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Science Talk

Carl Zimmer on Rats, Cats, Viruses and Tattoos

In part 2 of our interview, award-winning author Carl Zimmer talks about his latest books, and a new study that shows how Toxoplasma influences the behavior of rats--and maybe of us

In part 2 of our interview, award-winning author Carl Zimmer talks about his latest books, and a new study that shows how Toxoplasma influences the behavior of rats--and maybe of us.

Podcast Transcription

Steve:          Hi, Steve Mirsky here for Scientific American's Science Talk. Welcome back for part 2 of our conversation with Carl Zimmer. In part 1, we discussed evolutionary research in the urban environment. Next we talk about Carl's latest books as well as a study that came out last week about toxoplasma infections and the amazing effects that they have on rats.

You've also got a new book coming out and another book coming out soon that's a little bit a departure for you. You want to talk about those?

Zimmer:          Yeah, so in May I published a book called Planet of Viruses. I wrote a dozen essays on a dozen viruses looking at different aspects of virus biology and how viruses have affected humans, and also trying to make the point that, you know, viruses are incredibly abundant on Earth, and in fact are the most abundant life form on Earth.

Steve:          So are they a life form? I mean, I'm not even sure this question is important, but are they alive or are they almost alive?

Zimmer:          I think that they're alive. You know, there's certainly a debate about that. I think that it's kind of an arbitrary distinction to say that you have to be exactly like us in order to be alive. Viruses do all sorts of things that can only be done by living things. You know, they replicate, they evolve, they adapt, they have all sorts of traits that let them survive in very strange habitats like our throats, for example, and droplets floating around in the air. And so, you know, and viruses have been controlling the environment, literally. I mean, viruses help to set the climate itself. They might seed the clouds, they do all sorts of very powerful things in the environment, so it's silly to think of them as somehow not being alive, just because they, say, can't metabolize a sandwich. So, yeah, viruses are alive, I'll say.

Steve:          So, I should have said that that book is already out, that came out in May, and you have a book coming out soon that's, sort of, I guess it's a picture book.

Zimmer:          Yeah it is, it is. So I have a blog called The Loom, and I once posted a picture of a tattoo on the shoulder of a neurobiologist I know at Harvard. I just one day noticed that he has a DNA tattoo on his shoulder, and he explained to me how he actually had designed it so that it spells out his wife's initials in genetic code. And I thought, wow, and I wondered if there are other scientists out there who are hiding tattoos from the rest of us. So, I just asked that question in my blog and promptly got flooded with an endless supply of tattoos with all sorts of wild stories attached to them about every branch of science, from neuroscience to anthropology. And you know, after a while, maybe after the 300th or the 400th picture showed up, I said, you know, this seems like it would be a really fun idea for a book. So I picked out some of my favorites and then wrote kind of longish captions or shortish essays, however you want to think about it, about the subjects of their tattoos or about the people themselves, and so on. And yeah, so now it's called Science Ink--Tattoos of the Science-Obsessed, and that will be coming out in November.

Steve:          Really looking forward to that. I'm going to assume that the Darwin branching tree diagram is a pretty popular one.

Zimmer:          That is a very popular one, yeah. There must be four or five people who have the tree that Darwin drew in a notebook while he was working out his theory. In the notebooks, he draws this little sketch of a tree, and actually, he writes next to it, "I think" which is wonderful, and actually a number of these scientists have the tree and "I think" next to it because that really speaks to the scientists as well, the way you, sort of, play around with ideas and try to figure out what's actually going on.

Steve:          So, let's spend another two minutes, if you have them.

Zimmer:          Sure.

Steve:          We both covered a story last week that came out that talked about the fact that it been well known that rats who have a toxoplasma infection will actually become attracted to the scent of cats, rather than flee from that smell, which would be the evolutionarily prudent thing to do. So they get attracted to the smell and, you know, presumably, often wind up in the cat's mouth. But a study came out last week that showed exactly what kind of attraction it is, and it's pretty interesting.

Zimmer:          Yeah, I mean, scientists have been puzzling over toxoplasma for, I'd say, about 15 years now because it's got this really interesting lifecycle where in order to reproduce sexually it's got to get inside of the guts of cats. So the cat infected with this parasite is totally healthy, it's fine; and if it's infected, it sheds eggs, as it were, of toxoplasma with its droppings. So, that's why pregnant women are not supposed to have a kitty there, because they might get a toxoplasma infection, and for fetuses, toxoplasma is very dangerous because they don't have an immune system to keep it in check. For the rest of us, it's not really a big deal. We might feel a little fever when we get infected, but it passes away quickly. We don't kill off the parasite though, it just forms cysts in weird places like our brains.

Steve:          Our brains. We should point out that people who are immunocompromised are also at a different level of jeopardy from toxoplasma.

Zimmer:          Yeah. That's right. So when AIDS started to spread, one of the most puzzling things that happened was that people started developing cases of this disease called toxoplasmosis where toxoplasma gets out of control, and it causes inflammation in the brain, dementia, and can be fatal. But you know, the doctors were totally baffled by it, because normally people just keep the parasite in check. And the thing is that HIV just tears down your immune system, so that things like toxoplasma can just take off. But, you know, in a sense it's not in the "interest" of the parasite to kill you or another intermediate host. In order to reproduce, its intermediate host has to get back inside of a cat, and what happens often is that a rat, for example, that's infected gets eaten by a cat and then the parasite just moves from the rat to a cat.

Steve:          Right--the rat will get infected because it'll touch the cat droppings.

Zimmer:          Right. These eggs are just floating around in the dirt; you know, rats are sorta rummaging around and looking for food and they pick up the eggs, and they end up in their mouth and they get infected. So yeah, toxoplasma, you know, where there a lot of outdoor cats, toxoplasma is supposed to be very common everywhere.

