0:00:13 > 0:00:15The tapeworm.
0:00:15 > 0:00:19It's one of nature's most fascinating parasites.
0:00:21 > 0:00:24There are thousands of different kinds of tapeworm,
0:00:24 > 0:00:28each evolved to live in a different host species.
0:00:30 > 0:00:33And there are several species of tapeworm that live in humans,
0:00:33 > 0:00:36including one called Taenia saginata.
0:00:38 > 0:00:41Like many parasites, it has a complicated life cycle,
0:00:41 > 0:00:45which involves passing through more than one species.
0:00:47 > 0:00:48So imagine you're in rural Kenya
0:00:48 > 0:00:51and you've got a great big tapeworm inside you.
0:00:51 > 0:00:54This tapeworm is producing eggs.
0:00:54 > 0:00:58One day, you go down to a field and you excrete there,
0:00:58 > 0:01:00and you contaminate the grass.
0:01:00 > 0:01:05Then along comes a poor unfortunate cow who eats that grass.
0:01:08 > 0:01:11Along with grass, the cow eats the tapeworm eggs,
0:01:11 > 0:01:15which hatch into larvae and then enter the cow's bloodstream...
0:01:17 > 0:01:19..spreads through the blood to the muscle,
0:01:19 > 0:01:21perhaps the shoulders or the tongue,
0:01:21 > 0:01:24and there it forms a really tough cyst.
0:01:24 > 0:01:28And it hangs around, just waiting for the next stage,
0:01:28 > 0:01:31which is for the cow to be eaten by a human.
0:01:36 > 0:01:40The beef tapeworm can only grow to adulthood inside a human,
0:01:40 > 0:01:42and for that reason, it's hard to study.
0:01:44 > 0:01:47So I've decided to infest myself,
0:01:47 > 0:01:51as my contribution to research into these shy, retiring creatures.
0:01:53 > 0:01:56So I need to find infected beef.
0:01:59 > 0:02:01But in Britain, it's extremely unlikely
0:02:01 > 0:02:03you'll find a contaminated cow,
0:02:03 > 0:02:05which is why I've come to Kenya.
0:02:12 > 0:02:14With the help of a local meat inspector,
0:02:14 > 0:02:16we found some infected meat.
0:02:19 > 0:02:22Then we cut three cysts out of the meat,
0:02:22 > 0:02:24ready for me to swallow.
0:02:26 > 0:02:28Cheers.
0:02:30 > 0:02:32It's like jumping off a cliff. Here we go.
0:02:35 > 0:02:36Ah!
0:02:38 > 0:02:40There they go.
0:02:42 > 0:02:46In just a few seconds, the tapeworm cysts are in my stomach.
0:02:46 > 0:02:51That's full of acids that normally help protect us from disease.
0:02:54 > 0:02:58But the tapeworm uses our defences to its advantage.
0:02:58 > 0:03:02The acids dissolve the outer case of the cyst,
0:03:02 > 0:03:03releasing the worm inside.
0:03:10 > 0:03:15If they survive, each cyst will release a tapeworm scolex,
0:03:15 > 0:03:19equipped with four suckers, which it will use to latch onto my gut.
0:03:22 > 0:03:27Then it starts to grow, new segments emerging from the scolex.
0:03:31 > 0:03:36As it grows, the segments will get bigger and more mature,
0:03:36 > 0:03:39whilst new segments appear near the head.
0:03:41 > 0:03:43To see what a grown worm looks like,
0:03:43 > 0:03:48I met up with tapeworm expert, Prof Phil Craig.
0:03:50 > 0:03:55That is the adult tapeworm, and this tapeworm is exactly ten weeks old.
0:03:55 > 0:03:58We know that because it was from a previous volunteer.
0:04:01 > 0:04:05By ten weeks, the mature segments are full of eggs...
0:04:06 > 0:04:10..and they break off from the tapeworm and emerge from your body...
0:04:12 > 0:04:14..ready to infect a cow.
0:04:17 > 0:04:22So it comes out of your bottom, and it can actually crawl out?
0:04:22 > 0:04:24Unfortunately, yes.
0:04:24 > 0:04:27Psychologically, that upsets quite a few people that are infected.
0:04:27 > 0:04:29I can imagine.
