Researchers whose work was published today in the journal Science say they have found infectious prions - the agents that are believed to cause chronic wasting disease - in the leg muscles of infected mule deer, a finding that's likely to raise concerns among many of the hundreds of thousands of deer hunters in Wisconsin.
"This shows muscle contains infected material," said senior author Glenn Telling, an associate professor of microbiology at the University of Kentucky. "Anybody who may be handling or eating infected deer may be inadvertently exposed."
The finding does not change the state's recommendation for hunters, said James Kazmierczak, an epidemiologist with the Wisconsin Department of Health. No part of an infected animal should be consumed, and all animals killed in the chronic wasting disease endemic zone should be tested for the disease, he said.
"Whether people choose to follow that is another issue," Kazmierczak said.
Since 2002, 99,308 deer in Wisconsin have been tested. Of that total, 557 deer have tested positive for the disease, according to the state Department of Natural Resources.
The new report suggests that all so-called transmissible spongiform encephalopathies, ranging from chronic wasting disease to mad cow disease, should be taken more seriously, said Michael Hansen, senior scientist with the Consumers Union.
"No hunter should eat anything until after an animal has been tested," said Hansen, a biologist. "This finding, along with reports last year that CWD can move into primates, should raise some serious questions for hunters."
Study found jump to primates
In November, researchers at Creighton University in Omaha, Neb., and the University of Wisconsin-Madison reported that chronic wasting disease could infect squirrel monkeys, a member of the same order as humans.
"This is the first reported transmission of CWD to primates," the authors wrote in the Journal of Virology.
The two squirrel monkeys were injected in the brain with the infectious material from mule deer. Within 34 months they developed degenerative brain disease. After the animals were euthanized, the classic sponge-like lesions common to prion diseases were found in their brains.
The actual experiment was conducted in the 1980s by the late UW researcher Richard Marsh.
The authors said they presented the findings last year, along with some additional laboratory analysis, because of the emergence of chronic wasting disease in North America and its potential to infect humans.
But whether chronic wasting disease can infect people remains an open question. Some earlier laboratory research suggests it is possible, although there is a species barrier that likely would make such a jump more difficult, researchers say.
In August, researchers at Case Western Reserve University in Cleveland offered at least some reassurance that chronic wasting disease was not easily transmissible to humans.
The researchers used a type of "humanized" mice that had been genetically engineered to express the human form of normal prion protein in their brains. Prion protein comes in a normal and abnormal form. The abnormal form, also known as prions, is believed to cause normal prion protein to conform to its abnormal shape, resulting in invariably fatal brain disorders.
The brains of the humanized mice were injected with brain tissue samples from elk that had chronic wasting disease. But the mice did not develop prion disease.
However, mice that had been genetically engineered to express normal elk prion protein did get sick.
The researchers said the findings suggest there is a substantial species barrier for the transmission of elk chronic wasting disease into humans.
"There is some kind of barrier," said senior author Pierluigi Gambetti, a professor of neuropathology at Case Western Reserve University School of Medicine. "Whether it's surmountable is another story."
Because prions from deer and elk appear to be indistinguishable, it is likely that chronic wasting disease from mule deer and white-tailed deer "are of low or no transmissibility in humans," the Case Western researchers wrote.
Researchers use animal models such as humanized mice and squirrel monkeys in an attempt to mimic what might occur in people. However, it is possible that neither approach is an accurate model.
For instance, in both studies, the animals were injected in the brain with infected material, rather than the more normal route of potential infection, feeding.
"There are always caveats and limitations," said former UW researcher Jason Bartz, senior author of the squirrel monkey paper and now an assistant professor of microbiology and immunology at Creighton.
To show that chronic wasting disease prions were present in skeletal muscle, the University of Kentucky researchers used mice that had been genetically engineered to express normal deer prion protein in their brains.
The mice then were injected with extracts of leg muscle that was taken from mule deer that had chronic wasting disease.
Between 360 and 492 days, all 36 of those mice developed brain disease.
None of the control mice that were injected with muscle extract from deer that did not have chronic wasting disease got sick even though the mice were watched for as long as 589 days.
Laboratory mice have a normal life span of about two years.
Prion diseases often have very long incubation periods. In people, it can take 30 years for symptoms to develop.
