Is this the beginning of the end? Will humanity, as we know it, unravel and collapse as we are ripped apart by man-eating corpses? Probably not. At the very least, our cultural obsession with the world ending in a gruesome manner would finally be put to an end – no need for renewals of The Walking Dead. As interesting as an imminent zombie apocalypse may sound, fear not: – we are still safe – for now.
‘Zombie deer disease’ is the clickbait term for chronic wasting disease (CWD) – a transmissible, neurodegenerative disorder. Currently, the CWD outbreak appears to be limited to cervids, which are deer, elk, moose and the like. The symptoms are progressive weight-loss, decreased social interactions, lack of coordination, loss of fear for humans and more aggressive behaviour. It’s easy to see why the term ‘zombie deer disease’ caught on, although the truly frightening fact is that CWD is an infectious prion disease which is incurable and ultimately fatal. We will delve into what a prion disease is in a moment – does ‘mad cow disease’ sound familiar?
This isn’t the first time that CWD cases have been reported. In fact, it was first identified in the late 1960s in mule deer and black-tailed deer held in captivity in Colorado and later in Wyoming. The reason behind the sudden furore now, is that currently the number of CWD cases detected is on the rise and is spreading to areas previously devoid of the disease, as well as being identified now in both captive and free-ranging cervids.
As of March 6, 2019, CWD in free-ranging deer, elk and/or moose has been reported in at least 24 states in the continental United States, as well as two provinces in Canada. In addition, CWD has been reported in reindeer and moose in Norway and Finland, and a small number of imported cases have been reported in South Korea.”– Centers for Disease Control and Prevention
There is a rapidly growing concern regarding the transmission from the infected cervids to other wild animals and possibly to humans. As mentioned before, CWD is a prion disease to which there is no vaccine or treatment present if the disease were to jump from cervids to humans.
So, what is a prion disease?
Prions are infectious agents which are responsible for a group of always fatal neurodegenerative disorders. The term ‘prion’ is an acronym derived from proteinaceous and infectious particle. These protein-only particles were initially hypothesised, by Stanley B. Prusiner, to be the infectious agents of scrapie – a neurodegenerative disorder, which causes sheep to pathologically scrape against fences, among other altered behavioural traits. Prusiner accumulated data supporting his ‘protein only’ hypothesis which was only accepted years later by the scientific community – a common trend – and he was awarded a Nobel prize for his work on prions.
The Nobel Prize in Physiology or Medicine 1997 was awarded to Stanley B. Prusiner “for his discovery of Prions – a new biological principle of infection.”– Nobel prize organization
So now that we have clarified that prions are simply infectious proteins, it is equally important to understand that the normal non-infectious version of the prion protein is also referred to as a prion. It is unfortunate, that the normal cellular prion protein has been given the same name as the infectious agent associated with a group of fatal neurodegenerative disorders.
Normal prion proteins are produced by mammalian cells, secreted and localised at the cell surface. The highest levels of the protein are located in neurons of the brain and spinal cord – indicating a possibly important physiological role. Even though much research has gone into prion diseases, the actual function of normal prions remains unknown. It has been suggested, through various studies, to be involved in the protection of cells against internal and environmental stresses.
When normal prion proteins misfold, the properties of the protein changes and the misfolded form is associated with disease. Not only does the misfolded protein begin to accumulate and clump together, it also interacts with other normal prion proteins and causes them to misfold. Since the highest levels of normal cellular prion proteins are in neurons, the spread of the infectious prion between neuronal cells leads to brain damage.
How is it transmitted?
To investigate how transmission occurs between cervids, Mathiason et al. exposed 4 cohorts of 6-month-old CWD-naive hand-raised white-tailed deer (Odocoileus virginianus) fawns to saliva, blood, a combination of urine and faeces and brain homogenate from CWD-positive deer. The infectious CWD proteins were detected in deer inoculated with saliva, blood and brain homogenate. Their results suggest that deer in high density populations and in captivity would most likely facilitate salivary transmission of prion disease. This result does however need to be viewed with caution as a small sample size was used and therefore factors such as individual susceptibility may account for these results.
Is it transmissible to humans?
The answer is, unfortunately, not a simple yes or no as the debate is still on-going. On the one hand, there are some who strongly advocate that transmission will occur.
