The Guests Inside Us

Question: How do parasites alter the course of evolution?

Carl Zimmer: Well parasites are a huge menace to any free living organism. For one there are just a whole bunch of them. There are a lot more of those parasites than there are of us, so perhaps four, five, six, seven parasites for every free living species. That’s one estimate I’ve seen, although I bet there is a lot more and so these things, these parasites they’re trying to use us and other hosts to make more copies of themselves. That’s what they do and in the process we can get pretty sick or die, so any kind of mutation that might give us a little bit of resistance, might be able to let us evade these parasites, is going to be incredibly valuable. It’s going to be strongly favored by natural selection. And so you can see the effect of parasites in lots of different ways. I mean you can just go through the genome, the human genome and see that, the fingerprints of parasites there. They’re all over the place. They have shaped genes that make our immune system better, able to recognize certain kinds of parasites for example. They have… In some parts of the world they have made people resistant to malaria by making their blood cells harder for the malaria parasite to invade and on the other hand we have also kind of reached kind of an uneasy kind of a detente with some parasites as well. If you go to a jungle and look at the people who live there and this can be in the jungles of Venezuela or in Central Africa or what have you, places far from medical care, people are loaded with parasites, particularly intestinal worms, but it generally doesn’t harm their health all that much and what is also interesting is that people in these parts of the world also don’t have a lot of allergies whereas if you were to go to a city in Venezuela for example you would find people who have very few of these parasites, but have lots of allergies and other kind of autoimmune disorders. So there is a theory that over the past century we have rid ourselves of a lot of these nasty parasites. I mean nobody wants hookworm, but the problem is that our immune systems had evolved to be in a kind of a balance with these parasites, so they would sort of hold them in check, but they would not attack them too severely and it appears that our immune systems have to learn to find this balance, but if we live a life that is in a sense to clean we can’t… our bodies don’t learn that balance and so we get thrown off and we might attack some meaningless thing like a piece of cat dander or a piece of mold and we totally overreact because our immune systems haven’t been trained to basically calm down. So parasites have affected us on all sorts of different levels.

Question: How can parasites affect brain chemistry?

Carl Zimmer: Now if you are a parasite very often you don’t just want to make your host sick. You actually have other things in mind. So for example, there is a fungus that lives inside of ants that’s called Cordyceps. Now the fungus has to get from one ant to the next and what it does is it is going to shower its spores down on healthy ants. Well what that means is it’s got to get up above those ants, so how does it do that? What it does is it infects an ant, so a spore gets into an ant and it sort of burrows its way in and it starts to branch out inside of the ant and if you cut these poor ants open they’re like… after awhile they’re just pure fungus inside, but they’re still alive and they look pretty normal. They’re going around their business, but eventually when the fungus is ready they get a signal and they get this urge to climb upwards and what they do is they climb up plants, typically trees or small bushes and they’ll go about two feet up and then they’ll find a leaf and they will go and they will perch on the underside the leaf, usually amazingly enough facing north by northwest. These ants they’re heeding this call that’s coming from within them to do this very specific thing and the reason they’re doing that is because it’s good for the fungus. So what it does is it puts the fungus up in the air above other ants and now the fungus can get ready to spread. What it does is it produces a stalk that pops out of the ant’s head. So this poor ant has this giant spike that grows up out of its head that’s covered in spores. The fungus meanwhile is spreading out of the ant’s legs and abdomen and is attaching itself to the leaf, so the ant is now being cemented to the leaf and the fungus is also hardening the exoskeleton of the ant in a sense. It’s making this very durable case, this shelter, kind of a house where it can live and where it can survive and by being on the underside of that leaf it’s protected from rain and it’s protected from intense sunlight. It’s in this perfect place for being a parasitic fungus and now the spores from the ant coming out of that spike can shower down on the ants below and infect them and make them suffer that same horrible fate. 

