Knowing Neurons
Neurological and Psychiatric Disorders

Chronic Wasting Disease and Prions: The Next Big Thing?

By J. LaValley

A disease is sweeping the nation. As of April 2024, it can be found in 33 states and 5 Canadian provinces (National Wildlife Health Center, 2024) with cases climbing since 1967. Multiple states have strict testing protocols and surveillance programs in place to ensure humans are not exposed to this deadly disease (Chronic Wasting Disease (CWD), 2020). Haven’t heard about this before? Well, that’s probably because it commonly only infects hoofed herbivores like deer. The disease is Chronic Wasting Disease or more simply: CWD.

Deer and elk are the most commonly hunted large game in North America and around 65% of Americans are thought to have consumed their meat products at some point in their lifetime (Sowers, 2020; Abrams, 2011). Current deer and elk population estimates range between 30 to 36 million deer in the United States alone. In 1967 a concerning new disease was noted that affected the behavior and appearance of captive Colorado and Wyoming populations. It was termed Chronic Wasting Disease. It was not until 1978 that CWD was discovered to be a prion disease (a disease caused by a protein that has taken on the wrong shape and then causes other proteins to misfold) (Sigurdson & Aguzzi, 2009). In 1980 CWD was found in free-roaming deer populations, making it the only prion disease found in captive and wild animals (Sigurdson & Aguzzi, 2009). Since then, it has spread significantly. CWD is now found in more than half the states in the USA and Canada in deer, elk, and moose (i.e., the Cervidae family) (National Wildlife Health Center, 2023; Sigurdson & Aguzzi, 2009). 

So, what’s the big deal? Why is this seemingly obscure deer disease so closely monitored and talked about? The reason comes down to two things: what a prion is and what could happen if humans are susceptible to it. 

Figure 1: Current and past distribution of CWD in Canada and the United States as of April 2024. Light gray boxes indicate current free-ranging populations whereas dark gray indicates free-ranging populations before the year 2000. Yellow dots indicate captive populations that have been culled due to CWD and red indicates captive populations where CWD has been found but not culled as of yet (National Wildlife Health Center, 2024).
Figure 2: Distribution of hunters in the continental United States who hunted deer in some counties with CWD in Wisconsin from 2016-2017. This demonstrates how easily CWD-infected meat could potentially be distributed (National Wildlife Health Center, 2024).
What is a prion?

A prion (pronounced ‘pree-on’) is essentially a protein that has ‘misfolded,’ or in other words, has an abnormal shape or configuration, that causes more and more proteins to misfold (Kovacs & Budka, 2008). Prion is shorthand for proteinaceous infectious particles—in other words, an infectious protein. These prions cause prion diseases, also called ‘transmissible spongiform encephalopathies’ (TSEs). In other words, it is a transmissible disease that causes a sponge-like pattern to appear in infected brain tissue. Encephalopathy is any disease that affects the functioning of the brain. Within this umbrella category, there are several diseases that affect a variety of animals. The most common and best characterized are scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle (also known as mad cow disease), and CWD in deer, elk, and moose (Prion Diseases, 2021). In humans, the known prion diseases are Creutzfeldt-Jakob Disease (CJD), Gerstmann-Straussler-Scheinker Syndrome, Fatal Familial Insomnia, and Kuru (Weissmann, 2002; Collinge, 2008). Each of these diseases is caused by the prion protein (PrP) misfolding causing more and more proteins to misfold. These misfolded PrP then aggregate together and damage the neurons. How exactly this damage occurs is currently up for debate, with dozens of proposed mechanisms (Kovacs & Budka 2008).

Prion diseases are fatal, untreatable, and have an extremely long incubation period.

Prion diseases are fatal, untreatable, and have an extremely long incubation period (the time between infection and when symptoms begin). This means that if you contract a prion disease it could take between one and fifty years to show symptoms. This variability depends on the infected species, method of infection, and what prion disease you are dealing with (Sigurdson & Aguzzi, 2009; Collinge, 2008). These very long incubation periods are dangerous because they make identifying and controlling outbreaks extremely difficult. Additionally, testing policies may be different from state to state and the protocol for hunters coming in from out of state can be ambiguous (Chronic Wasting Disease (CWD), 2020). 

To make things even more difficult, prions are extremely hard to destroy. Normal sterilization and cooking techniques do next to nothing. Even when heated to temperatures of 600 degrees Celsius there is some degree of infectivity left (Saunders, 2008). The most effective manner of destroying a prion is to incinerate it at the temperature of 1000 degrees Celsius. Other established methods (e.g., composting of carcasses and enzymatic cleaners) are less likely to destroy prions (Saunders, 2008). 

But how do you get a prion disease?

The transmission of prions is not completely understood. In cases such as Scrapie in sheep and CWD in Cervidae, the transmission seems to travel through the herd. This could be due to methods including consumption of body fluids or waste such as milk, saliva, skin, urine, feces, and materials from birthing females (Gough & Madison, 2010). 