It is probable that human cases of chronic wasting disease associated with consumption with contaminated meat will be documented in the years ahead.”– Michael Osterholm, director of the Center for Infectious Disease Research and Policy
Support of this claim comes from a previous case similar to that of CWD. There was an outbreak of Bovine Spongiform Encephalopathies (mad cow disease) in the United Kingdom, over two decades ago, resulting in 4.4 million cattle being slaughtered. As with CWD now, mad cow disease was also believed to not be able to spread to humans. John Gummer (Minister of Agriculture at the time) had announced that beef was “completely safe”. It turned out that the prion disease was in fact transmissible to humans resulting in Variant Creutzfeldt-Jakob disease (or vCJD), a fatal condition.
Since 1995, when it was identified, 178 deaths have been attributed to vCJD. It’s thought that one in 2,000 people in the UK is a carrier of the disease. But it appears that relatively few who catch the infectious agent that causes the disease then go on to develop symptoms.”–BBC news
Even though mad cow disease is transmissible to humans, that does not necessarily dictate that CWD will be as well. The transmission of prion diseases from animals to humans varies with different species and prion proteins. Several studies have investigated the possibility of transmission of CWD to non-human primates, such as in cynomolgus macaques. The cynomolgus macaque monkeys are used as a surrogate model for CWD transmission to humans.
A study published in the Journal of Virology looked at transmission from cervids to monkeys from two different genera. They found that oral inoculation of brain homogenate resulted in 92% of squirrel monkeys (Saimiri sciureus) to develop disease, at an average of 69 months post-inoculation, but none of the cynomolgus macaques (Macaca fascicularis) developed disease. This interesting result suggests that there is a species barrier between cynomolgus macaques and cervids but not between squirrel monkeys and cervids. Compared to squirrel monkeys, the prion protein of cynomolgus macaques is more similar to humans. It was suggested that the differences in the prion protein sequence between the squirrel monkey and cynomolgus macaques is linked to disease susceptibility. Of the 5 amino acids that differ between the two monkey genera, 3 of those that are found in cynomolgus macaques are identical in humans. Even after 13 years post-inoculation, there was no evidence that CWD transmission had occurred to the macaques using clinical detection and using highly sensitive prion disease-screening assays. This study agrees with various other studies suggesting humans are at a low risk of contracting CWD.
The media has the distasteful tendency to sensationalise the disease, causing panic to get traffic, while simultaneously providing little information which is often misguided, misquoted or simply exaggerated. Unlike the data regarding the mad cow disease which showed compelling evidence that prions from cattle with BSE have infected humans and caused fatal neurodegeneration, the data regarding CWD involves multiple studies, including seven tube-based experiments, five studies on transgenic mice, two epidemiologic studies and nine case studies all showing no conclusive link between CWD exposure and the development of a prion disease in humans. With that being said, beware of misquoted science and fake news and revel in the zombie apocalypse-free world.
To date, there have been no reported cases of CWD infection in people.”– The Centers for Disease Control and Prevention
- Mathiason, C.K., Powers, J.G., Dahmes, S.J., Osborn, D.A., Miller, K.V., Warren, R.J., Mason, G.L., Hays, S.A., Hayes-Klug, J., Seelig, D.M. and Wild, M.A., 2006. Infectious prions in the saliva and blood of deer with chronic wasting disease. science, 314(5796), pp.133-136.
- Prusiner, S.B., 1998. Prions. Proceedings of the National Academy of Sciences, 95(23), pp.13363-13383.
- Race, B., Williams, K., Orrú, C.D., Hughson, A.G., Lubke, L. and Chesebro, B., 2018. Lack of transmission of chronic wasting disease to cynomolgus macaques. Journal of virology, 92(14), pp.e00550-18.
- Race B, Meade-White KD, Phillips K, Striebel J, Race R, Chesebro B. 2014. Chronic wasting disease agents in nonhuman primates. Emerg Infect Dis 20:833–837. https://doi.org/10.3201/eid2005.130778.
- Scott, M.R., Will, R., Ironside, J., Nguyen, H.O.B., Tremblay, P., DeArmond, S.J. and Prusiner, S.B., 1999. Compelling transgenetic evidence for transmission of bovine spongiform encephalopathy prions to humans. Proceedings of the National Academy of Sciences, 96(26), pp.15137-15142.
- Zabel, M.D. and Reid, C., 2015. A brief history of prions. Pathogens and disease, 73(9).
Photo of Winter Herd – Elk, in National Elk Refuge, Jackson Hole, Wyoming, courtesy Thomas D. Mangelsen (mangelsen.com)