Research Links Parasite In Cats To Mental Illnesses

What microorganism can make mice attracted to cats or make humans more likely to have car accidents or even develop a mental illness like schizophrenia? Host Guy Raz talks with infectious disease researcher Robert Yolken about Toxoplasma gondii and how it might affect human behavior.

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GUY RAZ, host:

If you or someone close to you has ever been pregnant, you've probably heard that pregnant women shouldn't change cat litter. That's because of a parasite some cats carry. It's called Toxoplasma gondii. Humans can become infected by handling cat litter. And if a pregnant woman becomes infected, toxoplasmosis can cause brain damage to the fetus.

Well now, a growing body of research is connecting toxoplasma to mental illness in adults - things like schizophrenia and bipolar disorders.

Dr. Robert Yolken, from Johns Hopkins Children's Center, has been studying the parasite's connection to mental illness. And he joins me from Baltimore.

Welcome to the program.

Dr. ROBERT YOLKEN (Neurovirologist, Johns Hopkins Children's Center): Thank you very much.

RAZ: First of all, how did cats themselves get infected with the parasite?

Dr. YOLKEN: You know, I should say for toxoplasma's point of view, the world is divided into two parts. So it's cats or non-cats. Toxoplasma would like to be in a cat. The cat is where it can complete its life cycle, can undergo sexual reproduction. So that's really where it wants to be.

Now toxoplasma gets into another animal, it's still alive, but it's not very happy. What I'm fond of saying is that it's kind of like a young person living in New Jersey. The person is alive but perhaps would rather be somewhere else...

(Soundbite of laughter)

Dr. YOLKEN: ...so in New York City or Philadelphia or Washington. Somewhere...

RAZ: Apologies to our listeners in New Jersey.

Dr. YOLKEN: I'm from New Jersey and my wife is, so I hope my fellow New Jerseyans won't mind my poking fun at my own state. But if you're a toxoplasma organism, you can't move so you have to change the motion of your host. And it appears that what toxoplasma does is, it actually changes the behavior of a host so it's more likely to get into a cat.

And how does it - does this? Well, if a toxoplasma happens to be in a mouse or a rat, what it does is, it actually alters the behavior of the rodent in quite a specific way, to make it more likely to get eaten by a cat. And it does this by actually having the animal lose its fear of cats and actually get attracted to cats. So this is a quite amazing effect of a parasite on animal behavior.

RAZ: Now, when humans are infected with this parasite, you have found a correlation between toxoplasmosis and changes in human behavior. Tell us what you found.

Dr. YOLKEN: Basically, we found that having toxoplasma raises the risk of schizophrenia about twofold, compared to the rest of the population. Toxoplasma probably functions through a pathway called dopamine. We know that dopamine is abnormal in schizophrenia, but the reason why it's abnormal is not really completely clear.

Another behavior which appears to be altered is the individuals with toxoplasma appear to take more risks, in terms of driving a motor vehicle and also being a pedestrian.

RAZ: Obviously, a lot of people listening to this who have cats - myself included - and small children might be a little bit terrified. What does this mean for people who own cats? I mean, should they keep their kids away from them?

Dr. YOLKEN: No. I have cats. I have two - one that's ours, and one that's our daughter's, that we're cat-sitting for while she's in college. And I think cats are wonderful pets, and I would encourage people adopting cats. I think there are a number of things that one can do to lower the risk of toxoplasma.

One is, if a cat is kept indoors, it's much, much less likely to get infected because it's not going out and eating rodents that might be outside. Secondly, I think that individuals who do have a litter box should be careful in terms of changing the litter and wearing gloves.

RAZ: How would you know if your cat had it?

Dr. YOLKEN: A veterinarian can generally do a test for a cat, and I'm told by my own veterinarian that that's not an uncommon request for people.

RAZ: So do your cats have it?

Dr. YOLKEN: My cats do not have it.

RAZ: So you don't have it, probably.

Dr. YOLKEN: No. I am toxoplasma positive. So it's largely a silent infection. There are some symptoms - a headache, people not feeling well - but these usually resolve. So most people that have toxoplasma would not really have any reason for knowing it.

RAZ: That's Robert Yolken. He's professor of pediatrics and a researcher at Johns Hopkins Children's Center in Baltimore.

Dr. Yolken, thanks so much.

Dr. YOLKEN: Thank you very much.

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