JAD ABUMRAD: Hello, I'm Jad Abumrad.

ROBERT KRULWICH: I'm Robert Krulwich.

JAD: This is Radiolab. Today's topic, parasites.

ROBERT: Where we have already learned that parasites can be good sometimes. Parasites can, of course, be very bad. But also parasites can affect human behavior making us—some of us a little ...

JAD: Lethargic.

ROBERT: Or solving our allergies.

JAD: And here's another question to consider though: Can they not just affect our behavior, can they control our behavior?

ROBERT: A different question entirely.

JAD: Yeah, and you know, we were—we were thinking about this question, you know, in the abstract doing some research. But then things got kind of real when our producer, Ellen Horne called in late to work one day.

[ARCHIVE CLIP, Ellen Horne: Hey, Lulu, it's Ellen. I just got home from the vet. I've been waiting on chest X-rays. blood work from my cat. She managed to scratch me ...]

ELLEN: This is my cat, Moose. Big, lovely affectionate kitty. She's like the sweetest cat you will ever meet.

JAD: I've met Moose.

ELLEN: She's a very sweet cat.

JAD: A darling.

ELLEN: But Moose has digestion problems. And this one day, I had to take her to the vet. And as I was putting her into the kitty carrier ...

[ARCHIVE CLIP, Ellen Horne: She managed to scratch me with her back claws and I have, like, a bloody wound on my hand. Her back claws are, like, totally poop covered, so I'm kind of worried. I have—I am six months pregnant.]

ELLEN: The very first thing that they tell you when you get pregnant is stay away from cat poop. So after it happened I called my midwife.

ELLEN: Are you ready for me?

ELLEN: She told me to rush right down to her office.

MIDWIFE: So it bled pretty profusely?

ELLEN: It did

JAD: Wait a second, why? What's so scary about cat poop?

ELLEN: Well, it turns out that cat poop can have in it this tiny parasite. It's called Toxoplasma gondii.

ELLEN: So what is the threat to the baby?

ELLEN: And if it gets to the baby ...

MIDWIFE: It can cause miscarriage. It can cause stillbirth. And it can also cause seizures, blindness.

JAD: So you're freaking out at this point?

ELLEN: Yeah, I'm kind of freaking out at this point.

MIDWIFE: Small cranium, small head.

ELLEN: But my midwife said there's probably nothing to worry about. So she took my blood ...

ELLEN: That's probably the better arm.

ELLEN: ... and she sent me home.

MIDWIFE: The turnaround time for the test is between two and three days.

ELLEN: Okay, so I'm looking on the internet ...

ELLEN: At home, I proceed to get myself even more freaked out.

ELLEN: A bunch of things about Toxoplasmosis.

ELLEN: And one of the things that I found was this lecture by Robert Sapolsky.

[ARCHIVE CLIP, Robert Sapolsky: Now the example I'm talking about here ...]

ELLEN: He's a neuroscientist who we've had on the show a lot.

[ARCHIVE CLIP, Robert Sapolsky: It has to do with a parasite called Toxoplasma.]

ELLEN: And I just decided that I was gonna call him up.

ROBERT SAPOLSKY: Hello?

ELLEN: And ask a few questions.

ROBERT SAPOLSKY: Okay, so what's the deal with Toxo?

ELLEN: When he proceeded to tell me one of the most amazing feats of mind control I'd ever heard.

JAD: What did he tell you?

ELLEN: Well, the first thing he told me is that Toxo doesn't actually want to be in me.

ROBERT SAPOLSKY: Yes, it really has wandered off into the wrong county if it winds up in a human.

ELLEN: It wants to be inside Moose.

ROBERT SAPOLSKY: For totally mysterious reasons, at least to me, Toxo can only reproduce sexually in the gut of cats.

ELLEN: So it's there in Moose's intestines that the Toxoplasma meet and hook up. Then they lay eggs. Next, Moose takes a trip to the backyard, where she ejects those eggs in her poop.

ROBERT SAPOLSKY: So it's out there now it the cat feces.

ELLEN: Step two, says Sapolsky, is that, you know, maybe a week later, a rat will come along and eat the cat poop. Now Toxo has a problem: it's stuck inside a rat. It really wants to be inside a cat, but rats totally freak out whenever they so much as even smell cat.

ROBERT SAPOLSKY: It's—it's a hardwired aversion. Toxo's evolutionary challenge now has been to figure out how to get rodents inside cats' stomachs.

ELLEN: Here is where the mind control comes in. And it's kind of hard to believe, but this is what Sapolsky says happens ...

ROBERT SAPOLSKY: Toxo starts off in the stomach of the rodent. Takes about six weeks to migrate its way up to the brain.

ELLEN: And once it's in there, it finds this particular region ...

ROBERT SAPOLSKY: Called the amygdala ...

ELLEN: Which is like command central for fear ...

ROBERT SAPOLSKY: And anxiety ...

ELLEN: And terror ...

ROBERT SAPOLSKY: All of that ...

ELLEN: It also finds this other region kind of right next door, where a very different emotion lives.

ROBERT SAPOLSKY: Sexual arousal.

ELLEN: And what Toxo seems to be able to do is somehow cross the wires.

ROBERT SAPOLSKY: This may be some horrifically simplified soundbite, but what I think is going on is that Toxo knows how to make cat urine smell sexy ...

JAD: What?

ROBERT SAPOLSKY: To rodents.

JAD: [laughs] God, that is so evil!

ROBERT SAPOLSKY: Which is, like, totally bizarre, but Toxo makes rodents like the smell of cats, and thus they approach, and thus they're more likely to wind up in the cat's stomach.

JAD: That's rough.