Judd Aiken, a UW prion disease researcher, said the University of Kentucky study clearly shows for the first time that prions are present in deer muscle.
Had the study found the opposite, "We could start arguing for the potential safety of venison," said Aiken, a professor of animal health and biomedical sciences.
While the study does not prove that people can get brain disease from infected deer meat, it suggests that caution should be practiced, he said.
Alan Crossley of the Wisconsin DNR said that some people who shoot positive deer go ahead and keep the venison and presumably eat it. During the 2004 hunting season, he estimated that 40% of those who had positive deer said they planned to keep their venison.
Crossley is the DNR's project leader for chronic wasting disease, and it's his job to call people who shoot deer that test positive. He estimated that he has contacted more than 500 hunters with positive deer.
"I feel like I don't run into folks who haven't thought about it already," Crossley said.
Since the discovery of the disease west of Madison in February 2002, he said most hunters have made their own risk assessment: Some go ahead and eat their deer before test results have been returned, and others wait to see what the test turns up.
Hunters who killed a deer during the nine-day gun season might have to wait up to eight weeks to get test results because of the high volume, according to Crossley. The state tested nearly 24,000 deer during the 2005 season, DNR figures show. Thus far, 87 deer have tested positive.
The Wisconsin Veterinary Diagnostic Laboratory uses a so-called "quick test" for disease detection, and if the deer is positive, a more elaborate test is used to confirm the presence of the disease, he said.
Aiken said he would not eat any venison from an area where the disease is known to exist, even if the animal tested negative for chronic wasting disease. That's because the available tests may not be sensitive enough to find the disease in early stages, he said.
Researchers said the study also was a reminder of the tragic mad cow debacle in Great Britain. When that diseased first appeared in cattle in the 1990s, officials assured the public that British beef was safe.
Eventually, more than 170 people contracted the human form of mad cow disease, said Jiri Safar, an associate professor of neurology at the University of California, San Francisco.
"We know mad cow disease is transmissible to people," Safar said. "We know very little about chronic wasting disease. We don't want to be proven wrong a few years down the road."
Lee Bergquist of the Journal Sentinel staff contributed to this report.
From the Jan. 27, 2006, editions of the Milwaukee
"This shows muscle contains infected material," said senior author Glenn Telling, an associate professor of microbiology at the University of Kentucky. "Anybody who may be handling or eating infected deer may be inadvertently exposed."
The finding does not change the state's recommendation for hunters, said James Kazmierczak, an epidemiologist with the Wisconsin Department of Health. No part of an infected animal should be consumed, and all animals killed in the chronic wasting disease endemic zone should be tested for the disease, he said.
"Whether people choose to follow that is another issue," Kazmierczak said.
Since 2002, 99,308 deer in Wisconsin have been tested. Of that total, 557 deer have tested positive for the disease, according to the state Department of Natural Resources.
The new report suggests that all so-called transmissible spongiform encephalopathies, ranging from chronic wasting disease to mad cow disease, should be taken more seriously, said Michael Hansen, senior scientist with the Consumers Union.
"No hunter should eat anything until after an animal has been tested," said Hansen, a biologist. "This finding, along with reports last year that CWD can move into primates, should raise some serious questions for hunters."
Study found jump to primates
In November, researchers at Creighton University in Omaha, Neb., and the University of Wisconsin-Madison reported that chronic wasting disease could infect squirrel monkeys, a member of the same order as humans.
"This is the first reported transmission of CWD to primates," the authors wrote in the Journal of Virology.
The two squirrel monkeys were injected in the brain with the infectious material from mule deer. Within 34 months they developed degenerative brain disease. After the animals were euthanized, the classic sponge-like lesions common to prion diseases were found in their brains.
The actual experiment was conducted in the 1980s by the late UW researcher Richard Marsh.
The authors said they presented the findings last year, along with some additional laboratory analysis, because of the emergence of chronic wasting disease in North America and its potential to infect humans.
But whether chronic wasting disease can infect people remains an open question. Some earlier laboratory research suggests it is possible, although there is a species barrier that likely would make such a jump more difficult, researchers say.
In August, researchers at Case Western Reserve University in Cleveland offered at least some reassurance that chronic wasting disease was not easily transmissible to humans.
