Chronic wasting disease (CWD) was first recognized in 1967, making it nearly six decades old as a known disease. A research team in Colorado identified it in a captive herd of mule deer, though at the time they didn’t fully understand what they were dealing with. Since then, CWD has spread from that single facility to 36 U.S. states, four Canadian provinces, South Korea, and parts of Scandinavia.
First Detection in Captive Deer
The story of CWD begins at a wildlife research facility in Colorado, where scientists noticed captive mule deer wasting away without an obvious cause. The animals lost weight, became listless, and eventually died. Researchers documented this unusual syndrome in 1967, but it took years before anyone understood the underlying biology. CWD turned out to be a prion disease, part of the same family of brain-destroying conditions that includes mad cow disease in cattle and scrapie in sheep. Unlike infections caused by bacteria or viruses, prion diseases are driven by misfolded proteins that essentially convince normal proteins in the brain to misfold too, creating a chain reaction that riddles brain tissue with tiny holes.
Jump to Wild Populations
For its first 14 years on record, CWD appeared to be confined to captive deer. That changed in 1981, when the disease was identified in wild elk in Colorado. This was the moment CWD went from a curiosity in research herds to a wildlife management crisis. Once prions circulate in free-ranging animals, containment becomes extraordinarily difficult. Infected deer shed prions through saliva, urine, and feces, and those prions bind to soil particles where they can remain infectious for years. A Wisconsin study tested soil samples from a depopulated deer farm 13 years after the animals were removed and still found reason to investigate whether prions persisted.
That environmental durability helps explain why CWD has been so hard to stop. Healthy deer can pick up prions by grazing on contaminated ground, drinking from contaminated water sources, or through direct nose-to-nose contact with infected animals.
Decades of Slow, Steady Spread
Through the 1980s and 1990s, CWD expanded outward from its original range in northeastern Colorado and southeastern Wyoming. The movement of captive deer and elk between farms accelerated that spread considerably. By 2001, South Korea detected CWD in elk imported from Canada, marking the disease’s first appearance in Asia. It has since been found in parts of the Scandinavian Peninsula as well.
As of April 2025, CWD has been confirmed in free-ranging deer or elk in 36 U.S. states and four Canadian provinces. Captive deer and elk facilities in 22 states and three provinces have also reported cases. The known susceptible species include white-tailed deer, mule deer, black-tailed deer, Rocky Mountain elk, and moose.
Why It Took So Long to Diagnose
One reason CWD spread quietly for years is that infected animals look perfectly healthy for a long time. The incubation period averages 18 to 24 months, meaning a deer can carry and shed prions for well over a year before showing any visible signs. When symptoms do appear, they include dramatic weight loss, stumbling, excessive drooling, and a blank or listless demeanor. By that point, the animal has likely already spread prions across its home range.
Reliable testing didn’t arrive until 2003, when the first rapid post-mortem test was licensed. It used tissue from lymph nodes in the throat and could return results in about 24 hours. A second-generation test followed later that year. It wasn’t until 2008 that researchers developed a test capable of detecting CWD in living elk, a significant step forward since earlier methods required killing the animal first.
Risk to Humans
Given that mad cow disease can infect people, the question of whether CWD poses a similar threat has been a concern since the 1990s. So far, the evidence is reassuring but not definitive. In laboratory studies, CWD prions readily infected squirrel monkeys, but squirrel monkeys are a distant relative of humans. When researchers tested CWD in macaques, a species much more closely related to us, no animals showed signs of prion disease even 13 years after being deliberately exposed. By contrast, mad cow prions transmitted to macaques easily and caused rapid, severe neurological decline.
More recent work using lab-grown human brain tissue has also failed to show CWD transmission. These findings suggest a significant species barrier between deer prions and human biology, though wildlife agencies still recommend that hunters have their deer tested before eating the meat in areas where CWD is present, and that no one consume meat from an animal that tests positive.
Where Things Stand After 58 Years
CWD has gone from an unexplained illness in a single Colorado research facility to one of the most persistent wildlife diseases in North America. No state or country has successfully eradicated it from a wild deer population once it has become established. The prions that cause it don’t break down easily in the environment, there is no vaccine, and no treatment exists for infected animals. Wildlife managers have tried culling, banning the transport of live deer, and restricting carcass movement across state lines, but the geographic footprint continues to grow. What started as a local oddity in 1967 is now a continental problem with no clear solution on the horizon.