ROBERT SAPOLSKY: Yeah.

ELLEN: In all other ways the rodent is totally normal.

ROBERT SAPOLSKY: Normal olfaction. Normal social behavior.

ELLEN: Just hot for cats.

ELLEN: Hi good kitty.

ELLEN: And I start to wonder ...

ELLEN: Moose really likes the microphone.

ELLEN: I love cats. Is it possible that Toxo ...

ELLEN: Moose is wonderful.

ELLEN: ... is what's been drawing me to cats?

ELLEN: It's why we let her put her fur everywhere.

ELLEN: And I ask him.

ROBERT SAPOLSKY: Pure speculation, but people who think about this stuff view it as not just purely speculative. The notion that Toxo can produce some sort of attraction to cats in humans, they don't think that's all that crazy.

JAD: Wait, so you're saying that like the crazy cat lady could be Toxoplasma?

ELLEN: Well, no one's really studied that yet.

ELLEN: Testing, testing.

ELLEN: But there are scientists out there that are making the case that Toxo can really change you.

ROBERT SAPOLSKY: Probably the most interesting established link is between Toxo and schizophrenia.

ELLEN: Are you Dr. Torre?

FULLER TORRE: Yep.

ELLEN: Nice to meet you. How are you?

FULLER TORRE: There's actually been at last count 54 studies on Toxoplasma in people with schizophrenia and other psychoses.

ELLEN: That's Dr. Fuller Torre. He works at the Stanley Medical Research Institute that sponsors a lot of these studies.

FULLER TORRE: Well, I've been doing research on schizophrenia since the early '70s.

JAD: And he thinks there's a link?

ELLEN: Yeah.

FULLER TORRE: Not a huge effect. Very, very small risk of schizophrenia, simply because schizophrenia is very rare.

JAD: But why would it cause schizophrenia to begin with? Is it trying to cause schizophrenia?

ELLEN: You imagine if the Tox was sort of lost in the brain, it thinks it's in a rat brain. Maybe it's just trying to do what it usually does to rats, but in humans it has a very different effect.

JAD: I see.

ELLEN: And one of the reasons he thinks this might be true, this connection between Toxoplasma and schizophrenia, is because of a historical link.

FULLER TORRE: The fact that what we now call schizophrenia was quite rare until the late part of the 18th century. And then during the 1800s, schizophrenia increased very rapidly.

ELLEN: Why?

FULLER TORRE: This was the first time when we started to keep cats as pets. They first were adopted by the kind of East Greenwich Village types in Paris, the artists. And it was really considered kind of weird, but it's the kind of thing that if you were an artist or writer or something like that you started to do that. And then it kind of spread to London, where the writers and artists kept it there. And then starting in about the 1840s, it started to become a little bit more popular. And then in the 1860s, and '70s, there was what was called a cat craze. Cats were all over greeting cards. The first cat show was in London in 1870, and in Madison Square Garden, I think 1880. It became very fashionable to have a cat.

ELLEN: We should say—I mean, he'll agree—at this point, it's just a theory.

JAD: Okay, but is there any evidence that Toxo can actually control our behavior like it does with rats?

ELLEN: Well, there are some scientists out there who believe that Toxo may affect something more common to all of us.

ROBERT SAPOLSKY: Here's another one of those give me a break ...

ELLEN: That's Robert Sapolsky again.

ROBERT SAPOLSKY: ... science fiction branches to this story. Two different groups independently have seen people who are Toxo infected have two to four times the likelihood of dying in car accidents.

JAD: Really?

ELLEN: Yeah. And I asked him why.

ROBERT SAPOLSKY: Insofar as Toxo makes rodents get really imprudent about cat smells, maybe Toxo is making all sorts of mammals get imprudent about anything that they're normally skittish about, like your body hurtling through space at a high speed.

ELLEN: So in the end, it might be possible—might be possible—that Toxo is guiding our emotions, changing who we are in some basic way. And if you consider that Toxo might just be one of thousands of tiny little parasites inside us pulling our strings from the inside, well, that thought is pretty creepy.

ROBERT SAPOLSKY: Even if the entire lesson with Toxo is a small subset of infected people now have one half of one percent more likelihood of wanting to drive really recklessly, even lurking in that one half of one percent are some serious implications for thinking about free will. We haven't a clue the biology lurking in the background that makes free will seem a little bit suspect.

JAD: Either way, whatever happened with your test?

ELLEN: Well, this is me with my midwife, Berry. And she's giving me the news.

ELLEN: What did you find out from the Toxo test?

MIDWIFE: That you have had past infection with Toxoplasma. Positive.

JAD: You're positive?

ELLEN: Yeah. But my midwife says that the baby's gonna be okay.

ELLEN: Does the baby look like she's small?

MIDWIFE: No, she looks like she's a nice size to a little bit on the larger size. So not a baby I'd be worried about.

ELLEN: And I believe her.

JAD: Thanks, Ellen.

ELLEN: Sure.

JAD: If you want to hear more about anything you're in this hour, check our website Radiolab.org.

[ELLEN: Hi there. This is Ellen Horne, and I am calling with my cat Moose, who is just recovering from surgery and doing very well. And we're calling to say that Radiolab is produced by Lulu Miller and Jad Abumrad. Our staff includes Soren Wheeler, Michael Raphael ...]

[ROBERT SAPOLSKY: Ellen Horne, Ann Heppermann, Jonathan Mitchell and Amanda Aronczyk. With help from ...]

[ELLEN HORNE: Jessica Benko, Charles Choi and Emma Jacobs. Special thanks to Elizabeth Givens, Pat Walters, Karen Havlick, Lauren Sessions and Charles Michelet.]

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