An infectious brain disease that has been killing deer, elk and moose both in the wild and on “captive farms” continues to stalk the land, expanding its domain to 23 states and two Canadian provinces since it was first identified in captive mule deer in a Colorado research facility in 1967.
Known as chronic wasting disease, or CWD, it has baffled scientists for decades. Where did it come from, and why is it spreading across the landscape? What health risks might it pose to humans who eat parts of infected animals? And can cattle get it from infected deer, elk, and moose, thus introducing it into the human food chain?
Recent research, primarily funded by the National Institutes of Health, has opened the door to one of the reasons this disease is so pervasive — and actually endemic in a large swath of land in northeastern Colorado and southeastern Wyoming.
According to researchers at The University of Texas Health Science Center at Houston (UTHealth), grass plants can bind, uptake and transport infectious prions. Why this is so important takes some understanding of what prions are.
Much smaller than bacteria, prions are single proteins that cannot be destroyed by typical “kill strategies” such as extreme heat or ultraviolet light.
“With prions, nothing like that works,” said Claudio Soto, Ph.D., a UTHealth researcher and lead author of an article about the topic published May 26, 2015, in Cell Reports.
These protein-based infectious agents cause the characteristic spongy degeneration of the brain, leading to emaciation, abnormal behavior, loss of bodily functions, and death. As such, they are responsible for a group of fatal diseases referred to as transmissible spongiform encephalopathy (TSE). The group includes so-called “mad cow disease” (bovine spongiform encephalopathy, or BSE) in cattle, scrapie in sheep, and variant Creutzfeldt-Jakob disease in humans, which, according to the World Health Organization, has been “strongly linked” to eating beef products contaminated with central nervous system tissue, such as spinal cord and brain, from cows infected with mad cow disease.
Soto’s team analyzed the retention of CWD and other infectious prion proteins and their infectivity in wheat grass roots and leaves that had been incubated with prion-contaminated material. They discovered that even highly diluted amounts of the material can bind to the roots and leaves. From there, they fed the wheat grass to hamsters, which became infected with the disease.
The team also found the infectious prion proteins in plants that had been exposed to urine and feces from prion-infected hamsters and deer.
In addition, the team found that plants can uptake prions from contaminated soil and transport them to different parts of the plant. By doing this, the plants can act as a carrier of CWD.
This means, Soto said, that plants may play an important role in environmental prion contamination and the horizontal transmission of the disease. (Horizontal transmission occurs when an infectious agent is transmitted between members of the same species.)
Scientists already knew that these CWD prions are good at binding to soil, especially clay-based soils, and that they can persist there. Soto said that when some of the soil where an infected dead animal had been buried was injected into research animals several years after it had been buried, the injected animals came down with prion disease.
The question then became: Can these prions bind to plants?
“Surprisingly, we found that they do bind to plants very efficiently,” he said. “Even more surprisingly, plants infected with the prions were able to transmit the disease when animals were fed the contaminated plants.”
Soto warns that there is a good possibility that prions have been progressively accumulating in the environment.
“We have to be careful about the potential dangers of this,” he said. “We need to take precautions.”
Again, these prions are persistent. For example, in 1985 when the Colorado Division of Wildlife tried to eliminate CWD from a research facility by treating the soil with chlorine, removing the treated soil, and applying an additional chlorine treatment before letting the facility remain vacant for more than a year, they were unsuccessful in eliminating CWD from the facility.
‘Nasty nature of this disease’
Matt Dunfee, coordinator of the Chronic Wasting Disease Alliance, told Food Safety News that the new research findings about grass plants provide laboratory and experimental documentation that prions do attach to plants.
“It underscores the nasty nature of this disease and the challenge it is to manage it on a natural landscape,” he said. “It’s hard to contain, especially when it spreads through the soil or on plants. We haven’t been able to eliminate it on a natural landscape known to be infected.”
He pointed out that an infected animal can shed the disease a lot over a year (via urine and feces). And its decomposing body can further infect the soil and therefore the plants.
What about humans?
Can humans who eat deer, elk or moose meat from animals infected with CWD get a human variant of this brain disease? That’s not a far-fetched question considering the strong link between mad cow disease and variant Creutzfeldt-Jakob disease in humans.
According to the Center for Food Safety’s timeline for mad cow disease, the first diagnosis of BSE as a new disease in cattle, similar to scrapie in sheep, occurred in 1986. In the early 1990s, the British government said that the disease presented no threat to humans — that cattle were a “dead-end host.”
But in 1995, three people died of variant Creutzfeldt-Jakob disease. The next year, the British government announced that mad cow disease could indeed be transmitted to humans. After that point, 4.5 million cattle were destroyed.
Livestock experts said the cows likely got BSE from eating ground-up sheep that had been infected with scrapie, which is also a prion-caused brain disease. Many people who believed that cows are vegetarians were horrified to learn that beef cattle were actually being fed ground-up dead animals and chicken litter.
