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A Primer on Chronic Wasting Disease

CWD is a threat to whitetail herds everywhere. What does it mean for hunters?

A Primer on Chronic Wasting Disease

Pictured is a healthy white-tailed deer in Texas, where CWD also has been found in mule deer and elk. (Shutterstock image)

Editor’s Note: Articles on Chronic Wasting Disease in white-tailed deer and other cervids were featured in the regional editions of the October issue of Game & Fish Magazine.

Read about the status of CWD in many states across the country, and what’s being done to combat the disease.


Aggressive programs for controlling the spread of CWD have been implemented by wildlife agencies throughout the United States and Canada. Here, the red shading indicates Eastern states that have confirmed cases of CWD.

By Carolee Anita Boyles

Many hunters may regard chronic wasting disease, or CWD, as a problem primarily in Western deer herds, hunters in the East know better. Some Northeastern states have been managing CWD for a number of years and are doing a reasonably good job of keeping track of it and slowing the spread to other parts of the region. Although strict surveillance and reporting programs won’t eradicate the disease, they do give officials a powerful tool to help limit its spread.

How and where CWD originated is something of a mystery. It was first identified in 1967, in research mule deer herds in Colorado. By the 1970s, researchers had become aware that CWD is a transmissible spongiform encephalopathy (TSE) similar to mad cow disease and the human variant, Creutzfeldt-Jakob Disease. Gradually spreading from those first research herds into the general cervid population, CWD has now been found in whitetails, mule deer, black-tailed deer, moose and elk, and is now in at least 25 states and 2 Canadian provinces. Despite recent media coverage suggesting that CWD may be caused by bacteria or other agents, the scientific consensus remains that the disease is caused by a mutated protein known as a prion.

>>Find the best day and time to hunt whitetails in your zip code

The prion is a naturally occurring protein that becomes misfolded, so the body is no longer able to break it down in the way normal proteins are broken down. The misfolded protein multiplies by causing the animal’s normal, healthy proteins to misfold. This causes damage to the animal’s nervous system. The process may take up to two years before the deer starts to show any symptoms, so most of the time a hunter cannot tell if a deer is infected when it’s in the woods.


In the Northeast, states with reported cases include New York, Pennsylvania, Maryland, Virginia and West Virginia. Cases in the latter four states comprise a cluster of reports that runs from west central Pennsylvania south through Maryland and West Virginia and into the northernmost counties in Virginia.

CWD was discovered in Pennsylvania in 2012; since then a total of 51 deer have tested positive for the disease. The most recent reports were in February of this year, when a doe on a Fulton County breeding farm tested positive for CWD, as did a buck on a Clearfield County hunting preserve. Neither of them showed any symptoms of CWD prior to their deaths; they were tested routinely under the state’s mandatory surveillance program. Both facilities will now remain under quarantine for five years. Both deer were from an area of the state where wild deer have tested positive for CWD since 2015 and captive deer have tested positive since 2017.

To help combat the spread of CWD in Pennsylvania, the Pennsylvania Game Commission recently updated its executive order prohibiting the importation of high-risk deer parts into Pennsylvania. Pennsylvania hunters who harvest deer anywhere in New York, Ohio, Maryland or West Virginia—not only in counties with known CWD—no longer may bring deer home without first removing the parts of the carcass most likely to transmit CWD.

“Just because CWD appears confined to a specific area, doesn’t mean it won’t turn up somewhere completely new, miles away,” said Game Commission Executive Director Bryan Burhans. “Tightening up this order puts teeth in the Game Commission’s ability to enforce it, allowing us to better protect our deer and elk from CWD.”

Although Maryland didn’t have any reports of infected deer during the 2018-2019 hunting season, it did have infected deer the previous year. In February 2018, the Maryland Department of Natural Resources found 10 cases of CWD in the existing Chronic Wasting Disease Management Area. Six of the deer were hunter-harvested during 2017, three were road-killed deer, and one was a reported sick deer. The department has been sampling for CWD since 2002, with the first confirmed case in February 2011. The cases in Maryland all seem to stem from an outbreak that started in West Virginia in 2005.


