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Toxo Terror: Are Our Brains Controlled by Cat-Loving Parasites?

Teeming within your body alone are trillions of microorganisms—bacteria, fungi, viruses, protists—that make up 90 percent of all the cells in your body. You are more them, than you are you. In fact, there’s not really a you.
Photo illustration by: Kamran Samimi

The sheer ubiquity of microscopic organisms is staggering. Microbiologist Tom Curtis recently compared the size of the microbial population to the size of the universe: The number of microbes in the world is billions of times larger than the number of stars in the sky. Think about that for a second.

And welcome back. Teeming within your body alone are trillions of microorganisms—bacteria, fungi, viruses, protists—that make up 90 percent of all the cells in your body. You are more them, than you are you. In fact, there's not really a you. Your body is more of an ecosystem than a discrete organism. The sum effect of this fact is unnerving: In a way, you are not one thing, but a codependent mass of countless organisms, moving in unison toward shared goals. Through hundreds of thousands of years of co-evolution within this host-microbe habitat, these organisms have developed a complex scheme of cat-and-mouse survival strategies that affect the overall system.


So then: Whenever you do something, whenever you make one of the myriad, inane daily choices that ultimately define who you are, who is it making the decision? You? Or is it them?

The idea that microscopic flora living inside our bodies can directly manipulate our behavior and personality is perhaps surprising. It's also exactly what a growing number of scientists is suggesting. And it's absolutely terrifying.

For the past few decades, an obscure scientist in Prague, Jaroslav Flegr, has been studying the influence of a particular brain parasite that co-opts the behavior of its hosts in surprising ways. The goal of this parasite, a protozoan known as Toxoplasma gondii, is simple: It wants whatever host it's infected (typically a rodent) to get eaten by a cat.

Why? Because Toxoplasma can only reproduce in the digestive tracts of cats. So it's devised a clever strategy: The parasite alters signals in the rodent's brain, making it behave in ways that increase the possibility it will be caught and eaten by a cat— namely by removing the rodent's instinctual fear of cats, slowing its reaction time, and actually making it attracted to the smell of cat urine.

"It's the most damn amazing thing you can ever see," said Robert Sapolsky, a neurobiologist at Stanford University, in an interview with "Toxo knows how to make cat urine smell attractive to rats. And rats go and check it out and that rat is now much more likely to wind up in the cat's stomach. Toxo's circle of life completed."


The fact that a parasite has learned to sneak into brains, hack some biochemical wiring, and effectively reverse a deep-seated, primordial fear that's been beaten into the inherited psyche of every living rodent is, in a word, bonkers. But Toxoplasma also infects humans. In fact, it's a fairly prevalent parasite, found in roughly one-third of the world population. The parasite has been known for over a century, and the screening of pregnant women for Toxoplasma is standard practice, as infection is fatally dangerous for developing babies. But the vast majority of infections in healthy adults are latent (or asymptomatic), meaning infected humans typically go about their business with no apparent ill effects.

It turns out, of course, that there are some effects. Many of them are still poorly understood, but new research is beginning to illuminate the many subtle but significant ways Toxoplasma influences human hosts. According to the Czech researcher Flegr, Toxoplasma tries to do the same thing in humans that it does in rats: control our brains to increase the likelihood of our getting eaten by a cat.

We're not hunted by cats of course. So what does Toxoplasma do when it wants us to get eaten? Well, the behavioral influence plays out in a number of strange ways. Toxoplasma infection in humans has been associated with everything from slowed reaction times to a fondness toward cat urine—to more extreme behaviors such as depression and even schizophrenia. And here's the kicker: Two different research groups have independently shown that Toxo-infected individuals are three to four times as likely of being killed in car accidents due to reckless driving.


Wait. What?

Just last month, Flegr's group published a paper in the scientific journal Public Library of Science with the chillingly cool title "Fatal Attraction Phenomenon in Humans." The study shows that some people infected with Toxoplasma find the typically gag-inducing odor of cat urine "pleasant," compared with non-infected individuals. And just to make sure it's not some sort of general urine kink, the researchers included other animal urine samples (horse, hyena and tiger!) in the study as well. But the infected group was horny for only the cat urine. In other words, Toxoplasma thinks we're rats, and it wants us to be eaten by a cat.

Such findings have spawned all sorts of fun speculation and cute stories about the shared evolutionary histories with our microscopic life partners. It's been suggested that Toxoplasma could even explain the eccentric behavior of so-called "cat ladies." You know, cases of irrational hoarding of like 20+ cats, often in squalor, and a tendency toward reclusive, nonsocial behavior. "It's totally cool cocktail party fodder," Sapolsky suggested in an email to me, "But, as far as I know, no empirical evidence."

