shows a deer with ribs showing through it's patchy fur, head hanging
This deer visibly shows signs of chronic wasting disease. Photo credit: Terry Kreeger, Wyoming Game and Fish and Chronic Wasting Disease Alliance/ USGS

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By Emily Davis

It’s been a while since Suzanne Kennedy-Stoskopf, a research professor at North Carolina State’s School of Veterinary Medicine, last held a lecture on chronic wasting disease, a transmissible, chronic, progressive neurological disease in deer.

The disease is spreading through many parts of the country but has not yet been detected in North Carolina. When Kennedy-Stoskopf recently checked in on the disease’s spread, she was shocked.

“We always used to joke that eventually, it would get here, but it’s clearly moving faster than anybody predicted,” she said.

Chronic wasting disease is just what it says: The disease slowly kills animals in the cervid family, primarily deer, but also moose and elk. The disease is wiping out some deer populations across the United States, but slowly.

According to the North Carolina Wildlife Resources Commission, modeling shows big declines, or even localized extinction of affected animals, over a period of 50 to 200 years following the arrival of the disease.

Since first showing up in the American West, CWD has inched its way into other parts of the country, including the Southeast. Positive cases have been found as close as western Tennessee and northern Virginia, and state lawmakers, conservationists and hunters are getting worried.

Part of their worry is whether the disease will ever make the jump to humans.

Sci-fi strangeness

“My presence here … tells us about the importance of everyone wrapping their head around this pretty significant disease problem,” said Mark Ruder, a researcher from the College of Veterinary Medicine at the University of Georgia, who gave a presentation to the House Wildlife Resources Committee at the North Carolina General Assembly in May.

The exact origin of CWD is unknown, but it was first observed in a Colorado deer in 1967. Ruder explained it wasn’t until years later that researchers recognized it as a form of transmissible spongiform encephalopathy (TSE), similar to bovine spongiform encephalopathy, also known as mad cow disease. There’s an exceedingly rare human form of genetic TSE too, known as Creutzfeldt-Jakob disease.

Top map shows the known distribution around 2000, however CWD may have been in other regions already, undetected.
2005: After CWD was detected in Wisconsin in 2002, states ramped up surveillance efforts, aided by federal assistance.
After 2011, federal support for state-based surveillance greatly diminished, states had to fund their own surveillance efforts. Some states were more aggressive than others.
2019: The latest CWD distribution map. Map, data credit: Bryan Richards, USGS

At present, only BSE has made the jump from cows to humans.

What these diseases have in common is they’re all caused by prions – what Ruder described as “abnormal forms of proteins.” He characterized prions as possessing a “sci-fi type strangeness.” They’re not viruses, they don’t carry genetic material. They’re not bacteria, they can’t be killed.

“Prions are not your typical infectious agents,” Ruder said. “In simple terms, this can kind of punch holes in the brain.”

Most CWD infected deer appear completely healthy, sometimes for years, until they’re not. He said clinical signs include a  lack of fear, weight loss, copious salivation or hair standing up straight.

Prions are not only shed by deer carcasses but can spread in the environment through urine, feces, saliva and even antler velvet. An infected deer is likely shedding contagious prions months, if not years before clinical signs appear, Ruder said. CWD can only be detected in the blood when animals are in the terminal stages of the disease or post-mortem sampling.

Getting those prions out of the environment is next to impossible. A study in Colorado found two years after a CWD-infected carcass had completely decomposed, animals coming in contact with the soil could still contract the disease.

“If you were to design a pathogen that you want to be the most formidable pathogen, there’s not a whole lot that you would do very much different,” Ruder said.

Spreading fast

Wildlife biologist Bryan Richards has been tracking CWD’s spread for 15 years, since he was hired by the U.S. Geological Survey’s National Wildlife Health Center, following the initial detection of CWD in Wisconsin. His work in mapping the distribution of the disease has shown a dramatic spread over the last 19 years.

“We know from experience that once disease becomes established in a free-ranging population, the odds of eliminating that disease are pretty negligible,” Richards said. And once the disease is in one environment, it can easily spread to another.

This photomicrograph of a brain tissue specimen, reveals the tissue changes that were found in a case of bovine spongiform encephalopathy (BSE), also referred to as mad cow disease. Note the presence of numerous vacuoles, or microscopic holes in the gray matter, imparting the spongy appearance to the brain of BSE-affected cows. Photo credit: U.S. Dept. of Agriculture – Animal and Plant Health Inspection Service, APHIS

According to Richards, those tiny prions have two primary ways of getting around: by animals and by people. Deer directly spread the disease to one another, in a slow diffusion that Ruder compared to a wildfire. When people get involved, Ruder said the spread is more like a “bus ride,” especially when humans move animals across state lines. Interactions with humans through activities such as deer farming and hunting allow the disease to  move long distances quickly.

Richards said that as the geographic footprint of the disease has changed and grown, people are taking the disease more seriously.

Rep. Jay Adams (R-Hickory), chair of the House Committee on Wildlife Resource Management, said he ranks the disease as the number one issue for which the committee must prepare.

