Do Cats Really Make People Go Crazy?

Cat making a woman crazy
Daniel Fishel/Thrillist

Ah yes, the "crazy cat lady" of modern folklore: see her eschew human contact for the company of her feline friends, donning a rotating ensemble of novelty cat shirts and sweatpants coated in cat hair. Listen in as she delivers a lively monologue to her cat posse while clipping coupons for wet food and kitty litter. Double-tap all of Taylor Swift's cat-centric Instagrams, and then tweet a shruggie when others condemn her to a life of loneliness because her love of cats must mean she's nuts.

Reports over the past several years of a cat-borne parasite called Toxoplasma gondii -- thought to control human brains and known to cause a wide array of maladies -- have only managed to give credence to the stereotype. But could it be that simple? Can cats actually make people -- particularly, people attracted to cats, i.e., "cat ladies" -- crazy?

Let's talk parasites and cat poop

Yes, it's true that cats are the source of an extremely common parasite, but there's a lot more to this protozoan than chalking its effect up to meow-mania.

Toxoplasma gondii are single-celled parasites whose only definitive primary hosts are cats. In other words, the parasite must infect a cat if it wants to lay eggs -- and it does, because it's a parasite, and perpetuation is its evolutionary function. Toxoplasma gondii eggs, or oocysts, are then excreted by infected cats in their feces, and can be consumed by humans and other mammals in a number of ways. An estimated 2 billion people worldwide -- 60 million in the United States -- are infected with Toxoplasma gondii.

That's a lot of cat poop, and the parasites embedded within bear a long shelf-life.

"Wait: consume cat feces?" you're probably asking in revulsion. Yes, consume cat feces, by way of drinking contaminated water, eating undercooked meat (of animals that grazed or hunted in soil where an infected cat defecated), handling and eating unwashed produce (planted in a garden visited by an infected cat), or cleaning an infected cat's litter box. The danger is particularly high for kids playing in sandboxes, the McMansions of litter boxes for cats on the go. Kittens are most vulnerable for infection, and a single cat infected by Toxoplasma gondii can shed up to 500 million oocysts in two weeks.

"At any given time, it has been estimated that approximately 1% of cats are excreting T. gondii oocysts," reports the Stanley Medical Research Institute, adding that American backyards and communities contain between 3 and 400 oocysts per square foot. "Insofar as these communities are representative of the United States population," the Institute continues, "the 81.7 million owned cats would produce 856,930 tons of outdoor cat feces each year. Assuming there are only 25 million feral cats, these would produce another 360,459 tons of cat feces, resulting in a total accumulation of 1,217,389 tons deposited annually in the environment of the United States."

That's a lot of cat poop, and the parasites embedded within bear a long shelf-life. Oocysts can survive in warm climates with moist soil or seawater for one year. Even one oocyst is enough to infect a human.

So can this parasite actually control human brains?

Not everyone infected with Toxoplasma gondii suffers toxoplasmosis, a serious illness that can cause miscarriage, fetal development disorders, retinal damage, brain damage, and death. The majority of the 2 billion people infected by the parasite don't know they have it, and never will.

But for those who do develop toxoplasmosis, the results can be devastating. "It's the most common cause of infection of the back of the eye," says Dr. Rima McLeod, professor of ophthalmology & visual science and pediatrics, and medical director of the Toxoplasmosis Center at the University of Chicago. "It can cause epilepsy, it can be devastating for a baby, and it can be devastating for someone with a compromised immune system." Those most at risk are pregnant women, who can transmit the infection to fetuses in utero, children, individuals undergoing immunosuppressive therapy like cancer treatment, and individuals suffering immunosuppressive diseases like AIDS.

Most people, of course, won't be eaten by cats.

The medical dangers of Toxoplasma gondii have been known for decades; the mental health implications of Toxoplasma gondii are comparatively newer on the scene, but gaining more credibility in the field. Czech parasitologist and evolutionary biologist Jaroslav Flegr, long considered fringe for his theory that the parasite can manipulate human personality, found in two separate studies that subjects testing positive for Toxoplasma gondii were over twice as likely to be in a car wreck.