0:04:29 > 0:04:31Yeah, it can come out under its own steam, as it were,
0:04:31 > 0:04:34so once the segment is in the rectum,
0:04:34 > 0:04:35it'll move around and that causes
0:04:35 > 0:04:39a sort of a strange fluttering sensation,
0:04:39 > 0:04:43and then it will move through the rectum, through the anus,
0:04:43 > 0:04:45and crawl around between the buttocks,
0:04:45 > 0:04:49down the legs and out onto the floor.
0:04:49 > 0:04:50- Right.- Can be embarrassing.
0:04:50 > 0:04:52It might surprise my wife or friends.
0:04:52 > 0:04:54I think it probably would. Yes.
0:04:56 > 0:05:00After six weeks, I decided to take a look at my tapeworm,
0:05:00 > 0:05:03using a technique known as capsule endoscopy.
0:05:05 > 0:05:06Down the hatch.
0:05:06 > 0:05:08It involves a tiny camera which you swallow.
0:05:08 > 0:05:11Ah!
0:05:11 > 0:05:12Live from my stomach.
0:05:13 > 0:05:17Several hours later, the capsule had passed into my intestines,
0:05:17 > 0:05:21and using a tablet, I could see live pictures from my gut.
0:05:21 > 0:05:25Aah, that is it.
0:05:25 > 0:05:27That is the tapeworm.
0:05:27 > 0:05:31I can just see its tail and its segments waving around.
0:05:31 > 0:05:34I don't know if I'm absolutely disgusted or...
0:05:34 > 0:05:36I'm actually quite excited. Wow.
0:05:37 > 0:05:39Because it would have been hugely disappointing
0:05:39 > 0:05:41to have gone all the way to Kenya,
0:05:41 > 0:05:42come back and seen nothing.
0:05:42 > 0:05:45But what's amazing is, I've not experienced anything
0:05:45 > 0:05:47and yet you can see it there.
0:05:47 > 0:05:51That is so weird.
0:05:51 > 0:05:54This footage provides scientists with a rare chance
0:05:54 > 0:05:58to see a live worm in its natural habitat.
0:05:58 > 0:06:02So we sent the complete video to a team at Salford University.
0:06:02 > 0:06:05Just at the back there, you can just see the worm,
0:06:05 > 0:06:07the thinnest part of the worm.
0:06:07 > 0:06:08This is sort of the front end.
0:06:08 > 0:06:10The segments are getting more mature
0:06:10 > 0:06:12as you're moving down the intestine here,
0:06:12 > 0:06:15they're getting bigger, becoming sexually mature.
0:06:15 > 0:06:20Probably, they're starting to get fertilisation taking place,
0:06:20 > 0:06:23so the worm is actually starting to reproduce at this stage.
0:06:23 > 0:06:27Can you tell at this point how big it is? I have no idea of the scale.
0:06:27 > 0:06:32- By the looks of this, it's certainly over a metre in length.- So a metre?
0:06:37 > 0:06:41That's another worm, yeah?
0:06:41 > 0:06:45- You have more than one worm, definitely.- Right.
0:06:46 > 0:06:47I'll stop there. OK.
0:06:47 > 0:06:51- And guess what? You've got three worms.- Oh, blimey.
0:06:51 > 0:06:54This is the very front end, the head end.
0:06:54 > 0:06:56This is the bit that attaches onto the intestine.
0:06:56 > 0:07:00There are actually four muscular suckers around that sort of head,
0:07:00 > 0:07:04and those are what's latching onto the wall of your intestine,
0:07:04 > 0:07:06- keeping the worm in place. - Right. Wow.
0:07:06 > 0:07:11100% hit rate. I have to say, a very successful experiment.
0:07:14 > 0:07:16Humans are home to many parasites.
0:07:16 > 0:07:20The ones that live on our skin are called ectoparasites,
0:07:20 > 0:07:25and this is one of the most familiar ectoparasites -
0:07:25 > 0:07:27the head louse.
0:07:30 > 0:07:33To find out more about this very common parasite,
0:07:33 > 0:07:37I've come to a delousing salon in north London.
0:07:41 > 0:07:44What I'd love you to do, then, is infect me.
0:07:45 > 0:07:48Head lice can only survive by drinking human blood,
0:07:48 > 0:07:51which they do several times a day.
0:07:52 > 0:07:56To keep these lice alive, I need to feed them...on me.
0:07:59 > 0:08:01These little pots should keep them safe,
0:08:01 > 0:08:03while allowing them to bite me.
0:08:04 > 0:08:07- Thank you very much for infecting me.- You're welcome.
