In Our Time - Parasitism

Episode Date: January 26, 2017

Melvyn Bragg and guests discuss the relationship between parasites and hosts, where one species lives on or in another to the benefit of the parasite but at a cost to the host, potentially leading to disease or death of the host. Typical examples are mistletoe and trees, hookworms and vertebrates, cuckoos and other birds. In many cases the parasite species do so well in or on a particular host that they reproduce much faster and can adapt to changes more efficiently, and it is thought that almost half of all animal species have a parasitic stage in their lifetime. What techniques do hosts have to counter the parasites, and what impact do parasites have on the evolution of their hosts? With Steve Jones Emeritus Professor of Genetics at University College, LondonWendy Gibson Professor of Protozoology at the University of Bristoland Kayla King Associate Professor in the Department of Zoology at the University of OxfordProducer: Simon Tillotson.

This is the BBC. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoy the programmes. Hello, all humans play a host to countless parasites. If not visibly on the surface, then internally and between the cells or inside the cells themselves. The parasites gain some advantage and we in turn are disadvantaged.

The food we eat nourishes us and we in turn nourish the organisms that depend on us. Usually we don't notice, but at worst we weaken, become diseased or die. This relationship is found in all of the animals and plants, and it's as old as evolution. The fittest to survive are those strong enough to reproduce, despite their burden of parasites. And this is all very finely balanced. For if the parasites destroy their host, they may lose their home, as well as their source of food. Overall, it's a very successful strategy for parasites, and theirs is the most common life cycle of living organisms.

With me to discuss parasitism, Steve Jones, a maritalist professor of genetics at University of College London, Wendy Gibson, Professor of Proteozoology at the University of Bristol, and Kayla King, Associate Professor in the Department of Zoology at the University of Oxford. Steve Jones, can parasitism be defined or something that's hard to pin down? Well, I think you've defined it pretty well, actually. It's an asymmetric relationship, like most relationships in nature, between generally a smaller organism

that lives on or within a larger organism takes nutrition from that larger organism and gives nothing in return. That's a nice neat definition, but as usual in biology, it's very blurred around the edges, and all parasitic relationships are on a knife edge all the time.

And one can win, the other can win, parasitism can turn into cooperation, cooperation can turn into parasitism, but it's a convenient bag into which to put the great majority of all living organisms. I mean, I tend to think, and I suspect a few listeners who tend to think, that parasites are a few things crawling around,

and there you go, I have to put up with them. But in fact, you say they're half the world's biomass. Humans have 400 parasites knocking about them, most of us, and they're a menace. Can you develop that? Well, yes. I mean, again, you have the definition problem. I mean, as most people know,

most of the cells within your skin are not your cells. They're the cells either of parasites or of bacteria, which, for the time being, are being helpful rather than parasitic, but can switch. And if you do that calculation, the proportion of cells in your body, which are human, are equivalent to your left leg from the knee to the toe. That's the only human cells. All the rest are other cells, many of which are parasitic, not all, but that gives you some hint of the extent to which that we are ourselves ecosystems, and ecosystems which are themselves always evolving. How do these half a leg of humans survive against all these parasites then, Steve? Well, it's a race. It's often said it's, you know, it's the famous Red Queen race where both sides are evolving. Alice in Wonderland.

Alice in Wonderland. And it's a dynamic race which goes nowhere. Because if the parasites are too successful, then their host disappear. And that's bad for the parasites. If the hosts are too successful, what you often find, actually, for example, if you grow mice in the lab, which you can do with great difficulty. and ensure there is absolutely no other living creature, parasite or bacteria or anything else, within these mice, they're called axemic mice.

Then they're very, very feeble and weak and they die. So it's difficult to draw the line. And many things which have been thought to be harmful parasites almost inevitably turn out to be doing good things as well as bad things. So, as I said, it's not simple. Can you give us some practical range of the parasites that are knocking about? Well, it's an enormous range. I mean, you can start off with things which are obvious and large, things like tapeworms.

