How a Cat Parasite Affects Your Behavior, Mental Health, and Sex Drive

We used to believe that a healthy human could control the Toxoplasma parasite indefinitely. New evidence suggests the opposite. Through a delicate finessing of the neurotransmitters in our brains, it is us who are being controlled.

Roc Morin

Roc Morin

Dr. Jaroslav Flegr, a Czech researcher who studies parasites and their effect on humans. Photo by the author

Dr. Jaroslav Flegr, a Czech researcher who studies parasites and their effect on humans. Photo by the author

Parasitic mind-control is common in the animal kingdom. The rabies virus produces a delirious rage in its dying host, causing the animal to infect new victims with its bite. The hairworm Spinochordodes tellinii manipulates the brains of crickets into committing suicide by leaping into water, where the worm can breed. When the protozoan Toxoplasma gondii enters a rodent, the animal's natural fear of cat urine is reversed. The rodent becomes attracted to the odor of its predator, and when eaten, the parasite is able to spawn inside the feline's intestines.

Although Toxoplasma is primarily a rodent parasite, human beings are not immune. Our cohabitation with cats ensures ample opportunity for toxoplasmosis to occur through fecal contact. Since its discovery in the early 1900s, the protozoan had been widely viewed as a relatively benign passenger in humans. The only perceived threat was to patients with compromised immune systems (such as people with AIDS) and pregnant women whose fetuses are often deformed or aborted by the pathogen. It was believed that a healthy human host could control the parasite indefinitely. New evidence suggests the opposite. Through a delicate finessing of the neurotransmitters in our brains, it is us who are being controlled.

Dr. Jaroslav Flegr was the first to make these claims in 2002. Analyzing traffic data, the Czech parasitologist discovered that toxoplasmosis-infected drivers are 2.6 times more likely to be involved in car crashes. Flegr sees a parallel between the risk-taking behavior of infected rodents and the risk-taking behavior of the infected motorists. It was a pattern the scientist had noticed first in himself.

While attending Charles University in Prague, the usually conscientious Flegr realized that he had suddenly become bolder. The student frequently found himself crossing the street without looking, oblivious to the blaring of car horns around him. He also began openly criticizing the Communist government, at a time when dissent was a crime. It wasn't until he tested positive for toxoplasmosis as part of an unrelated research project that Flegr began to make sense of his recklessness. If the parasite could alter the behavior of rodents, he reasoned, why not humans too?

Flegr tested his theory by administering personality inventories to toxoplasma-positive and -negative populations. In test after test, the results were consistent. Infected men were "more likely to disregard rules," and were more "suspicious, jealous, and dogmatic." The traits of the women were the exact opposite. They were more "warm-hearted," "outgoing," and "moralistic." A further decade of research has uncovered links to a variety of conditions as far-ranging as ADHD, OCD, schizophrenia, and suicidality.

I sat down recently with Dr. Flegr at his office in Prague to discuss how a tiny parasite living inside of at least 10 percent of Americans and 30 to 50 percent of the world's population can so profoundly alter who we are.

Why is there such a different response to infection in men and women?
Dr. Jaroslav Flegr: It's known that men and women react in opposite ways to stress. So, it's possible that Toxoplasma induces chronic stress, and that men and women react in opposite ways to the same effect.

It's interesting that the characteristics of infected women are generally perceived to be positive.
When women feel stressed, they start to be friendly. They seek company. It's the reason that we suppose that it's nice to be infected. [Laughter] But it's not true. It's just a defensive strategy.

I've read that some women have actually wanted to become infected.
Yes, but I don't recommend it.

Have they asked you to infect them?
Sometimes I'll get an email like that, but it's mostly men interested in infecting their girlfriends.

Because it makes women more promiscuous?
It's not true, actually. It's just journalists extrapolating my discoveries. My recent research shows that it decreases the sexual drive of women.

What does it do to a man's sex drive?
It seems that it does nothing. It is strange because there is a very a strong effect on women and no effect on men. I suppose that there are two processes that cancel each other out. One is they are ill, so that decreases sexual drive; the other is that Toxoplasma is known to increase the concentration of testosterone in males. So you would think that would increase the sex drive.

Are we just collateral damage in the life cycle of the parasites, or do the changes we undergo actually benefit the parasite in some way?
A few thousand years ago we were part of the life cycle of Toxoplasma. Even now a lot of people die due to tigers and lions in other parts of the world. It's actually possible that the [parasite's] manipulation is primarily aimed not at rodents but at apes.

Do you think that the effects of toxoplasmosis makes a human more likely to be eaten by a lion or tiger?
Yes. Several effects of toxoplasmosis really increase this risk. In our questionnaire, infected people say that they are less afraid than people who are not infected. We asked how much they are afraid of being in dark woods, for example, and they say that they are not so afraid. They also have weaker startle reactions. When infected people cross the street and a horn blows, they don't skip away. [Laughter] It's not a good strategy when we are endangered by tigers or lions.