0:08:09 > 0:08:11There aren't many people who would be pleased to hear
0:08:11 > 0:08:13their visitor is infested with lice.
0:08:13 > 0:08:17- Hi, James.- Hello. - I come bearing gifts.
0:08:17 > 0:08:18Oh, excellent.
0:08:18 > 0:08:21But James Logan is clearly delighted.
0:08:21 > 0:08:23Let's have a look. Oh, that's brilliant.
0:08:23 > 0:08:25James studies lice and other parasites
0:08:25 > 0:08:28at the London School of Hygiene and Tropical Medicine.
0:08:28 > 0:08:31- OK, so here we have one. - Hm-hm.
0:08:31 > 0:08:33Shall we put it on your arm?
0:08:33 > 0:08:34Why not?
0:08:34 > 0:08:37See if he scurries around.
0:08:37 > 0:08:39Using a hand-held microscope,
0:08:39 > 0:08:41we can study my head louse as it settles in.
0:08:43 > 0:08:46You can really see the blood here, can't you?
0:08:46 > 0:08:48So this one's quite recently fed on your blood,
0:08:48 > 0:08:52but you can see it being sort of pumped down here through its gut.
0:08:53 > 0:08:57Like all lice, the head louse has retractable mouth parts
0:08:57 > 0:08:59that can puncture my skin.
0:09:01 > 0:09:04But what really surprised me is how active they can be.
0:09:05 > 0:09:08- It's going so fast, I can't keep up. - This one's quite active, isn't it?
0:09:08 > 0:09:10It certainly is.
0:09:10 > 0:09:13Look at that, it's gone right to the last hair on your arm.
0:09:13 > 0:09:17Look at it clinging onto the edge of that hair with two of its claws.
0:09:17 > 0:09:18Look at those claws.
0:09:18 > 0:09:20It's got another claw out, and I wonder whether it's sat there
0:09:20 > 0:09:22waiting for another host to come past.
0:09:22 > 0:09:25So if I were to rub my head against yours,
0:09:25 > 0:09:29it would just grab a hold of your hair with the other claws
0:09:29 > 0:09:33- and it'd be across in an instant, would it?- Exactly right, yeah.
0:09:33 > 0:09:35Although they feed on your skin,
0:09:35 > 0:09:39head lice always lay their eggs on hair,
0:09:39 > 0:09:43which is why they can only complete their life cycle on your head.
0:09:44 > 0:09:48But there is another species that lives on humans,
0:09:48 > 0:09:52and that can tell us something fascinating about our distant past.
0:09:53 > 0:09:55There's another type of louse called the body louse,
0:09:55 > 0:09:59and I've got an example here to show you what an infestation looks like.
0:09:59 > 0:10:03And this, remarkably, is highly adapted to clothing.
0:10:03 > 0:10:07This was actually... I believe, this was from a homeless person...
0:10:07 > 0:10:09- Yeah.- ...who had a very heavy infestation.
0:10:09 > 0:10:12And I think, it looks like trousers to me, with a seam here,
0:10:12 > 0:10:15and this is exactly where they lay their eggs.
0:10:15 > 0:10:17- Aah. That is revolting. - You can see...
0:10:17 > 0:10:21Yeah, and those are eggs, so that is a massive infestation.
0:10:21 > 0:10:23- It makes you feel quite itchy, doesn't it?- It does.
0:10:25 > 0:10:28The body louse behaves very differently to the head louse.
0:10:28 > 0:10:31But if you compare their bodies' shapes,
0:10:31 > 0:10:34you see something significant.
0:10:34 > 0:10:36OK, so the one on the right, the dark one,
0:10:36 > 0:10:37- is my friend, the head louse?- Yep.
0:10:37 > 0:10:40He's looking dead at the moment, I have to say, not looking good.
0:10:40 > 0:10:42And the one on the left, they're body lice?
0:10:42 > 0:10:44That's exactly right.
0:10:44 > 0:10:46And what you can see is, they look remarkably similar.
0:10:46 > 0:10:47When you look at their DNA,
0:10:47 > 0:10:50what you find is that they diverged around 100 to 200,000 years ago,
0:10:50 > 0:10:53so not actually that long ago in sort of evolutionary terms,
0:10:53 > 0:10:56but enough to make them different species.
0:10:56 > 0:10:58And so that tells us something about when we, as humans,
0:10:58 > 0:11:00started to wear clothing.