The thing we all studied when we did our little biology, and we all thought, oh, God, I'm not going to work on those. They're horrible. And a tapeworm, you know, can be several feet long. You can go all the way down through the size range, through mites that everybody listening to this program, almost everybody has got mites on their eyelashes. They don't know that they're there. you can only just see them with the naked eye, but they're there. And you can go all the way down to elements in the cell,

things like the famous mitochondria, which burns all the power in the cell, which absolutely clearly and absolutely certainly started life off at the dawn of time as an intercellular parasite has been reduced by this battle between the host and the parasite to become an essential part of the cellular machinery, but can now and again itself, tiny though it is, turn on its owner and destroy it. definitely saying that sometimes parasites can have a positive and good effect. Well, except the trouble is,

you know, words like positive and good don't really belong in biology. It turns into theology then. Well, keeping you alive, let's keep it at that, okay? Does that belong in biology? Certainly, one of the most bizarre discoveries is if you take creatures

which seemed to be absolutely unnecessary and parasitic and you kill them off, there's a famous intracellular parasite, which I'll not doubt we'll talk about a bit later, and you kill them off. It turns out that the animals and humans

who bear them, if you kill off the parasites, the host then suffers. So you can't really call them parasites anymore. They become mutualists or commensals or any of these complicated words that don't actually mean very much. Kayla King, can you

distinguish the relationship between a parasite and its host and a predator and this prey? Yeah, certainly. initially though they are very similar in that they have an antagonistic relationship with their victim so they benefit predators and parasites benefit while the victim suffers but certainly there are differences between them their differences in their biology that we use to

distinguish them so firstly parasites generally tend to be much smaller than their hosts in contrast of course predators in general are often larger than their prey Like a cat and a mouse. Yes, exactly, exactly. But an exception to that rule is the parasitic cuckoo bird, which is quite much larger than the host or the host babies and even the host nest even.

And it's a hard time even fitting in. And another difference between them is that a given parasitic individual will attack and infect one host individual, whereas one predator individual can eat multiple, multiple prey items. And this leads to another difference, which is a bit of an evolutionary difference, in that host parasite interactions, there's a symmetry in the fitness effects. So what I mean by that is if a host becomes infected by a parasite, its reproductive potential goes down almost to zero over time. And if a parasite doesn't infect the host, then it can die as well.

So it needs the host to be able to proliferate. In contrast, there's an asymmetry in the fitness effects between predators and prey. And that's best illustrated by something called the life dinner principle, which essentially means that a predator is running for its dinner, but a prey is running for its life. So if the interaction is unsuccessful for the predator, the predator can just go off and find another prey item. And that leads to another difference,

which is that predators kill their prey, and that's how they benefit from the interaction. That's not necessarily true with parasites. In fact, it's quite rare for parasites to find it necessary to kill their hosts. They often just sort of sap host resources to be able to reproduce. But of course, as Steve mentioned, there is a continuum probably in place where you, can have predators on one side and parasites on the other.

And so you have lots of exceptions and lots of organisms that occupy some middle space. How do parasites get into us or onto us? Right. So all sorts of different ways. So we can think about categorizing the myriad types of transmission. Well, so let's show the outside predators and parasites and the inside parasites. Yes. So you can have ectop parasites, which, for example, might be fleas or ticks, which tend to occupy the outside of hosts. So don't even bother to get in.

They settle on. Yep. They just settle on and are able to sap host resources that way. Or taking a blood meal, for instance. Sounds like going up than dinah, doesn't it? I'll take a blood meal. Sorry.

And then, of course, you can have endoparasites. Steve mentioned tapeworms, which are really interesting example of parasites that can grow within us. But these kinds of parasites, they can transmit what's called horizontally, so between host individuals. So for me to use, Melvin, or Melvin to Wendy or Wendy to Steve,

or vertically. So parasites can transmit from mother to baby, and that's a kind of vertical transmission. Horizontally, however, you can get parasites that transmit directly between hosts. So, for instance, parasites actively jump, from one host to another. So ticks, flees, this sort of thing. Sexual transmission is another type of direct horizontal transmission,

where parasites are passed from one host to the other during the active mating. You can also have parasites that exploit food webs. So they transmit what we call trophically from prey to predators. So when predators consume prey, parasites are passed from prey to then predators. Wendy Gibson, can we just continue that in a way? Some of the other tiny. They've been there for a long time, very, very long time. People think, oh, they've been there for a thousand years.

