Chronic wasting disease (also called Zombie deer disease) is a transmissible brain disease somewhat related to Mad Cow Disease that affects deer, elk, reindeer, sika deer, and mooses. The disease has been detected in the United States, Canada, Norway, Finland, and South Korea.
Since the disease was first reported 50 years ago, sightings have expanded to several areas. According to the CDC, this disease is present in at least 24 states in the United States and has also spread internationally — there are likely more cases we don’t know about yet.
The disease was first noticed as veterinarians were reporting deer just “wasting away” — suffering from weakness, dehydration, weight loss, and confusion, among others. Deer suffering from this dreadful disease lose their fear of humans and coordination, appearing like “walking zombies”, hence the name.
Animals with chronic wasting disease may not show symptoms for several months or even years. But when the symptoms do develop, they can be devastating. Dramatic weight loss is the most striking one, along with stumbling and drooling. There is no real cure for the disease.
The disease isn’t caused by a virus or a bacteria, it’s caused by prions, misfolded proteins most commonly found in the central nervous system. Prions can cause several neurodegenerative diseases in animals and in humans, including Mad cow disease.
The problem is that these malfunctioning proteins can also be transmitted to other individuals. To make matters even worse, individuals with the disease tend to salivate and urinate more often, making them more likely to transmit the disease. Individuals with the disease are also more susceptible to hunting.
Jumping to other species
So far, the prion disease doesn’t seem to be able to jump to humans or livestock — but we’ve seen with the COVID-19 pandemic just how quickly the situation can change when diseases jump from one species to another. Prions don’t spread as viruses do, but prion diseases are still relatively understudied and there’s much we’ve yet to discover about them. Still, the less close contact there is between infected animal populations and humans, the lower the chance of the disease making the leap.
While we’re not exactly sure how big the risk is, taking steps to minimize it is always a good idea. In the meantime, several studies are already underway to help us better understand this disease, its spread, and how it can be tackled.
The US Department of Agriculture (USDA) is in the second phase of a study on Zombie deer disease. Along with researchers from Texas A&M University and Texas Parks and Wildlife, the USDA is working to identify deer in wild and farm herds that are more genetically susceptible to the disease.
“While the research is still in an early stage, it’s the first of its kind,” according to a June post on the USDA blog. “The hope is that continued analysis will enable us to effectively categorize animals as being minimally susceptible, moderately susceptible or highly susceptible to CWD. This will help herd owners make management decisions that may eventually help to breed the disease out of existence or at least significantly reduce its presence.”
A 2004 study found that the species barrier may not offer much protection in the long run.
“The foodborne transmission of (mad cow disease) to humans indicates that the species barrier may not completely protect humans from animal prion diseases,” the study states. “Conversion of human prion protein by CWD-associated prions has been demonstrated in an in vitro cell-free experiment, but limited investigations have not identified strong evidence for CWD transmission to humans.”
Animal studies also suggest that some species may be susceptible to the disease. In a study, monkeys that were fed meat from CWD-infected animals or came in contact with brain or body fluids from infected deer or elk could become infected, and the WHO warns that in general, all agents of known prion diseases should be kept from entering the human food chain. If the disease were to spread to people, the most likely pathway would be through eating infected deer or elk.
“These [protective] measures include the ban on meat and bone meal in animal feed, testing of slaughtered animals, systemic removal of “high-risk material” from carcasses and destruction of suspect and confirmed bovine cases as well as the control of animals potentially exposed at the same time,” the WHO says.
Another reason for concern is the American crow, which was recently evaluated as a potential vector for the disease. Prions, a 2013 study found, remain viable after passing through the bird’s digestive tract, and could therefore spread over extensive geographical ranges (since crows themselves cover large areas).
We’re not really sure just how dangerous Zombie deer disease can be in the long run for deer species or for humans, but for now, it’s best to reduce the risk as much as possible
Preventing the spread
As is often the case, minimizing interaction with wild animals (and this includes hunting) is a good way to prevent the spread of disease. The CDC offers the following tips to avoid exposure:
don’t shoot, handle, or eat meat from deer that behave unusually or seem sick — including roadkill;
if you must interact with an animal, wear rubber gloves;
don’t use household knives for cutting deer meat;
check state wildlife and public health announcements;
strongly consider testing the meat for disease before eating it;
if an animal tests positive for the disease, don’t eat its meat.
Other agencies in the US are trying to take steps such as instituting bans on salt licks and feeding wild deer. Prevention is extremely important as the disease is already starting to appear in other parts of the world.
Elsewhere, in Europe, the first case was observed in 2016 in a wild reindeer herd in southern Norway. The source of the Norway disease is unknown, especially as Norway has strict legislation that doesn’t allow the importation of live animals and deer into the country. In a few months, three other cases had been confirmed in the country. In March 2018, the Finnish Food Safety Authority also discovered the first case in the country. Sweden followed suit in 2019.
The European Union regulated chronic wasting disease surveillance runs between 2018 and 2020, testing a few thousand animals, both free-ranging, farmed, and semi-domesticated animals.
There’s no easy way out, and while there’s no reason to panic, researchers are keeping a cautious eye on the spread of this disease, trying to prevent and counter it as much as possible. Researchers are looking for ways to better diagnose the disease and maybe even develop vaccines for it. Since prions can persist in soils, disposing of infected carcasses is also a matter of concern.
Hunters also have to play their part, authorities warn. If we want to stop or at least stall the spread of the disease, hunters need to be much more careful when interacting with deer, and if possible, only doing so when they are certain the animals are disease-free.
Andrei's background is in geophysics, and he's been fascinated by it ever since he was a child. Feeling that there is a gap between scientists and the general audience, he started ZME Science -- and the results are what you see today.