In Virginia, the story is similar. Since 2002, the Virginia Department of Game & Inland Fisheries has tested more than 14,500 deer for CWD. The first positive case was in 2009, with a total of 68 positive cases between then and April of this year. Here again, a statewide sampling and quarantine program is aimed at keeping the number of new cases and the amount of spread to a minimum.

>>Read More: Can We Predict CWD Occurrences?

West Virginia has been battling CWD since 2005, when the first infected deer were found in Hampshire, Hardy and Morgan counties. Since then the state has found a total of 340 infected deer in Hampshire County, and six in Hardy County. Swift containment and quarantine action on the part of the West Virginia Division of Natural Resources, and restrictions on the transport and disposal of deer carcasses in those counties have helped retard the spread of the disease.

In early 2018, the WVDNR confirmed three cases of CWD outside the original containment zone, two road-killed deer in Berkeley County and one reported sick deer in Mineral County. In July 2018, these counties were added to the list of counties with restrictions.

New York has been largely successful in preventing the spread of CWD. The first case of CWD in the state was confirmed in 2005, in five deer from two captive breeding facilities in Oneida County. The NYDEC imposed an immediate containment area, with a mandatory deer check and a prohibition on moving intact carcasses outside the area, among other precautions. Since then, the state has maintained very strict quarantine and containment policies, and no new cases have been reported in New York.

For the remainder of the Northeast, the news remains good, with no infected deer reported in New Jersey, Delaware, Connecticut, Rhode Island, Massachusetts, New Hampshire, Maine or Vermont. Programs are in place in every state to monitor deer-herd health and stay ahead of the progression of CWD across the landscape.


CWD has been documented in parts of most states in the Midwest with the exception of Ohio, Indiana and Kentucky. In Ohio, CWD has been found in captive facilities, but not in the wild. It has been reported within 30 miles of the Indiana border but not in the state itself.

By Dr. James C. Kroll

If you hunt deer and haven’t heard about chronic wasting disease (CWD), you’ve been on another planet or in a coma this past decade. It seems everyone has an opinion on the true significance of CWD, yet those not based on science are simply that, opinions. Good scientists don’t believe anything they see, hear, smell, taste or feel, until tested and re-tested hundreds, or even thousands of times by other scientists. Science is never settled. This piece presents facts based on peer-reviewed, published science.


CWD belongs to a group of diseases called transmissible spongiform encephalopathies (TSEs)—communicable diseases that can turn portions of the brain into a sponge. The infective agents are prions (pree-ons), simple proteins normally beneficial to the nervous system, that can change shape and over 1 to 4 years become destructive to the brain. Several mammal species including humans have unique versions, such as Creutzfeldt-Jakob disease (CJD) and kuru.


CWD first appeared in 1967, when symptoms of a mysterious disease appeared in mule deer in the Colorado Division of Wildlife Research Facility at Fort Collins. There is no evidence CWD originated in the facility. In humans, CJD can develop spontaneously.


Most diseases can be divided into two types: density dependent and density independent. Density-dependent diseases require a highly dense population to spread. Where would you expect the next swine flu outbreak? New York City or a rural small town? Density-independent diseases require frequent direct contact; impact depends on the number of exposures. Because CWD isn’t density dependent, culling large numbers of deer won’t work. All attempts to “eradicate” CWD through culling have failed.

So, how are deer infected? Again, there isn’t much solid science to answer this deceptively simple question. The assumed mode of infection would be multiple contacts with an infected individual or infected materials. Studies often involve what I call “Frankenstein” experiments, in which a poor animal (mice, deer, ferrets, monkeys, etc.) has infected material force-fed or injected directly into their bodies or brains—not exactly a natural exposure.

>>Hot Spots: Where CWD is most prevalent

Experiments have involved very small sample sizes, sometimes three or less. In one study, a single whitetail fawn was injected with 250 mL of blood from CWD-positive deer; meanwhile, two more were injected intra-peritoneally (into the body cavity) with 250 mL of infected blood. All three became CWD positive after 18 months; yet, how many deer are exposed this way in the wild? Researchers force-fed three fawns each with 50 mL of infected urine and feces. One died right away; the others were negative after six months.