Then it turns out they've been there for 10,000 years. Then it turns out they've been there for millions of years. Can you give us an idea of how people develop the notion that parasites have been there for a very long time? Okay, so we can, for example, with human parasites, you can look at our closest living ancestors, so chimps, gorillas and so on, and study the parasites that they've got and see how they're related to the ones that we've got today. So, for example, if you take the malaria parasite,

people have studied the malaria parasites from chimps and gorillas and compared them to what we have, and they've actually found that one of our main malaria parasites, Plasmodium falcipram, is actually is most closely related to what we find in the gorilla rather than what you'd expect in the chimpanzee. It's the power of the malaria.

To use that as an example, in one cell malaria can, as it were, have a parasitic effect, infect the whole body, in which there are 10 to 14 zeros, a number of cells, millions and millions and millions of cells. So this is a powerful agent, isn't it? Yes, I mean, the thing with the malaria parasite is that once it infects you, it starts to multiply.

And first of all, it gets into your liver cells, multiplies in those, bursts out of those cells, and then starts to go in this blood cycle where it's in the red blood corpuscles. And then you've got this cycle, and it's the classic fevers and chills that people have experienced for, you know, years with malaria. So that was observed by doctors and the patients themselves that you got this classic fevers and chills. And that's basically the malaria parasite, infecting the red cells, multiplying up, and then bursting out. And that's synchronous such that you suddenly get this burst of parasites.

And so, of course, because those parasites can just multiply within us like that, we can get huge numbers of parasites in our bloodstream. basically. What are you used can parasites make of other animals on their way to us? Well, quite often parasites have more than one host. So a lot of the parasites that we have as humans are we're actually getting from other animals that are carrying those parasites. So those animals are also, sorry, they are also got parasites in them, if you see what I mean. So, for example, let's go back to malaria. So the mosquito is carrying the malaria parasite

and by its blood-sucking habit, and remember, it's only the female mosquito that sucks blood. So the female mosquito passes, picks up the parasite from an infected human from their bloodstream and then passes it on to another human when it next takes a blood meal. when at school we're told, and the word parasite means something degenerate, impoverished in a way, in its ability. It's not high up the chain, as it were, but you don't find that, do you?

No, indeed, I was taught this, you know, looking back in my biology books from school, and we had the example of a tapeworm, and it was shown as a degenerate creature because basically it's just lost. pigment because it lives inside the body it doesn't need pigment and it's got according to what I learned no sense organs so it's got no eyes no nothing and so I was fed this story that it was degenerate but in fact of course like all organisms is actually well adapted to where it lives and what it needs to do so and if you look at the complicated life cycles of some parasites in their sort of journey from one host to another,

then they're extremely... Hosts being people, yeah. Well, hosts being people and mosquitoes are also hosts and in the predator prey cycles we were talking about with tapeworms, those prey animals are also hosts. So you're finding

the parasites in all those different hosts. In different forms. And it's basically one organism doing that. adapting to different hosts. I mean, if you take the example, again, come back to the malaria parasite, it's in a human at 37 degrees in our bloodstreams, a very stable environment.

The next minute, mosquito comes and takes a blood meal, and it's then gone into the stomach of a mosquito at whatever is environmental temperature and a totally inhospitable environment, because in the mosquito stomach, well, that's there to digest the blood. So the parasite has to adapt in seconds, really, from being in the bloodstream of the human to going into the gut of an insect and survive. It sounds as if there's this massive attack going on all the time, Steve.

