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It's one of nature's most fascinating parasites.
There are thousands of different kinds of tapeworm,
each evolved to live in a different host species.
And there are several species of tapeworm that live in humans,
including one called Taenia saginata.
Like many parasites, it has a complicated life cycle,
which involves passing through more than one species.
So imagine you're in rural Kenya
and you've got a great big tapeworm inside you.
This tapeworm is producing eggs.
One day, you go down to a field and you excrete there,
and you contaminate the grass.
Then along comes a poor unfortunate cow who eats that grass.
Along with grass, the cow eats the tapeworm eggs,
which hatch into larvae and then enter the cow's bloodstream...
..spreads through the blood to the muscle,
perhaps the shoulders or the tongue,
and there it forms a really tough cyst.
And it hangs around, just waiting for the next stage,
which is for the cow to be eaten by a human.
The beef tapeworm can only grow to adulthood inside a human,
and for that reason, it's hard to study.
So I've decided to infest myself,
as my contribution to research into these shy, retiring creatures.
So I need to find infected beef.
But in Britain, it's extremely unlikely
you'll find a contaminated cow,
which is why I've come to Kenya.
With the help of a local meat inspector,
we found some infected meat.
Then we cut three cysts out of the meat,
ready for me to swallow.
It's like jumping off a cliff. Here we go.
There they go.
In just a few seconds, the tapeworm cysts are in my stomach.
That's full of acids that normally help protect us from disease.
But the tapeworm uses our defences to its advantage.
The acids dissolve the outer case of the cyst,
releasing the worm inside.
If they survive, each cyst will release a tapeworm scolex,
equipped with four suckers, which it will use to latch onto my gut.
Then it starts to grow, new segments emerging from the scolex.
As it grows, the segments will get bigger and more mature,
whilst new segments appear near the head.
To see what a grown worm looks like,
I met up with tapeworm expert, Prof Phil Craig.
That is the adult tapeworm, and this tapeworm is exactly ten weeks old.
We know that because it was from a previous volunteer.
By ten weeks, the mature segments are full of eggs...
..and they break off from the tapeworm and emerge from your body...
..ready to infect a cow.
So it comes out of your bottom, and it can actually crawl out?
Psychologically, that upsets quite a few people that are infected.
I can imagine.
Yeah, it can come out under its own steam, as it were,
so once the segment is in the rectum,
it'll move around and that causes
a sort of a strange fluttering sensation,
and then it will move through the rectum, through the anus,
and crawl around between the buttocks,
down the legs and out onto the floor.
-Can be embarrassing.
It might surprise my wife or friends.
I think it probably would. Yes.
After six weeks, I decided to take a look at my tapeworm,
using a technique known as capsule endoscopy.
Down the hatch.
It involves a tiny camera which you swallow.
Live from my stomach.
Several hours later, the capsule had passed into my intestines,
and using a tablet, I could see live pictures from my gut.
Aah, that is it.
That is the tapeworm.
I can just see its tail and its segments waving around.
I don't know if I'm absolutely disgusted or...
I'm actually quite excited. Wow.
Because it would have been hugely disappointing
to have gone all the way to Kenya,
come back and seen nothing.
But what's amazing is, I've not experienced anything
and yet you can see it there.
That is so weird.
This footage provides scientists with a rare chance
to see a live worm in its natural habitat.
So we sent the complete video to a team at Salford University.
Just at the back there, you can just see the worm,
the thinnest part of the worm.
This is sort of the front end.
The segments are getting more mature
as you're moving down the intestine here,
they're getting bigger, becoming sexually mature.
Probably, they're starting to get fertilisation taking place,
so the worm is actually starting to reproduce at this stage.
Can you tell at this point how big it is? I have no idea of the scale.
-By the looks of this, it's certainly over a metre in length.
-So a metre?
That's another worm, yeah?
-You have more than one worm, definitely.
I'll stop there. OK.
-And guess what? You've got three worms.
This is the very front end, the head end.
This is the bit that attaches onto the intestine.
There are actually four muscular suckers around that sort of head,
and those are what's latching onto the wall of your intestine,
-keeping the worm in place.
100% hit rate. I have to say, a very successful experiment.
Humans are home to many parasites.
The ones that live on our skin are called ectoparasites,
and this is one of the most familiar ectoparasites -
the head louse.
To find out more about this very common parasite,
I've come to a delousing salon in north London.
What I'd love you to do, then, is infect me.
Head lice can only survive by drinking human blood,
which they do several times a day.
To keep these lice alive, I need to feed them...on me.
These little pots should keep them safe,
while allowing them to bite me.
-Thank you very much for infecting me.
There aren't many people who would be pleased to hear
their visitor is infested with lice.
-I come bearing gifts.
But James Logan is clearly delighted.
