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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.

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.

  • 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 :
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