And what inside us do we have as our defence? Well, both inside and outside. I mean, the skin itself is remarkably impermeable. You know, the parasites tend to get in through cuts and that kind of stuff rather than actually burrowing through the skin. So that itself is an effective barrier. Things like tears have got stuff called lysosyme in them, which actually was a kind of predecessor, predecessor of penicillin

because it kills bacteria of things like, I think most people... So, you know, I've got to get that straight. But you can't think I've got some parasites, so I'm going to cry. You don't have to think. I mean, you know, I at the moment, I think most of us at the moment, have got a mild cold. And that gives you a blocked up stuff he knows. And the stuff, sordid though it is,

in your blocks up stuff he knows, has some fairly strong protective virtue against the parasite going any further. But there are some other, there's a thing that's just come up very recently, which I find quite amazing. It's, you know, a question has long been, why does the zebra have its stripes? and zebras, you've got stripes, why they're there, they're pretty,

what's the point of them? It turns out that biting insects like sexy flies, for some reason, unknown, as yet unknown to science, don't like stripes. So if you put on stripy trousers in the tropics, you're going to be bitten less than if you put on white trousers or black trousers. So that's a useful hint for our listeners, no doubt. We don't know why that is, but it seems to be quite a strong effect.

So you can see our kind of hierarchy. And we've mentioned briefly the cuckoo, And what the cuckoo does is lay eggs in its host's nests. And one of the defenses for the hosts is to go and blow away into somewhere that's so tightly closed in that the cuckus can't get in. So all these are blindingly obvious, but they do a lot of the job. But of course, once the parasite is in, then you've got a bigger problem. But we do have an immune system, which summons up, it recognizes outsiders

and then attacks them. And generally does the job very well. So we're all full of parasites, but generally they're either subdued or doing so little harm that we don't notice. But is the parasite attacking them immune system constantly? There's constantly an interaction between the two. And there are sometimes problems, subtle problems, which is that the parasite evolves to mimic, as we say, rather as the kuku does, to mimic your own cells so that your own cells don't. recognize the parasite as being foreign,

this kind of molecular mimicry, as it's called, so that they circumvent the parasites your line of defense. But then there are usually further lines of defense going on. You see that most beautifully in plants more than anything else, where plants have various fungal diseases and things like the potato blight, which isn't a fungus, but it's kind of like a fungus. They attack the potatoes, and they kill off lots of them,

but some of the potatoes have got genes which resist that particular variant because they can't get in, and you get this evolutionary race, and that goes on constantly within our own bodies between our immune systems and our parasites. Kayla, Kana King, what range of impacts can a parasite have

without killing its host? Let's stick to humans, its host, its persons. Okay. Well, there are some very fascinating examples of the impacts that parasites can have on their hosts, certainly without killing them. one that comes to mind, one effect, the parasite doesn't kill the host in terms of host mortality, but from an evolutionary perspective, does.

So you can get parasites that effectively castrate their host. Certainly we don't see this at humans. You bet you're humans. No, no, no. This sterilization tends to be seen in host parasite interactions involving worm parasites and an invertebrate hosts. So we see a lot in snails where the reproductive organs

make up a large mass or a large proportion of the body mass of the host. And so the parasites get in, they go and infect the reproductive organs, extract resources particularly from that region, and they effectively castrate the host. Now this is a good...

Barnacles and crabs, a rather spectacular example, aren't they? I'm not quite sure Are they I interrupted No please go No no no So but why would parasites do this

Why would parasites sterilize their Their host Because the host can't Then go on to reproduce To make more host So this tends to be One hypothesis Is that this is a strategy

That some parasites can use When they can afford to sort of sit around for a while And where host's longevity Is really at a benefit to the parasite And if If I may, I'd love to talk about another example that parasites can have on their host, which is really fascinating. It's one of the most fascinating examples of the effects of parasites. Parasites can actually act as sort of puppet masters to their hosts.

They can manipulate their host behavior to be able to facilitate transmission from one host to another. So I mentioned previously these trophically transmitted parasites, parasites that go from prey to predator. So there are some instances, an increasing number being observed, in fact, where parasites, when they infect their prey, can manipulate the behavior of the prey to increase the chance the predator might see them. So one really cool example is a worm parasite called dichrocyllium. I think it's in the UK, but I know for sure it's in Canada, where I'm from originally. It's one of the classic examples of behavior manipulation. So this worm parasite requires snails, ants, and sheep as hosts.

