We’re not ending 2025 without a last bit of awful news: Wildlife officials are sounding the alarm about a dangerous brain illness spreading among deer in North America.

Over the last several weeks, officials in Arkansas, Wisconsin, and Manitoba, Canada, have reported cases of chronic wasting disease (CWD) in new areas. The mad cow-like illness can spread easily between deer and has a 100% fatality rate. In addition to its ecological impact, the infection may also pose a potential health threat to humans, particularly hunters.

The zombie protein

Prions are perhaps the strangest pathogen around.

They’re the misfolded form of a normally harmless protein found in the brain (also confusingly called the prion protein). When a rogue prion encounters its regular counterpart, it somehow forces the latter to misfold as well, almost like a zombie infection. Over time, this amassing army of bad prions will destroy the animal’s brain, leading to death.

Prion diseases have been found in many animals, humans included. The most infamous one might be bovine spongiform encephalopathy, better known as mad cow disease. Much like mad cow, deer and related cervids afflicted with CWD will experience neurological symptoms like confusion, trouble walking, and rapid weight loss (the titular wasting).

CWD was first discovered in the U.S. over 50 years ago, though it took longer for scientists to determine that prions are behind it. And it’s been spreading further across North America ever since. Cases have now been documented in at least 36 U.S. states and four Canadian provinces.

In mid-December, the Arkansas Game and Fish Commission reported three cases of CWD in hunter-harvested white-tailed deer. These cases were found in Grant and Sevier counties, where CWD had never previously been detected (the first ever case in Arkansas was reported in 2016). Last week, the Wisconsin Department of Natural Resources confirmed its first positive CWD test result of a deer in La Crosse County. That same week, the Manitoba Natural Resources and Indigenous Futures reported five new cases of CWD, two of which were in previously undetected areas (the first CWD case in Manitoba was documented in 2021).

Why CWD matters

The increasing presence of CWD in North America is concerning for several reasons.

CWD can be transmitted relatively easily, compared to other prion diseases like mad cow and Creutzfeldt-Jakob disease (CJD), the most common, but still rare, prion disease that affects humans. CWD can spread directly through contact with infected animals and bodily fluids like saliva, and might also spread through prions left behind in the environment by urine or feces. That means CWD has the potential to cause sustained outbreaks of illness in both free-ranging and captive deer populations.