Popular zombie fiction often blames the shambling hordes on an infection that alters brain function. In this case, fact has fiction beat: the T. gondii microbe hijacks human brains already.
Timothy Verstynen and Bradley Voytek take on the neuroscience of the zombie brain in Do Zombies Dream of Undead Sheep? (Princeton University Press, 2014) with witty analyses of the characteristic lumbering gait, insatiable hunger for brains and memory-less existence of the undead. Through this exploration they shed light on fundamental neuroscientific questions while bringing in examples from zombie popular culture. The following excerpt from Chapter 11, “Fighting the Zombie Apocalypse…With Science!” offers an example of a microbe that can hijack human brains.
Humans have a long history of having their brains manipulated by parasitic organisms.
It turns out that cat poop can hijack your brain. Well, not the poop per se, but a tiny little organism that lives there: Toxoplasma gondii. T. gondii is a single-cell organism that has an interesting life cycle. It all starts when two microbes really like each other and undergo sexual reproduction. Apparently, the only place that the T. gondii organism can “get in the mood” happens to be the intestinal lining of cats.
As can happen in a microscopic love story, eventually new little microbes are born and go out into the world with the rest of the things in the intestinal tract. In this case they’re packaged in little cysts that can survive in the brutal world outside the gut. The hope is that another cat will step in the poop and the little cyst will make its way up to the cat’s mouth when the cat is eating, and from there, it’s back to the gut for more love making.
Now here is where the T. gondii life cycle gets more interesting. What happens if the cyst is ingested by an animal other than a cat? Rather than just live out a celibate life without reproducing, the T. gondii microbe goes in for self-love and starts reproducing asexually (i.e., cloning). This can give the host animal a flu-like illness as the infection grows, producing a fairly harmless condition called toxoplasmosis. Usually the symptoms pass and the infected animal (human or otherwise) appears to recover fully (though we have to note that toxoplasmosis is quite dangerous to human fetuses, which is why pregnant women are advised not to take care of the litter box or otherwise be in contact with cat feces).
At least it seems like the person or animal recovers.
In reality, the T. gondii microbe doesn’t give up trying to get back into feline intestines. It just changes tactics and engages in a little (really little) guerilla warfare. By rewiring the brain of the unsuspecting host.
Let’s say that you’re a rat that has been infected by the T. gondii microbe. Being a rat, you normally don’t like being around cats. Because they might eat you. In fact, rodents such as rats and mice appear to have an inherent fear of felines (Zangrossi and File 1994). Evolution has punished those who aren’t fearful.
This feline fear is not a good thing for the little T. gondii microbes that call cats their home. What’s a microbe to do? Well, the T. gondii alters the neural function of the infected hosts (you) to make them (you) less fearful in general and make riskier and riskier decisions (Webster 2001). You become more bold and less risk aversive. You are less likely to care if there’s a cat standing next to you, and thus more likely to become that cat’s lunch. Bad for you, but good for a microbe who’s looking for a little nookie in a feline’s gut.
But surely this won’t happen to you if you’re not a rodent but a human, right?
While we humans might not be cat food anymore (at least not as much as our ancestors might have been), as we just said, humans can become infected with the T. gondii microbe. Humans with a latent T. gondii infection, just like rodents, start to have altered personalities. They exhibit an emotional detachment and reduced risk aversiveness. Basically, they just stop caring about risky behaviors. According to a review by Joanne Webster in 2001, some studies found that,
“Based on questionnaires measuring personality factors, difference between [T. gondii] infected and uninfected groups were detected. For example infected men had lower “superego” and higher “pretension” scores, which the authors concluded implied that they had a higher tendency to disregard rules of their society and were more suspecting, jealous and dogmatic. [p. 1041]”
We don’t exactly know how the microbe effects this change in its host’s behavior to cause these personality shifts. But it is clear that these changes are linked to changes in the brain. Thus it is entirely possible for a microscopic, single-cell organism to take over control of our complex neural circuitry.
Of course, the symptoms of T. gondii infection are nothing like zombie behavior. But the infection does make it clear that it is possible for an external pathogen to hijack our brains and alter our behavior.
Reprinted with permission from Do Zombies Dream of Undead Sheep? A Neuroscientific View of the Zombie Brain by Timothy Verstynen and Bradley Voytek and published by Princeton University Press, 2014.