The chance of a human getting CWD from a deer is very tiny and the public should work with game management officials in the effort to control the disease, a University of Wisconsin doctor said Wednesday.
About 100 people, including doctors, nurses, medical students and journalists gathered at the Lake Wisconsin Country Club in Prairie du Sac for a talk by Dennis Maki, an infectious disease specialist with the University of Wisconsin Hospitals and Clinics. The program, Chronic Wasting Disease in Deer/ Prion Diseases in Humans, was sponsored by the Sauk County Medical Association.
The chronic wasting disease that was spotted in Wisconsin’s deer herd this year is an example of a class of diseases called transmissible spongiform encephalopathies, he said. Other examples include: kuru, a disease found in cannibals in the South Pacific; bovine spongiform encephalopathy, or mad cow disease; and Creutzfeldt-Jakob disease, a rare form of dementia in older humans.
TSE disease are identified by microscopic examination of the brain of the victim. Tiny holes are seen that damage the brain and give it the appearance of a sponge, Maki said.
CWD and other TSEs are spread by a disease-causing agent called a prion. It is a kind of distorted protein discovered only in the last decade or so, he said. Prions are unlike infectious agents such as bacteria or viruses that are more familiar.
Maki said prions were discovered because they will survive disinfectant and levels of heat that will kill other disease agents. “It was extraordinarily resistant to anything that would kill a virus,” he said.
When a prion comes into contact with normal proteins in the brain it causes their normal shape to change, he said, and the changed proteins can distort other proteins in the brain. The amount of distorted proteins builds up, gradually killing more and more brain cells of the animal or human that’s infected.
“Once they get started it’s a rapidly progressive dementia with death in one to two years,” Maki said. “Not a pretty picture, a bad disease, there’s no treatment.”
Human TSEs such as Creutzfeldt-Jakob disease are rare, the doctor said. However, the United Kingdom had an unusual increase in what was eventually called new variant Creutzfeldt-Jakob disease during the 1990s. This involved more than 150 cases of CJD that appeared in young people, such as those in their 30s.
Researchers eventually tied the British cases of new variant CJD to an outbreak of mad cow disease on British farms during the 1980s. The prion disease in cows was also tied back to a similar disease, scrapie, found in sheep.
The BSE outbreak is tied back to a practice of rendering down slaughtering wastes and dead animals, including sheep that died of scrapie, to produce meat and bone meal for cattle feed, Maki said. Because of changes in the rendering process, the scrapie prion wasn’t killed and spread to cows, and eventually to humans.
Despite the concern about TSEs stimulated when CWD was found in Wisconsin’s deer, Maki said he does not see a situation similar to Britain’s in the United States. About four cases of CJD have been found in young people in the U.S., but none appear to be tied to eating deer infected with CWD.
“We feel pretty confident we are not seeing (a British-style spread of) this disease in the United States,” he said.
In a laboratory test tube, human prions convert other human proteins to the distorted form fairly easily. Proteins from BSE infected cows convert human proteins much less effectively, he said. CWD prions are even less effective on human proteins than are those from cows.
Furthermore, prions have not been found the muscle of infected animals, said Maki. The parts that are risky are parts that the Department of Natural Resources has recommended that people not eat, such as brain and nerve tissue, lymph nodes and tonsils and the endocrine glands.
Despite 150 people getting new variant CJD in Britain over the 1990s, the risk of getting a TSE disease in that country less than one in 10 million, said Maki, and the risk of a person getting a prion disease from eating venison is 10 fold less.
Maki said he has regularly eaten venison from deer taken in the area around Mount Horeb where chronic wasting disease has been detected. And he said he would still eat venison taken from outside that area.
It’s important for hunters to cooperate with the DNR plan to lower the population of deer in the affected area, he said, and there should be close monitoring on control of game farms were deer and elk are being raised.
“It’s absolutely essential to contract the herd and conduct surveillance statewide,” he said. “And there should be much, much closer surveillance of deer farms.”
Maki cautioned the public that the test being used to detect CWD in deer is not perfect and a negative test does not prove that someone who eats that venison is safe from a prion disease. However, the chance of a person getting disease is very small.
“I have no qualms about eating venison from outside of the (eradication) zone,” he said.