>>CWD Frequently Asked Questions

One of the early CWD research teams confined nine healthy mule deer, three each in three different pens; each contained carcasses of deer that died from CWD. Only 19 percent became infected. Then, three deer each were placed in three sets of three pens: three where infected deer had been present two years prior, three with a CWD-infected carcass, and three with a live infected deer. Only 16 percent of the CWD-free deer became infected, showing resistance to infection. It’s also been proposed it would take 33,000 gallons of infected urine to equal 1 gm of infected brain. Yet, some states have banned use of urine scent products.


In a 2018 publication (available at, Drs. Don Davis, Kenneth Waldrup, Greg Stewart and I combined 150-plus years of scientific experience to answer this and other questions. There are 3,144 counties in the United States, and in 2018, 196 (6.2 percent) had CWD-positive deer. Hence, 93.8 percent of the U.S. in 2018 was free of CWD. In the 23 states with CWD in 2018, there were 1,714 counties, 11.3 percent, with infections; so, 88.6 percentof these states’ counties were CWD free. Fifteen of these states have less than 10 counties with CWD.


Since CWD first appeared in the Midwest (Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, North and South Dakota, Nebraska, Ohio and Wisconsin), there have been 661,623 CWD tests, with 7,715 positives (1.17 percent). However, 66.7 percent of positives have been from Wisconsin, mostly from four counties. This should relieve some of your concern, as CWD indeed is a fairly rare disease on the whole.


I would never minimize the importance of any wildlife disease, much less CWD! But, it’s important to put disease into perspective. We concluded from these data, CWD (as with most TSEs) is a fairly rare disease at the national level, with some local areas having higher levels. In order for CWD to be a significant problem, it must have potential to devastate deer populations, and it must have the ability to infect humans.

Unless the answer is “yes” to each of these, CWD cannot be as devastating as some have reported. True deer populations (whitetails, mule deer and blacktails) have declined by about 19 percent since 2000; but, causal factors have not included CWD as a major factor (

Only two studies (Wyoming) to date report CWD as a significant mortality factor in whitetails or mule deer. Both involve modeling, and both received significant criticisms about interpretation of results. Both report a relatively low “assumed” mortality from CWD (14 percent). Most mortalities were attributed to lions and “undetermined.” The science is anything but settled.

>>Read More: Wisconsin Whitetails Under Siege: Chronic Wasting Disease Update

Recently, a Minnesota scientist said it’s a matter of time until humans are infected. This was said in spite of absence of published science to support his claim, just a single unpublished, un-replicated Canadian study. The National Institutes of Health found cynomolgus macaques (primates thought to be closely related to humans), cerebrally and orally exposed to infected brain matter, didn’t contract CWD after 13 years. Fifty years of known CWD in deer, from which millions of pounds of venison were consumed, has not led to one infected human.

Yet, there has been an important human impact on hunter recruitment. When CWD appeared in Wisconsin in 2001, three outdoor magazines published pieces claiming three men had died from eating CWD-tainted venison. We lost up to 12 percent of Wisconsin hunters, and they’ve never returned. I must question motivations of those who draw false conclusions from unpublished and unscientific studies.

The material presented here is from published science as of April 2018. I hope this has answered some of your questions about CWD.

Editor’s Note: Follow all applicable state and local guidelines for transportation of harvested elk, moose and deer, as well as any CWD-specific hunting regulations.In many areas where CWD has been found, state wildlife agencies offer CWD testing for harvested deer. Most agencies recommend testing harvested deer in areas with a high incidence of CWD. In some, it might be required. The Centers for Disease Control and Prevention (CDC) recommends that, to be cautious, any deer that tests positive for CWD not be consumed.


The states in red have confirmed cases of CWD in wild deer. Oklahoma has cases confined to captive elk herds. Widespread in the Rocky Mountain states and midwest, CWD has also been found in Virginia, West Virginia and several other eastern states.

