Scientists watch deer disease for species jumps

Chronic wasting disease has now spread to 25 states, including Missouri. Infected animals show drastic weight loss (wasting), lose coordination, become listless and weak.Photo courtesy Missouri Department of Conservation.

For the second year in a row, chronic wasting disease made inroads into Southwest Missouri.

Not quite the Joplin area. Not yet, anyway.

And not in any dramatic numbers.

But this hunting season, two more cases were confirmed in Stone County and one more in Taney County on top of the single case each that those counties reported in the 2018-19 season, which marked the first time the deer disease had been picked up in this part of the state.

Just across the state line, though, the story is different.

Nearly 800 CWD cases have been reported in deer and elk in Northwest Arkansas since the first confirmed positive there in 2016.

Boone County, Arkansas, just south of Branson, has reported more than 100 CWD cases in deer; nearby Carroll County has had nearly as many.

Missouri experts, who have been keeping a close eye on the border since the Northwest Arkansas outbreak, believe it is just a matter of time before more CWD cases appear in more counties in Southwest Missouri.

While they attempt to limit the disease in Missouri and Arkansas, other experts have been making inroads on another front — trying to understand just what CWD means for hunters and others who eat venison. Does this disease have the ability to jump species — to spread from cervids (deer, elk and moose) to humans who might eat venison from an infected animal?

One of those experts is Brent Race, a veterinarian and scientist with the National Institutes of Health's Laboratory of Persistent Viral Diseases in Montana who has been leading some of the latest studies.

Race, who is a hunter who regularly eats venison, said that with everything he knows, he's comfortable eating deer and elk as long as it is not positive for CWD. Yet he acknowledges there are unanswered questions, particularly as the disease continues to spread around the country as well as other parts of the world.

"As the range and prevalence of CWD increases, so will the potential for human exposure to CWD prions," he wrote in the Journal of Virology in 2018. "It is currently unknown if CWD poses a risk to human health. However, determining this risk is critical to prevent a similar scenario as what occurred when mad cow disease was found to be transmissible to humans."


"Unknown" — that's really the word that follows hunters into their deer blinds and hangs around with them at night in their deer camps. Race not only used it in his scientific report but also during an interview last week.

CWD, according to the federal Centers for Disease Control and Prevention, is one of a group of prion diseases — a type of brain disease that, while rare, is fatal in humans and animals.

The diseases are caused by an infectious agent called a prion, "abnormal, pathogenic ... transmissible and are able to induce abnormal folding of specific normal cellular proteins called prion proteins that are found most abundantly in the brain," the agency warns.

In animals, the diseases include bovine spongiform encephalopathy, or mad cow disease, and there is a link, according to federal experts, between BSE in cattle and variant Creutzfeldt-Jakob disease in humans who ate infected beef. Creutzfeldt-Jakob is a progressive neurodegenerative disease that is always fatal in humans.

"Prion diseases," according to the National Institutes of Health, "are a significant public health concern and have been known to spread from animals to people and, in the case of variant Creutzfeldt-Jakob disease, from human to human through blood."

CWD was first detected in Colorado in 1967, and since then, it has spread to 25 states. It showed up in Missouri a decade ago in captive deer at a private breeding and hunting operation in the northern part of the state. Since then, it has appeared in two other clusters, one around Ste. Genevieve County along the Mississippi River and a handful of times in Cedar, St. Clair and Polk counties, just a few years ago.

And then, in the fall of 2015, a hunter near Ponca, Arkansas, bagged an elk that was the descendant of Rocky Mountain elk brought to that state in the early 1980s, some from southern Colorado and some from Nebraska, and reintroduced in the Ozarks. The Ozarks had historically been home to a different subspecies, eastern elk, that was ultimately hunted to extinction in the 19th century.

That elk in 2015 tested positive for the disease, which was the first confirmed case in Arkansas.

Since then, there have been 770 confirmed cases in that state, as of Dec. 3, with more than half of those in Newton County, Arkansas, and the rest in mostly surrounding counties, including Boone and Carroll.

Jenn Ballard, state wildlife veterinarian for the Arkansas Game and Fish Commission, said last week that the "high prevalence" of the disease in their state in such a short time indicates it was probably on the landscape — meaning in the state's deer and elk herd — for a while before it was detected.

An Arkansas map of those infected deer shows some positives about as close to the Missouri line as possible, and during the 2018-19 hunting season in Missouri, it showed up for the first time in the southwest part of the state, in Stone and Taney counties. There also were five positives in Oregon County, farther east but also on the state line.

Species barrier

As the disease spreads, scientists have been exploring CWD's ability or inability to jump species barriers, and one of those leading that research is Race.

In a 2009 scientific paper, he noted that tests showed cross-species transmission was possible, with CWD spreading to ferrets and minks when it was injected into the brain but not when fed to them. Hamsters, too, were susceptible, as are some "transgenic" mice, meaning the mice had their genome altered to include the DNA of other animals, including deer.

While mice modified with deer DNA were susceptible to CWD, those mice modified to produce human prion protein have not yet been susceptible to CWD. In the 2009 study, Race and his colleagues called that "a finding that provided evidence for a human species barrier against CWD infection."

However, work that began in the 1980s and was published in 2005 by other scientists showed that squirrel monkeys infected with the disease from a mule deer came down with the disease in less than three years.