Let's have a look. Oh, that's brilliant.
James studies lice and other parasites
at the London School of Hygiene and Tropical Medicine.
-OK, so here we have one.
Shall we put it on your arm?
See if he scurries around.
Using a hand-held microscope,
we can study my head louse as it settles in.
You can really see the blood here, can't you?
So this one's quite recently fed on your blood,
but you can see it being sort of pumped down here through its gut.
Like all lice, the head louse has retractable mouth parts
that can puncture my skin.
But what really surprised me is how active they can be.
-It's going so fast, I can't keep up.
-This one's quite active, isn't it?
It certainly is.
Look at that, it's gone right to the last hair on your arm.
Look at it clinging onto the edge of that hair with two of its claws.
Look at those claws.
It's got another claw out, and I wonder whether it's sat there
waiting for another host to come past.
So if I were to rub my head against yours,
it would just grab a hold of your hair with the other claws
-and it'd be across in an instant, would it?
-Exactly right, yeah.
Although they feed on your skin,
head lice always lay their eggs on hair,
which is why they can only complete their life cycle on your head.
But there is another species that lives on humans,
and that can tell us something fascinating about our distant past.
There's another type of louse called the body louse,
and I've got an example here to show you what an infestation looks like.
And this, remarkably, is highly adapted to clothing.
This was actually... I believe, this was from a homeless person...
-...who had a very heavy infestation.
And I think, it looks like trousers to me, with a seam here,
and this is exactly where they lay their eggs.
-Aah. That is revolting.
-You can see...
Yeah, and those are eggs, so that is a massive infestation.
-It makes you feel quite itchy, doesn't it?
The body louse behaves very differently to the head louse.
But if you compare their bodies' shapes,
you see something significant.
OK, so the one on the right, the dark one,
-is my friend, the head louse?
He's looking dead at the moment, I have to say, not looking good.
And the one on the left, they're body lice?
That's exactly right.
And what you can see is, they look remarkably similar.
When you look at their DNA,
what you find is that they diverged around 100 to 200,000 years ago,
so not actually that long ago in sort of evolutionary terms,
but enough to make them different species.
And so that tells us something about when we, as humans,
started to wear clothing.
So before that, the idea is, humans were naked,
then they started to wear clothes, maybe a head louse drops down,
-thinks it's actually quite a nice place to live...
..and begins to evolve, a whole new species starts to evolve on clothes.
Exactly right, yep.
So it's quite incredible to think you can look at the louse
to find out more about our own ancestry and our own evolution.
Some parasites cause relatively little harm to their host.
But this one is deadly.
It's a microscopic single-celled parasite...
It causes malaria,
a disease that kills more than half a million people a year.
I want to find out why it is so deadly,
so I've come to the National Institute of Medical Research,
where they will infect my blood with plasmodium.
So, I've got a sample of my blood here, looking nice and red. Healthy?
Yes, yes, and warm, straight out of your body.
-I won't shake hands.
-Thanks very much, Fiona.
-Right, I guess we're not allowed in that room.
-That's right, yeah.
What's Fiona up to at the moment, then?
So, what she's going to do now is
she's going to take a sample of Plasmodium falciparum.
So this is the most dangerous form of the parasite
that causes most deaths in, for example, Sub-Saharan Africa.
We've grown this in the lab,
we can culture it continuously in human red cells,
and she's now going to take a sample of those parasites
-and inoculate them into your blood.
I must admit that of all the diseases,
malaria is high on the ones I want to avoid.
Well, you should try to avoid it if you can.
To see the malaria parasite multiplying in my blood,
they're using a new photographic time-lapse technique.
Although I have treated people with malaria,
I have never seen the malaria parasite in action before.
Very intrigued to see what happens to my blood,
but what is chilling is thinking that out there in the world,
mosquitoes are infecting children.
1,000 children every day are dying.
That's why this sort of work is so important.
Here it is. My blood infected by the deadly malaria parasite.
So the parasite goes through this life cycle in the infected red cells.
Once the infection is complete,
Mike Blackman shows me the finished movie,
which reveals how the parasite kills its host.
So the little white blobs here, they are the parasite, right?
They are the parasite, yeah.
So a single merozoite, this invasive form of the parasite,
binds to a red cell, invades it, grows within it,
digests the haemoglobin of the red cell,
this is the red protein that is used to carry oxygen via red cells,
and then eventually forms around about 16 to 32 daughter merozoites.
Does it? Oh, wow, look at that one go.
-That one just exploded, didn't it?
-That's, right, exactly, yes.
And suddenly you're seeing lots. Oh, wow, they're really going.
The whole thing is destroyed in a single, very rapid process.
The merozoites are released and they immediately invade a new cell,
-and these things...
-I had no idea it was going to be that violent.
