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For thousands of years, humans have alternately adored or abhored cats. The reason for the our mass preoccupation with this feline species may be that cats have literal power over our brains, through brain parasites with which they infect us.

If you spend any time at all on the Internet--and the fact that you have found this article suggests that you do--you know that there is no kind of photo more likely to go viral on Facebook than a kitty photo and no kind of video more likely to go viral on YouTube than a kitty video.


The simple fact is that most people find cats to be fascinating. We pet them. We groom them. We feed them Fancy Feast. We build them kitty climbing gyms and we talk to them in kitty talk. Americans alone spend over $15 billion per year on cat food and other products to make cats comfortable. Charities are organized to provide for stray cats. Kitty videos secure literally billions of views.

It's all enough to make you wonder whether cats somehow have control over our minds. And one Czech scientist thinks that they do.

Ants, Cats, and Brain Control

Jaroslav Flegr is an extraordinarily productive biological researcher in the Czech Republic. Because he has focused on biology rather than on mastering the English language, he seldom attends conferences, and aside from a story in Atlantic Monthlya few years ago, relatively few people outside of Eastern Europe are aware of his work.

But about 20 years ago, he told a reporter for Atlantic Monthly, Dr. Flegr began to wonder if one-celled protozoans had invaded his brain and started making him do crazy things.

Flegr had read the work of the English evolutionary biologist Richard Dawkins, who described the ways in which a flatworm can enslave an ant by invading its nervous system. Instead of heading into its anthill when temperatures drop, the flatworm-infected ant climbs a blade of grass and bites it, hanging on until both grass and ant are consumed by a sheep. Once the ant is digested along with the blade of grass in the sheep's stomach, the flatworm is able to reproduce and release its offspring in the sheep's droppings.

The Czech biologist wondered if some kind of parasite might be doing something similar to him. Growing up in Czechoslovakia during the Communist era, he often made statements that could have landed him in trouble with the political authorities. He often wandered off into heavy traffic, he told Atlantic Monthly, cars honking at him. Caught in a gun battle during a visit to eastern Turkey, Flegr said he was not worried at all, even though his fellow travelers were terrified. Flegr did not have a clue as to what could be wrong with him until in 1990 he was appointed to do research at Charles University in Prague, where there happened to be a major research project on the parasite Toxoplasmodium gondii, also known as T. gondii.

A Parasite That Hides Out in the Human Brain

The toxoplasmosis parasite, it turns out, goes through a life cycle very similar to the flatworm described by Dr. Dawkins. T. gondii is only capable of reproducing when it is inside the small intestine of a cat. After mating, the parasites and their offspring are released into kitty poop, and have to find a new host.

Toxoplasmosis parasites can't reproduce in humans and other animals. However, this banana-shaped single-celled microorganism is capable to slipping our of the kitty litter and into the tissues in warm-blooded animals, including humans who change litter boxes, to find the perfect place to "hang out" until it is eventually eaten by a cat so it can mate with another T. gondii to reproduce. That perfect hang out for the parasite is the brain. 

The Toxoplasmosis gondii parasite finds its way into brain tissue and encases itself in a cyst, waiting for the opportunity to be eaten by a cat so it can reproduce. As a cyst, the parasite can survive for many years in a not-quite dormant state, active only to the extent, Flegr found, of influencing its host's behavior so that the host's brain is eaten by a cat. The effects of the parasite turn out to be quite specific.

How Cat Parasites Control Our Brains

Doing his research in the last days of Communist regime in Czechslovakia, Flegr had to work with limited funding. He wasn't able to grow the parasite in the lab to study its life cycle. However, he was able to use students as, he told Atlantic Monthly, "very cheap experimental animals," to use standard psychological tests to look for differences between students who had been infected with the parasite, as 30 to 40% of Czechs were, and students who were parasite-free.


Flegr discovered sex-specific differences in the effects of the parasite on the human brain. In men, getting an infection by the parasite made them:

  • Less likely to "follow the rules.
  • More prone to injury.
  • More suspicious of others.

In women, getting an infection by the parasite made them:

  • Less suspicious of others.
  • Less prone to jealousy.
  • More relaxed.
  • More open to other people, more warmhearted.

Dr. Flegr's findings suggest that the reason more women seem to be "cat people" than men is that men who love cats tend to walk off into heavy traffic and get run over by a truck. Later researchers explained that the reason men and women react differently to cat parasites is testosterone levels. Their findings of psychological changes in people infected by the parasites may reflect cultural differences.

In France, men who have the parasite were described as "more dogmatic, less confident, and more orderly" than others, while women who have the parasite were described as "more conscientious, less secure, more sanctimonious, and more persistent" than others. Other studies in which brain cells infected by the parasite have been grown in the lab find that infected brains grow in response to the production of dopamine, a reward chemical that in men is often elevated after risk-taking, and in women is often associated with "nesting" behaviors.

But is possible that these findings, and the findings of 14 other studies, aren't really just pushing the border between science and science fiction?

Probably it really is the case that cat owners develop different personalities as the result of cat parasites. Scientists have observed changes in the behavior not just of humans but also in many other species. The T. gondii parasite doesn't just infect cats and people. It also infects mice, birds, buffaloes, grizly bears, elephants, chinchillas, goats, sheep, pronghorn antelopes, sea lions, owls, pigeons, koalas, and wombats. Most of these animals show changes in behavior after infection. And studies have found that infections with this cat parasite in humans are linked to higher risk of both schizophrenia and brain cancer.

It is possible that cats really do control our minds, or at least our brains, by transmitting a parasite to us. If you live in the USA, there's about a 1 in 10 chance you already have it, and if you live in Europe, your chances of already having the parasite are about 1 in 3. The good news is, for more than 90% of people who catch the disease, the most serious symptom is an unusual appreciation for cats, but your history of owning cats (or being owned by cats, as the case may be) is something you should be sure to mention to your doctor if you have to take any kind of medication that reduces the strength of the immune system, or if you develop any form of immune deficiency disease.

Additional Facts About Feline Toxoplasmosis

How can you recognize symptoms of toxoplasmosis that require medical treatment? See your doctor if you experience:

  • Swollen lymph glands,
  • Night sweats,
  • Sore throat,
  • Difficulty breathing, and especially if you experience
  • Visual disturbancs with these symptoms.

There are many other conditions that also cause these symptoms, but not many that cause this particular combination. Your doctor would recognize changes in eye tissue caused by the parasite just by looking in your eye. Treatment options are somewhat limited and always require medical supervision.

Read full article

  • Fond G, Capdevielle D, Macgregor A, Attal J, Larue A, Brittner M, Ducasse D, Boulenger JP. [Toxoplasma gondii: a potential role in the genesis of psychiatric disorders].Encephale. 2013 Feb
  • 39(1):38-43. doi: 10.1016/j.encep.2012.06.014. Epub 2012 Aug 21. French.
  • McAuliffe, K. How Your Cat Is Making You Crazy. Atlantic Monthly. March 2012.
  • Photo courtesy of Wayne Noffsinger by Flickr :
  • Photo courtesy of by Flickr :

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