What is Chronic Wasting Disease?

Chronic wasting disease, often referred to as CWD, is a prion disease that affects all members of the Cervidae (deer) family, including white-tailed deer (Odocoileus virginianus), elk (Cervus canadensis), and moose (Alces alces americana). The causative agent of CWD is a prion, an infectious misfolded protein, which binds to the normal prion proteins and causes them to convert to the misfolded shape. Prion proteins occur in higher concentrations in nervous and lymphoid tissues as well as the spleen, where it is believed that the proteins transport copper-an essential mineral that is used to make new blood cell, maintain brain hormone balance, and in many other functions. Over time, these misfolded proteins and the lack of copper cause cell death, which leads to holes to develop in the brains of infected animals.

As the disease progresses, so many holes form in the brain of infected animals that the brains have the texture and appearance similar to that of a sponge (a healthy brain is solid). This is what defines CWD as a transmissible spongiform encephalopathy (TSE). An encephalopathy is any disease that attacks the brain and spongiform (sponge-like) refers to the state of the brain after the disease has taken its course. Other TSEs include bovine spongiform encephalopathy (BSE, commonly called mad cow disease) a cattle disease, Creutzfldt-Jakob disease (CJD) found in humans, and scrapie which is a sheep disease.

CWD was first discovered in 1967 in a mule deer at a research facility in Colorado. To date, it has been found in 26 states, 4 Canadian provinces, South Korea, Norway, Finland, and Sweden. Some of the spread may be attributed to human activities, such as moving unknowingly infected live cervids for agricultural purposes. In 2012, CWD was found in a captive white-tailed deer in Pennsylvania, and two months later it was found in wild whitetails.

To minimize human-assisted spread of CWD, the Pennsylvania Game Commission has banned the importation of high-risk parts from anywhere outside of Pennsylvania. It is also illegal to export high-risk parts from Pennsylvania's Disease Management Areas (DMAs) or Established Area. People can take steps to reduce CWD transmission like harvesting additional deer using DMAP tags, not feeding deer, and not moving live cervids or carcasses/high-risk parts. DMA's have additional regulations like it is illegal to feed deer or use urine-based attractants.

Infected animals spread these prions to each other and across the environment in their saliva, urine, and feces. Other individuals can become infected through social contact with infected individuals or indirectly through prion-contaminated environments. Prion can exist in the soil for at least two years, though it is likely that it can survive longer. Research has that cervids can be infected with CWD by eating plants that have taken in CWD prions from the soil.

In cervids, CWD is always fatal and there is no cure. Some cervid genotypes can survive longer with CWD, but this is not a "cure". Infected animals will still die, and because they survived longer with CWD they may be able to spread more prions than individuals that died earlier. Regardless of genotype, and it is impossible to control mating in wild animals and animals die from many other causes (humans, starvation, and predators), so breeding immunity in wild cervids is unlikely.

The clinical signs of CWD are easy to identify—animals appear starved, do not respond to stimuli, do not display natural deer behavior, and drool excessively—these symptoms generally take 16-24 months to develop. Infected animals may not display symptoms until late in the course of infection. However, these signs are not exclusively unique to CWD and could be the result of other types of diseases or even trauma from being hit by vehicles. The only way to identify a CWD infection is to sample and test the brain tissue or lymph nodes of animals, which cannot be done on live animals.

Some prion diseases are capable of infecting more than one species. For example, in the 1990s in England, humans and zoo animals (several species of ungulates including kudu, elands, and oryxs) developed two new prion diseases: variant CJD and exotic ungulate encephalopathy, respectively, after eating beef or feed tainted with bovine spongiform encephalopathy.

Although there are no known cases of CWD infecting humans, experiments have been undertaken to determine if non-human primates or other animals to determine if humans can be infected with CWD. In these experiments several species of monkeys, hamsters, and lab mice were experimentally expose animals to CWD prions by feeding them infected meat, exposing them to infected blood (to simulate handling infected meat or blood), and injecting them with infected material. Additionally, an experiment found that in a lab setting, healthy human-like prion proteins (harvested from lab mice that were modified for neurological studies) did bind with CWD prions and turn previously functioning proteins into non-functioning diseased proteins. It is important to note that this experiment occurred in a lab setting with prion proteins collected from transgenic mice, so it is possible that human prion proteins in a brain respond differently. These experiments show mixed results, with some experiments showing CWD can infect animals other than cervids and human-like proteins; while some fail to infect study animals, so it is unknown if CWD can infect humans or will be able to infect humans in the future.

Figure 1. A map of current DMA’s as of 6/29/2022 (photo credit: Pennsylvania Game Commission). This may change, and it is always best to check the PGC website for the most up-to-date information.

Given the possibility of CWD crossing the species divide, it is important to be cautious about CWD. When hunting in areas known to have CWD, like Pennsylvania's DMAs (see Figure 1), it is always best to wear rubber gloves when handling deer, submit the head of harvested animals using Game Commission guidelines for CWD testing, and properly dispose of high-risk parts (areas high in prion density like the brain, eyes, lymph nodes, head, tonsil, spine, and spleen). This can be done by disposing of carcasses or high-risk parts at a Pennsylvania Game Commission CWD dumpster or in trash destined for a lined landfill. If neither of those options are possible, leave high-risk parts on-site, to not infect new areas. If an animal tests positive, do not eat it! Cooking does not make meat safe, as the prion can withstand temperatures up to 1,500°F. Certain soil compounds like humic acid can in lab conditions potentially decrease prion infectivity in the soil, but application across the landscape is impossible and it does not eliminate the prion so animals can still get infected. For more information about CWD regulations and DMA's visit the Pennsylvania Game Commission.