I mean, that was utterly destroyed.
Each explosion obliterates one of my oxygen-carrying red blood cells,
and releases new parasites into my blood to infect yet more cells.
Suddenly, you've gone from a situation where
there are relatively few, they're everywhere,
and they're just swarming.
I mean, you do appreciate just what a terrible thing they're doing.
Yeah, if this were going on inside you, you'd be in a pretty poor way.
You soon become anaemic from lack of blood cells,
and the debris from all those shattered cells can
block your blood vessels, which in turn can be fatal.
So our interest here is exactly how this goes on,
how does the parasite actually do that? We don't really know.
The parasite is very, very good, very smart.
It's evolved with its human host for a long time.
This is Toxoplasma gondii.
It's a microscopic parasite that looks like a wriggling comma,
and it's possibly the most prolific parasite that lives on humans.
Its life cycle always passes through a cat,
where the parasite breeds.
An infected cat passes oocysts,
tiny capsules containing the parasites.
The oocysts can survive on the ground for months,
waiting to be eaten by a rat or mouse,
which then becomes a carrier.
But for the life cycle to complete,
the parasite has to get back into a cat,
which means the mouse has to be caught and eaten by a cat.
So, toxoplasma has to encourage its mouse host to commit suicide.
A few years ago,
Joanne Webster set up a series of elegant experiments to see
how toxoplasma might go about altering a rodent's behaviour,
making it more likely to get eaten.
She placed infected and uninfected rats in a chamber
where she had liberally doused one corner with cat urine.
And we simply plopped the rat in and let them tootle about over
each four-hour, ten-hour night, and simply watch where they went.
Alice here is an uninfected female.
She smelled the cat area and she shot off here.
She seems to be avoiding the cat-smelling one.
She's hanging around in different areas.
Putting in a rat infected with toxoplasma
produced a very different result.
This is Felix here.
He's infected and he's actually in the cat zone.
Which is not what you'd expect. A normal mouse or rat would run...
-Yeah, smell it and absolutely hide.
-And he seems to be just enjoying it.
Yep, he's out, he's active, he's tootling about.
So presumably, if you're the parasite, what you want is,
you want the rat, Felix the Rat, to hang around
-near where cats are because you want to be eaten.
Joanne had shown that infected rats are attracted to cat smells.
They're also generally more fearless
and have slower reaction times.
All these factors made them far more likely to become cat victims.
What is rather disturbing is that this parasite, toxoplasma,
also infects us.
Around a quarter of the British population are infected
without knowing it.
You can catch it from cat litter.
Or infected soil.
Or from eating undercooked meat
from an animal that was itself infected.
Since toxoplasma can manipulate rat brains,
can the parasite also change our behaviour?
There's intriguing evidence that it can.
Scientists have analysed blood samples from people
involved in traffic accidents, and they found they are more than
twice as likely to be infected with toxoplasma than the average.
The claim is that infected people are slower to react
and take greater risks -
just like infected rats.
Quite scary, isn't it,
the thought of a parasite manipulating your behaviour?
Absolutely, and I think the fact that it almost brings on
the concept of free will,
because how much of your behaviour you're expressing
is yours and how much is it the parasite within you?
Does toxoplasma change human behaviour?
At the moment, there's no direct evidence.
If there is an effect, it will be, in the vast majority of cases,
But the fact that it can alter a rat's behaviour
is nothing short of remarkable.
It shows just what extraordinary abilities evolution has
equipped parasites with to ensure they spread to a new host.
This is a leech.
For thousands of years they were used to treat
everything from skin diseases to fevers.
Those cures almost certainly did more harm than good.
But in the right hands, the leech can be a useful surgical assistant.
Iain Whitaker is a reconstructive surgeon who is pioneering
the modern use of leeches.
I normally use them on extremities, for example a finger,
or in rare instances, a nose or an ear.
-You're reattaching the end of a finger, something like that?
-Yes, that's right.
It is technically possible with microsurgery to attach the artery
so you've got blood flowing in,
but the blood flowing out via the veins is much more difficult.
-When things start to swell up?
-That's exactly right, that's the basis of it.
And the leeches are quite good at sort of controlling the flow,
-Yeah, it's almost as if they're custom made.
You know, they remove a fairly predictable amount of blood,
it's self contained,
and they promote bleeding after they're removed as well.
It's the ability to remove blood that makes the leech
so useful in surgery.
But how much blood can a leech consume?
Well, there's one way to find out.
First, we have to see how much the hungry leech weighs.
-About point four.
-So shall we put it on, are you ready?
-I'm ready, yeah.
They only need to feed once a year.
It takes about three months to digest a blood meal.
I can feel it, yeah.
I can definitely feel something going on there.
At the moment, it's attached by its head end,
where the jaws and teeth will have made a hole.