Steve:          So, what's really interesting is the researchers found out what precisely, the precise kind of attraction that the parasite endows in the rat to make them want to approach the cat odors.

Zimmer:          Right, right, because initially some scientists at Oxford had done experiments in some big enclosures where they would put different odors in different corners, and they would just see what happened to the rats. And they were just observing the rats and once they looked at the numbers, they could see that the rats tended, healthy rats as soon as they smelled the cat odor, they stayed away from that corner. And in fact they explored a lot less in general; they got very anxious; whereas the infected rats, they didn't change, and in some cases, they seemed to be exploring that corner more. So you have that observation on the outside. So, then a scientist named Robert Sapolsky from Stanford University said, "Well let's try to see what's happening on the inside." And so they looked at rat brains to see where the parasite was accumulating, and it was accumulating in one particular part of the brain, in the limbic system, where a lot of what we would call emotions are regulated. And then just recently they did a very cool study where they were able to look at what areas of the brain were becoming more or less active in infected rats when they smelled cat urine, and it was quite mind blowing. So there are two, kind of, circuits, you can think of them, in this part of the brain, and they're pretty close to each other; and one of them, basically, you know, regulates fear and withdrawing from danger and being defensive. In these infected rats, that area of the brain was less active, specifically when they smelled cat urine, which is amazingly cool enough as it is; but they found another part of the brain right nearby that was actually becoming more active than usual, and that is an area of the brain that is responsible for sexual attraction; where, you know, a rat will approach a smell because it thinks maybe it's going to find a mate. So somehow it seems that this parasite is actually making this rat kind of, in a sense--I'm going to be metaphorical here but--fall in love with cats. It is feeling a pull towards the odor of cats and therefore towards its own doom.

Steve:          That's amazing. What a clever parasite. So, now the obvious question, let's talk about me personally. You know, where this is going.

Zimmer:          I do and I'm not sure I'm looking forward to the trip. You're not a cat lady, are you?

Steve:          I have two cats.

Zimmer:          Oh boy!

Steve:          And I'm just wondering: Have I been manipulated--I grew up with a cat--have I been manipulated by toxoplasma to be in love with cats?

Zimmer:          So, people ask this all the time, and you know, they've asked Robert Sapolsky, and he'll, sort of, shrug and go, "Oh, who knows?" Let me just put it this way. There's no evidence at all that you know, "cat ladies" or other cat lovers are more infected with toxoplasma than other people. A third or so of all people on Earth are infected with toxoplasma. And so. you know, someone would have to, like, do a really strange, very large-scale study that really nails this down and no one has done it. Now that being said, there are some very tantalizing studies, by no means the final word, but these studies show that there are potentially some differences in personality that arise when people get infected with toxoplasma. You know, it changes, women for example, when they take personality tests--if they're infected, they tend to be more open-hearted, and that's one of the measurements of personality tests. Men tend to be more distrustful authority. People who are infected with toxoplasma tend to get into more traffic accidents. This doesn't mean that people who are infected with toxoplasma are zombies running around, you know, just throwing themselves in front of cars, but, you know, there is something happening and that does not necessarily mean that, you know, it's some well-adapted strategy of toxoplasma. You know, it's possible that toxoplasma is really adapted for affecting the brains of rodents and other, you know, common prey of cats. The fact that you're giving your cat food and so on is not really, that's not really helping the parasite. The parasite would really be happy if your cats ate you, okay--little bit different. And so it's possible that, you know, that these parasites have all these very interesting brain drugs that they're secreting, and they just have a different effect on people than they do on rats.

Steve:          Interesting. And it also brings up the question of who knows what else we might be infected with that is controlling behavior in various ways.

Zimmer:          Yeah, now that's an important point because it's not necessarily just parasites that are doing this to us; you know, we are home to, you know, over a thousand species of bacteria; a lot of them are in our guts, and they're releasing molecules all the time and some of those molecules are getting into bloodstream and some of them are getting into our brains. And so researchers are doing some studies on how our resident bacteria affect behavior. And they are seeing, when they look at mice, you know, that have been experimentally inoculated with different kinds of harmless bacteria, they can see different levels of anxiety and so on. They can see changes in behavior. You know, the scariest one to me actually is what happens when mice eat a lot of food and get really fat. The diversity of the bacteria changes, and if you take those species of bacteria living in the obese mouse's gut and you put them in a mouse that has no bacteria in it at all, that mouse with that transplanted bacteria will actually become unusually hungry. And the only difference, the only thing, the only manipulation is that it's got these bacteria from an overfed, overweight mouse. And so it really looks like it's the bacteria that are sending messages to the brain that are making the mouse hungrier, in order to feed the bacteria.

Steve:          Wow!

Zimmer:          And you know that could very well be happening to us.

Steve:          So we're going to be looking as an obesity treatment at antibiotics followed by fecal transplants.

Zimmer:          That sounds weird, but I bet something like that will be happening within the next decade.

Steve:          Wow! Hey, Carl this was really fun. It's a pleasure to talk to you, and everybody read his article in the New York Times July 25th, available at the Times' site, and at Carl's site and go out and buy his books.

Zimmer:          Thanks, Steve.

Steve:          If you haven't heard about fecal transplants, google it; it's fascinating, and I don't want to talk about it. Get your science news at our Web site--that's www.ScientificAmerican.com--where you can also check out various exclusive Web features related to our September single-topic issue all about cities. And follow us on Twitter, where you'll get a tweet each time a new article hits the Web site. Our Twitter name is @SciAm, S-C-I-A-M. For Science Talk, I am Steve Mirsky. Thanks for clicking on us.

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