"This evidence that at least one species of nonhuman primate was susceptible to CWD weakened the conclusion that humans may be protected from CWD by a species barrier," the study by Race and colleagues concluded.

Race said it was not surprising that squirrel monkeys, which are known to be vulnerable to other prion diseases, tested positive.

He argued that cynomolgus macaques are evolutionarily closer to humans than squirrel monkeys and offer a more accurate model to test that human species barrier.

His 2009 study also showed no signs of CWD passing to the lab's macaques: "At nearly six years post-inoculation, no macaques have shown clinical signs of CWD. Intracerebral inoculation of cynomolgus macaques with BSE causes disease in three years; human variant CJD requires two to three years, and human sporadic CJD requires five years."

That again gave them hope that a barrier existed.

In 2014, Race and his colleagues published a follow-up study, having observed the macaques for four more years.

Among those conclusions: "No evidence of prion infection was detected. ... The lack of CWD transmission during (greater than) 10 years suggests that a substantial species barrier exists between cervids and cynomolgus macaques.

"Although these nonhuman primates are not exact models of human susceptibility, they support the data ... that suggest humans are at a low risk of contracting CWD. Nevertheless, it remains sensible to minimize exposure to tissues potentially contaminated with the CWD agent."

Canadian warning

However, in 2017, researchers in Canada studying CWD in the same kinds of macaques presented preliminary findings from their work at a conference and noted there was some evidence CWD might have passed to macaques from infected deer.

That work is ongoing, and the team of researchers has not published its findings in a peer-reviewed journal, which is the traditional route of vetting scientific work, and whether their preliminary finding holds up to scrutiny remains to be seen.

Attempts to speak to an expert at the U.S. Centers for Disease Control and Prevention about these studies were unsuccessful, but the agency instead of an interview forwarded copies of a report from Health Canada, a department of the Canadian government responsible for national public health. While that report noted there has been no known transmission of CWD from cervids to humans, it noted that both intercranial injections and being fed muscle tissue from infected deer may have resulted in CWD infections in the macaques in Canada.

"While extensive disease surveillance in Canada and elsewhere has not provided any direct evidence that CWD has infected humans, the potential for CWD to be transmitted to humans cannot be excluded. In exercising precaution, (the agency) continues to advocate that the most prudent approach is to consider that CWD has the potential to infect humans," the report said.

Yet two years later, the study has yet to be published and put through scientific paces, which means that so far, the only published scientific studies looking at CWD and macaques have not found evidence of transmission.

Meanwhile, Race and colleagues continued their study, updating it again in late 2018, with ongoing observations of macaques that had been exposed to infected venison.

The findings: "No evidence for the transmission of CWD to (cynomolgus macaques) using a broad range of data including clinical, pathological and biochemical observations."

Those findings, as well as six studies involving transgenic mice that produced human proteins, has led Race to conclude: "We can say there is evidence of a strong species barrier," at least where CWD and humans are concerned.

Those 'unknowns'

Race's advice, as both a scientist and a hunter:

• Have any deer or elk that has been killed tested for the disease, and do not eat meat from animals that test positive for CWD.

That advice is echoed by experts in Missouri.

To date, Missouri has sampled more than 160,000 deer, according to Jasmine Batten, wildlife disease coordinator with the Missouri Department of Conservation. So far, there have been 141 positives statewide, including those five in Stone and Taney counties.

"Based on our surveillance, we feel it is relatively rare in the state," Batten said.

They also feel confident, based on surveillance, they are picking it up early when it appears.

"The longevity of the surveillance we have done since 2002 gives us some confidence that we know where the disease is. However, we are taking aggressive management and surveillance actions to keep the disease as rare as possible because we believe it is a significant threat to our resources," Batten said.

• Hunters who want to be extra cautious can hunt in areas that remain free of the disease.

• Wear latex gloves when field dressing the animal.

Race also notes that in the more than 50 years the disease has been on the landscape, there has been another experiment of sorts. An unknown number of hunters and others have eaten an unknown number of infected deer and elk, over a long period of time, but there has been no human infection.

"Every year that goes by without another human case, I feel more comfortable," he said in an interview.

Still, other "unknowns" persist.

Race, Batten and Ballard all say that the incubation period can be long for a deer or elk after exposure to CWD. Race said it could be as long as two years, yet in the very early stages of the disease in cervids — perhaps the first month or so — CWD may not be detectable, which means that a negative test doesn't always guarantee the animal hasn't been exposed.

Race said some states are using the phrase "did not detect" rather than saying the test is negative.

The other unknown: new strains of CWD. All three scientists and wildlife experts — Race, Batten and Ballard — said that is a new concern.

"We are learning more and more all the time about strains," Ballard said. "That is something that has really come to light in the last few years."

And the possibility of different strains, coupled with the fact that CWD continues spreading, creates a scenario that makes it possible that one day the country might "stumble into one with an unfortunate outcome."

"It increases the statistical chances," Ballard said.

These scientists agree on one thing: For hunters, right now, it is important that they understand they are assuming some risk regarding CWD because of those unknowns.

"I don't think there is any reason for hunters to panic," Batten said, but added: "CWD has been changing hunting as we know it. There's no going back."

Andy Ostmeyer is the metro editor at the Globe. His email address is

Recommended for you