The science of leeches

DR TIM COCKERILL: Leeches. For most people, they're the stuff of nightmares. But there's a lot more to them than just slimy bloodsuckers.

Now, the leeches in this tank are medicinal leeches, and they've got this name for a really good reason. So for about 2,000 years up until quite recently, it was thought that the body's health was kept in balance by substances called humours which flowed through the body. Now, blood was one of those humours. And if you had bad blood, or too much blood, then it needed to be drained. And that is where these guys came in.

Draining the blood from the body was known as bloodletting. And using leeches to do this was the height of fashion in Victorian times. In fact, millions were used every year across Europe.

Leeches are actually worms related to the ones you find in your back garden. But these are parasites that suck the blood of other animals. Now they find a host by detecting shadows and movement in the water. So if I dip my fingers in the tank and wiggle them around, the leeches should come to me.

And there we are. This is Larry the leech. Or it could be Lucia. Leeches are hermaphrodites, they've got both male and female bodyparts. Now this one is hungry. You can see it's desperately trying to find somewhere to attach. So I'm going to let it feed on my arm.

When leeches are feeding, they inject anti-inflammatories and an anticoagulant called hirudin that stops the blood from clotting. And it's because of these that they're still used in medicine today. They drain blood clots and promote blood flow in skin grafts.

People say that leeches inject a kind of anaesthetic so the host can't feel it feeding. Now there's actually no good evidence to support this, and I can actually feel this. It feels a bit like a mild nettle sting.

Now what it's actually doing is using three rows of teeth in a kind of Y shape. There's about a hundred teeth in each row, and it's using these to slice through the skin to suck the blood. And it could take up to a couple of hours until it's fully engorged, so all we can do is sit and wait.

The leech has been feeding for about 2 hours, and as you can see, it's absolutely massive. But it's showing no sign of stopping yet. Now I don't want to try and pull it off, because if the leech feels threatened, it might regurgitate its stomach contents back into the wound.

TITLE: Another hour passes...

COCKERILL: So after nearly three hours, the leech has finally dropped off, and the wound is going to continue to bleed for at least two hours afterwards. Now the leech can last on this single meal for over twelve months. So I'll see you the same time next year.