And so you might think, okay, well, how do they get from ants to sheep? Because ants aren't the normal food, right? So what this parasite does is when it's infecting the ant, it's localized in the head region of the ant and causes the ants to bite down hard on the very tips of grass blades. So you can imagine this scene where you have a field and you see a bunch of ants on the tips of grass. It's a bit strange.

But the thought is that this increases the opportunity for sheep to pick them up while they're foraging in the field. Can we develop that, Wendy Gibson? I mean, the idea of parasites moving from one host species to another. Yes, I just want to give another example of what Kayla's talking about because there's another fantastic one that listeners might like to look up on the web If you search zombie snails on YouTube, you will come across this other parasite, another fantastic name, Leuco Claridium, which gets into, again, its main host is a bird,

but it's got an intermediate host, which is a snail, and it gets into the snail, and its larval stage in the snail goes into the ice stalks of the snail, and basically is brightly coloured and pulsates. And also changes the behaviour of the snail, a little bit like Kayla was talking about with the ant, whereby the snail, instead of going to hide in the day, actually stays in full view on leaves during the day. And so it's open to predation by birds,

partly because it's exposed during the day and partly because of these pulsating ice stalks. and that's the parasite manipulating that host in order to get into its next host. And the bird host is important because the bird host is the one in which that parasite is going to do sexual reproduction and actually then reproduce. I like to Steve to take up snails back. This word host gets in the way of everything, doesn't it? Host is such a friendly, welcoming word. and these things are in an attempt to kill you.

Right, Steve, snails. Well, actually, I wasn't, we're a kind of an evolutionary arms race between the three of us here to look at more and more bizarre cases. But the most bizarre behavioral one, which is of interest to humans, is a parasite which makes an interaction between cats and mice. It's a thing called toxoplasma. Now, mice strangely don't like cats, and if they can smell cats from a long way away,

and if they smell cat urine, they'll run away. but if they're infected with this thing toxoplasma, suddenly the cat's urine smell becomes absolutely endearing and marvellous and they run to the cat and the cat then eats them. So you've got a complete shift in behaviour. But the thing which is really quite remarkable is that at least a third of the human population is infected with this toxoplasma, gets into their brains.

What happens that? Do we run to cats? Well, no, it's hard to believe this, but it seems to be true. It's one of these stories that's always been around. and nobody's ever really believed. But there's a strong tie between various psychiatric conditions, things like schizophrenia, depression and so on. People with those conditions are much more likely to have toxoplasma in their brain. And there are claims, and the figures are fairly impressive,

that if you look at people who have been killed by dangerous driving, they're twice as likely to have been infected by toxoplasma, as is the general population. So strangely and bizarrely, although the parasite-carriage, can't go further in humans, it's changing our behavior and the way it's changing the mouse behavior. Top that. I'm not going to even begin to attempt to, Steve. Kayla, what extent does the existence of these parasites as encourage sexual reproduction by their host?

Yeah, very interesting question. So this is a question that I've been tackling for a lot of my career. Excuse me. So this hypothesis that Steve mentioned, which is the Red Queen, Alice in Wonderland, this idea that hosts and parasites can co-evolve, evolved together and reciprocally because of their interactions, can favor the existence of sexual reproduction. So firstly, what do I mean by co-evolution specifically? So we've been talking about all the negative things that parasites can do to their hosts.

So this encourages then the evolution of host to be able to resist those parasites. But then this then feeds back and encourages the evolution of parasites to then be able to infect host. So this kind of co-evolutionary battle that goes on and on and on and continuously can make sexually reproducing organisms advantageous. And why is that? So sexual reproduction results in the generation of genetically diverse offspring. offspring that possess novel gene combinations that parasite simply can't specialize on,

can't adapt to infect. So simply put, if you're an asexual mother, then your offspring are genetically identical to you. So if a parasite can infect you, it certainly has a good chance of infecting your offspring. If you're a sexual mother, then your offspring are not genetically identical to you. So if a parasite can infect you,