In other cases, such as Gerstmann-Straussler-Scheinker Syndrome and Fatal Familial Insomnia in humans, genetics seems to be the main factor (Kovacs & Budka, 2008). These seem to be exclusively inherited diseases that don’t have any recorded transmission through consumption or interaction. 

In the case of Kuru, the prion disease that was found in Papua New Guinea, it was passed to other humans through the cannibalistic consumption of diseased human brains. It is thought that it may have originally been caused by someone consuming the brain of a Creutzfeldt-Jakob Disease (CJD) infected individual (Collinge, 2008). 

Finally, in Creutzfeldt-Jakob Disease (CJD), ‘infection’ seems to be entirely random. The main cause seems to be spontaneous misfolding of a protein that somehow is able to then cause other proteins to misfold. There is a variant version of CJD, termed vCJD, in which transmission is achieved through the consumption of meat across species (Cleeland, 2009). Specifically, humans consume BSE-infected cow meat. Currently, this is the only known animal-to-human transmission method identified. This transmission method also brings up an important point for CWD. If humans do at some point become sick by consuming CWD-infected meat we may not necessarily develop CWD just like Cervidae do. Instead, infection may be like BSE where we develop a condition like variant Creutzfeldt-Jakob Disease in response to CWD. 

So, what is special about chronic wasting disease?

CWD is a prion disease that affects deer, elk, and some species of moose (Sigurdson & Aguzzi, 2009). It is found predominantly in the United States and Canada (National Wildlife Health Center, 2023). The symptoms of the disease are characterized by extreme weight loss (hence “wasting”), loss of coordination and stumbling, dull behavior, salivation, increased thirst and urination, drooping ears, and a lack of fear of humans (CWD in Animals, 2021). 

CWD is suspected to be the most transmissible prion disease we know of.

One of the biggest concerns is how easily transmissible CWD is; currently, it is suspected to be the most transmissible prion disease we know of (Sigurdson & Aguzzi, 2009). CWD may also persist in the soil and foliage where individuals who are infected decompose or even where an infected individual urinated or defecated. This makes the very land the deer are living on a possible source of infection for CWD (Sigurdson & Aguzzi, 2009). In turn, this makes the possibility of eradicating the disease next to impossible in free-ranging animals.

Luckily, testing is available. In the United States, the USDA (United States Department of Agriculture) policy is to test animals susceptible to CWD via immunohistological testing of certain regions of the brain stem or the lymph nodes (Chronic Wasting Disease (CWD), 2020). Immunohistochemistry is a common process for disease diagnosis in which scientists use antibodies joined with fluorescent labels to target specific proteins that relate to the disease they are testing for. This is good news for the ability to identify the disease.

The bad news is that since deer can have the disease and spread it before showing symptoms, it can be hard to effectively identify new outbreaks of CWD until it has already taken root in the population. Additionally, since funding was cut in 2012, there is no national surveillance program for CWD. Though the USDA still works closely with state wildlife and agriculture agencies, it is ultimately the burden of the state to ensure testing is being done (Chronic Wasting Disease (CWD), 2020). This can make it hard for some states, and in particular private ranches, to access testing sites without large financial burdens. 

Are humans at risk for CWD?

The short, reassuring answer is: probably not. 

The longer answer is a little more complicated, and a little less reassuring. Humans have most definitely consumed CWD-infected meat (Sigurdson & Aguzzi, 2009). That is not even slightly up for debate. That being said, currently, there is no evidence that a human has contracted CJD in response to consuming venison (Sigurdson & Aguzzi, 2009). However, scientists have been able to induce prion disease in some non-human primates through ingestion.

It should be noted that not all primates introduced to these prions ended up infected (Race et al, 2014). In other words, scientists intentionally tried to get different species of primates sick by exposing them to CWD in various manners, but only some species, specifically spider monkeys, became ill. Other species, like macaques, did not become ill no matter what the scientists did. However, considering the risk of what could occur if humans were susceptible to CWD (or will someday become susceptible), scientists are not ready to claim there is zero danger to humans.

So, what should you do?

In general, the advice is: do not knowingly consume meat infected with, or suspected to be infected with, CWD. It is not worth the risk. 

In general, the advice is: do not knowingly consume meat infected with, or suspected to be infected with, CWD. It is not worth the risk. 

It is not likely that CWD causes disease in humans but scientists say it isn’t worth the potential danger to knowingly expose yourself to CWD (Race et al, 2014). If you are accepting meat from another individual, ask where they hunted and whether they got the animal tested (Prevention, 2021). Hunters specifically should be aware and get their animals tested and can find resources online for what the protocol is for their particular state or region. If you are handling a non-tested animal, wear gloves and try to be as clean as possible. If you see an animal acting oddly, report it to State Wildlife Authorities to help them monitor these cases effectively.

In this spirit, consider supporting state or federal legislation that funds surveillance programs for these types of diseases. Regardless of whether CWD is something humans should worry about, it is something that can hurt our deer, and preventing the spread is of huge importance. While deer are often seen as a pest species they are actually considered a keystone species (an organism that helps to keep an entire ecosystem running) in many areas. They manage the vegetation in forests, distribute seeds and nutrients as they migrate, and serve as a major source of food for a multitude of species (Ramirez, 2021). If deer were to disappear the American continent would become a very different landscape. 

~~~

Written by Julia LaValley
Illustrated by Yang-Sun Hwang
Edited by Mary CooperChris Gabriel, Alli Lindquist

~~~

Become a Patron!

