Common cat parasite linked to personality changes in humans

Scientists say Toxoplasma gondii can modify the brain — and a third of the world's population is infected with it.

A parasite found in cats can also cause changes in personality and behavior, according to research.

Toxoplasma gondii, also known as T. gondii, is a protozoa that typically infects cats, but can make its home in any warm-blooded animal. People can pick up the parasite by coming into contact with cat feces, drinking contaminated water or eating unwashed vegetables or undercooked meat.

What does it do?

Although T. gondii can be deadly to people with weakened immune systems, healthy people who contract it typically experience nothing worse than flu-like symptoms. Until recently, scientists assumed that after the immune system fought off the protozoa, T. gondii was dormant inside brain cells.

But Jaroslav Flegr, an evolutionary biologist at Charles University in Prague, believes that the parasite is modifying the connections between neurons in the brains of infected people, causing them to behave in strange and self-destructive ways. He says that, in addition to increasing suicide attempts, T. gondii can also change our personalities, contribute to mental disorders and play a role in car wrecks.

"Toxoplasma might even kill as many people as malaria, or at least a million people a year," Flegr said in an interview with The Atlantic in 2012.

The parasite infects up to a third of the world’s population, but is more common in certain parts of the world. For example, according to Flegr, the French have infection rates as high as 55 percent, likely due to their taste for undercooked meat. Americans, on the other hand, have an infection rate of 10 percent to 20 percent.

Flegr began researching the protozoa in the 1990s because he suspected that something was manipulating his personality and causing him to engage in risky behavior.

He told The Atlantic that he would walk into traffic "and if cars honked at me, I didn’t jump out of the way." And during a visit to eastern Turkey, where gunfire was a common occurrence, he says he was calm while "my colleagues were terrified. I wondered what was wrong with myself."

He later discovered that he’s infected with T. gondii, and he began to study the parasite.

Effects of T. gondii on the brain

A 3-D CG illustration of Toxoplasma gondii Toxoplasma gondii can alter a person's behavior and personality. (Photo: Kateryna Kon/Shutterstock)

Flegr’s first study involved administering personality tests to infected people and parasite-free people, and he was surprised to find many sex-specific changes in personality. Males with the parasite were more introverted, suspicious, likely to disregard rules and oblivious to people’s opinions of them. Infected women were more outgoing, trusting, rule-abiding and concerned with their image.

His findings were so strange that he thought his data was flawed, so he tested civilian and military populations and got the same results. Still unconvinced, he brought subjects in for further observation and found the same evidence.

But why would men and women be affected by the same parasite in different ways? He suspects that heightened anxiety might be the cause. When under stress, women find comfort in bonding with others. Men, on the other hand, are more likely to withdraw and exhibit hostile behavior.

Flegr also used a computer test to assess study participants’ reaction times, and he found that subjects who tested positive for T. gondii had significantly delayed reaction times. Curious as to whether the parasite could have an adverse effect on driving, he conducted two studies on Czech citizens and discovered that those with the parasite were two and a half times as likely to be in a car accident.

Two Turkish studies have replicated his research linking T. gondii to traffic accidents, and Flegr estimates that the parasite is a likely factor in several hundred thousand auto deaths a year due to delayed reaction times and an abnormal fear response.

But changes in reaction time and personality aren’t the only bizarre effects T. gondii might have on the brain. Studies have linked it to schizophrenia, finding that schizophrenic patients with the parasite have reduced gray matter in the brain.

Flegr even found that it makes men rate the scent of cat urine more favorably. Why cat urine?

T. gondii can reproduce only in cats, and neuroscientist Robert Sapolsky of Stanford University discovered that the parasite disconnects fear circuits in rats’ brains, making the animals attracted to cats’ odor and therefore more likely to be eaten by one. In fact, infected rats were so drawn to cat urine that Joanne Webster, a parasitologist at Imperial College in London, has dubbed the effect "fatal feline attraction."

Flegr reasons this could be why infected men are more attracted to the scent than men without the parasite.

There is one odd, but surprising, benefit

While T. gondii has been shown to cause several personality disorders like schizophrenia and bipolar disorder, a new study shows that it can also have a positive effect on students who major in business and business professions.

A survey of 1,300 college students in the U.S. found that those infected with the parasite were 1.7 times more likely to major in business — specifically management and entrepreneurship. It doesn't stop there. Infected individuals who attend business events are twice as likely to start their own business, and countries with a high infection rate have more entrepreneurs. Researchers believe the reason is because an infection causes people to have less fear and to be more likely to take a high-risk, high-reward strategy in life.

While an infection may sound like a good thing if you have a business idea rolling around in your head, you should still take proper precautions to avoid infection.

How to avoid infection

Most people have no idea they’re infected with T. gondii because the parasite can only be detected through a blood test, but scientists say we shouldn’t be too concerned.

"In the vast majority of people, there will be no ill effects, and those who are affected will mostly demonstrate subtle shifts of behavior," Webster told The Atlantic.

And although T. gondii has been called the "cat lady parasite," Flegr says people are more likely to be infected from drinking contaminated water or eating unwashed vegetables or undercooked meat than by a cat.

In fact, Flegr, who has two outdoor cats, says indoor cats pose virtually no threat because they likely don’t carry T. gondii. Cats that spend time outdoors likely do have the parasite, but they shed it in their feces for only three weeks of their life, typically when they’re young and have just begun hunting.

Editor's note: This article has been updated since it was originally published in August 2012.

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