0:11:00 > 0:11:02So before that, the idea is, humans were naked,
0:11:02 > 0:11:06then they started to wear clothes, maybe a head louse drops down,
0:11:06 > 0:11:08- thinks it's actually quite a nice place to live...- Yep.
0:11:08 > 0:11:11..and begins to evolve, a whole new species starts to evolve on clothes.
0:11:11 > 0:11:13Exactly right, yep.
0:11:13 > 0:11:15So it's quite incredible to think you can look at the louse
0:11:15 > 0:11:19to find out more about our own ancestry and our own evolution.
0:11:22 > 0:11:27Some parasites cause relatively little harm to their host.
0:11:27 > 0:11:29But this one is deadly.
0:11:30 > 0:11:34It's a microscopic single-celled parasite...
0:11:35 > 0:11:37..called plasmodium.
0:11:39 > 0:11:41It causes malaria,
0:11:41 > 0:11:45a disease that kills more than half a million people a year.
0:11:49 > 0:11:53I want to find out why it is so deadly,
0:11:53 > 0:11:56so I've come to the National Institute of Medical Research,
0:11:56 > 0:11:59where they will infect my blood with plasmodium.
0:12:03 > 0:12:06So, I've got a sample of my blood here, looking nice and red. Healthy?
0:12:06 > 0:12:09Yes, yes, and warm, straight out of your body.
0:12:10 > 0:12:12- Hi there.- Hi. - I won't shake hands.
0:12:13 > 0:12:18- Thanks very much, Fiona. - Right, I guess we're not allowed in that room.- That's right, yeah.
0:12:18 > 0:12:20What's Fiona up to at the moment, then?
0:12:20 > 0:12:22So, what she's going to do now is
0:12:22 > 0:12:25she's going to take a sample of Plasmodium falciparum.
0:12:25 > 0:12:28So this is the most dangerous form of the parasite
0:12:28 > 0:12:31that causes most deaths in, for example, Sub-Saharan Africa.
0:12:31 > 0:12:33We've grown this in the lab,
0:12:33 > 0:12:35we can culture it continuously in human red cells,
0:12:35 > 0:12:37and she's now going to take a sample of those parasites
0:12:37 > 0:12:39- and inoculate them into your blood.- Right.
0:12:39 > 0:12:41I must admit that of all the diseases,
0:12:41 > 0:12:44malaria is high on the ones I want to avoid.
0:12:44 > 0:12:46Well, you should try to avoid it if you can.
0:12:48 > 0:12:52To see the malaria parasite multiplying in my blood,
0:12:52 > 0:12:55they're using a new photographic time-lapse technique.
0:12:55 > 0:12:57Although I have treated people with malaria,
0:12:57 > 0:13:01I have never seen the malaria parasite in action before.
0:13:02 > 0:13:04Very intrigued to see what happens to my blood,
0:13:04 > 0:13:09but what is chilling is thinking that out there in the world,
0:13:09 > 0:13:11mosquitoes are infecting children.
0:13:11 > 0:13:131,000 children every day are dying.
0:13:13 > 0:13:16That's why this sort of work is so important.
0:13:18 > 0:13:22Here it is. My blood infected by the deadly malaria parasite.
0:13:23 > 0:13:28So the parasite goes through this life cycle in the infected red cells.
0:13:28 > 0:13:30Once the infection is complete,
0:13:30 > 0:13:33Mike Blackman shows me the finished movie,
0:13:33 > 0:13:36which reveals how the parasite kills its host.
0:13:36 > 0:13:39So the little white blobs here, they are the parasite, right?
0:13:39 > 0:13:41They are the parasite, yeah.
0:13:41 > 0:13:45So a single merozoite, this invasive form of the parasite,
0:13:45 > 0:13:49binds to a red cell, invades it, grows within it,
0:13:49 > 0:13:51digests the haemoglobin of the red cell,
0:13:51 > 0:13:55this is the red protein that is used to carry oxygen via red cells,
0:13:55 > 0:14:01and then eventually forms around about 16 to 32 daughter merozoites.
0:14:01 > 0:14:04Does it? Oh, wow, look at that one go.
0:14:04 > 0:14:06- That one just exploded, didn't it? - That's, right, exactly, yes.
0:14:06 > 0:14:10And suddenly you're seeing lots. Oh, wow, they're really going.