By Carolee Anita Boyles

In the South, states with reported cases include Texas, Oklahoma, Arkansas, Tennessee and Mississippi. In Tennessee, for instance, a total of 24 deer tested positive for CWD this past hunting season, all from Fayette and Hardeman counties.

“The instance of more positives was fully expected, and this doesn’t change our plan of response or recent regulation changes made by the Tennessee Fish and Wildlife Commission,” said Chuck Yoest, CWD coordinator, in a statement earlier this year. “We do expect to find even more positives in the CWD Management Zone since we have increased sampling and the disease occurs there. Increased sampling is to determine disease prevalence and spatial distribution.”

>>See photo examples of animals stricken with CWD

In Oklahoma, the news is better. During the past five years of testing, no wild deer have been identified with CWD. In April, however, one elk from a farmed herd in Lincoln County tested positive for CWD. The 2-year-old bull elk died because of an injury and was routinely tested per the facility’s certified health plan. The facility was immediately quarantined, and the Oklahoma Department of Wildlife Conservation began testing wild deer near the facility for the disease. This is the second farmed elk herd in Oklahoma to have a confirmed case of CWD. The first one was in 1998; the disease did not spread from that herd into the general deer population.

In Mississippi, CWD is becoming fairly widespread in the northern and western part of the state. The Mississippi Department of Wildlife, Fisheries and Parks (MDWFP) has established two CWD Management Zones to try to manage the spread of the disease. During the 2018-2019 hunting season, the MDWFP received more than 7,000 test samples of hunter-harvested, road-killed and reported diseased deer. Of those samples, 19 were positive for CWD, including deer from Panola and Tallahatchie counties, new locations for the disease.

In Arkansas, CWD was first detected in Newton County on February 23, 2016, when a hunter-harvested elk tested positive. Two weeks later, on March 3, 2016, a deer from Newton County also tested positive. Although the Arkansas Game and Fish Commission (AGFC) established a containment zone, the disease has continued to slowly spread. During December 2018, a hunter-harvested deer from Scott County tested positive for CWD; this county was outside the original containment zone.

Dr. Jenn Ballard, state wildlife veterinarian for the AGFC, said this shows how CWD can slowly spread even when management regulations are in effect.

“We know bucks tend to carry a higher prevalence of the disease than does, and we know bucks can disperse long distances, potentially moving the disease across the landscape,” she said. “That is why the ACFC has partnered with taxidermists to help us collect samples as a free service to hunters.”


The situation with CWD in Texas is a bit more complex than it is in other states because of the variety of cervid species found there.

“CWD has been confirmed in the Waco Mountains of far west Texas,” said Mitch Lockwood, big-game program director for the Texas Parks and Wildlife Department. “That’s in mule deer. We only have mule deer in that part of the state. CWD has also been confirmed in mule deer, whitetail deer and elk up in the northwestern portion of our Panhandle. We’ve also found it in captive deer herds in south central Texas. It’s also been found on some release sites, what some states call shooting preserves. These are basically high-fence pastures that are adjacent to or surrounding their breeding facilities where they put the deer for hunting purposes.”

As with other states, Texas has rules in place to try to limit the spread of CWD, but Lockwood said completely preventing its spread is very unlikely.

>>Learn more about CWD in Texas

“We’re trying to minimize or prevent the geographic spread of this disease, but deer and elk can make some pretty large movements,” Lockwood said. “So, there is a risk of the disease expanded beyond the current area.”

Research into CWD and how to reduce its spread is ongoing, Lockwood said. “We’re in the process of designing a research project to see if we can utilize some other management techniques to further help in that effort,” he said. “There’s still a whole lot that’s unknown about the best way to manage this disease and keep its prevalence low. We’d like to see the CWD zones shrink, and that’s a possibility, but we also won’t be surprised if we see additional positives in those particular areas in a few years.”

For the rest of the states in the South, however, the news remains good at this time. No infected deer have been reported in Louisiana, Alabama, Georgia, the Carolinas, Kentucky and Florida. How long that will be the case remains to be seen, even with programs in place in every state to monitor deer-herd health and stay ahead of the progression of the disease.

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