it can't necessarily infect your babies. So if you have a population of asexual individual, that are genetically very similar, then parasites have the potential to evolve and rapidly spread and wipe out that population. But what sex does is it creates organisms which are a bit of a moving target for parasites over evolutionary time in contrast to that. Additionally, there's a hypothesis called Hamilton-Zook hypothesis, where parasites, where females should be able to select males. which appear to be resistant to parasites, which signals that they have good genes. So it's thought then that infection in the environment,

paired with females being able to choose males, can lead to the evolution of really elaborate ornamentation. So males can sort of show off the peacock tail, for instance. I'm really fit. I'm resistant to infection. Steve, I mean, there are some unfortunate parallels between sexual reproduction itself and the interations between parasite and host. And of course, in the sexual reproduction,

it's the males who are the parasites. They're forcing their female hosts, as it were, concubines, to copy their male genes. You have a strangely, strangely restricted view of a person. Forcing concubines, right, okay, Steve. But you can see the race between males and females where there's constant pressure on the male to show, oh, I'm really great, I've got an enormous horn,

I've got a huge tail. I can resist parasites. And the females who get more and more choosy and can resist the blandishments of all but the best males. So it's a rather unfortunate parallel, but I think there's something in it. So, you know, we owe our sex lives to our parasites. Right. Wendy Gibson,

what can parasites tell us, what can I tell archaeologists about the past? Yeah, so this is a very fascinating field of study. So a lot of parasites, for example, some of the worms that infect our guts, lay their eggs and they come out in our feces and they go into the environment. And because those eggs then have to last survive very well in the environment in order for the next host to pick them up, they're very resistant in the environment.

So they've got like hard shells around them and so on. So that means that quite often they're very well preserved in archaeological remains. So parasitologists, archaeoparasatologists, can look at those remains, just the same way that they look for pollen grains and identify the different plants that were there. They're looking at the worm eggs that are there and they can identify those two species. So, for example, in the recent discovery of Richard III in the Leicester car park, they were able to take soil samples from, how will I put that politely, his lumber region where his guts had basically been. So they took the soil from there and then looked through it for parasite eggs. so they're not viable anymore of course

but they were identifiable by microscopy so we can say that Richard VIII definitely had roundworms he was infected with those but then probably at that time in England most people actually had were infected with worms like those and round these round worms are actually about I have to go back to old style, 10 inches long or so. So quite big worms.

You can go back much further in time and people have looked in, for example, Egyptian mummies and again found the parasite eggs in those. So again they can build up a picture. If that person was infected with that parasite, well, what kind of life must they have led? what were they eating in order to have acquired that parasite? The same thing with mummies on the other side in Chile and so on,

where those have been preserved. So you're going back then thousands of years, maybe 9,000 years or so, and you can actually identify what parasites those people have. Another example that appealed to me is looking at dwellings, ancient dwellings and you can find the places where people did particular things like picking off their parasites because of course it's only in the modern day where we've had you know insecticides and drugs etc to rid ourselves of these parasites in the old days people just had lice fleas all those

ectoparasites feeding on their blood so so there's been nice studies of Inuit dwellings where you can see that they've actually, there's the accumulation of lice that they've presumably picked off one another, delousing, and put them, thrown them into the entrance tunnel of their dwelling. I mean, we've already thanked parasites for the evolution of sex, but we can do the same for the evolution of society because one of the claims that anthropologists make,

anthropologists make lots of claims, I have to tell you, is that actually if you look at primary, groups, chimpanzees and the like, the way that they hold themselves together is by picking off each other's parasites. They're grooming, mutual grooming, and they groom in groups. And if you're a member of the group, you will be groomed and you will groom in return. And some suggest that this is the beginning of social groups in primates. So, you know, not just sex, but also society. Carely, I'm sorry briefly if you don't mind. Do we know why some parasites are more harmful

than others? Right. Well, this is a question that's puzzled biologists since the time of Darwin, really, and certainly probably before that. So a few decades ago, the conventional wisdom was that for a parasite to be virulent, it was sort of an ancestral state of the parasite. Why would you bite the hand that feeds you? But a more modern approach is to think about something called a virulence transmission trade-off. So one of the costs of being really, really harmful to your host is you might kill them before you get the chance to actually transmit to the next host. So this tends to be the case for most parasites where they have a trade-off. But when there is a lack of a trade-off, this is where you can get, in theory, really, really scary infections. So there's something called the curse of the pharaoh hypothesis.