Still, it's tempting to draw a connection. If you've ever visited a home where cat hair covers every possible surface and the wretched stink of cat waste is heavy in the air, the possibility of toxoplasmic cat lovers doesn't seem so far-fetched. This is, after all, a parasite that encourages an unhealthy feline attraction, an immunity to cat piss, and eccentric (perhaps schizophrenic) behavior.


To take it a step further, maybe cat ladies are more likely to be eaten by cats. Imagine someone leading a reclusive life, dying alone, surrounded by now-starving cats. It's not absurd: In 2008, a woman in Romania described as a cat lover was "eaten head to toe" by her pet cats after dying of natural causes.

The phenomenon even has its own technical name: postmortem predation. At a 1992 American Academy of Forensic Sciences conference in New Orleans, a forensic pathologist told the following story, paraphrased by an editor at The Straight Dope: "Sometimes, when an individual living alone dies unexpectedly, several days may pass before anyone takes notice. Some of these individuals may own a dog or a cat, which will go unfed. In my experience, a dog may go for several days before finally resorting to eating the owner's body. A cat, on the other hand, will only wait a day or two."

For now, though, the proposition that parasites are controlling our minds to suit their own survival needs is a radical one. Many of the scientists I spoke with view Flegr's conclusions in his human studies with raised eyebrows, cautious enthusiasm, polite euphemisms, or incredulousness. When Flegr began looking at this complex parasitic relationship, most of his papers where rejected by scholarly journals.

But that's changing. Toxoplasma research has become something of a bona fide subfield in parasitology over the last few years. The number of scientific papers published with findings about the effects of Toxoplasma on human behavior or personality has exploded. "After nearly 20 years of research in this field, I think most parasitologists and evolutionary biologists finally accept this effect," says Flegr. The challenge now is to get researchers from other disciplines, such as psychologists, to understand this effect on human behavior in order to get a clearer picture of how the parasite might influence personality types. "When I send my papers to psychological journals, they're usually rejected," says Flegr, "for the same reasons I encountered 15 years ago with parasitology journals."


"Flegr is very creative and pushing the envelope," says Kevin Lafferty, a researcher studying parasites for the US Geological Survey. "It is hard to do the sort of work he does with humans, so the results are open to alternative interpretations. But I think he is on to something aboutToxoplasma."

Research on this curious parasitic manipulation has gone increasingly mainstream. There are now several groups at top-tier science institutions pushing the investigation forward. Joanne Webster, a parasite epidemiologist at Imperial College London, leads a team teasing apart the behavioral components of Toxoplasma, and Glenn McConkey's lab at University of Leeds is focusing on the parasite's genome.

The real fruit of this research may be what the ancient critter can teach us about modern neuroscience. Sapolsky, for instance, is a highly regarded professor of biology and neurology at Stanford. A baboon social scientist turned neuroendocrinologist, Sapolsky has recently become fascinated with the bizarre story of Toxoplasma and is now leading the charge to unpack the parasite's neurobiology. In his interview with Edge, he explained what drives his interest in Toxoplasma and other emerging parasite interactions, "this utterly bizarre world of parasites manipulating our behavior.":

What does Toxo do to humans? There's some interesting stuff there that's reminiscent of what's going on in rodents. Some literature is coming out now reporting neuropsychological testing on men who are Toxo-infected, showing that they get a little bit impulsive. And then the truly astonishing thing: two different groups independently have reported that people who are Toxo-infected have three to four times the likelihood of being killed in car accidents involving reckless speeding. In other words, you take a Toxo-infected rat and it does some dumb-ass thing that it should be innately skittish about, like going right up to cat smells. Maybe you take a Toxo-infected human and they start having a proclivity towards doing dumb-ass things that we should be innately averse to, like having your body hurdle through space at high G-forces. Maybe this is the same neurobiology.

On a certain level, this is a protozoan parasite that knows more about the neurobiology of anxiety and fear than 25,000 neuroscientists standing on each other's shoulders, and this is not a rare pattern. Look at the rabies virus; rabies knows more about aggression than we neuroscientists do. It knows how to make you rabid. It knows how to make you want to bite someone, and that saliva of yours contains rabies virus particles, passed on to another person."


Sapolsky recently followed up in an email: "I think that the really fascinating and creepy thing about the whole Toxo story is that there have to be a gazillion other examples like this out there, ones that we haven't a clue about —viruses, bacteria, protozoa, multicellular parasites."

In other words, this is just the beginning of a new chapter in biology. One that stands to reshape how we view discrete organisms, neurochemistry, and even free will. One in which the human body is a cornucopia of alien life, and we are merely vessels guided by the competing whims of microscopic creatures imprinted with coded instructions through eons of evolution.

And maybe, just maybe, it helps explain how we ended up with so many cats.