0:14:12 > 0:14:16The whole thing is destroyed in a single, very rapid process.
0:14:16 > 0:14:20The merozoites are released and they immediately invade a new cell,
0:14:20 > 0:14:23- and these things...- I had no idea it was going to be that violent.
0:14:23 > 0:14:25I mean, that was utterly destroyed.
0:14:25 > 0:14:30Each explosion obliterates one of my oxygen-carrying red blood cells,
0:14:30 > 0:14:35and releases new parasites into my blood to infect yet more cells.
0:14:37 > 0:14:39Suddenly, you've gone from a situation where
0:14:39 > 0:14:41there are relatively few, they're everywhere,
0:14:41 > 0:14:43and they're just swarming.
0:14:43 > 0:14:46I mean, you do appreciate just what a terrible thing they're doing.
0:14:46 > 0:14:51Yeah, if this were going on inside you, you'd be in a pretty poor way.
0:14:51 > 0:14:54You soon become anaemic from lack of blood cells,
0:14:54 > 0:14:57and the debris from all those shattered cells can
0:14:57 > 0:15:00block your blood vessels, which in turn can be fatal.
0:15:02 > 0:15:05So our interest here is exactly how this goes on,
0:15:05 > 0:15:08how does the parasite actually do that? We don't really know.
0:15:08 > 0:15:10The parasite is very, very good, very smart.
0:15:10 > 0:15:14It's evolved with its human host for a long time.
0:15:16 > 0:15:18This is Toxoplasma gondii.
0:15:20 > 0:15:24It's a microscopic parasite that looks like a wriggling comma,
0:15:24 > 0:15:29and it's possibly the most prolific parasite that lives on humans.
0:15:31 > 0:15:34Its life cycle always passes through a cat,
0:15:34 > 0:15:35where the parasite breeds.
0:15:38 > 0:15:42An infected cat passes oocysts,
0:15:42 > 0:15:46tiny capsules containing the parasites.
0:15:46 > 0:15:49The oocysts can survive on the ground for months,
0:15:49 > 0:15:53waiting to be eaten by a rat or mouse,
0:15:53 > 0:15:55which then becomes a carrier.
0:15:58 > 0:16:00But for the life cycle to complete,
0:16:00 > 0:16:03the parasite has to get back into a cat,
0:16:03 > 0:16:08which means the mouse has to be caught and eaten by a cat.
0:16:11 > 0:16:17So, toxoplasma has to encourage its mouse host to commit suicide.
0:16:20 > 0:16:21A few years ago,
0:16:21 > 0:16:24Joanne Webster set up a series of elegant experiments to see
0:16:24 > 0:16:30how toxoplasma might go about altering a rodent's behaviour,
0:16:30 > 0:16:32making it more likely to get eaten.
0:16:34 > 0:16:37She placed infected and uninfected rats in a chamber
0:16:37 > 0:16:41where she had liberally doused one corner with cat urine.
0:16:41 > 0:16:45And we simply plopped the rat in and let them tootle about over
0:16:45 > 0:16:49each four-hour, ten-hour night, and simply watch where they went.
0:16:49 > 0:16:51Alice here is an uninfected female.
0:16:51 > 0:16:54She smelled the cat area and she shot off here.
0:16:54 > 0:16:57She seems to be avoiding the cat-smelling one.
0:16:57 > 0:17:00She's hanging around in different areas.
0:17:00 > 0:17:03Putting in a rat infected with toxoplasma
0:17:03 > 0:17:05produced a very different result.
0:17:05 > 0:17:07This is Felix here.
0:17:07 > 0:17:09He's infected and he's actually in the cat zone.
0:17:09 > 0:17:12Which is not what you'd expect. A normal mouse or rat would run...
0:17:12 > 0:17:16- Yeah, smell it and absolutely hide. - And he seems to be just enjoying it.
0:17:16 > 0:17:19Yep, he's out, he's active, he's tootling about.
0:17:19 > 0:17:22So presumably, if you're the parasite, what you want is,
0:17:22 > 0:17:26you want the rat, Felix the Rat, to hang around
0:17:26 > 0:17:29- near where cats are because you want to be eaten.- Yes.
0:17:30 > 0:17:35Joanne had shown that infected rats are attracted to cat smells.
0:17:35 > 0:17:37They're also generally more fearless
0:17:37 > 0:17:40and have slower reaction times.
0:17:40 > 0:17:45All these factors made them far more likely to become cat victims.
