“Zombie deer disease” spreads through deer, humans could be next
This morning, thousands of deer across the United States woke up with a little less of their brains. Chronic wasting disease, a neurological disease in deer, elk, and moose, has reached 27 U.S. states and two Canadian provinces. Experts worry that the disease could make the leap to humans.
White-tailed deer are susceptible to chronic wasting disease, a fatal disease that fills their brains with holes.
What is chronic wasting disease?
Chronic wasting disease, also known as ‘zombie deer disease’ causes drastic weight loss, dehydration, drooling, listlessness, stumbling, drooping ears, and aggression. Like a fictional zombie, this disease goes after the brain, leaving animals with porous, spongy brain tissue. And there’s no saving an animal once it’s infected, chronic wasting disease always ends in death, as there are no vaccines or treatments for the disease.
Photo by Terry Kreeger, Wyoming Game and Fish and Chronic Wasting Disease Alliance, public domain
This deer is showing signs of chronic wasting disease, such as weight loss and drooping ears.
Chronic wasting disease has torn through deer populations like the most terrifying zombie apocalypse the movies have to offer. But instead of being spread through the bite of an infected animal, this disease spreads through infected food and water, bodily fluids, and tissue. Symptoms can take a year to develop, giving animals ample time to pass on the disease before they ultimately succumb to the holes in their brain tissue.
Mad deer disease?
Chronic wasting disease is caused by a prion, the same kind of infectious agent that causes bovine spongiform encephalopathy, commonly known as mad cow disease. Prions are infectious proteins that cause abnormal folding of normal prion proteins in the brain. Scientists still don’t know the function of normal prion proteins, but we know that when the infectious prions alter them the results are devastating. Mad cow disease causes the central nervous system of cattle to become spongy, just like chronic wasting disease does to deer.
Photo by pikaluk on flickr, CC BY 2.0
Mad cow disease is caused by prions, the same kind of infectious agent as chronic wasting disease.
Since both chronic wasting disease and mad cow disease are caused by prions, the troublesome story of mad cow disease causes concern for the future of chronic wasting disease. Humans can contract a version of mad cow disease called variant Creutzfeldt-Jakob disease.
In the U.K. 1980’s and 90’s, an outbreak of mad cow disease caused a subsequent outbreak of variant Creutzfeldt-Jakob disease. The outbreak is thought to have been caused by British farmers feeding infected cow byproducts to living cattle, introducing the mad cow prion into their livestock.
While experts at the time doubted that mad cow disease could spread to humans, variant Creutzfeldt-Jakob disease infected people who ate the meat of cows infected with mad cow disease. Worldwide, 232 people are known to have ever contracted variant Creutzfeldt-Jakob disease. Unfortunately, they have all died, as the disease is always fatal. On the bright side, variant Creutzfeldt-Jakob disease is not transmissible between humans, so the disease is not as challenging as airborne diseases to control.
However, experts remain concerned that chronic wasting disease could follow the same trajectory as mad cow disease.
Are humans at risk?
While scientists aren’t sure the disease will infect humans, the risk increases the more infected meat is eaten. No humans have been infected yet, but studies show that chronic wasting disease can be transmitted to certain primates. With about 7 to 15 thousand infected animals eaten each year in the form of venison according to the Alliance for Public Wildlife, it may be a matter of time until chronic wasting disease makes the leap from deer to human.
Concern is rising over infected deer meat eaten by hunters.
What’s being done?
The Center for Disease Control recommends that sick or strange-looking deer be avoided and that hunters test deer meat from infected areas before eating it. State policies are also in place to limit the amount of infected meat consumed. North Carolina has strict packaging and processing guidelines for imported deer meat, and Indiana has increased its monitoring for the disease although testing is not required by law. Frequent testing of hunted deer carcasses could save the lives of deer and hunters alike if this disease moves between the species.
Despite the recommended caution, no one wants hunters to stop hunting. In fact, hunting can prevent the spread of chronic wasting disease as it lowers the population density of deer. Additionally, we need infrastructure to prevent human infection, like testing hunted deer for the disease and culling to keep population densities lower. Culling might sound like it would make hunting harder, since there would be fewer deer. However, hunting opportunity has not been reduced in states that cull their deer. In fact, protecting deer populations from crashing due to disease will keep hunters bagging their prize deer for generations.
It seems like chronic wasting
disease could be the next possible human prion disease on the horizon. Time
will tell whether the risk can be mitigated with proper policy, cautious
hunting, and increased awareness. Either way, we can’t afford to wait until an
outbreak to take action.
References:
Barria, M. A., Libori, A., Mitchell, G. & Head, M. W. Susceptibility of human Prion protein to conversion by chronic wasting disease Prions. Emerg. Infect. Dis. 24, 1482–1489 (2018).
Blanchong, J. A. et al. Effects of chronic wasting disease on reproduction and fawn harvest vulnerability in Wisconsin white-tailed deer. J. Wildl. Dis. 48, 361–370 (2012).
Chronic Wasting Disease (CWD). (2022, May 8). Retrieved from https://www.cdc.gov/prions/cwd/index.html
Davenport, K. A. et al. Insights into Chronic Wasting Disease and Bovine Spongiform Encephalopathy Species Barriers by Use of Real-Time Conversion. J. Virol. 89(18):9524-9531 (2015).
Denkers, N. D. et al. Very low oral exposure to prions of brain or saliva origin can transmit chronic wasting disease. PLoS One 15, e0237410 (2020).
Edmunds, D. R. et al. Chronic wasting disease drives population decline of white-tailed deer. PLoS One 11, 1–19 (2016).
Geist, V. et al. The challenge of CWD: insidious and dire. Alliance for Public Wildlife: Living Legacy White Paper Series (2017).
Koutsoumanis, K. et al. Update on chronic wasting disease (CWD) III. EFSA J. 1-63, (2019).
Manjerovic, M. B., Green, M. L., Mateus-Pinilla, N. & Novakofski, J. The importance of localized culling in stabilizing chronic wasting disease prevalence in white-tailed deer populations. Prev. Vet. Med. 113, 139–145 (2014).
Moreno, J. A. & Telling, G. C. Molecular mechanisms of chronic wasting disease prion propagation. Cold Spring Harb. Perspect. Med. 8, (2018).
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