With cases of the human form of mad cow disease being reported from other countries, including the U.S. and Japan, governments around the world passed bans on feeding anything to cattle that contained infectious materials, such as those from spinal cords and brains. Many slapped import bans on beef from countries that had cases of BSE.
As for CWD and any risks to humans, the alliance says there is currently no solid evidence that humans can get it from deer, elk or moose. Most researchers agree with that. According to the group, the diseases (BSE and CWD) are “distinctly different.”
The World Health Organization has also weighed in, stating that after reviewing the available scientific information, its conclusion is that “currently there is no evidence that CWD can be transmitted to humans.”
Even so, the alliance notes that public health officials recommend that human exposure to the CWD infectious agent be avoided as agencies continue to evaluate any potential health risk.
Dunfee added that Wyoming did a 13-year study in which cows were put in deer pens heavily infected with CWD, and none of the cows became infected.
The U.S. Centers for Disease Control and Prevention (CDC) reviewed clinical records and pathology studies of three cases of sporadic Creutzfeldt-Jakob disease in 1997-98 in young adults who had eaten venison, and that review failed to find a causal link to CWD.
The same was true for other cases involving people who had eaten deer or elk meat, although the CDC report contains these words of caution: “Surveillance for human prion diseases, particularly in areas where CWD has been detected, remains important to effectively monitor the possible transmission of CWD to humans.”
The report also pointed out that “because of the long incubation period associated with prion diseases, convincing negative results from epidemiological and experimental laboratory studies would likely require years of follow-up.”
Soto would agree. He said that even though there have been no confirmed cases of infections in humans from CWD, the public should know that “it’s a possibility that needs to be explored.”
“I don’t want to scare people,” he said, “but these (CWD) prions are accumulating, and prions have a long incubation period — sometimes as long as 30 to 40 years in humans.”
Food safety tips
CDC advises hunters that “to minimize the risk for exposure to the CWD agent,” they should “consult with their state wildlife agencies to identify areas where CWD occurs and continue to follow advice provided by public health and wildlife agencies.”
The agency also advises hunters to avoid eating meat from deer and elk that look sick or test positive for CWD. They should wear gloves when field-dressing carcasses, bone-out the meat from the animal, and minimize handling of brain and spinal cord tissues. As a precaution, they should avoid eating deer and elk tissues known to harbor the CWD agent (e.g., brain, spinal cord, eyes, spleen, tonsils, lymph nodes) from areas where CWD has been identified.
In addition, hunters should wash their hands and instruments thoroughly after field dressing is completed, and they should request that their animal is processed individually, without meat from other animals being added to the meat from their animal.
While out in the field, hunters should be on the lookout for animals that are unusually thin and exhibit behavior such as having trouble walking, as well as those acting tame around humans and allowing someone to approach them. Such circumstances should be reported to a state wildlife agency.
Dunfee said that hunters in most CWD areas are required to turn in the heads of any deer, elk or moose they kill so it can be examined for signs of CWD.
“Hunters are massively helpful in providing surveillance for wildlife agencies,” he noted.
Some states also prohibit “baiting and feeding,” which would include putting out salt for animals because that entices the deer, elk or moose to congregate in an area, which in turn can make them more susceptible to infection with CWD. Also, many states test deer or elk that have been killed by cars primarily because animals with advanced CWD are particularly vulnerable to that sort of accident.
It’s not just hunters who can be exposed to CWD. There are many “captive farms” in the U.S. and elsewhere where deer and elk are raised for their meat, antlers and hides. In some cases, the animals are confined in large fenced areas on ranches or farms so hunters can come in and shoot them.
“It’s a huge industry,” said Dunfee.
The meat and other products are sold both domestically and overseas. But precautions need to be taken. In 2008, elk meat sold at a farmer’s market in Longmont, Colorado, was found to come from a captive elk infected with CWD.
But CWD can also be endemic in some wild herds, and though states have tried various methods to “wipe it out,” nothing has worked, although progress has been made in monitoring it.
“We’re probably going to have to deal with this (CWD) in perpetuity,” Dunfee said. “There probably will never be a silver bullet. The reality is that you’d never get all the infected animals on the landscape.”
Even so, states without CWD are taking measures to try and keep it out of their state.
Kristin Mansfield, state wildlife veterinarian for the Washington State Department of Fish and Wildlife, told Food Safety News that back in the 1990s, Washington passed legislation prohibiting most deer and elk farms and imposing strict regulations on importing deer, elk or moose.
“We’re so fortunate they did that,” she said. “It was probably the best thing we could have done. I think by having these rules, we lowered the risk for bringing CWD into the state.”
And though some states with captive farms haven’t found any cases of CWD in wild animals, Mansfield said that animals often escape from these farms.
Despite the absence of CWD in her state, Mansfield said that wildlife officials know they need to keep “an ear to the ground.” They check out any reports of sick animals and keep informed about where the disease is located. Fortunately, it hasn’t been been reported in surrounding states or British Columbia to the north.
“So far, we’re feeling as secure about this as possible,” she said.