Now that is just one example. I know dozens. There are probably hundreds of examples that scientists have documented of parasites manipulating the behavior of their host. Sometimes it’s going from one host to another in the same species. There are a lot of cases where what the parasite wants to do is get from one species to another species. So for example, there is a parasite called toxoplasma. It’s very common in birds and rats and so on. Lots of mammals can carry it, but they are not the host where toxoplasma can really reproduce or produce the next generation of toxoplasma. That host, what is called a final host is cats, so you’ve got toxoplasma in something like a rat for example and in order for it to complete its lifecycle it has got to get into a cat and rats are very good at not getting into cats. I mean that’s sort of what they’re on earth for, to avoid getting eaten by cats. They have an incredibly sensitive sense of smell and if they get the faintest whiff of cat odor they get very anxious and they look around. They take evasive action. They don’t want to get eaten. What is really remarkable about a rat infected with toxoplasma is that it looks totally normal except that it is no longer afraid of the smell of a cat. In fact, sometimes when toxoplasma gets into rats they actually seem to get a little curious. They say what is this, this looks interesting, I’m going to investigate this and so it appears that the toxoplasma is very precisely manipulating the rat brain, maybe by secreting certain kinds of chemicals to affect its whole network for processing fear and other kinds of emotions and so it may be making it an easier target for cats. 

What makes this really intriguing is that about a quarter of all people on earth are infected with toxoplasma. We get it from lots of different places. So for example, the soil is actually a place where you can pick it up if a cat has left its droppings in the area. This is why pregnant women are not supposed to handle kitty litter because kitty litter may be loaded with toxoplasma. Now if you get infected with toxoplasma generally it’s not a big deal because what happens is you may feel a little fever, but then after awhile you recover and you’re fine. What has happened is that the toxoplasma has formed little cysts in your brain, thousands and thousands and thousands of cysts in your brain and it’s just hiding there. It’s just hanging out. It’s still alive. In fact, every now and then they might break out, but your immune system can sense that they’ve broken out and they attack them again and they go back into their cysts, so there is this balance that we strike. The reason that pregnant women aren’t supposed to handle kitty litter is because if the toxoplasma gets into their babies that’s when the trouble starts because the babies don’t have a mature immune system yet, so there is nothing there to keep toxoplasma in check and so it will replicate like crazy. It can cause brain damage. It’s not a good thing. So there we are, a billion people maybe, maybe two billion people with toxoplasma in their brains and we know that it can affect mammal brains and the fact is that a rat brain and a human brain aren’t all that different. In the really basic ways they’re quite similar. So is it possible that the toxoplasma is affecting people? It’s possible. The evidence is really… I’d say it’s pretty sketchy at this point, but it’s evidence that can’t be just dismissed out of hand. So for example, people with toxoplasma have been reported to get into more traffic accidents. So does this mean that they’re being more reckless, that they’re not being as anxious as a normal human or rat might be? I don’t know the answer to that, but the fact remains that we have these mind altering parasites in our brains and so I think we ought to figure out what they’re doing.

Question: How was the parasite that causes sleeping sickness almost eradicated, and what can still be done about it?

Carl Zimmer: There are lots of diseases in the tropics that we fortunately don’t have to contend with in places like the United States and we should consider ourselves fortunate because some of them are really horrendous. A particularly horrendous one is called Sleeping Sickness. Sleeping Sickness is caused by a single celled organism called a trypanosome and it looks like a little fluke or a kind of a flatworm under a microscope, but it’s obviously much tinier than that. They are carried inside certain kinds of flies in Africa and these flies will bite people and they will inject these parasites, the trypanosomes into these humans who then start to get sick and they develop something called Sleeping Sickness, which you know not surprisingly makes you very tired and rundown. The real problem is that unless you treat it, it is quite fatal and so it’s a serious problem in… particularly in the belt just below the Sahara. Now a hundred years ago it was quite a serious problem, much more serious than it is today, but it’s been gradually eradicated from a lot of places where it was a big problem. It was eradicated through good public health, through treating people and through trying to attack the populations of the flies to basically break the cycle of transmission and it has worked in a lot of places. And this is a story that has been replicated with a number of parasites, with for example, a parasite that causes something called River Blindness. It’s a worm that actually like gets into your system and can get into your eye and inflames your eye and scars it until you’re blind. That is being very nicely eradicated and other diseases as well. Sleeping Sickness was getting close to that kind of eradication or at least being really driven down to very tiny levels, but unfortunately the war in Sudan has given it a new lease on life and so there have been in the past ten or twenty years new flare-ups of Sleeping Sickness and so it’s a real testament to the devastation that war can have. It isn’t just people being killed by bullets. It’s also people being killed by parasites as well.

Recorded on January 6, 2010
Interviewed\r\n by Austin Allen