Chronic wasting disease (CWD) is an unusual disease of deer that has been making headlines for all the wrong reasons. Chronic wasting disease is interesting because it is not caused by a virus or bacteria, but instead an infectious particle called a prion. Prions are proteins that propagate themselves in the body by triggering a normal protein to misfold itself into the infectious form. Mad cow (BSE), scrapie, and CWD are all prion diseases and are all 100% fatal to those that actually contract them, though the process can take years.
Prions are of particular concern to humans because some prions have demonstrated an ability to infect and kill humans. The proteins that can be misfolded into prions sometimes misfold spontaneously, leading to a terrible syndrome called Creutzfeldt-Jacob Disease (CJD) that inevitably leads to a terribly painful death. Humans can also contract similar diseases to CJD by consuming beef infected with BSE or human flesh from someone with CJD, leading to vCJD or Kuru respectively.
CWD is menacing because it has been experimentally shown capable of infecting primates and mice and there is little reason to believe humans could not be infected. Like scrapie, the disease can be spread through feces and is persistent in the environment, lasting for years in the soil. Scariest of all, plants can uptake prions into their tissues and then infect the animals that eat the plants.
For those hypochondriacs among us or anyone hankering for societal collapse, this leads to thoughts of a potential nightmare unfolding.
A variant of CWD emerges that is efficient at infecting humans, one capable of being spread like a virus through the air. Prions can also be transmitted through feces, so even a few deer defecating and breathing on a piece of farmland could be enough to introduce it into the food supply chain. As the new variant spreads through deer and human populations, vast numbers of infectious particles would be created and contaminate the land for years to come. Because prions usually take decades to show symptoms in humans, we might not even know until it was too late.
What’s worse are the infected themselves. Prion-addled humans, like infected animals, eventually lose control of their minds and bodies. They become disoriented and aggressive, attacking others and spreading the plague further. Any time spent in an infected area could prove deadly, through inhaled dust, consumed plants, wildlife encounters, human interaction, or a bite from one of these ‘zombies.’ Eating and drinking literally anything becomes potentially deadly, and whole swaths of land become essentially uninhabitable. You can’t hide in nature nor can you seek shelter with other people. It’s the end of humanity as we know it.
News outlets practically salivate over any mention of a zombie virus and CWD is no exception. The idea of a soil-borne pathogen that persists for years and can infect people who eat crops grown on it is frightening for anyone that grows or eats food. Is a scenario even one-tenth as bad as this likely, though? In case you stop reading here, no. It’s nearly impossible.
Human to Human Transmission
Prions are so far very unlikely to cause a massive pandemic. For that kind of spread you would need the ability to infect humans and the ability to spread from human to human, aka sustained human to human transmission. A lot of things need to happen before a pathogen can be transmitted from human to human, let alone become a global threat, and CWD so far fulfills neither criterion.
A pathogen capable of causing a pandemic needs to be capable of infecting humans in the first place. So far only BSE (mad cow) and kuru seem to be capable of doing it with any regularity, though it seems CWD is capable of infecting some primates experimentally. The susceptible primates were squirrel monkeys, while macaques (more closely related to humans) and transgenic mice (engineered to be more similar to humans) were essentially immune. This is probably why no confirmed cases of spillover to humans have actually been documented beyond anecdotes and urban legends. CWD just isn’t very good at infecting humans.
Both the genes that encode prion proteins and individual lines of infectious prions do evolve, and theoretically could evolve to infect humans, but given the longer generation time for host animals and the slow courses of the disease it likely would take much longer than it would for a virus. There is no guarantee a lineage prions would ever evolve to infect humans.
So let’s assume that one does evolve. Once a line of prions has the ability to infect humans once they enter the body it would then need to make its way out of an infected human and reach an uninfected human, which is easier said than done. Most prions don’t seem to be spread by saliva or semen, nor are they spread through respiratory particles. CWD seems to be infectious through large amounts of saliva, though this result may not carry over to the real world given the amount of saliva used in the experiment. Infected nervous tissue and sometimes blood needs to end up in mucous membranes and wounds or prions needs to enter the digestive tract. Or, in very unusual circumstances, inhaled if the tissue become aerosolized.
In order for a proper pandemic to occur, repeated transmission from wildlife to humans won’t cut it. Doomsday isn’t going to happen from deer alone. An apocalyptic prion pandemic needs sustained human to human transmission, and prions just don’t fit the bill. There are a few ways diseases can be transmitted, but airborne is perhaps the scariest. Fortunately, the mere idea of a prion becoming an airborne pathogen flies in the face of everything we know about prions, and the fearmongering around airborne prions tends so center on one misunderstood study, which made aerosolized prions and exposed mice to the deadly mist. The mice were easily infected, as one might expect. After all, they made a mist out of a highly infectious agent and forced animals to breath in concentrated doses of it.