Flegr maintains that those infected with the parasite are less fearful and exhibit slower response times, a conclusion that connects neatly with parasitologist Joanne Webster's findings on "fatal feline attraction," wherein Webster and her team observed that Toxo-infected rats were drawn to cat urine, despite the inevitability of that attraction essentially equating with suicide.

"For evolutionary reasons, the Toxoplasma gondii parasite cannot complete its life cycle in the intestine of cats," says research psychiatrist Dr. E. Fuller Torrey, executive director of the Stanley Medical Research Institute and schizophrenia specialist who has investigated Toxoplasma gondii for some time. "So that if you are a parasite, and you're trying to be a smart parasite and succeed in life, evolutionary-wise the theory is that the parasite has learned better ways to make it more likely to get back into a cat."

OK, but is there any hard evidence that the parasite is rewiring our brains?

The question is this: if a cat parasite can rewire a rat's brain so that it's attracted to its own predator, can that same parasite rewire a human's brain so that it's attracted to cats, too? Possibly! As Motherboard puts it, "Toxoplasma thinks we're rats, and it wants us to be eaten by a cat," adding, "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..."

Most people, of course, won't be eaten by cats. But the parasite may also rewire the human brain in even more serious ways, indicating that there's far more interesting things going on with Toxoplasma gondii than explaining why cat people like cats.

It's important to stress that nobody should go home and shoot their cat.

More than 80 studies, including those led by Dr. Torrey and co-investigator and Johns Hopkins University neurovirologist Dr. Robert Yolken, detail links between Toxoplasma gondii and schizophrenia, including four studies reporting "that individuals with schizophrenia, compared to controls, have had more contact with cats during childhood." A 2015 study at the Academic Medical Center in Amsterdam reported by CBS News found that "an individual infected with T. gondii was almost twice as likely to develop schizophrenia."

Scientists are quick to caution against jumping to conclusions that these scientific studies yield definitive evidence of the crazy cat lady theory in action. "Well, first of all, we haven't proven anything," says Dr. Torrey. "What we've shown is that there are certain aspects of Toxoplasma gondii that would be consistent with some of the symptoms of schizophrenia, but none of us would say that we've shown Toxoplasma gondii causes schizophrenia, or Toxoplasma gondii causes crazy cat ladies, etc."

Schizophrenia is a serious and chronic mental illness, affecting 1% of the general population. "Crazy" is an insensitive word for describing individuals afflicted with mental illness. No matter the probability of a parasite found in cat poop leading to schizophrenia, the label "crazy" can and should be scrubbed from the lexicon surrounding Toxoplasma gondii.

But what about the "crazy cat lady" stereotype?

Women are not at greater risk for infection by Toxoplasma gondii, by the way. While Flegr maintains that the parasite yields different behavioral manifestations for men versus women, there's no data indicating that one sex is more likely than the other to contract the infection. On top of that, Flegr's observed personality distinctions contradict much of the classic "crazy cat lady" personality type that such women are antisocial recluses. As reported in The Atlantic's 2012 profile of Flegr, "[...] males who had the parasite were more introverted, suspicious, oblivious to other people's opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women."

Not only is the stereotype both inaccurate and offensive; it also diminishes the seriousness of the disease theorized to explain it.

"It's a very significant source of damage to people in a way that matters," says Dr. McLeod of toxoplasmosis. "There's a lot of hype about things it might do, but we know the things that it can do. [An infected] baby can end up without sight, without cognition, all the things that Zika can do, this virus can do, and it's here with us right now."

And despite the fact that the parasite is specific to cat feces, the indoor house cat is, more than likely, not at fault, says the Cornell Feline Health Center at Cornell University's College of Veterinary Medicine. Feral cats and outdoor cats are the more probable culprits; even still, you can get toxoplasmosis whether you interact with cats regularly or not.  

"It's a very common parasite, it's in the environment," says Dr. McLeod. "You just have to live in the world to get it."

"Cats can make nice pets," adds Dr. Torrey. "But there are some aspects to what has come out so far that makes some of us uneasy, and would suggest that, especially for children, you may want to take certain precautions" -- such as covering sandboxes when not in use. Otherwise, he says, cat companionship is not cause for alarm or panic.

"I think it's important to stress that nobody should go home and shoot their cat or whatever."

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Stephie Grob Plante is a writer and cat lover based in Austin. Follow her: @stephiegrob.