The science behind real life zombies

In the spirit of Halloween we bring you the science fact and fiction behind the undead. Zombies, those brain loving little guys, (and girls) are everywhere. We are all familiar (if you are horror fans, or  at least not living on an Amish compound) with the classic  zombie. But did you know that we aren’t the only zombie lovers out there? It turns out that nature has its own special types of zombies, but this isn’t a science fiction movie, this is science fact!

Things like the Ophiocordyceps unilateralis, which is a fungus that can literally take control of ants. Once this happens it will climb down from its habitat, or after falling to the forest floor will climb underneath a leaf, and clamp down with its mandibles to secure itself to the leaf vein. This will be where the ant dies, in this “death grip.” Here is where it gets even more exciting (well not for the ant), the fungus breaks out of the ants head and spreads its spores.

Not quite right… but still funny.

That is not even the scariest of zombies, Gordian worms (also known as horsehair worms) can live in crickets for long periods of time. When this little guy is ready, it causes changes in protein expressions in the brain that make the cricket extremely thirsty. The cricket, basically possessed, heads to a pool of water where it drowns itself trying to quench its thirst. This isn’t the end however, the worm exits the cricket to mate (hint: they live in the water).

But more complex animals can become zombies too, mice for example can be invaded by zombie creating parasites. Toxoplasma makes rats lose their fear of cats — or more specifically, it causes the rat to be attracted to cat urine. This means the mice won’t try to run away when a cat tries to eat them because they are infatuated with the smell of the cat urine. Of course, this is exactly what the parasite wants, when the cat eats the rat the Toxoplasma can then pass into the cat and mature. Afterwards the cat passes it by its feces and the cycle continues.

And what horror story would be complete without a twist? This little guy has even shown up infecting humans — and while it is thought to not do anything to humans  — it has been recently implicated in serious mental health changes like depression and even suicide. While nothing is conclusive quite yet, it would not be completely shocking — given the similarities between the mouse brain and the human brain — that it has some unintended effects that may not even be seen in the mouse models.

Another scary fact, toxoplasma is very common, even in humans. If you’ve owned a cat, or have been raised with one, it may already be inside you and you may not even know it. Typically unless you are immunocompromised you would never notice Toxoplasma and the body usually takes care of it, but we still don’t know what the long lasting effects are from the infection, even in cases where it is temporary.

Yummy, yummy cat urine!!

Not to be left out, snails get in on the zombie action. Leucochloridium paradoxum [a mouthful I know], causes the snail to develop eyes that look like caterpillars. Snails, which prefer the dark, head out in daylight while infected, which causes them to be seen then consequently get eaten by birds and the life cycle continues.

I’m hideous!… ly delicious to birds.

That is not even all the examples that are out there (from the ones we know about). But with all these zombies in nature the real question is, what about humans?

What if some mind controlling parasite started turning us into brain eating zombies?

Well sadly for you zombie fans, besides the possible implications from a toxoplasma infection, there aren’t any zombie parasites that affect humans (that we know about). However, if you just cannot wait for the zombie apocalypse, it would not be a surprise if something — possibly soon in a life of the earth timescale — could hijack a human brain… if it has not already happened.

Just because human physiology is complex, that does not mean we are invincible. Think of all the nasty things that we have going around already, HIV/AIDs loves to hijack the immune system (which is very different from zombie I know). Think of it like this, we may be complicated, but the more complex the system, the more ways there are to alter it.

So what could come our way?

Well things like that are tough to predict, it’s hard because when we talk about mutations in existing virus we have trouble guessing what would happen. That is why I wrote this post on the genetically created flu virus. Of course, there is the alternative, a genetically modified human-created virus that would do the job, perhaps something like in Extinction: The GMO Chronicles (which turned out to be a pretty good low budget flick).

The movie revolves around a type of virus (they have one subtle hint it is actually a plasmid, those creepy, creepy things) that cross breeds genes from plants, animals and even humans. However, that kind of future would still be a ways off, if it were even remotely possible to do.

Lab help: You can read about Plasmids here

That is because genetic engineering is super complex, it would be like me randomly trying to replace a single word in, say Atlas Shrugged (which would be easier to do since it is vastly shorter than your DNA). It would be really hard to find the single word I wanted to replace without accidently replacing all the other incidences of this word. This makes gene editing hard, because if I replace every instance of the word gene in this page with the word chip it would only make sense some of the time and the rest of the time it would just look like gibberish.

Now I could go through and instead just look for a certain word, I look for a whole sentence and then replace the word that I want. This type of searching is almost perfect for things like text because we have so many combinations so the chances of finding the same sentence more than once or twice is small. DNA on the other hand only has 4 different base pairs (4 letters) and is much, much longer so the same strings pops up multiple times.

This means the word chip would get thrown in the correct sentence, but all other instances of that sentence. So again we would have gibberish, and while in some cases this does nothing — maybe the sentence isn’t important, or is just some random detail. In other cases it could cause big issues — what if we accidentally changed the whole point of a book by changing that one word and suddenly Frankenstein’s monster turns into Frankenstein’s hunger.

This is unfortunately how genes work, most of the time if we edit something by mistake it will not do much (and that is a good case). Most of the time incorrect edits will cause cell death, or other really screwy things because the gene we wanted to change also caused other instances of that string to be replaced. So needless to say, we are dealing with very complex machinery. Personally, a gene swapping, zombie creating virus would be a pretty cool scientific feat, but alas not likely to happen until we get better at gene editing (at which point if we were that good, fixing the problem would be just as easy).

A little gene humor

Zombies in the undead sense would not happen either unfortunately, but humans with an insatiable appetite for human flesh or even brains could happen. This would not be too hard to manage with right genetic modifications. There already is a real life condition that has similar results called, Prader-Willi syndrome. This is a condition where you would not only have an incredible hunger, you would not be able to tell when you are full. People who suffer from the syndrome and don’t get professional help are typically dangerous because having a constant hunger can in fact make pretty much anyone violent.

The bottom line is that while we should not worry about zombies anytime soon, it is fun to talk about all the weird ways things like this could happen.

On a side, note I want to point out that the term genetically modified (or GMO) is one of those terms that does not really mean anything. Everything is genetically modified from when it was first created, me, you, your mother, that “all-natural” and organic steak you ate, it’s all genetically modified. It is like saying organic, I would love to see some non-carbon based food [which is what the term organic means after all], because if it is not organic you probably should not be eating it. Furthermore, whether it is modified in the lab, or by nature, it is all the same Lego set, so to speak.

In any case, Zombie apocalypse or not:

Happy Halloween!!

Sources: 
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Vyas A, Kim SK, Giacomini N, Boothroyd JC, & Sapolsky RM (2007). Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. Proceedings of the National Academy of Sciences of the United States of America, 104 (15), 6442-7 PMID: 17404235

Passamonti L, Crockett MJ, Apergis-Schoute AM, Clark L, Rowe JB, Calder AJ, & Robbins TW (2012). Effects of acute tryptophan depletion on prefrontal-amygdala connectivity while viewing facial signals of aggression. Biological psychiatry, 71 (1), 36-43 PMID: 21920502

Thomas, F., Schmidt-Rhaesa, A., Martin, G., Manu, C., Durand, P., & Renaud, F. (2002). Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts? Journal of Evolutionary Biology, 15 (3), 356-361 DOI: 10.1046/j.1420-9101.2002.00410.x

W. Wesołowska T. Wesołowski (2014). Do Leucochloridium sporocysts manipulate the behaviour of their snail hosts? Journal of Zoology , 292 (3), 151-155 : 10.1111/jzo.12094