And the idea here is that the longevity of a parasite in the environment, the longer it can survive in the environment without a host, the greater the chance that it'll be really virulent to its host, because it can afford to burn through host resources and wipe out the host much faster, and then hang about in the environment for a while and sit and wait for the next host. For a while, it could be thousands of years. Exactly. So the hypothesis itself is named after the case,

the mysterious death of Lord Carnarvon that was the first to enter the tomb of King Tut. So one of the thoughts was that he acquired an infection whilst in the tomb that had been sitting around and waiting for a host. I see. So, Steve, Is there a very good examples of successful combating of these parasites? Successful combating the parasites? A few.

And it's notable that actually one of the ones which we have won is smallpox, which is actually one of these parasites, which can live out in the outside world. And that's one reason why it was so devastating. And that, quite amazingly really, has been driven out. There are samples in Moscow and samples in the state. but that's all there is. We've done very well with polio,

and again, that's water, and even better with many parasitic worms. There's one called Draconculus, the little dragon, the guinea worm, which is a particularly unpleasant one, which moves around under the skin. And we've basically down from about 300,000, 10 years ago, to, what was it last year?

20 or 30? 25. Do you want to take this up, Wendy? Yes, so this example, Guinea worm is a really good one of how just simple measures, and so no high tech, but a good knowledge of the parasite life cycle has actually got rid of it, almost. We're just wanting to declare that it has been eliminated,

but maybe this year, maybe next year. So it's the Carter Centre, so ex-president Jimmy Carter from the States, has this program on guinea worm eradication. And what they're doing is very simple. How the parasite gets into you is by that you basically drink some water contaminated with the larval stage of the parasite. So what's one of the measures is to just use filters on the water. Obviously these are people, the guinea worm is occurring in countries where you don't have water coming out of taps, so you're having to get your drinking water from stagnant pools and so on,

which are then contaminated with the larval parasites. So when people drink that water, they get infected. But if you can filter out that larval stage, then you eliminate transmission to the humans. And that has brought, as Steve said, it's brought the prevalence of that parasite down from millions in the 1980s down to just 25 cases last year and those were in just three countries in Africa that sounds very hopeful see but there's still it's a very very small indentation to the mass of the great parasitic population isn't it that's true but certainly when it comes to the big parasites

I think there's hope in many ways I mean the snail parasites that cause things like elephantiasis what's the problem there is again dirty water and dirty water is very often the problem. We all know, of course, about cholera and John Snow and the pump. It's a restatement of the truth about medicine is that all health is public

health. What matters is what you do societally rather than what you do with... I mean, good sewage systems and washing your homes. I mean, the irony is that when flush toilets first came in to Britain, what do they do to cholera? They made it worse because what happened was they just pumped it straight into the River Thames

which meant that there was an immediate guarantee that the cholera and bacillus was going to find a host immediately, and the virulence, the nastiness of the disease got worse. But once you begin to control it, so the cholera has to keep its hosts alive, then the virulence goes down. So I think that's the way we need to look ahead, which isn't to say that things like antibiotics and so on haven't been successful because they have. But common sense is the thing that really wins. Kayla, is there any way, can you give us some, is there any way that parasites can play a positive row? Because from some of your notes, you say get rid of all parasites and that's a good thing. But it has been mentioned two or three times in this conversation that parasites can be helpful.

Where are we on that? Yes, absolutely. So I would imagine that if we were to eliminate all parasites, although as an evolutionary biologist, I would sort of question whether that's possible, given that we know parasites can evolve quite rapidly in response to barriers to their establishment. antibiotic resistant being one example. If we were to remove all parasites, quite simply, in nature, nature would be less diverse.

We would see less diversity at the genetic level, looking at traits and behaviors. We've been talking about all the different ways that parasites can shape host behaviors and traits and appearance and also genes, but also at the ecosystem level. So it's been shown recently in the tropics that if there's one particular host species, which is quite dominant in the community,

that parasites can target that species, knock it back, and that would let otherwise inferior competitors to flourish, that is until they themselves become common, and parasites can go after them. So simply put, the environment, the nature would have less diversity. It would be way more homogeneous.

