance will require the most intensive testing effort ever undertaken for a wildlife species in Wisconsin. It will require the efforts of an estimated 800 to 1,000 men and women to register deer, collect tissues and transport this large number of samples for laboratory analysis. Deer heads will be collected at county registration stations and then transported to regional processing centers where brain stem and lymph node tissue samples will be removed and packaged before shipping to a testing laboratory.

In all cases, hunters will be asked to allow researchers to sample tissue from harvested deer for testing. Plans are being worked out for sampling trophy bucks shot in the intensive harvest zone that will allow hunters to keep the heads for taxidermy. Wildlife officials point out that it could take up to six months to receive all sample results.

Hunters will be able to access confidential CWD testing results via an Internet site or a toll free telephone number to get test results for their individual deer. Information on how to access the service will be given at the time of sampling.

Judging risks to people

The World Health Organization and the Centers for Disease Control have never documented an instance of chronic wasting disease being passed from deer to humans. There is agreement among human health officials, however, that following an outbreak of mad cow disease in Europe, humans did develop a new variant of Creutzfeldt-Jakob disease after consuming beef from infected cattle. The human cases were linked to possible consumption of organ meat and tissues known to concentrate the infection within the cattle. Mad cow disease has never been found in North American cattle.

Creutzfeldt-Jakob disease is known to occur spontaneously in the human population at a rate of one in one million. In small areas of western states and Canada where CWD has existed in wild deer, captive deer and elk herds for nearly 30 years, the background level of CJD has remained stable at the one-in-one-million level in the face of an unknown number of likely exposures.

The state's lead agency for human health questions, the Wisconsin Department of Health and Family Services, stresses however that no occurrences does not mean there is zero risk of developing CJD from eating venison. They do however, feel that the risk is very small as long as people avoid eating certain deer tissues including the brain, spine, spleen, lymph, eyeball and tonsils, which are known to concentrate the infectious agent. The prion has never been found in deer muscle tissue.

You can further reduce the health risk by carefully processing venison according to guidelines published by the Department of Agriculture, Trade and Consumer Protection. (See our August 2002 "CWD update.") Additionally, deer that look sick should not be eaten. The World Health Organization and Centers for Disease Control advice is similar.

"While there is no evidence that properly processed venison poses any known measurable health risk, it is human nature to want to know if an animal might be infected before deciding to consume any part of it," says Hauge.

Familiar comparisons that often liken health risks of doing something to your chances of being struck by lightning or dying of heart disease can't be made for CWD. Since the disease has never been documented in humans, there's no basis for saying your chances of infection or subsequent disease are one-in-a-million or one-in-ten-million. This makes it difficult to understand and place this outbreak in context. But consider that 5,000 people die in the United States each year from food poisoning and 50,000 die in automobile accidents, while no one, to the best of medical science's knowledge, has ever died of CWD. While these risks are of a different nature than consuming venison, they do point out that we assume risks every day.

Quicker tests, more testing capacity and the importance of the hunt

Even with available testing, it is possible that an animal may have some level of abnormal prions in its body that is still too low for the current test to pick up, according to prion scientists. Several research laboratories across the country are working on developing more sensitive test methods that can be completed in less time. The current test takes about 50 hours to complete from the time the sample arrives at the laboratory.

"Certification of laboratories to test for this disease takes time. By this fall, we expect to be able to test up to 30,000 deer per year with in-state facilities being developed at the Wisconsin Veterinary Diagnostic Laboratory," explains Hauge. "USDA has assured us that certified laboratories in other states will provide the additional testing capacity for our increased statewide sampling."

Hunters are needed now, more than ever, to continue their traditions. We need to define the spread and scope of this disease and motorists, homeowners and citizens-at-large have a strong stake in suppressing the deer population. On a landscape capable of supporting many more deer than the current 1.6 million, a decline in hunting would lead to a rapid growth in herd numbers. A larger herd would likely add significantly to the existing 40,000 per year vehicle-deer collisions already recorded. Farmers and homeowners would see more grazing damage and forests would have a tougher time regenerating as succulent young trees are browsed by a hungry herd.

Wildlife ecologist John Cary of the University of Wisconsin estimate that if CWD were left unchecked as many as 50 percent of the deer population in the core area would be infected within ten years and in 15 years every deer would have it, signaling the start of a population collapse. In 20 years the populations would collapse completely. The effect of such a population drop on Wisconsin's estimated $1.4 billion deer hunting and viewing economy could be significant. Cary cautions, however, this estimate is based on a number of assumptions that could prove him wrong.

Maintaining interest in hunting is important to protect the future of whitetails in Wisconsin. Hopefully, the extensive testing program laid out by state wildlife managers will maintain the confidence of hunters and their families in venison as healthy food and hunting as a valuable family tradition.

For more information on CWD, visit Department of Natural Resources. For questions regarding captive deer and elk, contact the Department of Agriculture, Trade and Consumer Protection, (608)224-4882. For questions on human health and disease contact the Department of Health and Family Services, (608)266-1120.

Robert Manwell is a senior public affairs manager with the Department of Natural Resources and is DNR's chief communications officer managing the CWD outbreak.