0:17:45 > 0:17:50What is rather disturbing is that this parasite, toxoplasma,
0:17:50 > 0:17:53also infects us.
0:17:53 > 0:17:55Around a quarter of the British population are infected
0:17:55 > 0:17:57without knowing it.
0:17:57 > 0:17:59You can catch it from cat litter.
0:18:01 > 0:18:02Or infected soil.
0:18:04 > 0:18:07Or from eating undercooked meat
0:18:07 > 0:18:09from an animal that was itself infected.
0:18:13 > 0:18:16Since toxoplasma can manipulate rat brains,
0:18:16 > 0:18:21can the parasite also change our behaviour?
0:18:21 > 0:18:23There's intriguing evidence that it can.
0:18:26 > 0:18:29Scientists have analysed blood samples from people
0:18:29 > 0:18:32involved in traffic accidents, and they found they are more than
0:18:32 > 0:18:37twice as likely to be infected with toxoplasma than the average.
0:18:38 > 0:18:41The claim is that infected people are slower to react
0:18:41 > 0:18:43and take greater risks -
0:18:43 > 0:18:45just like infected rats.
0:18:48 > 0:18:50Quite scary, isn't it,
0:18:50 > 0:18:52the thought of a parasite manipulating your behaviour?
0:18:52 > 0:18:55Absolutely, and I think the fact that it almost brings on
0:18:55 > 0:18:56the concept of free will,
0:18:56 > 0:18:59because how much of your behaviour you're expressing
0:18:59 > 0:19:02is yours and how much is it the parasite within you?
0:19:02 > 0:19:06Does toxoplasma change human behaviour?
0:19:06 > 0:19:10At the moment, there's no direct evidence.
0:19:10 > 0:19:14If there is an effect, it will be, in the vast majority of cases,
0:19:14 > 0:19:15very subtle.
0:19:15 > 0:19:19But the fact that it can alter a rat's behaviour
0:19:19 > 0:19:22is nothing short of remarkable.
0:19:22 > 0:19:25It shows just what extraordinary abilities evolution has
0:19:25 > 0:19:30equipped parasites with to ensure they spread to a new host.
0:19:36 > 0:19:39This is a leech.
0:19:39 > 0:19:41For thousands of years they were used to treat
0:19:41 > 0:19:44everything from skin diseases to fevers.
0:19:44 > 0:19:47Those cures almost certainly did more harm than good.
0:19:49 > 0:19:53But in the right hands, the leech can be a useful surgical assistant.
0:19:56 > 0:19:59Iain Whitaker is a reconstructive surgeon who is pioneering
0:19:59 > 0:20:01the modern use of leeches.
0:20:02 > 0:20:06I normally use them on extremities, for example a finger,
0:20:06 > 0:20:08or in rare instances, a nose or an ear.
0:20:08 > 0:20:12- You're reattaching the end of a finger, something like that? - Yes, that's right.
0:20:12 > 0:20:15It is technically possible with microsurgery to attach the artery
0:20:15 > 0:20:17so you've got blood flowing in,
0:20:17 > 0:20:20but the blood flowing out via the veins is much more difficult.
0:20:20 > 0:20:24- When things start to swell up? - That's exactly right, that's the basis of it.
0:20:24 > 0:20:27And the leeches are quite good at sort of controlling the flow,
0:20:27 > 0:20:30- are they?- Yeah, it's almost as if they're custom made.
0:20:30 > 0:20:33You know, they remove a fairly predictable amount of blood,
0:20:33 > 0:20:35it's self contained,
0:20:35 > 0:20:38and they promote bleeding after they're removed as well.
0:20:41 > 0:20:43It's the ability to remove blood that makes the leech
0:20:43 > 0:20:46so useful in surgery.
0:20:46 > 0:20:50But how much blood can a leech consume?
0:20:50 > 0:20:53Well, there's one way to find out.
0:20:53 > 0:20:57First, we have to see how much the hungry leech weighs.
0:20:57 > 0:21:01- About point four. - That's perfect.- OK.
0:21:01 > 0:21:04- So shall we put it on, are you ready?- I'm ready, yeah.
0:21:06 > 0:21:09They only need to feed once a year.
0:21:09 > 0:21:12It takes about three months to digest a blood meal.
0:21:13 > 0:21:15I can feel it, yeah.
0:21:15 > 0:21:17I can definitely feel something going on there.