The issue is that just because you can be infected a certain way doesn’t mean it is likely to happen outside of the lab. Prions can be temporarily airborne in extenuating circumstances, usually from mechanical means like centrifuges, saws, meat grinders, and severe injuries (gunshots to the head might be able to release aerosolized prions). This should be of great concern to people who work in environments like slaughterhouses, where prions might be released through the cutting of infected tissue with saws or while slaughtering animals and in laboratories where prions might be aerosolized through the use of certain scientific equipment. Hunters who might be shooting a deer and then walking near the carcass also should be cautious. None of these mechanisms of transmission are particularly common, however, and there is no evidence it happens or is a viable way for prions to spread from host to host without some pretty serious leaps in logic.
Of course, that leaves the less efficient and slightly less scary fecal transmission. If it happens to deer it may be possible with humans as well. This is an issue because fecal contamination of food happens regularly and sickens thousands of people per year, and prions might be resistant to many of the technologies we use to process human waste. Because plants can carry prions in their tissue, both accidental fecal contamination and intentional use of human waste on crops are more likely routes of entry for prions into the human population as compared to aerosols. That’s why when we think of prions as a widespread threat, our concern should probably be focused on their environmental transmission.
Environment to human transmission
The other possible infection route, the one that often infects grazing mammals, is the consumption of infectious particles in the environment, usually bonded to soil. This usually happens when an animal defecates, or when it dies and decays, leaving behind the more persistent prions. These are then taken up by plants or splashed onto plants by the rain and eaten by other grazers, perpetuating the cycle of infection.
Unlike deer, however, we tend to bury or incinerate our dead. Cremated remains are hot enough that prions are destroyed, and it is unlikely we will be eating the soil remaining from a human grave or growing crops on burial plots within the time it takes for prion particles to break down naturally. If human waste is recycled and used on farmland and a significant number of people are infected, however, it might be possible for human feces to spread the disease through the food supply. This requires large numbers of infected people *and* the ability for human feces to carry infectious prions, something that has never been demonstrated to actually be possible. The prions also need to be able to survive the sewage treatment process and be applied to fields quickly enough so that the prions do not degrade. If it doesn’t happen within a couple years, most of the prions will have degraded. So deer feces to human transmission? It might be possible. Human to human? Pretty unlikely.
Once this barrier is broken, the dose transmitted needs to actually reach nervous tissue. There is no safe threshold below which a dose of prions won’t cause infection – even one prion can be enough. But statistically a single prion has a great chance of just passing through the digestive system, and prions in the blood are relatively scarce even in infected individuals (which is why the main issue with bloodborne prions is blood transfusions, because the exchanged amount of blood is very large). Of course accidents happen, but a lot of unlikely coincidences need to happen for an accident to lead to an epidemic.
It’s important to note that prions are not immortal, just very very tough. They do degrade with UV light and through decay processes, though they are remarkably persistent for a biological entity. In soil they are rather stable and can last a couple years, though a couple of reports put that number as long as 16 years in select circumstances. Once exposed to sunlight and rain they do degrade, though it can take a long time. It’s not like PFAS, where the molecule in question just build up in the soil until you can’t touch anything outside without being contaminated. Prions do break down.
Prions and the Garden
All this talk about prions and soil might leave a gardener or homesteader concerned. Because CWD appears to be transmissible through feces, deer do seem to enjoy pooping in gardens, and plants can uptake prions from the soil a garden just might be a place where CWD could cross over into humans, despite the *exceedingly* low risk of any spillover.
Prions bind very well to soil and, surprisingly, the infectivity of soil-bound prions is actually enhanced. This close association keeps prions from migrating deeper into the soil and maintains prions at the surface, where soil-human contact is more likely. Despite the general low risk of CWD to humans it does seem prudent to avoid situations where humans are eating prion-contaminated crops, as that is the good way to ensure there are plenty of opportunities for CWD to jump over to humans.
The good news is that you are probably taking these precautions anyways. Keep deer out of your garden. Put up a fence, adopt a dog, use repellant. Anything that keeps deer from eating your plants will keep them from exposing people to diseases.
Conclusions
The crux of the issue is that there is no good reason (aka no selective pressure) that would encourage a lineage of prions to evolve a sustained capacity for human to human infection. Humans are basically dead-end hosts, who are burned or buried and basically removed as a source of inoculum. The scariest possible scenario would be a line of CWD that stumbled onto the ability to consistently infect humans by accident, not because of selective pressure to infect humans. This line could theoretically infect people who ate the meat or even ate crops that grew on infected soil, but it would stop there. A terrifying, but ultimately localized, problem.
Prions are common in the environment. They are widespread in fungi and certain species of animal. There has never been sustained human-to-human transmission of any prion, ever, nor have their been prion epidemics in humans not caused directly by human actions (eating humans, feeding cows other dead cows). Only a couple kinds of prions infect humans with any regularity, and the number of humans infected are small.
If you hunt, test your deer and elk. If you eat beef, only eat high quality beef from cow populations that get tested. If you eat humans, probably stop. Fence your garden. CWD could definitely change and jump over to humans, but the impact of such a jump is still not even going to get close to the impact of an easily transmissible virus like COVID and is ultimately very unlikely to occur.