So is there a sense, Wendy, in eliminating parasites, we're going to do ourselves long-term damage rather than long-term, bring long-term benefit? Well, clearly if you talk to people in tropical countries who are suffering from malaria and these other unpleasant diseases, guinea worm, elephantias, etc., I think they would want those removed. But then there's this opposite idea that we in the developed countries have now become too clean. So there's this hypothesis called the hygiene hypothesis whereby the fact that we've eliminated, for example,

we would have had, like Richard III, we would have had worm burdens in our guts. That would have been the norm rather than being very unusual. And so there's an idea that our immune system co-evolved with the parasites to deal with them. And now, because those parasites are not there, we have this allergy epidemic. And they're actually clinical trials and so on

where people have taken worm, eggs and to infect themselves with those worms to see if it cures their inflammatory bowel disease, things like that. Now, I'm a little bit dubious on this because it seems that you're curing one thing and giving yourself another real problem. Well, thank you very much. That's Wendy Gibson, Kayla King and Steve Jones. Next week we'll be talking about the political philosopher Hannah Arendt and her idea of the banality of evil. Thanks for listening. And the In Our Time podcast

gets some extra time now with a few minutes of bonus material from Melvin and his guests. One thing we haven't talked about which is really remarkable is where do parasites come from? Where do our human diseases come from? Almost without exception they come from animals and from domestic animals. And you can see that again and again

I mean smallpox came from camels bizarrely. Everyone is an animal disease. So the thing which is most remarkable about that is a lot of diseases like leprosy, say, if you look, or plague, if you look at the genetics of the parasite, all these billions of parasites across the world are genetically almost identical, which means that they may once, 10,000 years ago, have infected one human being, just one time, and it's spread across the world

from that. I wish we'd manage to get that out of the program. That's the problem with doing this podcast. I spend the time shaking my head and thinking why in the programme? Yeah, so I was going to add to that the, because you said we're going to have parasites from domestic animals because there's the nice story about where we got our tapeworms, the beef tapeworm and pork tapeworm, that today we get by eating undercooked raw beef or pork.

And in fact, that's been linked to not our domestication of livestock 10,000 years ago, but actually to when we took up hunting, and meat eating big time on the African plains, so in our ancestors, because the tapeworms that we have are most closely related to those that you find today in lions and hyenas and so on, so the classic big carnivores of the African plains. So again, that's been of real interest to anthropologists because they can then see that those parasites are actually telling us something about our evolution. This is a bit of a different topic,

but when we were talking about the lines of defense that hosts can have against parasites, there's a new wave of research showing that the microbes residing within us and on us as part of our microbiome, the microbial community that we harbor, can actually provide a barrier of defense against parasitic infection. So if your listeners haven't heard of fecal transplants, perhaps they can look that up.

So that's a new treatment being piloted and I think actually used to treat infection with Clostridium difficile, and it's quite effective. And that's where the microbial community from the feces of a healthy person is transplanted into the gut of somebody suffering from C.Difacill infection. And essentially the microbes, the microbial community, play a really important role in clearing that infection and preventing the establishment of others.

If I can add an appendix to that story, there's a claim, you know, the appendix is a relic dog, take it out, it doesn't do any good. The claim is that the appendix is the last refuge of your microbial community, so that if you get a nasty, invasive thing like Lestridium, which can kill you, that may invade and knock most of your internal, helpful microbium out, microbial community out. But it stays there in the appendix.

So if you have your appendix out, you've lost what could have been your last line of defence. That's a keep, is it, in the fortress? It is, exactly right, yeah. You keep your appendix. Anything else? and the producers pouring the ground outside the door to come in and offer you. I'd just ask if you want tea or coffee. I'd have a quick tea.

Yes, tea would be lovely. There are many more science and discussion programmes from Radio 4 to download for free. Find these on the website at BBC.co.com.com.com. slash radio 4.