Robert Sapolsky Explains Toxoplasmosis - The Joe Rogan Experience

Hello, freak bitches. I found out about you several years back. I heard something about toxoplasmosis. Am I saying it right? Yeah. And I had seen a speech that you had given on it where you were talking about how many people had been infected by this cat parasite. I had had cats my whole life and I even had feral cats. And I've always wondered, I should probably get tested and I'm worried about the result. Perhaps not. Well, it was just fascinating to me that literally, I mean, what is the number that you estimate in Americans alone that might have been infected? I believe it's on the order of, well, I'm not sure with Americans, but worldwide it's something like 50% of humans is the best guess. 50% of humans worldwide. It's one thing on that scale. For people who've never heard of this, would you mind explaining what this parasite is and how it affects rats and then cats and then people? Okay, totally bizarre. So it's this protozoan parasite, toxoplasma gondii, and it's got one of these weird parasitic lifestyles. The only place on earth where it could reproduce sexually is in the gut of a cat. I don't know why there are people who know this, but so it reproduces there, comes out in the cat feces. Feces are eaten by rodents and now toxo's evolutionary challenge is to get that rodent into a cat's stomach. So what toxo has evolved is this ability. It slowly migrates to the brain of rodents and basically wipes out the innate fear that rodents have of cat smells. You take a lab rat who's been the descendant of lab rats for a thousand years and they've never seen a cat and put a little puddle of cat pee in its cage and the rat's going to go on the other side of the cage. Just hardwired instinctual aversion to cat pheromones and then put toxo in a rat and it loses that aversion. In fact in a subset of rats, they like the smell. So out in the natural setting, you now go and approach a cat and soon the rodents inside the cat's stomach and toxo is completely in its life cycle. Well, I had heard that it was a subset of rats that actually are gravitated towards it because I'd heard it actually rewires them sexually, right? Yes, that's actually work that we did in my lab that it basically crosses some of the circuitry in the brain and hypothalamus so that cat pheromones that used to be activating every alarm circuit in your limbic system and these rodents now instead sort of taps into sexual arousal pathways. In male rats, when they smell cat pheromones, they increase testosterone production. So toxo has just figured out the most brilliant way of doing it. It makes cat pee smell sexy. Is there any understanding at all of the mechanism of how a parasite can figure out how to rewire an animal's sexual reward system, the fear of predators? How does that work? Well, that's something my lab spent a bunch of years on trying to figure out. When you look at some of these parasites, this taps into this whole world of behavioral manipulation of hosts by parasites and turns out they've evolved unbelievably brilliant mechanisms for manipulating hosts for their own benefit. Think about it. You get rabies, you get a rabid dog and what that's about is a virus that has affected the nervous system of that dog so that it's now rabid and more likely to bite somebody with viral particles in its saliva, which it now passes on to the next individual. Like you take 10,000 neuroscientists and stick them in a convention on the neurobiology of aggression and rabies knows more about the neural wiring of aggression than we do. Wow. Toxo knows quotation marks, something about fear and aversion and the neurobiology of attraction. Part of what it seems to involve is toxo somewhere along the way has picked up a gene that is pertinent to the dopamine system in mammals. Dopamine is this neurotransmitter. It's about pleasure. There's no protozoan parasite for buineers that's had any use for this stuff except it's part of how toxo seems to be manipulating the reward system in rodents. And then a couple of years ago there's a paper showing that in chimpanzees, toxo makes you less afraid of the smell of leopards. Wow. So this appears to be a parasite that just has evolved like this spectacular insight into fear circuitry and attraction circuitry and it's all for its own benefit to wind up in a cat's gut. So it's specifically cats like the chimpanzees still have aversions to snakes and other things that can kill them? Yeah. Wow. And at one point my lab was full of like bobcat key and wolf pee and there's actually like a company you could buy urine from. I don't know why anyone would want it except for us but they sell urine from actually what they use it for is you can go spritz it around your garden to scare the deer away. Other makes sense. So there's, I don't know where they get the urine from but it's like come certified and all of that. And yeah, it's remarkably specific. So like other, have they ever done tests where they test like wolf urine or anything like that around chimps? Do they have any aversion to that? As far as I know the chimp study has only been with big cat urine but the rodent study's exactly that showing it's a fair specificity. The rodents lose a little bit of their general skittishness. They get a little bit disinhibited behaviorally so just in general they're out more and more exploratory more likely to get eaten but the most selective lasering effect is they're not scared anymore of cat pheromones. Now what's fascinating to me is that I've also read that there was a disproportionate amount of successful soccer teams that are in countries with high rates of infestation of toxoplasma. Okay, that one's new to me but that sounds like exactly the sort of epidemiological studies that are popping up about humans. Okay, so what about humans? There's two branches of interesting stuff with toxin in humans. One is a literature that's been around for quite some time showing that toxo seems to increase the risk of schizophrenia. There's a higher rate of schizophrenia of individuals who have antibodies against toxo. In other words, sometime in the past their body was dealing with it who had cats growing up whose mother had cats during pregnancy and like anybody who gets pregnant knows you immediately get all anxious about cat litter boxes because of the possibility of toxoplasmosis. It can attack the fetal nervous system, do all sorts of damage and a subtle version of it seems to be a sleeper effect of increasing the risk of schizophrenia. The other realm is toxo-infected humans get subtle changes in personality, neuropsychology, neuropsychological profiles, they get a little bit disinhibited. If you're