“If this were to arrive in North Carolina by a natural pathway, it would take a long time, but if somebody delivered it to us in a truck, it could be this afternoon,” Adams told the committee.

He said he’s looked deeply into CWD and has become increasingly concerned.

“I’m burdening you with the knowledge I’ve been carrying around for about four years,” Adams said. “The first moment that you decide that this is significant, you never back away from it, it becomes more and more and more significant.”

A growing concern

CWD has not been found in any West Coast states and lacks a strong presence in the Southeast. According to Richards, there can be two reasons for this absence.

“A, CWD has not gotten to those locations, or B, it hasn’t been detected in those locations,” he said.

Recently in western Tennessee, widespread testing was conducted after ten deer tested positive for the disease. According to a wildlife researcher in the state, the results at the end of an extended surveillance season revealed 186 positive deer in three counties out of 3,132 animals taken, for an overall prevalence of 8.66 percent in the affected area.

CWD had likely been spreading unseen in the state for a long time before it was identified.

For Richards, state surveillance is key to preventing the spread of CWD.

Jonathan Shaw has been a state deer biologist with the North Carolina Wildlife Resources Commission for four years.

Shaw said the commission is doing everything to prevent the disease from coming to the state, but they are also preparing for the possibility that it may arrive despite their best efforts. This preparation includes a response plan and hunting activity restrictions.

The Commission has a rule prohibiting the importation of whole carcasses and high-risk carcass parts into the state. These high-risk parts include the nervous system, brain and spinal tissue, where the prions concentrate.

According to Shaw, they are also looking into the possibility of prohibiting the use of urine in baiting deer.

Hard conversations

For Luke Weingarten, the chair of the N.C. chapter of Backcountry Hunters and Anglers, CWD is a constant concern.

“We’re super happy that we don’t have it here,” Weingarten said. “Although I think that every responsible, thoughtful person probably needs to finish that sentence with the word ‘yet.’”

Adams told the committee that implementing necessary measures against the disease will prove a challenge for lawmakers.

For example, restrictions on hunters baiting for deer, such as the urine baiting ban suggested by the NC Wildlife Resources Commission, are expected to be contentious issues.

“In North Carolina, I think baiting is something that’s been happening culturally for so long, that that’s going to be a heavy lift,” Weingarten said. “But it’s a conversation that absolutely needs to be had.”

Weingarten said he expects hard conversations between the stakeholders involved in state wildlife, including hunters, conservationists and state lawmakers, but he is happy to see outreach and prevention happening.

“If that turns out to be science-based data telling us baiting ain’t great, then we need to take a good hard look at what we’re doing in terms of baiting wildlife, and deer specifically,” he said. “I expect an argument, but that’s ok, we can argue.”

Humans at risk?

Species barriers refer to the natural obstacle for diseases that prevent them from spreading between different species. They’re the reason you can’t give the flu to your dog, and the reason why a deer in your backyard shouldn’t be able to give you CWD.

There is currently no evidence of CWD infecting humans, but a 2017 experimental study suggests the possibility.

The decade-long, $7.9 million study by Canadian researchers used macaques as genetically similar human stand-ins and studied the spread of CWD in the primates under lab conditions. The study, which is still in progress, wanted to mimic the kind of exposure humans have with deer, including consumption, field dressing, and transfusion.

The researchers have found evidence of successful transmission. Two-thirds of macaques orally fed muscle tissue of infected deer had CWD by years five and six of the study.

It should be noted that, although the tissues used in the research are raw, CWD prions can’t be cooked out of venison, according to Ruder.

Nonetheless, the results of the Canadian study have been concerning enough that the Centers for Disease Control and Prevention published stricter recommendations in response.

According to Ruder,  the macaque study does not mean CWD is going to infect people.

“It tells us that the species barrier that we’ve wanted to rely heavily on for many years is maybe not as robust as we once thought it was,” Ruder said. “The potential is there.”

A different, 2018 study with macaques found no evidence of transmission.

Kennedy-Stoskopf said although macaques are the closest you can get to humans, it’s inconclusive with just two experimental studies.

“I know that I’ve eaten sheep that have been infected with scrapie (another prion disease) when I was living in Michigan, and that was 40 years ago,” Kennedy-Stoskopf said.

Another study suggested the prions could have a decades-long incubation period in humans.

The 2017 review article on CWD research, published in the journal, Transboundary and Emerging Diseases, said despite little evidence of human transmission, researchers hypothesize “a CWD prion more readily transmissible to humans could emerge over time given sufficiently extensive human exposure.”

Limited long-term studies and contradicting evidence means it is hard to make any sure conclusions on the risk CWD poses to hunters and the general public.

“The bottom line is the research is not definitive,” said Richards. “It’s likely a small risk that CWD could cross over into a human host, but the risk is not zero.”

Emily Davis

Emily Davis was the NC Health News summer intern for 2019. She's a senior at UNC Chapel Hill in the School of Media and Journalism.