Hollywood and gory novels have made us totally obsessed with zombies. Among all the movies and fancy dress outfits, there’s a Zombie Squad, Zombie Research Society and even a zombie apocalypse emergency plan at the Pentagon. Needless to say, the fear is very real among some of us, but is the threat? As it’s Halloween, let’s take a look at some of nature’s nasties to see if we have the ingredients for your worst nightmares.
Why so angry?
So, zombies are typically depicted as angry fellas that will do just about anything to get their slobbery, gnarly chops around your juicy flesh. Brain injuries, of course, can make people aggressive; a number of brain regions, like the hypothalamus, have been linked to aggressive behavior. But unless a significant proportion of the world all falls over at the same time and bonks themselves on the head, smacking just the right area, we need something contagious to have zombies in the making.
A virus seems the most plausible agent here; several viruses have been linked to aggression in animals, but perhaps the most famous is the rabies virus. To fit in nicely with Hollywood, this virus can be transmitted by bites or scratches as it is present in the saliva, which often frothily exudes from victims.
Following initial infection, it makes its way towards the central nervous system and attacks the brain. Symptoms start off pretty nondescript, a bit like flu, but as the disease progresses some of your classic zombie-esque signs start to pop up: confusion, agitation, excessive salivation, and hyper-aggressiveness. The latter symptom is a wonderful example of an agent evolving ways to enhance transmission and thus success: aggressive individuals will be more likely to attack others and hence pass on the virus.
There simply aren’t enough mouths to go around
There are a couple of problems with the rabies virus acting as the tinder for a zombie apocalypse, though. First off: it definitely kills you, pretty hastily sometimes, which is actually poor form for any infectious agent as it’s detrimental to transmission. Second, it can sometimes hang around the body for a year without causing any symptoms, which again isn’t really conducive to a pandemic scenario. And finally, while transmission through biting makes a good movie plot, it probably isn’t going to result in a worldwide disaster, purely because of the logistics. Infected individuals would have to go around gnawing on a lot of people to make a dent.
So we need to throw in some genes from a more contagious agent. Flu? No, we can do better than that: measles, one of the most infectious diseases in history. Although if we’re thinking outside the box, maybe rotavirus, but given the fact that this causes gastroenteritis in infected individuals, zombies running around with horrific diarrhea is perhaps not your classic horror story scene, but we’re open to suggestion.
Like bacteria, viruses can actually swap genetic information, but fortunately for our nightmares this doesn’t occur between different viruses. It’s therefore extremely unlikely that a measles virus and a rabies virus would make sweet, sweet viral love and produce some wickedly infectious, rage-driving virus. But who knows, one day it might be possible to make the right concoction in the lab, should anyone desire to do so…
You’ll never catch me
So we have two key ingredients for our zombifying cocktail, but there are plenty of other things to consider. For one, we haven’t discussed how they end up meandering and stumbling around like mummies that have just crawled out of a sarcophagus. Here we may have to think a little outside the box again, but this time we’re moving away from viruses, and onto something even weirder: prions.
These are highly infectious protein particles that cause a variety of nasty neurodegenerative diseases, like Creutzfeldt-Jakob disease (CJD) and kuru, which are collectively known as transmissible spongiform encephalopathies. They spread like wildfire in the central nervous system, causing proteins to twist into the wrong conformation and clump together, resulting in portions of brain tissue dying and effectively turning into a sponge, hence “spongiform.”
CJD also brings with it some particularly nasty, zombie-like symptoms: twitching, muscle paralysis and slurred speech. Kuru presents similar symptoms, but you’ve probably never heard of it, likely because it’s only been found in one place: Papua New Guinea. Here, it spreads as a result of cannibalistic funeral practices in which bodies are cooked and eaten after death, with close female relatives being the lucky ones to gobble down the brain – the most infectious part.
So if, in some unlikely scenario, our measles/rabies virus hybrid ended up in someone that got a hankering for a prion-riddled brain, we could almost have a zombie apocalypse in the making.
Your mind is mine
We’ve covered three important zombie characteristics, but there is one last thing that definitely needs to be taken into account: mind control.
It may horrify you to find out that there is a plethora of mind-controlling organisms out there, straight from Mother Nature’s womb, not an evil scientist’s basement. There are fungi out there that turn ant victims into the walking dead, wasps that enslave cockroaches and worms that make crickets suicidal. You don’t have to look far to find zombifying parasites; but what about something capable of taking over the minds of humans? Preposterous, surely. Hmm, not so much.
Toxoplasma gondii, a single-celled parasite that causes a disease known as toxoplasmosis, infects animals and people all across the globe. As many as 60 million people in the U.S. may have it, but most don’t realize because it can be symptomless. In rodents, however, it does something super weird…
Many animals can play host to this organism, but it can only complete its life cycle in cats. So what to do if one ends up in the guts of a useless animal like a rat or a mouse? Force your way inside a feline, of course. But how does a lowly parasite do that? By taking away the poor rodent’s fear.
Infected rodents no longer show aversion to cat urine, making them easy prey. Weirdly, infected humans even find the odor of cat urine quite pleasant, but not the pee of other animals (yes, someone studied this). There has also been some evidence that T. gondii makes humans more reckless and likely to engage in risk-taking, but this is contentious.
So, while mind control is most certainly not a figment of Hollywood’s imagination, we would need T. gondii to re-jig its DNA quite a bit before it becomes a zombie threat: a mass of pee-sniffing humans hardly has a terrifying ring to it.
Can we actually make a zombie in real life, then? There may be some pretty terrifying ingredients out there, but none of them quite meet our very high, imagination-driven expectations of a true zombifying agent.
Second image in text: 306/Shutterstock