Here you can see how the leech's triple-jawed mouth is able to
cut through a membrane. That's what it's doing to my skin.
Once it's made a hole, it can start feeding on my blood.
It sucks by contracting its muscles
in a rhythmic movement called peristalsis.
Its digestive tract, visible here in red, can expand to
hold huge quantities of blood.
And its saliva contains proteins that help the blood keep flowing.
But it will be injecting some fairly ingenious things.
Hirudin is probably the most important,
which will prevent it clotting so it can feed more efficiently.
It's the most potent anti-coagulant known to man.
And how long will I go on bleeding for?
I think the average is about 12 hours.
In some instances, it can go up to 48, and 72 in extreme circumstances.
Oh, God, it's moved. It's come off.
-It fell off.
-Oh, there we go.
Shall we see how heavy it is?
-Hey, blimey, so it's now seven or eight times heavier.
That is phenomenal, I mean, on you that would be...
The equivalent of me, 115 to 120 stone after one meal.
By helping blood to flow freely into newly re-attached tissues,
the leech can save parts of the body that would otherwise die.
Because it's a very upsetting injury to lose a finger or a thumb,
and leeches are literally the answer in some cases when we
can't get any other way to remove the blood, you know.
So they literally will save people's careers.
Parasites that live inside us, endoparasites, face a challenge.
They have to avoid attack from our immune system.
Helena Helmby studies how parasitic worms manage this feat.
The thing that puzzles me is,
how do these worms evade the human immune system?
Because, I mean, some of them are huge.
Yes, and we have a very, very sophisticated immune system,
which is constantly on watch
guarding against any microorganism 24/7.
But these large parasites have actually developed
a very sophisticated way of dealing with our immune system,
because they have evolved with us for thousands
if not millions of years.
The worms have evolved mechanisms to dampen down our immune responses,
by secreting compounds that manipulate our immune system.
That enables the worms to survive,
but may also have benefits for us.
The worms are allowed to stay because the immune system realises
that the attack to kill them
would be far too dangerous, really, for the host.
Obviously, you don't want a massive inflammatory response
in your intestine because that'd cause a lot of damage to your intestine
and that would be dangerous for you as a host,
so there's some sort of an uneasy truce
between the worm and the host in this case.
So our bodies have evolved a kind of ceasefire with the worms.
But in a modern, technological society,
most people spend their lives worm free.
Some scientists believe this may have contributed to
the rise of allergies.
It's known as the hygiene hypothesis.
This theory states that having parasites in your body
can help your immune system work properly.
It's not proven, but the evidence is intriguing.
There's no doubt that there has been a huge rise in allergies
and so-called autoimmune diseases
as we've got richer and more hygienic.
And rates of these diseases are highest in countries
that don't have lots of parasitic worms.
But what's controversial is that some people are now using worms
to try and treat their diseases.
Daniel Heyman has Crohn's.
I got ill...
'It's a debilitating condition in which the immune system in the gut
'starts attacking the body's own tissues.'
..when I'd lost several stone in weight
and I was bleeding internally, and so it was pretty serious.
And it took me a while to kind of work out
what I could eat and what I couldn't.
And curry was no longer...
Curry wasn't on the menu at all, no, no,
I was limited to sort of fish, rice, very plain foods.
'Although there are drugs that help,
'they can have unpleasant side effects.
'So Daniel decided to infect himself with hookworm,
'a small blood-sucking worm that lives in the intestines.
'Daniel hoped they would suppress his gut's immune system
'for their own survival and, by doing so, reduce his symptoms.
'Daniel ordered a tube of live hookworm on the internet,
'and placed them on his skin, where they burrowed in,
'eventually finding their way into his guts.
'That was two years ago.'
Do you have any fear of food at the moment?
No, I mean, I'm really free, like, thanks to the worms.
I am basically free to eat as anyone else would.
'Daniel's story is fascinating,
'but to prove that hookworm treatment works,
'you need proper clinical trials.
'Without that evidence, Helena Helmby, like most scientists,
'finds the idea of self-infection very troubling.'
I mean, scientists have worked now for over 100 years
trying to eradicate these diseases, and now we want to start
reintroducing them into people again.
I mean, these worms actually live in the gut and they suck blood
from your intestinal mucosa.
And they move around a lot as they do that,
so they cause a lot of bleeding.
And if you have a high dose of hookworm in your gut,
you will become anaemic quite quickly.
Rather than infecting people with human parasites,
we should be really looking into
purifying these molecules that the worms produce,
and turn them into drugs.
Turn them into worm pills.
So parasites can teach us a great deal about our own bodies.
And they may even hold the key to future medical treatments.
Human parasites are a reminder that
we are just one part of a complex web of life
that has evolved with us,
making us who we are.