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. Also, parasites can affect human behavior, making some of us a little-

Jad: -lethargic-

Robert: -or solving our allergies.

Jad: Here's a question to consider though. Can they not just affect our behavior, can they control our behavior?

Robert: A different question entirely.

Jad: We were thinking about this question in the abstract, doing some research, but then things got real when our producer Ellen Horne called in late to work one day.

Ellen Horne: Hey, Lulu. It's Ellen. I've just got home from the vet. I'm waiting on chest X-rays and blood work for my cat. She managed to scratch me-- This is my cat, Moose. Hey, Moose. Big, lovely, affectionate kitty. She's the sweetest cat you'll ever meet.

Jad: I've met Moose.

Ellen: She's a very sweet cat.

Jad: She's a darling.

Ellen: Moose has digestion problems.

[laughter]

Ellen: This one day, I had to take her to the vet. As I was putting her into the kitty-carrier, she managed to scratch me with her back claws and I had a bloody wound on my hand. Her back claws are totally poop-covered, so I'm worried as I am six months pregnant. The very first thing that they tell you when you get pregnant is, "Stay away from cat poop." After it happened, I called my midwife. Stacy, are you ready for me?

Stacy: Yes. Come on in.

Ellen: She told me to rush straight down to her office. It bled pretty profusely.

Stacy: It did?

Jad: Wait a second. Why? What's so scary about cat poop?

Ellen: It turns out that cat poop can have in it this tiny parasite, called Toxoplasma gondii. What is the threat to the baby? If it gets to the baby-

Stacy: -it can cause miscarriage, it can cause stillbirth, and it can also cause seizures, blindness-

Jad: You're freaking out at this point.

Ellen: Yes, I'm freaking at this point.

Stacy: -small cranium, small head.

Ellen: My midwife said there's probably nothing to worry about. She took my blood-

Stacy: That's probably your better arm.

Ellen: -and she sent me home.

Stacy: The turn-around time for the test is between two and three days.

Ellen: I'm looking on the internet at home. I proceed to get myself even more freaked out. A bunch of things about Toxoplasma-- One of the things that I found was this lecture by Robert Sapolsky.

Sapolsky: Now, the example I'm talking about here--

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

Sapolsky: It has to do with a parasite called Toxoplasma.

Ellen: I just decided that I was going to call him up-

Sapolsky: Hello.

Ellen: -and ask a few questions.

Sapolsky: Okay. What's the deal with Toxo?

Ellen: He proceeded to tell me one of the most amazing feats of mind control I'd ever heard.

[music]

Jad: What did he tell you?

Ellen: The first thing he told me is that Toxo doesn't actually want to be in me.

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.

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

Ellen: 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.

Sapolsky: It's out there now in the cat feces.

Ellen: Step two, says Sapolsky, is that 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 a cat.

Sapolsky: It's a hard-wired 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. It's hard to believe, but this is what Sapolsky says happens.

Sapolsky: Toxo starts off in the stomach of the rodent, takes about six weeks to migrate its way up to the brain.

Ellen: Once it's in there, it finds this particular region-

Sapolsky: -called the amygdala-

Ellen: -which is like command central for fear and-

Sapolsky: -anxiety, and-

Ellen: -terror.

Sapolsky: All of that.

Ellen: It also finds this other region right next door where a very different emotion lives.

Sapolsky: Sexual arousal.

Ellen: What Toxo seems to be able to do is to somehow cross the wires.

Sapolsky: This may be some horrifically simplified sound bite, but what I think is going on is that Toxo knows how to make cat urine smell sexy-

Jad: What?

Sapolsky: -to rodents.

Jad: [laughs] Oh my God. That is so evil.

Sapolsky: Which is 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.

Sapolsky: Yes.

Ellen: In all other ways, the rodent is totally normal.

Sapolsky: Normal olfaction, normal social behavior.

Ellen: Just hot for cats. That's a good kitty. I started to wonder-- Moose really likes the microphone, "I love cats. Is it possible that Toxo-" This is wonderful. "-is what's been drawing me to cats?" Sorry, we let her put her fur everywhere. I ask him.

Sapolsky: Pure speculation, but people who think about this stuff, a view that as not just purely speculative, the notion that Toxo can produce some attraction to cats in humans. They don't think that's all the crazy.

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

Ellen: No one's really studied that yet- testing, testing- but there are scientists out there that are making the case that Toxo can really change you.

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

Ellen: Are you Dr. Torrey?

Dr. Torrey: I am. I am the beast.

Ellen: Nice to meet you.

Dr. Torrey: How are you?

Ellen: I'm good.

Dr. Torrey: There's actually been, the last count, 54 studies on Toxoplasma in people with schizophrenia and other psychoses.

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

Dr. Torrey: I've been doing research on schizophrenia since the early '70s.

Jad: He thinks there's a link?

Ellen: Yes.

Dr. Torrey: Not a huge effect. A very, very small risk of schizophrenia simply because schizophrenia is very rare.

Jad: Why would it cause schizophrenia to begin with? Is it trying to cause schizophrenia?

Ellen: Imagine if the Toxo was lost in the brain. It thinks it's 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: One of the reasons he thinks this might be true, this connection between Toxoplasma and schizophrenia, is because of a historical link.

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

Ellen: Why?

Dr. Torrey: This was the first time when we started to keep cats as pets. They first were adopted by the East-Greenwich-Village-types in Paris, the artists. It was really considered weird, but it's the kind of thing that if you were an artist, or a writer, or something like that, you started doing it. Then it spread to London where the writers and artists [unintelligible 00:07:38] there. Then, starting in about the 1840s, it had started to become a little bit more popular.

Then, in the 1860s, and 1870s, there was what's called a cat craze. Cats were all over. Greeting cards, the first cat show was in London in 1870, and in Madison Square Garden in 1880. It became very fashionable to have a cat.

Ellen: We should say, and 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 the rats?

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

Sapolsky: Here's another one of those give-me-a-break-

Ellen: That's Robert Sapolsky again.

Sapolsky: -science fiction branches to the 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: Yes. I asked him why.

Sapolsky: In so far 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 high speed.

Ellen: In the end, it might be possible that Toxo so is guiding our emotions-

[music]

Ellen: -changing who we are in some basic way. If you consider that Toxo might just be one of thousands of tiny little parasites inside us pulling our strings from the inside, that thought is pretty creepy.

Sapolsky: Even if the entire lesson with Toxo is a small subset of infected people now have 1/2 of 1% more likelihood to want to drive really recklessly. Even lurking in that 1/2 of 1% 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: This is me with my midwife, Berry. She is giving me her news. What did we find out from the Toxo test?

Berry: That you have had past infection with toxoplasma, positive.

Jad: You positive?

Ellen: Yes, but my midwife says that the baby is going to be okay. Does the baby look like she is small? Does she look--

Berry: Oh no she looks like she is a nice size to a little bit on the larger size, so not a baby I'd be worried about.

Ellen: I believe her.

Jad: Thanks, Ellen.

Ellen: Sure.

[music]

Jad: You want to hear more about anything during this hour check our website radiolab.org

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