References:

Abrams, J. Y., Maddox, R. A., Harvey, A. R., Schonberger, L. B., & Belay, E. D. (2011). Travel History, Hunting, and Venison Consumption Related to Prion Disease Exposure, 2006-2007 FoodNet Population Survey. Journal of the American Dietetic Association, 111(6), 858–863. https://doi.org/10.1016/j.jada.2011.03.015

Cleeland, B. (2009). The Bovine Spongiform Encephalopathy (BSE) Epidemic in the United Kingdom. International Risk Governance Council, Geneva, Switzerland. https://beta.irgc.org/wp-content/uploads/2018/09/BSE_full_case_study_web1.pdf

Collinge, J., Whitfield, J., McKintosh, E., Frosh, A., Mead, S., Hill, A. F., Brandner, S., Thomas, D., & Alpers, M. P. (2008). A clinical study of kuru patients with long incubation periods at the end of the epidemic in Papua New Guinea. Philosophical Transactions of the Royal Society B: Biological Sciences, 363(1510), 3725–3739. https://doi.org/10.1098/rstb.2008.0068

Chronic Wasting Disease (CWD). (n.d.). USDA APHIS. Retrieved October 1, 2023, from https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/nvap/NVAP-Reference-Guide/Control-and-Eradication/Chronic-Wasting-Disease

CWD in Animals. (2021, October 13). CDC. https://www.cdc.gov/prions/cwd/cwd-animals.html

Gough, K. C., & Maddison, B. C. (2010). Prion transmission. Prion, 4(4), 275–282. https://doi.org/10.4161/pri.4.4.13678

Kovacs, G. G., & Budka, H. (2008). Prion Diseases: From Protein to Cell Pathology. The American Journal of Pathology, 172(3), 555–565. https://doi.org/10.2353/ajpath.2008.070442

National Wildlife Health Center. (2023). Expanding Distribution of Chronic Wasting Disease. https://www.usgs.gov/centers/nwhc/science/expanding-distribution-chronic-wasting-disease#overview

Prevention. (2021, October 15). CDC. https://www.cdc.gov/prions/cwd/prevention.html

Prion Diseases. (2021, November 10). CDC. https://www.cdc.gov/prions/index.html

Race, B., Meade-White, K. D., Phillips, K., Striebel, J., Race, R., & Chesebro, B. (2014). Chronic Wasting Disease Agents in Nonhuman Primates. Emerging Infectious Diseases, 20(5), 833–837. https://doi.org/10.3201/eid2005.130778

Ramirez, J. I. (2021). Uncovering the different scales in deer–forest interactions. Ecology and Evolution, 11(10), 5017–5024. https://doi.org/10.1002/ece3.7439

Saunders, S. E., Bartelt-Hunt, S. L., & Bartz, J. C. (2008). Prions in the environment. Prion, 2(4), 162–169.

Sigurdson, C. J., & Aguzzi, A. (2007). Chronic wasting disease. Biochimica et Biophysica Acta, 1772(6), 610–618. https://doi.org/10.1016/j.bbadis.2006.10.010

Sowers, M. (2020). Most Common Game Animals in the U.S and Information on Intakes, Habitation, Hunt Frequency, and Human Consumption. Environmental Protection Agency. https://clu-in.org/conf/tio/commongame/slides/2Slide_Presentation_for_Maia_Sowers,_U.S._EPA_Intern.pdf

Weissmann, C., Enari, M., Klohn, P.-C., & Rossi, D. (2002). Transmission of Prions. The Journal of Infectious Diseases. https://academic.oup.com/jid/article-abstract/186/Supplement_2/S157/2191193

Williams, E. S. (2005). Chronic Wasting Disease. Veterinary Pathology, 42(5), 530–549. https://doi.org/10.1354/vp.42-5-530

Author

  • Julia LaValley

    Julia is a PhD student in the Neuroscience and Behavior program at UMass Amherst. Her research involves using florescent imaging paired with immunohistochemistry and in situ hybridization to investigate the neural centers responsible for sensory processing and motor control in invertebrate mollusks. In addition to her research, she is also active in the science policy and science communication fields. Outside of work, Julia enjoys going for hikes with her dogs, reading sci-fi, and trying new recipes. For more information, please visit her profile. (https://www.linkedin.com/in/julia-lavalley/)

Julia LaValley

Julia is a PhD student in the Neuroscience and Behavior program at UMass Amherst. Her research involves using florescent imaging paired with immunohistochemistry and in situ hybridization to investigate the neural centers responsible for sensory processing and motor control in invertebrate mollusks. In addition to her research, she is also active in the science policy and science communication fields. Outside of work, Julia enjoys going for hikes with her dogs, reading sci-fi, and trying new recipes. For more information, please visit her profile. (https://www.linkedin.com/in/julia-lavalley/)