And scientists are concerned that the infection could make its way to humans.
Chronic wasting disease — or "zombie deer disease" — was first observed in 1967 in Fort Collins, Colorado, and has since infected wild herds in 24 states and Canada, as well as in South Korea and Norway, NPR reported.
"CWD passes from animal to animal through prions, misfolded proteins that cause other proteins to misfold around them," NPR reported. "Different prion diseases tend to only harm certain species, but can evolve to overcome those limitations."
In some herds, as many as half of the animals carry prions.
But direct contact isn't the only way prions are transmitted. According to The New York Times, sick animals and cadavers can spread prions through plants and soil, which could be coated with deformed proteins for years, perhaps even decades.
An animal infected with the disease can live two years before signs of symptoms -- such as a vacant stare, thick saliva, exposed ribs or drooping heads -- become visible.
There have been no reported human illnesses due to the disease, and scientists don’t have conclusive evidence that infected meat has ever harmed people, suggesting there is a “species barrier” between humans and deer.
Researchers led by Mark Zabel, associate director at Colorado State University’s Prion Research Center, found that macaque monkeys who ate infected deer contracted the disease, the first time the disease was shown to spread to a primate through meat.
"While most research shows there's a robust species barrier, this recent study showed that barrier might not be quite as robust as we once thought," Matt Dunfee, head of the Chronic Wasting Disease Alliancein Fort Collins, Colorado, told NPR.
Zabel and his team also found that the prions involved in the "zombie disease," which scientists have only known about for 50 years, are probably still evolving, "which leads us to believe it's only a matter of time before a prion emerges that can spread to humans," NPR reported.
Mad cow disease, for example, is a prion disease that evolved from scrapie, a deadly disease that afflicts sheep. Once the prions were passed to cows, the cows developed a prion disease of their own (mad cow disease). And when humans ate the beef from those sick cows, they developed prions in their own brains. As of 2016, according to the Food and Drug Administration, 231 people had died from the condition.
Zabel believes the only way to get rid of CWD prions is to set controlled fires. But "there's a lot that we still don't know and don't understand about the disease," Zabel said in an interview with The New York Times.
According to Michael Miller, senior wildlife veterinarian for Colorado Parks and Wildlife, mule deer transmission more than tripled toward the end of 2017, and CWD continues to be prevalent in Colorado.
Public health officials in the area have been monitoring for CWD and human brain-wasting diseases, such as Creutzfeldt-Jakob disease.
But over the past 21 years, rising rates of both diseases haven’t impacted human health.
Still, as a precaution, Dunfee told NPR, "if you are hunting in an area where CWD is found, have your animal tested. If it comes back positive, don't eat the meat."
Read the full study published in the "Microbiology and Molecular Biology Reviews" at mmbr.asm.org.
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