What Is Prion Disease?

In the United States, deer and elk harbor chronic wasting disease

MarkBridger/Getty Images 

In early 2018, concerns about “zombie deer” spreading illness to humans gripped the attention of the public. Although possible, the chances of being infected with chronic wasting disease (CWD)—which is similar to mad cow disease—after eating venison is very low. To date, many deer populations have a low prevalence of CWD. Moreover, there has never been a verified case of chronic wasting disease being transmitted from deer or elk to humans.


In deer and elk, CWD results in a slow, agonizing death that eventually robs the animal of its ability to eat and drink. In humans, CWD slowly destroys the brain. It is a neurological disease that’s spread by reindeer, elk, deer, and moose. More generally, CWD can be classified as a “slow” infectious disease. Slow infectious diseases are due to viruses and prions; CWD is caused by prions.

Chronic wasting disease is best understood in the context of prion disease. Let’s start by taking a more general look at prion disease.

What Is Prion Disease?

Recent studies have illumined four points about prions.

First, prions are the only known transmissible pathogens that lack nucleic acid. Other infectious agents like bacteria and viruses contain DNA and RNA that guide their reproduction. Different strains of prions result in different types of disease.

Second, prions result in infectious, genetic, and sporadic disorders.

No other illnesses due to a single cause manifest in such a wide gamut of clinical presentation.

Third, prions are misfolded proteins that self-propagate in the brain. Typically, the prion protein is thought to play a role in nerve signaling. This normal protein called PrPC (prion protein cellular) has an alpha-helical formation.

In prion disease, this alpha-helical formation switches to a pathological beta-pleated sheet called PrPSC (prion protein scrapie). These PrPSC accumulate into filaments which disrupt nerve-cell functioning and cause cell death.

Prions propagate when beta-pleated sheets (PrPSC) recruit alpha-helical forms (PrPC) to become beta-pleated sheets. A specific cellular RNA mediates this change. Of note,  PrPSC and PrPC have the same amino-acid composition but different conformations or shapes. Analogously, the difference in these two conformations can be thought of as pleats or folds in fabric.

Prion-Mediated Disease in Humans

In people, prions cause “slow” infectious disease. These diseases have a long incubation period and take a long time to manifest. Their onset is gradual, and their course is progressive. Unfortunately, death is inevitable.

Prion-mediated diseases in humans are called transmissible spongiform encephalopathies (TSE). These diseases are “spongiform” because they cause the brain to take on a spongy appearance, riddled with holes in the brain tissue.

Five types of TSE occur in humans including the following:

  • Kuru
  • Creutzfeldt-Jakob disease (CJD)
  • Variant Creutzfeldt-Jakob disease (vCJD)
  • Gerstmann-Sträussler-Scheinker (GSS) syndrome
  • Fatal familial insomnia

The clinical presentation of CJD includes dementia, loss of body movements, jerking, visual loss, and paralysis affecting one side of the body. Although similar to kuru, which affects the Fore tribes in New Guinea after ingestion of human brains, kuru doesn’t result in dementia. Furthermore, CJD is found worldwide and not related to dietary habits, occupation, or animal exposure. In fact, vegetarians can develop CJD.  Overall, CJD affects one person in a million and occurs in countries where animals have prion disease as well as in countries where animals don’t harbor prion disease.

Chronic wasting disease is a type of vCJD. A more common form of vCJD is bovine spongiform encephalopathy or mad cow disease. The reason why CWD and mad cow disease are called “variant” CJD is that the disease occurs in patients who are a lot younger than those who typically present with CJD. Moreover, there are certain pathological and clinical findings that are different in vCJD.

In 1996, mad cow disease came into the limelight after a spate of cases was identified in Great Britain. People who fell ill had likely consumed beef mixed with cow brains. Additionally, only people with a certain type of prion—prion proteins homozygous for methionine—developed the illness. Apparently, prion proteins homozygous for methionine are more easily folded into the beta-pleated sheet form (PrPSC).

Chronic Wasting Disease

To date, there have been no known cases of transmission of CWD to humans. However, there is some circumstantial evidence. In 2002, neurodegenerative disease was diagnosed in three men who ate deer meat in the 1990s. One of these men was confirmed to have CJD. (Remember that CJD is “slow” and takes time to manifest.)

According to the CDC, as of January 2018, CWD in free-ranging deer, elk, and moose has been reported in at least 22 states and two Canadian provinces. In the United States, CWD has been identified in the Midwest, Southwest, and some parts of the East Coast. It’s also possible that CWD is present in areas of the United States that lack robust surveillance systems. Although mostly found in the United States and Canada, CWD has also been found in Norway and South Korea.