If toxoplasmosis correlates with schizophrenia, OCD, and suicidality—those would all seem to be things that would isolate in an individual from the safety of their social group and make them more vulnerable to being eaten by a large cat.
It's possible that could be the reason.

As with the rodents, is there actually a human attraction to cat urine itself?
Yes, we observed this fatal attraction phenomenon in humans. Infected men rated the smell of very diluted cat urine as more pleasurable. It was a double-blind study. The people didn't know whether they were infected, and they didn't know what they were smelling. Using 12 urine samples from different animals, they had to rate pleasantness of smell. The pattern was quite clear when we analyzed the results.

I was talking to a graduate student, Charlie Nichols, and he wondered if an attraction to the smell of cat urine is one of the reasons why people like to have cats around.
It's possible. At least when somebody dislikes the smell of cats they probably don't keep them. Smell plays a very strong role in our life. We don't realize this because it's mostly subconscious reactions, but love is a question of smell. To fall in love with somebody—very often smell is the reason for this.

Do you think toxoplasmosis plays a role in love in any way?
Toxoplasma changes our sense of smell quantitatively and qualitatively too. There is not enough data for this, but there is some indirect evidence for it. It's known that the smell of schizophrenics changes. A smell that was pleasurable starts to be unpleasurable. Many data show that a large percentage of schizophrenia is caused by toxoplasmosis.

What is the evidence for that?
There is a prospective study showing that antibodies against toxoplasmosis appeared in the blood of subjects from six months to three years before the start of schizophrenia. Many times schizophrenia is triggered by and may be caused by Toxoplasma. Of course, it's rare. The frequency of toxoplasmosis is about 30 percent and the frequency of schizophrenia is about 1 percent. So most people who are infected by toxoplasmosis do not get schizophrenia.

You yourself have toxoplasmosis, correct?

Do you know where you got it from?
There were several possible sources. I spent more than one year in Japan, and I ate a lot of raw meat, so maybe there.

How did you feel when you found that out?
I was not very happy about this. But a lot of people in the department were infected too—about 30 percent. Now, the prevalence of toxoplasmosis in our students is about 10 percent.

Is that because of better hygiene?
Possibly. Maybe it's better regulation of public sandboxes. Sand must be changed very often. There are other possibilities too. For example, our recent paper accepted for publication has shown that Toxoplasma is very probably a sexually transmitted disease. So maybe because of the AIDS epidemic, unprotected sex is not so popular, and it has decreased toxoplasmosis.

Did you find Toxoplasma in semen and vaginal fluid?
In some animal species we saw parasites in semen. And in about two-thirds of cases where a human fetus is infected, we weren't able to find any risk factor. The mother did not eat raw meat, she washed vegetables, she behaved very reasonably. There were no risks, and still she became infected. So it's quite possible that during unprotected sex with her husband, she acquired the infection.

You found that the husband was infected?
We have no data on this, but it should be tested.

Can women transmit it to men?
I believe that transmission goes only from men to women—or mainly.

Do you think that the increase in testosterone in infected men is the Toxoplasma trying to increase sex drive so that it can more easily spread?
It is possible. In The Selfish Gene, Richard Dawkins mentions the possibility of increased sexual drive of patients with syphilis.

I've read reports about HIV also increasing sex drive.
I believe chlamydia can do that maybe too.

I understand that there are very different rates of toxoplasmosis from country to country. Latin American countries have the highest, and South Korea has the lowest, I believe. Do you think toxoplasmosis could affect behavior on a national level?
I believe it can have this impact. It was already published by another parasitologist that national personality can be partially explained by frequency of toxoplasmosis. This year, we published a very important paper showing that the frequency of a lot of diseases can be explained by differences in the prevalence of toxoplasmosis. Our data show a strong relation with epilepsy and cerebrovascular diseases including infarcts [heart attacks]. In Europe, it explains about 16 percent or 17 percent of infarcts. So, if we were able to find a treatment for toxoplasmosis, or if we find a vaccine, we can save a lot of lives.


As the interview drew to a close, I asked Flegr if I could photograph him for the article. He agreed, joking, "It's a pity I have no cats to hold."

But, as I looked through the view-finder I could see his grin quickly fading. The mood had suddenly shifted.

"I think it's enough," he snapped. "I was told by my friend that if a journalist is taking your photo using this kind of lens, it means that they want you to seem ugly in the photos. It's the standard method for journalists taking pictures of politicians when they don't like them."

I forced out what I hoped would look like a reassuring smile. "Not at all, doctor. I'm just trying to capture your personality."

"But, you have fixed-focus camera," he countered. "We have data using different lenses that shows really large differences in the attractiveness of different people."

We shook hands tensely and said goodbye, but it seemed more like good riddance. I made my way out through the empty halls of Charles University, ruminating about that unexpected burst of suspiciousness. Where had it come from? Was it Flegr or was it the Toxoplasma? At this point, was there any difference?

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