Wwhen the buck died in October, it perished in what most people would consider the middle of nowhere, miles from the nearest road. But his last breath was not taken in an isolated corner of American geography. It succumbed to a long-feared disease in the backcountry of Yellowstone National Park, northwest Wyoming—the first confirmed case of chronic wasting disease in the nation’s most famous nature reserve.
for years, chronic wasting disease (CWD), which is caused by prions – abnormal, transmissible pathogenic agents – has been creeping across North America, with concerns raised mainly by hunters after seeing deer behaving strangely.
The prions cause changes in the hosts’ brains and nervous systems, leaving animals drooling, lethargic, emaciated, limping and with a distinct “blank stare” that has led some to call it “zombie heart disease.” It spreads through the cervid family: deer, elk, moose, caribou and reindeer. It is fatal, 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, said Dr. Thomas Roffe, a veterinarian and former chief of animal health for the Fish & Game ServiceA US federal agency.
“This case is putting CWD on the radar of widespread attention in ways it hasn’t been before—and that’s, ironically, a good thing,” he says. “This is a disease that has major ecological implications.”
Roffe has predicted for decades that CWD would reach Yellowstone, and warned that both the federal government and the state of Wyoming necessary to take aggressive measures to slow its spread. Those warnings went largely unheeded, he says, and now the consequences will play out in front of the millions who visit the park every year.
The area forms a large 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 move through Yellowstone and support populations of grizzly bears, wolves, cougars, coyotes and other scavengers.
The disease is a “slow-moving disaster”, according to Dr Michael Osterholm, an epidemiologist who is tracking the outbreak bovine spongiform encephalopathy, or “mad cow disease”.” – a related prion condition – in the United Kingdom, and is director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Dr. Cory Anderson recently earned his doctorate studying at Osterholm, focusing on pathways of CWD transmission. “We are dealing with a disease that is always fatal, incurable and highly contagious. Baked in the worry is that we don’t have an effective easy way to eradicate it, neither from the animals it infects nor the environment it infects.”
Once an environment is infected, the pathogen is extremely difficult to eradicate. It can last for years in dirt or on surfaces, and scientists report it is resistant to disinfectants, formaldehyde, radiation and incineration at 600C (1,100F).
Jumping the species barrier
In the USA and Canada, CWD has received attention not only because it affects large game animals, but also because of the possibility that it can jump the species barrier. Deer, elk and moose can infect livestock, other mammals, birds or even humans. Epidemiologists say the absence of an “overflow” case does not mean it will not happen. CWD is one of a group of fatal neurological disorders that includes BSE.
“The BSE [mad cow] outbreak in Britain provides an example of how things can get crazy overnight when a spillover event from, say, cattle to humans happens,” says Anderson. “We’re talking about the potential of something similar happening. 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 seen against a backdrop of dangerous emerging zoonotic pathogens moving back and forth on species barriers between humans, livestock and wildlife worldwide. Outbreaks occur as human settlements and agricultural operations push 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 diseased not be eaten.
The Alliance for Public Wildlife in 2017 estimated that 7,000 to 15,000 CWD-infected animals are unknowingly eaten by humans per year, and that number was expected to increase by 20% annually. In Wisconsin, where venison testing is voluntary, Anderson and Osterholm say many thousands of people likely ate 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 of 6,701 deer, elk and moose was tested. Disease was present in about 800 samples, suggesting infection rates are rising.
According to the US Geological SurveyCWD is now present in 32 states and three Canadian provinces.
Slow down the spread
In the wake of the confirmation of CWD in the park, the Yellowstone authorities are reviewing their strategy for surveillance and dealing with more sick animals in the future. Roffe says CWD virulence is “density dependent,” meaning infection rates are higher where large numbers of animals congregate.
Particularly problematic, he notes, are the controversial artificial feeding of wildlife by people. In Wyoming, the state and federal government operate nearly two dozen “feeding grounds” for elk, where more than 20,000 animals are given alfalfa to help them survive the winter. The practice is condemned by leading wildlife management organisations.
“The science of what is needed to help slow the spread of CWD is clear, and has been known for a long time,” says Roffe. “You don’t feed wildlife in the midst of a growing disease pandemic.”
Studies suggest that animals that some hunters consider rivals may actually be allies. Game predators such as wolves, cougars and bears are able to detect sick animals long before humans do, and will prey on them and remove them from the landscape. So far they have maintained immunity to disease.
A major policy discrepancy, conservationists say, is that Wyoming, Montana and Idaho, the three states that Greater Yellowstone Ecosystemwhich some estimate to span 90,000 sq km (35,000 sq mi), encourages the liberal killing of wolves and cougars for sport and livestock protection, even when it is unnecessary and may be counterproductive to controlling CWD.
“We’re still at the forefront of a scary disease event, and we don’t know where it’s going,” Roffe said. “There is a lot at stake for the Yellowstone ecosystem, and a lot at stake for all Americans who enjoy having healthy wildlife on the landscape.”
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