Does 'zombie deer disease' pose risks for humans?

'Zombie deer disease' is spreading among wildlife. But scientists haven't reached an agreement on whether the condition can spread to humans.

Chronic wasting disease is a type of prion disease, according to the Centers for Disease Control and Prevention in the United States. It’s a neurodegenerative condition that infects wildlife, including deer. No cases have yet been reported in humans, but it has most recently been confirmed in deer, as well as moose in Canada, after previous cases were reported in Yellowstone National Park in the United States.

However, as transmission increases across North America, Scandinavia and South Korea, some experts raise the concern that human transmission could be on the horizon. They're basing their concerns on experimental studies, the history of other prion diseases transmitting from animals to humans (which is extremely rare) and the potential impacts of climate change.

"As of yet, there has been no transmission from deer or elk to humans," says Jennifer Mullinax, associate professor of wildlife ecology and management at the University of Maryland. "However, given the nature of prions, CDC and other agencies have supported all efforts to keep any prion disease out of the food chain."

Chronic wasting disease, or CWD, is transmitted in cervids, which are hoofed ruminant mammals, such as deer, reindeer, elk and moose. Although it is an infectious disease, CWD is not caused by a bacterium or a virus, according to the US Department of Agriculture's Animal and Plant Health Inspection Service.

Instead, a misfolded prion protein causes the issue, but researchers don't yet know what causes the protein to become abnormal. Normal prion proteins potentially play a role in cell signaling. But when misfolded, they cause more proteins to misfold.

Misfolded prion proteins in the brain kill brain cells and cause bodily dysfunction, leading to unusual symptoms. Symptoms include weight loss, excessive drinking and urination, poor balance and coordination, drooping ears and difficulty swallowing. The difficulty swallowing can lead to drooling and eventually pneumonia and death. The classic symptoms – and the image of an uncoordinated, stumbling and drooling animal – have led to the term "zombie deer disease". The symptoms can take months or years to manifest, making a visual diagnosis difficult.

When prion proteins are misfolded, they become infectious, which leads to spread among wildlife. CWD spreads from animal to animal through direct contact with bodily fluids and waste and through indirect contact with contaminated soil, water and food.

The CDC estimates that in areas where the prion disease is endemic, infection rates range from 10% to 25%. In 2023, surveillance results from the Canadian province of Alberta suggest a 23% positivity rate for mule deer.

Current evidence does not show that CWD can be spread to humans when they eat the meat of an infected animal, encounter infected wildlife, or drink or touch contaminated soil or water. But researchers continue to investigate whether animal-to-human transmission is possible. "The current body of research is a mixed bag, meaning we don't know yet," Mullinax says.

Older research from the CDC, published in 2011 used the Foodborne Diseases Active Surveillance Network 2006-2007 population survey to assess exposure risk. The survey included results from more than 17,000 participants. More than 65% of the total respondents reported eating wild game at least some of the time. The researchers were looking only at the prevalence of potential exposure, but they reported that no evidence of human transmission had been found to date.

However, prion diseases, also called transmissible spongiform encephalopathies, are present in humans. These include Creutzfeldt-Jakob Disease, which is an inherited condition, and variant Creutzfeldt-Jakob disease. The latter, researchers have now confirmed, is caused by the same infectious agent that leads to bovine spongiform encephalopathy, or BSE, also called "mad cow" disease.

Variant Creutzfeldt-Jakob disease, or vCJD, was first discovered in 1996, in the United Kingdom. But mad cow in cattle was discovered about a decade prior. About 230 cases of vCJD have been reported globally across 12 countries.

"The situation with vCJD – caused by zoonotic transmission of the BSE prion from consuming infected beef – serves as perhaps the best example of what a potential crisis of CWD transmission to humans could look like, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota.

"However, it’s important to note that BSE and CWD prions differ structurally and we do not yet know whether the pathology and clinical presentation would be comparable if CWD transmission to humans were to occur," he adds.

In 2004, CDC researchers looked at cases of the human prion disease CJD in Wyoming and in Colorado from 1979 to 2000. The latter state is where CWD was first discovered in 1967. The researchers were partly investigating whether non-inherited cases were present and if they were related to zombie deer disease. Colorado reported 67 cases of CJD, and Wyoming 7. The incidence of CJD in these states proved similar to that of the rest of the United States. And the researchers report that, at the time, only two non-familial cases of CJD in people who ate venison in endemic areas had ever been reported.

Although CJD is generally an inherited disease, it was also previously transmitted through medical procedures. However, no cases of this type of transmission have been reported since the mid-1970s when better sterilization procedures were implemented, according to the CDC.

While older studies did not uncover definitive evidence of animal-to-human transmission of CWD, that doesn't rule out the possibility. Newer research has raised concerns.