The researchers used a type of "humanized" mice that had been genetically engineered to express the human form of normal prion protein in their brains. Prion protein comes in a normal and abnormal form. The abnormal form, also known as prions, is believed to cause normal prion protein to conform to its abnormal shape, resulting in invariably fatal brain disorders.
The brains of the humanized mice were injected with brain tissue samples from elk that had chronic wasting disease. But the mice did not develop prion disease.
However, mice that had been genetically engineered to express normal elk prion protein did get sick.
The researchers said the findings suggest there is a substantial species barrier for the transmission of elk chronic wasting disease into humans.
"There is some kind of barrier," said senior author Pierluigi Gambetti, a professor of neuropathology at Case Western Reserve University School of Medicine. "Whether it's surmountable is another story."
Because prions from deer and elk appear to be indistinguishable, it is likely that chronic wasting disease from mule deer and white-tailed deer "are of low or no transmissibility in humans," the Case Western researchers wrote.
Researchers use animal models such as humanized mice and squirrel monkeys in an attempt to mimic what might occur in people. However, it is possible that neither approach is an accurate model.
For instance, in both studies, the animals were injected in the brain with infected material, rather than the more normal route of potential infection, feeding.
"There are always caveats and limitations," said former UW researcher Jason Bartz, senior author of the squirrel monkey paper and now an assistant professor of microbiology and immunology at Creighton.
To show that chronic wasting disease prions were present in skeletal muscle, the University of Kentucky researchers used mice that had been genetically engineered to express normal deer prion protein in their brains.
The mice then were injected with extracts of leg muscle that was taken from mule deer that had chronic wasting disease.
Between 360 and 492 days, all 36 of those mice developed brain disease.
None of the control mice that were injected with muscle extract from deer that did not have chronic wasting disease got sick even though the mice were watched for as long as 589 days.
Laboratory mice have a normal life span of about two years.
Prion diseases often have very long incubation periods. In people, it can take 30 years for symptoms to develop.
Judd Aiken, a UW prion disease researcher, said the University of Kentucky study clearly shows for the first time that prions are present in deer muscle.
Had the study found the opposite, "We could start arguing for the potential safety of venison," said Aiken, a professor of animal health and biomedical sciences.
While the study does not prove that people can get brain disease from infected deer meat, it suggests that caution should be practiced, he said.
Alan Crossley of the Wisconsin DNR said that some people who shoot positive deer go ahead and keep the venison and presumably eat it. During the 2004 hunting season, he estimated that 40% of those who had positive deer said they planned to keep their venison.
Crossley is the DNR's project leader for chronic wasting disease, and it's his job to call people who shoot deer that test positive. He estimated that he has contacted more than 500 hunters with positive deer.
"I feel like I don't run into folks who haven't thought about it already," Crossley said.
Since the discovery of the disease west of Madison in February 2002, he said most hunters have made their own risk assessment: Some go ahead and eat their deer before test results have been returned, and others wait to see what the test turns up.
Hunters who killed a deer during the nine-day gun season might have to wait up to eight weeks to get test results because of the high volume, according to Crossley. The state tested nearly 24,000 deer during the 2005 season, DNR figures show. Thus far, 87 deer have tested positive.
The Wisconsin Veterinary Diagnostic Laboratory uses a so-called "quick test" for disease detection, and if the deer is positive, a more elaborate test is used to confirm the presence of the disease, he said.
Aiken said he would not eat any venison from an area where the disease is known to exist, even if the animal tested negative for chronic wasting disease. That's because the available tests may not be sensitive enough to find the disease in early stages, he said.
Researchers said the study also was a reminder of the tragic mad cow debacle in Great Britain. When that diseased first appeared in cattle in the 1990s, officials assured the public that British beef was safe.
Eventually, more than 170 people contracted the human form of mad cow disease, said Jiri Safar, an associate professor of neurology at the University of California, San Francisco.
"We know mad cow disease is transmissible to people," Safar said. "We know very little about chronic wasting disease. We don't want to be proven wrong a few years down the road."
Lee Bergquist of the Journal Sentinel staff contributed to this report.
From the Jan. 27, 2006, editions of the Milwaukee
