"Zombie deer disease" is spreading across the globe.
Known more formally as chronic wasting disease (CWD), the infection eats away at the brains of deer, elk, and moose. It causes the animals to show signs of dementia, manifesting as difficulty walking and eating, before they ultimately die — and experts are now warning that the disease could make the jump from animals to humans.
"Zombie deer disease" is spread by prions, pathogenic proteins that can't be killed because they aren't alive. They're neither bacteria nor viruses, but once they infect an animal, they cause its cells to fold abnormally and clump together. Essentially, the prions "turn the brain into Swiss cheese," evolutionary biologist Peter Larsen told VOX.
CWD spreads between animals through direct or indirect contact with an infected animal's saliva, blood, urine, or other bodily fluids. As of January, 24 states and two Canadian provinces had reported cases of CWD, as have Norway, Finland, and South Korea.
In some locations, as many as 25 percent of animals are infected, according to the Centers for Disease Control and Prevention.
To date, no human has contracted CWD, even if they've eaten meat from an infected animal. However, a study published in the journal Emerging Infectious Diseases in August did show that prions from an infected animal could infect human cells in a petri dish, so experts are concerned CWD could make the leap to humans.
"It is probable that human cases of chronic wasting disease associated with consumption with contaminated meat will be documented in the years ahead," Michael Osterholm, the director of the Center for Infectious Disease Research and Policy, told the Minnesota legislature earlier this month. "It’s possible the number of human cases will be substantial and will not be isolated events."
Another prion disease — bovine spongiform encephalopathy, better known as "mad cow disease" — did make the leap to humans following an outbreak in cattle in the 1990s, and people are still dying because of it.
As Osterholm told VOX, "We don’t want to find out 10 years from now that we should have been doing something in 2019 but didn’t."
More on disease outbreaks: The WHO Is on the Lookout for the Disease That May Be the Next Global Pandemic