0:21:17 > 0:21:21At the moment, it's attached by its head end,
0:21:21 > 0:21:24where the jaws and teeth will have made a hole.
0:21:25 > 0:21:30Here you can see how the leech's triple-jawed mouth is able to
0:21:30 > 0:21:34cut through a membrane. That's what it's doing to my skin.
0:21:38 > 0:21:41Once it's made a hole, it can start feeding on my blood.
0:21:45 > 0:21:47It sucks by contracting its muscles
0:21:47 > 0:21:51in a rhythmic movement called peristalsis.
0:21:53 > 0:21:56Its digestive tract, visible here in red, can expand to
0:21:56 > 0:21:59hold huge quantities of blood.
0:22:01 > 0:22:06And its saliva contains proteins that help the blood keep flowing.
0:22:09 > 0:22:12But it will be injecting some fairly ingenious things.
0:22:12 > 0:22:15Hirudin is probably the most important,
0:22:15 > 0:22:18which will prevent it clotting so it can feed more efficiently.
0:22:18 > 0:22:21It's the most potent anti-coagulant known to man.
0:22:21 > 0:22:23And how long will I go on bleeding for?
0:22:23 > 0:22:26I think the average is about 12 hours.
0:22:26 > 0:22:30In some instances, it can go up to 48, and 72 in extreme circumstances.
0:22:30 > 0:22:33Oh, God, it's moved. It's come off.
0:22:33 > 0:22:35- It fell off.- Oh, there we go.
0:22:35 > 0:22:37Shall we see how heavy it is?
0:22:37 > 0:22:40- Hey, blimey, so it's now seven or eight times heavier.- Yeah.
0:22:40 > 0:22:43That is phenomenal, I mean, on you that would be...
0:22:43 > 0:22:48The equivalent of me, 115 to 120 stone after one meal.
0:22:50 > 0:22:54By helping blood to flow freely into newly re-attached tissues,
0:22:54 > 0:22:58the leech can save parts of the body that would otherwise die.
0:22:58 > 0:23:03Because it's a very upsetting injury to lose a finger or a thumb,
0:23:03 > 0:23:06and leeches are literally the answer in some cases when we
0:23:06 > 0:23:09can't get any other way to remove the blood, you know.
0:23:09 > 0:23:11So they literally will save people's careers.
0:23:19 > 0:23:24Parasites that live inside us, endoparasites, face a challenge.
0:23:24 > 0:23:27They have to avoid attack from our immune system.
0:23:29 > 0:23:33Helena Helmby studies how parasitic worms manage this feat.
0:23:33 > 0:23:35The thing that puzzles me is,
0:23:35 > 0:23:38how do these worms evade the human immune system?
0:23:38 > 0:23:40Because, I mean, some of them are huge.
0:23:40 > 0:23:44Yes, and we have a very, very sophisticated immune system,
0:23:44 > 0:23:46which is constantly on watch
0:23:46 > 0:23:49guarding against any microorganism 24/7.
0:23:49 > 0:23:54But these large parasites have actually developed
0:23:54 > 0:23:57a very sophisticated way of dealing with our immune system,
0:23:57 > 0:23:59because they have evolved with us for thousands
0:23:59 > 0:24:02if not millions of years.
0:24:02 > 0:24:07The worms have evolved mechanisms to dampen down our immune responses,
0:24:07 > 0:24:12by secreting compounds that manipulate our immune system.
0:24:12 > 0:24:15That enables the worms to survive,
0:24:15 > 0:24:18but may also have benefits for us.
0:24:18 > 0:24:22The worms are allowed to stay because the immune system realises
0:24:22 > 0:24:25that the attack to kill them
0:24:25 > 0:24:29would be far too dangerous, really, for the host.
0:24:29 > 0:24:32Obviously, you don't want a massive inflammatory response
0:24:32 > 0:24:35in your intestine because that'd cause a lot of damage to your intestine
0:24:35 > 0:24:37and that would be dangerous for you as a host,
0:24:37 > 0:24:40so there's some sort of an uneasy truce
0:24:40 > 0:24:43between the worm and the host in this case.
0:24:45 > 0:24:49So our bodies have evolved a kind of ceasefire with the worms.
0:24:49 > 0:24:53But in a modern, technological society,
0:24:53 > 0:24:56most people spend their lives worm free.
0:24:56 > 0:24:59Some scientists believe this may have contributed to
0:24:59 > 0:25:02the rise of allergies.
