We won’t end 2025 without some terrible news: Wildlife officials are sounding the alarm about a dangerous brain disease spreading among North America’s deer.

In the past few weeks, officials in Arkansas, Wisconsin, and Manitoba, Canada, have reported cases of chronic wasting disease (CWD) in new areas. Mad cow-like disease spreads easily among deer and has a 100% mortality rate. In addition to its ecological impact, the infection may also pose a potential health threat to humans, especially hunters.

The zombie protein

Prions are perhaps the strangest pathogens around.

It is the wrong form of a normally harmless protein found in the brain (also called prion protein). When a rogue prion finds its regular counterpart, it somehow forces the latter to unfold as well, almost like a zombie infection. Over time, this gathering army of evil prions destroys the animal’s brain, leading to death.

Prion diseases have been found in many animals, including humans. The most famous is probably bovine spongiform encephalopathy, better known as mad cow disease. Like mad cow, deer and related cervids infected with CWD experience neurological symptoms such as confusion, trouble walking, and rapid weight loss (the titular wasting).

CWD first discovered in the US more than 50 years ago, although it took longer for scientists to determine that prions were behind it. And it has spread throughout North America since then. There are cases now documented in at least 36 US 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 has never been found before (the first case in Arkansas was reported in 2016). Last week, the Wisconsin Department of Natural Resources confirmed the first positive CWD test result in a deer in La Crosse County. In the same week, Manitoba Natural Resources and Indigenous Futures reported five new cases of CWD, two of which were in areas where it had not been found before (the first case of CWD in Manitoba was documented in 2021).

Why is CWD important?

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

CWD is easily transmitted, compared to other prion diseases such as mad cow and Creutzfeldt-Jakob disease (CJD), the most common, but still rare, prion disease affecting humans. CWD can be spread directly through contact with infected animals and bodily fluids such as saliva, and can also be spread through prions left in the environment through urine or feces. That means CWD has the potential to cause ongoing disease outbreaks in both free-ranging and captive deer populations.

There is also the daunting, if only theoretical, risk of human transmission. Mad cow has sparked outbreaks of CJD in humans, caused by people eating contaminated meat. There is a possibility of a similar phenomenon occurring in hunters who eat meat contaminated with CWD. Last year, for example, a group of researchers contended in a case report that two hunters who died from CJD may have been infected after eating CWD-infected deer.

These cases have not been completely tied to CWD, and other scientists skeptics about the connection. Experimental studies also provide mixed evidence for any risk of transmission to humans, at least for now.

However, CWD remains a real and growing threat to deer, as well as the human industries that depend on them. So even if CWD doesn’t directly harm us, it can cause a serious ecological disaster — large outbreaks can devastate local deer populations, which in turn affect other species in the area, or cause the closure of deer farms, a $7.9 billion industry. To that end, Wisconsin officials are expanding an existing bait and feeding ban in La Crosse County (bait can attract large populations of deer to an area). Hunters, meanwhile, are advised not to eat deer that have not been tested for, or confirmed to have, CWD.