Chronic wasting disease (CWD) is caused by prions, which are misfolded proteins that act as infectious agents. Unlike bacteria or viruses, prions contain no DNA or RNA. They are simply normal brain proteins that have folded into an abnormal shape, and they force other healthy proteins to do the same. This chain reaction gradually destroys the brain and nervous system, and it is always fatal.
How a Misfolded Protein Becomes Infectious
Every deer naturally produces a protein in its brain and nervous system called a cellular prion protein. Under normal circumstances, this protein folds into a specific three-dimensional shape and performs routine cellular functions. In CWD, a copy of this protein misfolds into a different, more rigid shape. That misfolded version then acts like a template: when it comes into contact with a normally folded protein, it forces it to refold into the abnormal shape too.
This process, called protein templating, is self-perpetuating. Each newly misfolded protein recruits more normal proteins, creating a snowball effect. The abnormal proteins clump together into dense deposits throughout the brain and lymph tissue. These deposits damage and kill nerve cells, producing the progressive neurological decline that defines the disease. Because prions are not alive, they cannot be killed with antibiotics, antivirals, or standard sterilization methods like heat or chemical disinfectants.
How CWD Spreads Between Animals
CWD prions spread through body fluids: saliva, urine, feces, and blood. A healthy deer can become infected through direct nose-to-nose contact with a sick animal, or indirectly by grazing on contaminated ground, drinking from contaminated water, or encountering prions left behind in the environment. Infected deer begin shedding prions before they show any symptoms, which makes containment extremely difficult. A deer can carry and spread the disease for over a year before anyone could tell it was sick.
The long incubation period, averaging 18 to 24 months, means infected animals move freely across landscapes and interact with healthy herds while silently spreading prions the entire time.
Prions Persist in Soil for Years
One of the most troubling aspects of CWD is what happens after prions enter the environment. Shed through saliva, urine, and decomposing carcasses, prions bind to soil particles and remain infectious for years. A study published in the journal Pathogens found that prions incubated with various soil types for over 55 weeks (more than a year) showed no significant decrease in infectivity compared to prions that had been in soil for just one week.
Certain soil minerals actually enhance prion infectivity by binding tightly to the misfolded proteins, while organic soil components like humic acids can reduce it somewhat. But the overall picture is stark: even when researchers could no longer detect the prion protein through standard laboratory methods after prolonged soil contact, the samples still caused infection in test animals. The prions essentially become harder to find but no less dangerous.
This environmental persistence means that even after an infected deer dies or is removed from an area, the ground itself can serve as a reservoir of disease for future animals that graze or bed there.
Which Species Are Affected
CWD affects members of the cervid family. Confirmed susceptible species include white-tailed deer, mule deer, elk, moose, reindeer, and muntjac. The disease has now been detected in 36 U.S. states and five Canadian provinces, in both free-ranging wild herds and captive cervid facilities. Cases have also been confirmed in Norway, South Korea, and other countries. The geographic spread has accelerated over the past two decades, driven partly by natural deer movement and partly by the transport of live captive cervids.
Genetic variation between species and even between individual animals within a species plays a role in susceptibility. Specific amino acid differences in the prion protein can make conversion from the normal form to the misfolded form more or less efficient. Some individual deer carry gene variants that slow disease progression, though no known genotype provides complete immunity.
What CWD Looks Like in an Infected Deer
Because the incubation period averages 18 to 24 months, most infected deer look perfectly healthy for a long time. When symptoms finally appear, the most obvious sign is progressive weight loss, often severe enough to leave the animal visibly emaciated. Behavioral changes follow: infected deer may become less social, lose awareness of their surroundings, and show a striking loss of fear toward humans. Increased drinking, excessive urination, and heavy drooling are also common. In the late stages, elk and deer develop a characteristic drooping of the ears along with extreme wasting. The disease is always fatal once symptoms begin, and there is no treatment.
Can Humans Get CWD?
No case of CWD in a human has ever been documented. However, the CDC notes that a related animal prion disease, bovine spongiform encephalopathy (mad cow disease), did cross the species barrier and caused a fatal brain disease in people. Some laboratory studies in primates have shown that monkeys can develop CWD-like illness after eating tissue from infected deer or elk. Because of this, scientists consider CWD a theoretical risk to people, and the CDC has been collaborating with state health officials in affected areas for decades to monitor for any potential human cases.
If CWD were ever to spread to humans, the most likely route would be eating meat from an infected animal. Many state wildlife agencies now offer free CWD testing for hunter-harvested deer in affected zones, and the CDC recommends having deer tested before consuming the meat in areas where CWD is present.
How Testing and Carcass Rules Help Slow the Spread
CWD can only be definitively diagnosed after death, by examining tissue from the lymph nodes near the throat and the brainstem. Two laboratory methods are approved for official testing: one looks for prion deposits within tissue slices under a microscope, and the other detects prion proteins in ground-up tissue samples. Both tissue types need to be tested for an accurate result, because prions may appear in lymph tissue earlier in the disease than in the brain.
State wildlife agencies have introduced carcass disposal rules to limit environmental contamination. In Texas, for example, hunters are encouraged to leave unused carcass parts at the property where the animal was harvested. If parts are transported, they must be disposed of through commercial trash service, returned to the harvest site, or buried at least three feet underground with three feet of soil cover. Deboned meat from different deer must stay in separate containers during transport. These rules exist because moving carcasses, especially brain and spinal tissue, can introduce prions to new areas where they persist in soil indefinitely.
Similar regulations are in place across most states with confirmed CWD. Some states also ban the importation of whole deer carcasses from other states and restrict the movement of live captive cervids. Despite these measures, CWD continues to expand geographically. The combination of a long, silent incubation period, environmental persistence measured in years, and the lack of any vaccine or treatment makes this one of the most challenging wildlife diseases to manage.