Interestingly, CWD was first identified in captive deer in the late 1960s. By 1981, it was identified in wild deer. Although the prevalence of CWD in wild deer populations is typically low, in some populations, prevalence of disease can exceed 10 percent, with an up to a 25 percent infection rate reported in the literature. Of note, in captive deer populations, prevalence of CWD can be much higher. Specifically, nearly 80 percent of deer in one captive herd of deer had CWD.

Animal studies suggest that CWD could be transmitted to non-human primates, such as monkeys, that consume deer meat tainted with brain or body fluids.

In deer and elk, it can take up to a year before symptoms of CJD manifest. These symptoms include weight loss, listlessness, and stumbling. There are no treatments or vaccines for CWD. Furthermore, some animals can die of CWD without ever developing symptoms.

In 1997, the WHO recommended that all agents that cause prion disease—including deer with CWD—be kept out of the food chain for fear of transmission.    


If CWD was to be spread to humans, the best way to prevent this transmission is by not eating deer or elk meat. The practice of eating venison is widespread in the United States. In a 2006-2007 survey conducted by the CDC, 20 percent of respondents reported hunting deer or elk, and two-thirds reported eating venison or elk meat.

With the consumption of deer and elk widespread and no tangible evidence of transmission yet documented, it’s unlikely that many venison and elk meat aficionados will stop their consumption. Therefore, it’s recommended that hunters take precautions when hunting.

Certain state wildlife agencies monitor the prevalence of CWD in wild populations of deer and elk using tests. It’s important to check with state websites and state wildlife officials for guidance and avoid hunting populations in which CWD is identified.

Importantly, not all states monitor CWD in wild deer and elk. Moreover, a negative test for CWD does not necessarily mean that an individual deer or elk is free of disease. Nevertheless, the chance that a deer or elk with a negative test doesn’t harbor CWD is high.

Here is some advice for hunters regarding CWD:

  • Don’t handle, shoot or eat deer or elk that look sickly or are acting in strange ways
  • Avoid road-kill
  • While field-dressing a deer, wear latex or rubber gloves, minimize your handling of brains or other organs, and don’t use household cutlery or kitchen utensils
  • Consider having the deer or elk that you plan to eat tested for CWD
  • If you’re having game processed commercially, ask that your meat be prepared separately from other deer and elk
  • Never eat a deer or elk that tests positive for CWD
  • Check with state wildlife officials to learn whether testing of deer or elk for CWD is recommended or required

With respect to commercial venison and elk meat, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service operates a national CWD herd certification program. This program is voluntary, and herd owners agree to subject their herds to testing. Not all commercial herd owners participate in the program. It’s probably a good idea to only consume deer or elk meat from commercial suppliers that participate in the program.

Prions in Certain Soil Types

In 2014, Kuznetsova and colleagues found that certain types of soil in southeast Alberta and southern Saskatchewan (parts of Canada) may harbor prions responsible for CWD.

According to the researchers:

In general, clay-rich soils may bind prions avidly and enhance their infectivity comparable to pure clay mineral montmorillonite. Organic components of soils are also diverse and not well characterized, yet can impact prion-soil interaction. Other important contributing factors include soil pH, composition of soil solution and amount of metals (metal oxides) …. The major soils in the CWD-endemic region of Alberta and Saskatchewan are Chernozems, present in 60% of the total area; they are generally similar in texture, clay mineralogy and soil organic matter content, and can be characterized as clay loamy, montmorillonite (smectite) soils with 6-10% organic carbon.

Animals eat soil to meet their mineral needs. This soil is re-introduced to the soil in the form of excrement or carcasses. Thus, prions can be cycled into the soil. It appears that prions attach quite well to clay.

A Word From Verywell

To date, there have been no proven transmissions of chronic wasting disease from deer or elk to humans; nevertheless, experts are concerned about the risk. Chronic wasting disease is similar to mad cow disease, which has been documented to spread from cows to humans.

When consuming venison or elk meat, it’s a good idea to take certain precautions and check for guidance from state wildlife officials. With wild game, never eat meat from a deer or elk that appears sickly. Furthermore, it’s a good idea to have wild deer or elk meat tested for CWD.

When buying commercial deer or elk meat, make sure that this meat is certified as being devoid of CWD.


Chronic Wasting Disease. CDC.

Kuznetsova A et al. Potential role of soil properties in the spread of CWD in western Canada. Prion. 2014;8(1):92-9.

Prusiner SB, Miller BL. Prion Diseases. In: Kasper D, Fauci A, Hauser S, Longo D, Jameson J, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 19e New York, NY: McGraw-Hill; 2014.

Slow Viruses & Prions. In: Levinson W. eds. Review of Medical Microbiology and Immunology, 14e New York, NY: McGraw-Hill.