If You Own Cats, You Might Have a Brain-Altering Parasite

A story of humans, an infectious parasite, and your cat.

Zulie Rane
Sep 9, 2019 · 5 min read
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Photo by Japheth Mast from Pexels

If you’re a cat owner, like me, you’re probably infected with a parasite.

In the last few decades, it became apparent that the vast majority of house cats were infected with a particular species of a tiny one-celled parasite, called Toxoplasma gondii. That on its own wasn’t news; we’re all hosts to a wide and rich variety of parasites.

What makes this one special is that scientists discovered that, when this tiny little parasite was present in rats, those rats became far less afraid of the smell of cat urine than normal.

Weird, right?

It turned out T. gondii was manipulating them. It changed their behavior, to make it more likely for them to be eaten by cats, which is the next species of host for T. gondii, and the only host in which the parasite can naturally reproduce.

Mind alteration in animals infected by parasites is a relatively new field of study. That being said, it’s been demonstrated to be a relatively common adaptation: parasites with more than one host need a way to go between the hosts. And what easier way than to just take control of your host by changing their behavior mentally?

The concern began when people realized that a parasite that could alter rodent behavior? Maybe it could change human behavior, too.

Does this parasite make humans love cats?

A rash of studies came out showing that people infected with the parasite (up to 84% of the human population in some countries) experienced significant changes in behavior. Infection with T. gondii was linked to everything from entrepreneurship to the sex ratio to bad driving. Not only that, but there were differences in how men and women were perceived to show the effects. To sum it up in one especially demonstrative 2006 headline, “Parasite makes men dumb, women sexy.”

But it wasn’t just that. According to some studies, men were more likely to be impulsive and jealous when infected. Women were more likely to be more warm-hearted, moralistic, concerned with their image.

In terms of popular science, these studies were like gold. A parasite caused by cat ownership that changed men and women in different ways by controlling their brain? A scientific study that backed up the Crazy Cat Lady stereotype? People ate it up. Not only that, but some men, in particular, started looking for ways to infect the women in their lives, intentionally, to reap these alleged positive characteristics.

A prime example of bad science.

One recent study published, the most comprehensive to date, found absolutely no correlation between toxoplasmosis in humans and weird behavior.

They note:

It has been observed that the ‘hotter’ the topic and as more studies are reported and accumulate, replication becomes more difficult [49]. If we accept that the findings reported in the present article represent scenario two, then views of the link between T. gondii and aberrant behavior may need to be tempered accordingly.

What this means is that people got way too excited about the possibilities of turning their girlfriends into sexually adventurous, warm-hearted T. gondii hosts and bought into the myth.

The only definitive link between toxoplasmosis and humans is the fact that if you become infected with this parasite when you’re pregnant, you’re at a much higher risk of a complicated pregnancy. There’s been some research on incidence of this parasite in causing schizophrenia, but that too is inconclusive as of yet.

It can also cause blindness and death in immuno-compromised sections of the population, meaning people who are already unhealthy. Of course, that’s not as fun a headline as “Parasite makes men dumb, women sexy,” so that sort of scientific finding is not as highly popularized.

Where did it go wrong?

What a lot of the population doesn’t normally realize is that if something is true in mice, or flies, or any other lab organism, it doesn’t mean it’ll be true in humans. The biological mechanisms governing our systems are vastly different in complicated ways. Sometimes they’re similar, but far more often, more testing and experimentation needs to be done before we know if the same thing will happen to humans as happens to any lab organism.

The problem is that doesn’t translate neatly to exciting, click-baity headlines.

It is very true that rats, when exposed to this parasite, significantly alter their behavior in a way that makes them more likely to get eaten by cats. But that makes sense — the parasite wants to get back into its primary host, the cat, which is the only place it can reproduce, making more of itself to pass on to more rats and cats in an endless cycle.

Humans, however, are rarely eaten by cats, so we’re kind of a dead-end for the parasite. The same mechanisms the parasite would have evolved to make them good at manipulating rats won’t be very effective in humans, because the parasite has no need for that kind of manipulation.

Why did this myth take off?

Frankly, people wanted to buy into the crazy cat lady hype. They wanted to believe that a parasite infecting an estimated 30–50% of the global human population was able to make people more likely to become entrepreneurs.

People are successful in their relationships and in their businesses for a variety of complex reasons. It’s in our nature to want to boil down these sorts of things to a single factor, which is what makes this myth so attractive to us. But, for now, it’s just that — a myth.

As a cat-owner with two degrees in biology, there were large parts of me that wanted this to be true, too — it’s cool, it’s fun! But the science never lies.

It’s possible I’m wrong and there is a direct link between infection and behavioral changes in humans. But I’m inclined to believe, like K. Sugden, that people want it to be true too much for the current studies to be believable. More studies, with better science, controlling for additional factors, need to be done to show behavioral changes. Until then? I’ll be hanging out with my cats.

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