0:25:03 > 0:25:06It's known as the hygiene hypothesis.
0:25:10 > 0:25:13This theory states that having parasites in your body
0:25:13 > 0:25:17can help your immune system work properly.
0:25:17 > 0:25:20It's not proven, but the evidence is intriguing.
0:25:21 > 0:25:24There's no doubt that there has been a huge rise in allergies
0:25:24 > 0:25:26and so-called autoimmune diseases
0:25:26 > 0:25:29as we've got richer and more hygienic.
0:25:31 > 0:25:34And rates of these diseases are highest in countries
0:25:34 > 0:25:38that don't have lots of parasitic worms.
0:25:42 > 0:25:46But what's controversial is that some people are now using worms
0:25:46 > 0:25:49to try and treat their diseases.
0:25:51 > 0:25:53Daniel Heyman has Crohn's.
0:25:53 > 0:25:55I got ill...
0:25:55 > 0:25:59'It's a debilitating condition in which the immune system in the gut
0:25:59 > 0:26:02'starts attacking the body's own tissues.'
0:26:02 > 0:26:04..when I'd lost several stone in weight
0:26:04 > 0:26:09and I was bleeding internally, and so it was pretty serious.
0:26:09 > 0:26:11And it took me a while to kind of work out
0:26:11 > 0:26:14what I could eat and what I couldn't.
0:26:14 > 0:26:15And curry was no longer...
0:26:15 > 0:26:17Curry wasn't on the menu at all, no, no,
0:26:17 > 0:26:20I was limited to sort of fish, rice, very plain foods.
0:26:23 > 0:26:25'Although there are drugs that help,
0:26:25 > 0:26:28'they can have unpleasant side effects.
0:26:28 > 0:26:31'So Daniel decided to infect himself with hookworm,
0:26:31 > 0:26:36'a small blood-sucking worm that lives in the intestines.
0:26:38 > 0:26:41'Daniel hoped they would suppress his gut's immune system
0:26:41 > 0:26:47'for their own survival and, by doing so, reduce his symptoms.
0:26:47 > 0:26:51'Daniel ordered a tube of live hookworm on the internet,
0:26:51 > 0:26:55'and placed them on his skin, where they burrowed in,
0:26:55 > 0:26:58'eventually finding their way into his guts.
0:26:59 > 0:27:01'That was two years ago.'
0:27:01 > 0:27:03Do you have any fear of food at the moment?
0:27:03 > 0:27:08No, I mean, I'm really free, like, thanks to the worms.
0:27:08 > 0:27:10THEY LAUGH
0:27:10 > 0:27:13I am basically free to eat as anyone else would.
0:27:14 > 0:27:17'Daniel's story is fascinating,
0:27:17 > 0:27:20'but to prove that hookworm treatment works,
0:27:20 > 0:27:22'you need proper clinical trials.
0:27:22 > 0:27:27'Without that evidence, Helena Helmby, like most scientists,
0:27:27 > 0:27:31'finds the idea of self-infection very troubling.'
0:27:31 > 0:27:34I mean, scientists have worked now for over 100 years
0:27:34 > 0:27:37trying to eradicate these diseases, and now we want to start
0:27:37 > 0:27:40reintroducing them into people again.
0:27:40 > 0:27:45I mean, these worms actually live in the gut and they suck blood
0:27:45 > 0:27:47from your intestinal mucosa.
0:27:47 > 0:27:49And they move around a lot as they do that,
0:27:49 > 0:27:51so they cause a lot of bleeding.
0:27:51 > 0:27:53And if you have a high dose of hookworm in your gut,
0:27:53 > 0:27:56you will become anaemic quite quickly.
0:27:56 > 0:28:00Rather than infecting people with human parasites,
0:28:00 > 0:28:02we should be really looking into
0:28:02 > 0:28:05purifying these molecules that the worms produce,
0:28:05 > 0:28:07and turn them into drugs.
0:28:07 > 0:28:09Turn them into worm pills.
0:28:11 > 0:28:15So parasites can teach us a great deal about our own bodies.
0:28:17 > 0:28:20And they may even hold the key to future medical treatments.
0:28:21 > 0:28:25Human parasites are a reminder that
0:28:25 > 0:28:28we are just one part of a complex web of life
0:28:28 > 0:28:31that has evolved with us,
0:28:31 > 0:28:33making us who we are.