WWhen the mule deer died in October, it perished in a place most people would consider the middle of nowhere, miles from the nearest road. But the last breath was not breathed in some remote corner of American geography. The animal succumbed to a long-feared disease in the backcountry of Yellowstone National Park, in northwestern Wyoming – the first confirmed case of chronic wasting disease in the nation's most famous wildlife refuge.
For years, chronic wasting disease (CWD), caused by prions – abnormal, transmissible pathogens – spread stealthily across North America, with concerns mainly raised by hunters after noticing deer behaving strangely.
The prions cause changes in the brains and nervous systems of the hosts, causing animals to drool, lethargic, emaciated, stumbling and with a telltale “vacant stare” that leads some to call it “zombie deer disease.” It spreads through the cervid family: deer, elk, moose, caribou and reindeer. It is deadly, with no known treatments or vaccines.
Its discovery in Yellowstone, whose ecosystem supports the largest and most diverse range of large wild mammals in the continental US, represents an important public wake-up call, says Dr. Thomas Roffe, veterinarian and former U.S. chief animal health officer . Fish and Game Servicea US federal agency.
“This case puts CWD on the radar of widespread attention in a way it hasn't before – and that, ironically, is a good thing,” he says. “It is a disease that has enormous ecological consequences.”
Roffe has predicted for decades that CWD would reach Yellowstone and warned that both the federal government and the state of Wyoming needed to take aggressive action to slow its spread. Those warnings were largely ignored, he says, and now the consequences will be felt for the millions who visit the park each year.
The area provides a vast laboratory for observing what happens when CWD infiltrates an ecosystem with its original full complement of biological diversity. Hundreds of thousands of elk and deer roam Yellowstone, supporting populations of grizzly bears, wolves, cougars, coyotes and other scavengers.
The disease is a “slowly progressing disaster,” said Dr. Michael Osterholm, an epidemiologist who has studied the outbreak of bovine spongiform encephalopathy, or “mad cow disease” — a related prion condition — in Britain and is director of the Center. in infectious disease research and policy from the University of Minnesota.
Dr. Cory Anderson recently completed his PhD at Osterholm, focusing on the routes of CWD transmission. “We are dealing with a disease that is always fatal, incurable and highly contagious. The concern is deeply rooted in the fact that we have no effective, easy way to eradicate it, either from the animals it infects or the environment it contaminates.”
Once an environment is infected, the pathogen is extremely difficult to eradicate. It can remain in dirt or on surfaces for years scientists report that it is resistant against disinfectants, formaldehyde, radiation and combustion at 600C (1100F).
Jumping over the species barrier
In the US and Canada, CWD has received attention not only because it affects big game animals, but also because of the possibility that it could bridge the species barrier. Deer, elk and moose can infect livestock, other mammals, birds and even humans. Epidemiologists say the absence of a “spillover” case does not mean it won't happen. CWD is part of a cluster of fatal neurological disorders that also includes BSE.
“The BSE (mad cow disease) outbreak in Britain was an example of how things can get out of hand overnight if there is a spillover from, for example, livestock to people,” says Anderson. “We're talking about the possibility that something similar could happen. No one is saying it will definitely happen, but it is important that people are prepared.”
Dr. Raina Plowright, a disease ecologist at Cornell University, says CWD must be viewed against the backdrop of dangerous emerging zoonotic pathogens that are moving across species barriers between humans, livestock and wildlife worldwide. Outbreaks occur as human settlements and agricultural activities penetrate deeper into environments where contact with disease-carrying animals increases.
With hunting season underway in the US, the US Centers for Disease Control and individual states strongly recommend that harvested game animals be tested for disease, and that meat from cervids that appear sick should not be consumed.
The Public Wildlife Alliance estimated in 2017 that 7,000 to 15,000 CWD-infected animals were unknowingly eaten by humans per year, and that this number was expected to increase by 20% annually. In Wisconsin, where testing of venison is voluntary, Anderson and Osterholm say many thousands of people have likely eaten meat from infected deer.
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Wyoming serves as a reference point for other states. Since 1997, 92,000 tissue samples have been collected and tested there, said Breanna Ball of the Wyoming Game and Fish Department. Last year, the meat from 6,701 deer, elk and elk was tested. Disease was present in about 800 samples, indicating that the number of infections is increasing.
This is evident from the American Geological SurveyCWD is now present in 32 states and three Canadian provinces.
Slowing the spread
After the confirmation of CWD in the parkYellowstone authorities are reviewing their strategy for surveillance and dealing with more sick animals in the future. Roffe says the virulence of CWD is “density dependent,” meaning infection rates are higher where large numbers of animals congregate.
Particularly problematic, he notes, is the controversial artificial feeding of wildlife by people. In Wyoming, the state and federal government operate nearly two dozen elk “feedlots,” where more than 20,000 animals are fed alfalfa to help them survive the winter. The practice is condemned by leading nature management organizations.
“The science of what it takes to slow the spread of CWD is clear and has been known for a long time,” says Roffe. “You don't feed wildlife in the face of a growing disease pandemic.”
Studies suggest that animals that some hunters view as competitors may actually be allies. Wildlife predators such as wolves, cougars and bears can detect sick animals long before humans do, and they will hunt them and remove them from the landscape. So far, they have retained their immunity to diseases.
A major contradiction in the policy, conservationists say, is that Wyoming, Montana and Idaho, the three states that make up the region, Greater Yellowstone Ecosystemwhich some estimate extends over 90,000 square kilometers (35,000 sq mi), encourages liberal killing of wolves and cougars for sport and livestock protection, even when it is unnecessary and may be counterproductive to controlling CWD.
“We are still on the cusp of a terrifying disease event, and we don't know where it is going,” Roffe said. “There is a lot at stake for the Yellowstone ecosystem, and a lot is at stake for all Americans who want healthy wildlife on the landscape.”
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