toxo-infected you're more likely to die in a car accident involving reckless speeding. If you're toxo-infected and clinically depressed for the same severity of depression you're more likely to impulsively kill yourself. In other words, toxo is doing something kind of similar. If you're a rat, one of the hardest wired scary things in the universe out there is the smell of a cat. If you're a human, it's hurtling through space really fast and jumping out of windows and toxo seems to blunt a lot of those effects there. In the speech that I saw you give you were talking about your time working in a hospital and that there was a disproportionate amount of motorcycle victims. This was actually something I heard from a clinician, an old sort of parapsychology infectious disease doctor who sort of when I was first telling him about this sort of emerging toxo story he had like one of these bolts of memory saying, my God, I remember back when I was a resident there was this old doctor saying, you know, if you're ever harvesting organs from an accident victim, I don't know why, I don't know why, but if it's from somebody who was in a motorcycle accident, make sure you check to see if they have toxoplasmosis. I don't know why, but there's a high rate of that that you find in organs from people who were killed driving motorcycles recklessly. Totally anecdotal, n equals one kind of thing, but nonetheless, this was a guy who like studies infectious disease and toxoplasmosis and had not heard about sort of the behavioral findings before and that out of the recesses of his memory. So what initially seemed like, okay, this is a parasite that's very selectively developed this life cycle between cat stomachs and rodent brains and completing its life cycle and weird when it gets into humans and has some behavioral effects also, that's just kind of evolutionary spillover. But then you see if it's doing something similar between chimps and leopards suggesting that that life cycle manipulation has been selected for in primates as well. Very strange. It's very strange. And for me, the strangest thing is the certainty with which there's a gazillion viruses and bacteria and God knows what else out there that manipulate host behavior in ways we just haven't figured out yet. Or we just have this right. We haven't discovered the particular one. What does it do to women? A similar effect seems to have less severe effects on neuropsychological profiles of women. Again, the literature on this is pretty scanty in humans, but it seems to have some similar effects, but not as extreme. However, the story now gets a little bit more complicated. And this is actually this fabulous scientist, Ance Villas, who is my postdoc, who's now a professor in Singapore, who's continued to study this. OK, so normally one of the things animals have evolved to be really good at is picking up signals that somebody else is unhealthy. It's like a potential mate is unhealthy. There's sickness behavior. There's very olfactory cues. If your rodent makes perfect sense, the last thing you want to do is to be mating with somebody who's like rancid and infectious and rodent equivalent of STDs. So normally sick animals, parasite infected animals and such are detected by other rodents and avoided. Toxo does something different. You get a toxo-infected male and now he smells more attractive to female rodents. And when mating, toxo gets into the sperm and can be transmitted to the female. So suddenly we've got a different story here. We start off with a parasite story where toxo is just ruthlessly exploiting the poor rodents for its own reproductive benefit and its own evolutionary selfish gene well-being. But now instead it's got elements of, instead of parasitism, symbiosis. So you're a male rat infected with toxo, down side you're more likely to get eaten by a cat, up side you're more likely to pass on copies of your genes by increased sexual selection. So it might be in fact more of a balanced symbiotic relationship between male rats and toxo. You know, more research is needed, blah blah. It's just like cool sort of biology out there. It's crazy. And is it transferred sexually with men and women too as well as with rats? I don't know. I don't think it's been looked at. Oh wow, that seems like something I would want to look at right away. What about organ donors? Other than, again, pure anecdotalism that one elderly doc somewhere back when saying watch it when you're getting organs from people killing motorcycle accidents, beyond that, I don't know, people are looking at it I'm sure. Do they even test? I say if you got a liver and you know, like you needed a liver transplant, would they test? I suspect they do. And sort of in the clinical world of people who worry about toxo, toxo, pregnancy, scary alarms going off, toxo, anything else after an acute period of infection, you have a latent toxoplasma infection. In other words, the agreed upon sort of notion there is toxo has gone latent. It's formed sort of these cysts that are inert and you got nothing to worry about then. But the whole notion that meanwhile up in the nervous system, there's effects happening there, you know, infectious disease people are thinking about inflammation outside in the body there for them. Chronic toxo infection is not something you worry about a whole lot. But if it's having behavioral effects up there in the nervous system, maybe it is something to worry about. Well, it's just it's to me, it's unfathomable how this little thing figures out how to hijack a whole whole body, a whole biological system and work it to its own desires. It's very it's very hard for me to grasp. Well, if you think of it in terms of, I don't know, toxo has had like 100,000 more generations to evolve its ways of exploiting mammals than mammals have had ways of fighting it off. What's most remarkable is it turns out this is like a whole world of parasites that do bizarre, manipulative things to their host. Most of it's not in the realm of mammals. Instead, there's like some parasitic something or other that gets into barnacles and takes over their reproductive system so that the barnacle digs a hole for them, them, not the barnacle, but the parasite to lay eggs into. There's the aquatic worm that infects the grasshopper makes it commit suicide. Exactly. That one's bizarre. That one's bizarre. There's this wasp that gets into cockroaches and takes over. I'm fascinated by parasites beyond, but it's just it's so confusing to me how something I mean, obviously, you're dealing with hundreds of thousands of generations for it to get to this this current state, but like how something evolves to be so effective. Yeah, it's just it's so confusing. It's remarkable.