Chronic wasting disease, a deadly neurological disease affecting deer, elk and moose, is spreading across North America. Most recently discovered in North Carolina in March 2022, CWD has been confirmed in 30 U.S. states and four Canadian provinces, as well as in Norway, Finland, Sweden and South Korea. Dr. Alan Houston, professor of forest and wildlife ecology at the University of Tennessee’s 18,400-acre Ames Agricultural Research and Education Center in West Tennessee, explains what you know about CWD and what wildlife scientists are trying to understand.
How does chronic wasting disease affect animals?
Chronic wasting disease is contagious and relentless. There is no cure, no way to test live animals, and no realistic way to stop the spread of infections once they are introduced into wild populations.
Infected deer typically live between 18 months and two years. They have a long incubation period and usually show no symptoms, but as the disease progresses, the animals begin to experience listlessness and weight loss.
For the last six weeks or so, they will appear aimless, blind to danger, and become haggard and drooling. They often stand with outstretched legs like a sawhorse, as if trying not to tip over.
These so-called “zombie deer” often receive media attention, but as the disease progresses in the wild, the deer become more susceptible to other diseases, less able to protect themselves, more vulnerable to predation, and more likely to be hit by cars. They rarely live to the point of being zombies.
How long have scientists known about CWD?
Chronic wasting disease was first identified in the mid-1960s, when deer in captivity in Colorado began to show symptoms commonly described as “wasting.” Researchers attributed it to stress until the late 1970s, when wildlife veterinarian Beth Williams performed autopsies on deer that had died from a similar syndrome. She found brain lesions consistent with transmissible spongiform encephalopathy – a neurological disorder that afflicts animals and humans.
In 1978, Williams and neuropathologist Stuart Young co-authored the first scientific paper describing chronic wasting disease as TSE. But the root cause remains a mystery.
A year later, neuroscientist Dr. Stanley Prusiner, studying TSE disease, discovered that a very small protein can become misshapen and resist the body’s ability to break it down. It enters cells, tricks them into replicating it, and then enters the lymphatic and nervous systems. Eventually it moves to the brain, where small pieces aggregate and cause TSE. Prusiner calls this inanimate infectious protein a “prion.”
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Does CWD threaten humanity?
Prion diseases are always fatal, but they don’t all affect the same species. CWD prions are beneficial to deer or deer species. Other prions cause human diseases, such as Creutzfeldt-Jakob disease, which progresses similarly to accelerated Alzheimer’s.
Bovine spongiform encephalopathy, commonly known as “mad cow disease”, is a prion disease that infects cattle. In rare cases, people exposed to BSE develop a form of Creutzfeld-Jakob disease.
Human cases of CWD have never been recorded. However, some laboratory experiments have shown that CWD prions can be transferred to other mammals. The Centers for Disease Control and Prevention warns against eating meat from infected animals. Some infectious disease experts argue that “while the threat to humans from CWD is low, it is not zero” and that risk assessments must include the possibility of new strains emerging.
For example, while no one knows how or where CWD originated, some scholars believe that a mutated prion crossed the species barrier and became deer from a sheep infected with prion disease in another animal.
Why are prion diseases so hard to prevent?
Because of their structure and the fact that they don’t contain genetic material, prions like the ones that cause CWD are nearly indestructible. Breaking down or denaturing prions requires very high concentrations of chlorine solutions or heat in excess of 1,800 degrees Fahrenheit (980 degrees Celsius).
Once they are deposited in the landscape through urine or feces, CWD prions can persist for decades. Years after removing deer infected with chronic wasting disease from pens, other deer placed on contaminated soil in those pens also became infected.
In wildlife, prions spread as deer, which are highly social animals that groom and lick each other. During the fall mating season, bucks seek out mates, fight and breed. They also visit places known as scratches, where they lick the soil and overhead branches where other deer have left their calling cards. Because of these behaviors, bucks typically have twice the CWD of dos.
Since the disease does not kill quickly, infected animals are able to reproduce for a season or two, so there is no strong genetic selection pressure in favor of developing herd immunity. Recent research has shown that infected deer do occasionally transmit prions to their fawns before birth.
In some places where CWD occurs, the infection rate may be as low as 1% to 5%, and the disease’s impact on the entire herd may not be noticeable, even for hunters. Elsewhere, infection rates can reach 50 percent, and possibly even 100 percent. In these cases, population impacts will inevitably lead to smaller and younger herds.
What do hunters need to know?
Nobody wants a disease named after them. Since infected deer often appear healthy, even when viewed up close after harvest, the only way to ensure the animal is disease-free is to test it, usually using lymph nodes.
As CWD spreads and more people are tested for deer, it can take weeks to receive results. This makes meat processing more onerous.
It’s important to realize that once a deer is infected, it can start a fire like a tinder. Moving deer, live or dead, can introduce and spread disease.
For example, the transfer of deer between farms has been associated with its introduction. CWD can also be transmitted by disregarding state wildlife regulations explaining the proper way to transport and dispose of harvested deer, elk, or moose. Feeding and bait stations and salt licking can increase infection rates by concentrating deer and producing point-source accumulation of prions.
What would you like to know about CWD?
CWD was discovered in 2019 in northern Mississippi and western Tennessee where I work. Based on the relatively high infection rate at the time, it could have been around for years and was spreading rapidly. Currently, approximately 40 percent of the cattle at the Ames Research Station are infected.
Research on CWD has been conducted nationwide for decades. But it’s in the South now. At Ames, in collaboration with other scientists from across the country, we are analyzing soil and branch samples for prion concentrations and quantifying deer visits at scraping sites and salt stations. We are also working on ways to destroy the salt station to avoid continued exposure.
Other research includes training dogs to detect disease-related metabolites and developing early warning systems before prions can move across the landscape.
We are studying the responses of hunters when CWD is detected locally and comparing hunting success rates before and after infection in the herd. The hunter’s point of view is important because they love the outdoors and are the first stopgap for CWD to spread. Hunting is a major tool for deer management, especially to control overpopulation in places where CWD is rampant.
Our deer hunters are involved in research every step of the way and often collect samples from the deer they harvest. As one hunter put it, “We brought in everything but the rails.”
We hope we can expand our efforts with other scientists as we analyze diseased herds to provide insights into chronic wasting diseases for the benefit of animals and humans.