No Patterns of Prion Disease Among Hunters in CWD Zones, New Data Shows

July 10, 2023 By: Lindsay Thomas Jr.

Can eating the venison of a deer with chronic wasting disease cause a person to acquire a related, fatal prion disease? There’s no evidence it has ever happened, but prion diseases also have notoriously long incubation periods before symptoms appear. So, health experts monitor patterns of disease among hunters in areas of the United States where deer and elk are known to have CWD. Preliminary results from a new long-term study found no increase in prion disease deaths in hunters compared to normal annual levels in all Americans.

Dr. Joe Abrams of the Centers for Disease Control in Atlanta presented preliminary data from a survey of hunters in some of the oldest CWD outbreak areas in wild deer and/or elk in Colorado, Wyoming and Wisconsin. The Colorado and Wyoming studies reviewed data going back 30 years and examined records for over 2 million hunters, including hundreds of thousands who hunted in CWD endemic areas. It is likely that over decades many thousands of these hunters knowingly or unknowingly ate venison from CWD-positive animals.

“There was no indication of these hunters dying at higher rates of prion diseases than the general population,” Dr. Abrams said, speaking at the 4th International CWD Symposium held in Denver this summer. NDA was a sponsor of the Symposium, and I attended to hear the latest CWD science.

The Wisconsin study identified 642 hunters known to have consumed CWD-positive venison in the past 20 years. None of them had been diagnosed with a prion disease – though finding even one case in 642 hunters would far exceed the normal annual rate of about one in a million.

I’ll break down the numbers from these surveys in all three states, but first for important context let’s look at the normal background level of prion disease in Americans, according to the CDC. 

Monitoring the Human Version: CJD

Dr. Abrams shared the CDC chart below in his presentation. It shows annual deaths since 1979 in the United States from Creutzfeldt-Jakob disease (CJD), a human prion disease that is similar but not related to CWD in deer. The red line is the important index, because CJD normally occurs more often in people over age 55. As the average age of U.S. citizens has increased demographically, the total number of annual cases has risen, but the age-adjusted level (red line) shows the rate is still around 1 death per million people per year. The slight rise visible across 40 years, according to Dr. Abrams and other disease experts, is likely explained by increased awareness and diagnosis of the disease, not greater prevalence. Creutzfeld-Jakob disease remains an extremely rare, fatal disease.

Annual human deaths from Creutzfeld-Jakob disease (CJD), a rare human prion disease similar to CWD in deer. The age-adjusted level (red line) shows between 1 and 1.5 deaths per million Americans per year.

Rather than looking at the national trend, a better way to monitor for health issues among hunters is to study geographic regions where CWD has existed in deer and/or elk for years. Signs of a significant increase in prion diseases among hunters in such regions would be a red flag. No such pattern has yet emerged in any study. The CDC’s new data is the latest with similar findings.

Colorado Hunters

The first case of CWD discovered in a wild deer or elk was in Colorado in 1981, and it is now widespread in deer and elk across northern Colorado. The CDC obtained records for 1,587,078 deer and elk hunters dating back to 1995 and checked them against the national mortality database, examining the cause of death for any now deceased.

The data amounted to almost 14 million “person-years” of health information. A “normal” rate of prion disease death for that many person-years would be 11 to 28 cases, with a more specific “expected” 19.4 cases. There were exactly 19 cases among the Colorado hunters examined.

The CDC further refined the data to hunters known to have purchased a license to hunt in a CWD-endemic area of Colorado, about 38% of all the hunters. With a range of two to 12 and an expected 6.3 cases, this group revealed seven prion deaths. Again, no red flags here. “We saw no indication of hunters dying at higher rates of prion diseases than the general population,” said Dr. Abrams.

Wyoming Hunters

The Wyoming dataset included 533,987 hunters checked against the mortality database. That’s over 4 million person-years. Among all hunters, the CDC found seven prion deaths compared to an expected range of one to 10, or 5.4 cases. 

Among hunters with a statewide license or known to hunt in a CWD endemic zone, the study found four prion deaths compared to an expected confidence range of zero to seven, or 2.9 cases. Among hunters known to hunt in a CWD endemic zone: four prion deaths compared to an expected range of zero to five, or two cases.

Though the case numbers in Wyoming were in the upper end of the expected range compared to Colorado, Dr. Abrams emphasized they were still within the expected range. “There was no significantly increased risk of prion deaths among hunters in Wyoming,” he said.

Latest USGS map as of July 2023 showing the distribution of CWD in deer and elk in North America.

Wisconsin Hunters

Wisconsin DNR discovered CWD in wild whitetails in 2001. Since 2003, they surveyed hunters about various aspects of hunting and CWD specifically. From the dataset, CDC was able to identify 642 Wisconsin hunters who had submitted deer for testing, received results indicating their deer was CWD positive, and told surveyors they ate the venison anyway or intended to. These 642 hunters contributed 5,778 person-years to an analysis. Additionally, researchers identified 3,940 more hunters who harvested CWD-positive deer from 2020 to 2022 but were not asked about venison consumption.

There were no cases of prion-disease deaths among any of these Wisconsin hunters. That’s great news, but it’s also important to understand more study is needed here. Even one prion-disease death in a data set this size would far exceed “normal.” Altogether, both groups of Wisconsin hunters total less than 20,000 “person-years” of monitoring, where the expected rate is around one case in 1 million person-years. However, combined with the Colorado and Wyoming data, the entire study is encouraging. 

Because scientific certainty is not 100%, and prion diseases are still not fully understood, experts continue to recommend CWD testing of all deer harvested in known CWD areas, and avoiding consumption of CWD-positive venison.

The New York Sportsman’s Feast

This is just one of several studies monitoring prion disease in hunters or in all Americans. For example, in 2005 in New York, attendees of a local sportsman’s feast inadvertently consumed venison that was later determined to be CWD-positive. The event remains the only known large-group exposure to CWD-positive venison. 

Binghamton University and the Oneida County Health Department continue to maintain contact with and follow the health outcomes of 29 banquet participants who ate venison. A report at the 15-year mark in 2020 showed “no definitive health outcomes that indicate transmission of a neurological disease to humans.” Additionally, there were no diagnosed cases of CJD, Parkinson’s, Multiple Sclerosis or Alzheimer’s disease.

So Is CWD-Positive Venison Safe to Eat?

The evidence remains strong that CWD in whitetails and elk is not causing disease among hunters who consume infected animals. Additional studies at this year’s CWD Symposium reinforced the evidence for a strong species barrier, but a few laboratory experiments continue to suggest the theoretical possibility of human infection.

Because scientific certainty is not 100%, and prion diseases are still not fully understood, experts continue to recommend CWD testing of all deer harvested in known CWD areas, and avoiding consumption of CWD-positive venison.

The risk is obviously low, but it’s not zero. If you hunt in a CWD zone, you can minimize your risk through testing. NDA follows and recommends the advice of health experts. Testing of deer harvested in a CWD zone is also helpful to state wildlife agency monitoring efforts. It’s one of the primary ways hunters can contribute to the fight against CWD

If you kill a deer that tests positive for CWD, that’s a good thing. You removed an infected deer from the woods, which helps fight the disease. Don’t let it get you down. Collect your replacement tag from the state wildlife agency, keep hunting, and keep testing the deer you harvest.

About Lindsay Thomas Jr.:

Lindsay Thomas Jr. is NDA's Chief Communications Officer. He has been a member of the staff since 2003. Prior to that, Lindsay was an editor at a Georgia hunting and fishing news magazine for nine years. Throughout his career as an editor, he has written and published numerous articles on deer management and hunting. He earned his journalism degree at the University of Georgia.