TROUBLING TRAIL
Can chronic wasting disease jump from deer to humans?
Amid renewed concern about whether chronic wasting disease can jump from deer to people, a fatal human brain condition in the same family is showing up more often in Wisconsin and nationally.
It’s happening as state testing for the deer disease is down, and hunters routinely opt not to test deer killed in affected zones.
In 2002, the year CWD was discovered in Wisconsin, six cases of Creutzfeldt-Jakob disease were recorded, according to the state Department of Health Services. In two of the last four years, 13 cases have been recorded. That’s a 117% increase.
There has also been an increase nationally.
In 2002, there were 260 cases of Creutzfeldt-Jakob, compared with 481 in 2015, an 85% increase, according to data from the U.S. Centers for Disease Control and Prevention.
Creutzfeldt-Jakob is closely related to the form of mad cow disease that infected people, primarily in Great Britain, in the late 1990s and early 2000s, after they ate beef from infected cows. Indeed, human mad cow disease is known as variant-Creutzfeldt-Jakob. Both diseases attack the brain, and death usually occurs within a year.
The increase in the human disease comes as chronic wasting disease in deer — which, like Creutzfeldt-Jakob, is caused by infectious agents known as prions — continues to spread. Like its human counterparts, CWD also attacks the brain and is always fatal.
In Wisconsin, it appeared initially to be confined to a core area of western Dane County and eastern Iowa County. Today, there are 18 counties where CWD has been found in the wild deer population, according to state figures. Nationally, CWD is known to exist in at least 21 states.
That raises two questions: whether the rise in Creutzfeldt-Jakob cases is statistically significant, and whether it is linked to the spread of CWD.
Wisconsin officials are skeptical on both counts.
“The department believes the modest increase in the number of confirmed cases in the state is a reflection of our increased efforts to detect and confirm cases,” Jennifer Miller, a spokeswoman for the Wisconsin Department of Health Services, said in an email.
Since Creutzfeldt-Jakob occurs mainly in people over the age of 60, an aging population also may be factor, she said.
Miller said that the increased surveillance of Creutzfeldt-Jakob cases began in 2002, when it became a reportable disease with vigorous follow-up. Before that, all figures were based on death certificate data.
Nevertheless, the public health implications of CWD have gained some urgency.
A study in Canada, where the deer disease also is present, found that macaque monkeys had contracted chronic wasting disease after being fed meat from deer that had tested positive for CWD. The findings are the first known transmissions of the prion disease to a primate from eating infected venison.
“While no human cases of CWD have been reported to date, the new study findings raise concerns that people who hunt or consume meat from infected animals could be at risk for CWD infection,” Christine Pearson, a spokeswoman for the CDC, said in an email.
Concerns about possible transmission of the disease to humans have led to a two-pronged approach by the CDC: It is looking for unusual cases of human prion disease and attempting to find cases of prion disease in people who may have eaten meat from infected animals. Complicating that process, incubation periods for prion diseases can vary from several years to decades.
In the meantime, the CDC says meat from infected deer should not be eaten and hunters should have their deer or elk tested if it came from an area where the disease is known to exist.
For now, the CDC attributes the increase to an aging population, more awareness among neurologists and the use of the National Prion Disease Pathology Surveillance Center at Case Western Reserve University in Cleveland, Ohio. The lab is where the brains of people with suspected Creutzfeldt-Jakob disease are sent for analysis and to monitor for any potential new prion diseases in people.
Cause not known
Traditional Creutzfeldt-Jakob occurs in roughly one in a million people per year. With the exception of about 5% to 15% of cases that have a clear genetic link, its cause is not known.
With mad cow disease, almost all those who were infected were under the age of 55 with many in their 20s. Traditional Creutzfeldt-Jakob disease most often occurs after the age of 60. The youngest person to die of Creutzfeldt-Jakob in Wisconsin was 43. The mean age was 67, though 31 cases were confirmed in people under the age of 60.
The fact that none of the Wisconsin Creutzfeldt-Jakob cases were young people is reassuring, experts who study prion diseases said.
However, they point out that if chronic wasting disease has infected people, it is unknown what ages would be affected or what the disease would look like. If it were confined mostly to older people, that could be the result of an accumulation of years of eating infected venison.
Over the last five years, 52% of Creutzfeldt-Jakob cases in Wisconsin were people who had a history of eating venison at some time in their lives, said Miller, the DHS spokeswoman.
But she noted that a 2011 national food survey on hunting and venison consumption found that in Minnesota, a state that is demographically similar to Wisconsin, nearly 80% of residents had eaten venison during their lifetimes.
One major concern is that, over a period of years, the deer disease could mutate as it passes from animal to animal, eventually producing prions that could infect people, said Mark Zabel, a prion researcher and associate director of the Prion Research Center at Colorado State University.
Mad cow disease first showed up in cattle after they were given feed and bone meal from sheep, including some that had been infected with another prion animal disease — scrapie. Eventually, that led to a strain of prions in cattle that could infect humans.
Though there is no evidence so far, something similar could occur with deer prions, Zabel said. If that happened, the people most susceptible would be those who ate the most infected deer meat.
Chronic wasting disease in people might be indistinguishable from Creutzfeldt-Jakob disease, said Kurt Giles, a prion disease expert and associate professor of neurology at the University of California, San Francisco’s Institute for Neurodegenerative Diseases.
“All I can say is that currently I don’t know of any evidence of transmission of CWD to people, but that doesn’t mean it can’t happen in the future, or indeed has already happened but we can’t detect it,” he said.
Testing drop-off
As chronic wasting disease has spread in Wisconsin, the commitment to testing for it has dropped off.
In 2006, there were 30,272 samples taken, and less than 1% — 205 animals — tested positive, according to data from the state Department of Natural Resources. By 2015, there were only 3,145 samples taken, but 9% tested positive. In 2016, there were 6,129 samples and 7% tested positive.
Further, the agency has backed away from aggressive measures deployed in the early years to drive down the population, such as the use of sharpshooters and extended seasons.
In an email, DNR spokesman Jim Dick said the state was not backing off CWD management, adding that the agency’s work on the disease is taking place on many fronts. But officials also say that eliminating the
disease is unlikely because of the cost and difficulty of killing so many deer.
In fiscal 2004, the DNR spent $2.16 million on testing, agency figures show. By fiscal 2016, spending was down to $406,924. The cost of testing to the state is about $80 per deer, according to the DNR.
The drop in sampling has been affected by several factors, said Tami Ryan, the agency’s wildlife health section chief. They include the length of the deer season, changing regulations, hunter attitudes and the availability of funding.
The DNR lost federal dollars for the program beginning in 2012, Ryan said.
Despite less testing, the “DNR’s policy is, and always has been, to test deer at the hunter’s request,” Ryan said in an email.
The DNR provides free CWD testing to hunters statewide. To have an animal tested at no cost, hunters may drop off the deer head at a DNR office, CWD testing kiosk or at a participating taxidermist.
Ryan said factors influencing whether hunters want their deer tested can include how much venison they eat and their personal level of concern about the human health risk of CWD.
DNR figures show that tens of thousands of hunters are killing deer in areas where CWD is prevalent and are not submitting them for testing. In 2016, in a CWD-prone area that the DNR describes as the southern farmland zone, 442 deer tested positive for disease out of 3,760 samples.
More than 65,000 deer were killed in that zone and were not tested, according to DNR figures.
Paul Smith of the Journal Sentinel contributed to this report.