Why medicinal leeches are making a return

It has been announced that medicinal leeches are successfully breeding at London Zoo. The largest native leech species in the UK used to be abundant, but their  number went into decline when they began to be used less readily in medicine, and their natural habitats - such as wetlands - were drained. But these segmented worms seem to be back in vogue. James Tytko took a trip to London Zoo to find out why…

Naomi - Hi there. My name's Dr. Naomi Ewald from Freshwater Habitats Trust and I'm part of the project looking at leeches and their conservation in the UK. Medicinal leeches are one of the largest leeches we have in the UK. They can grow up to about 10 centimetres, sometimes bigger for really large adults. Not only are they big, but they're also really patterned. So they have this lovely red and yellow stripe down their bodies that looks sort of like a running stitch and on their bellies they're yellow and black patterned. And we'll look later but you can actually tell individuals apart based on the patterns on their bellies. So these are big leeches. They're blood sucking leeches and they're the only leech in the UK that can do that.

James - In terms of their role in fresh waters around the UK, what do they eat and what eats them? Apart from human blood, of course <laugh>.

Naomi - So medicinal leeches are really important as part of when we are thinking about conservation of ponds in the UK. So we have about 500,000 ponds in the UK. Unfortunately, most of them are polluted to some extent, normally because of surrounding land use change, agricultural runoff, or because they're no longer grazed by grazing animals. And so where we find leeches in the UK is because they're still within important freshwater areas. So these are the best of the best ponds that we have. And out of those 500,000, there are only about 150 ponds left in the wild in the UK that support medicinal leech.

James - Let's move to the medical history then, if you would.

Naomi - Yeah, so we've known that medicinal leaches have been used for what you might have in inverted commas called 'medicine' really since ancient Egyptian times. And there was something like a leech mania around the 17th and 18th century through into the 19th century, particularly in Europe. And essentially if you were feeling a bit under the weather, they thought that they could rebalance you by sticking a leech on you. So we know that St. Bartholomew's hospital in London was using up to 90,000 leeches a year, and in France they were importing 60 million leeches a year at the height of this leech craze. And you know, it's questionable whether actually that did any good or not because back in the days some of those practices were probably not very clean and they were probably taking leech from one person and sticking it onto another. So the risk of dying from infection and gangrene was probably as much as it was a cure.

James - Fortunately, bloodletting by leeches has been discredited as a medical practice for quite some time. So why the kind of conservation push now? Is it the other pressures to their environment that you mentioned?

Naomi - Yeah, that's right because we know that leeches are now only found in those very best of the best places. The first step is to try and see if we can rebuild those populations naturally. So obviously we're advising landowners that they have the leeches and what sort of management they could undertake to improve it, and we're trying to create new ponds adjacent to those important freshwater areas. So building out and creating a sort of a network of clean freshwater ponds across the landscape. If that doesn't happen, we then want to think about whether we can look at sites where they've been historically, but are now missing and work out whether we can recover them to those places as really a signal that the habitats are improving and that if they can support a healthy population of leeches, they're probably supporting a lot of other good stuff as well.

James - Indeed. What about this renewed medical interest in leeches and particularly their venom?

Naomi - So medicinal leeches have a really clever saliva that's designed to help them feed, and that essentially is a component made up of anticoagulants so that the blood will flow freely once they've bitten in. So they have three jaws with serrated teeth and they'll puncture a hole in their victim and then the blood will start flowing because of their saliva. They also have an antihistamine which stops the swelling and opens the blood vessels so that it flows even more. And then there's something of a sort of an antiseptic and also a way that will stop you feeling them even biting you. So essentially if a leech latches onto you, you don't know about it and you bleed for a really long time. And in the 1970s they realised that actually there was a whole new world of medicine that could benefit from what you could learn from leeches, particularly around the sort of cosmetics and also reattachment of limbs because one of the key things is to keep the new tissue alive. And if you've got blood being drawn through that by this saliva and it's very hard to recreate that synthetically. So the best bets are to find a leech, stick it on you, and then that works really well. But then we discovered that the leeches they've been using are Mediterranean medicinal leech and not our lovely native UK leech. And so really we wanted to see, well, can we use some of the techniques that they learned from the medicine trade to actually see if we could breed them in this country? And this is when we turn to London Zoo for their advice because obviously they have all the experience in the world of helping set up these breeding programs.

James - Without further ado, I think it's time to see the fruits of that labour.

Aaron - I'm Aaron Harvey. I am the aquarist here at ZSL London Zoo. I am the head of looking after the leeches here at ZSL.

James - Terrific. Well that makes you very well placed to talk me through what I'm looking at here. I've got three boxes full of leeches of kind of incrementally increasing size, I think it would be fair to say. So tell me about these first ones.

Aaron - So the first box we have here, so these are our smallest leeches. These are just coming up to about three months of age. They can live as adults for up to about 20 years.

James - Okay. So these are really at the start of their journey.

Aaron - Very start of their life. And then with our second box...

James - A little bit more placid, probably fair to say.

Aaron - So these are slightly older. So these are just over three months or about three months of age. They've had a fair few feeds now. So they're starting to chill and calm down a lot more. They're very sensitive to movement and body temperature. So this is generally the activity we'll see when we're working with them

James - He's looking at me as a potential meal.

Aaron - Maybe, these leeches again, these are older, so these are actually four months of age.

James - Tell me a bit about the kind of techniques Naomi was talking about that have translated from the renewed medical interest in leeches and how that's been applied here at London.

Aaron - We've taken a lot of evidence from wild caught data. We've looked at temperature fluctuations through seasonality, and we've tried to bring that here to London. So we control water temperature very closely, especially around this time of year, we start to drop their water temperature. Eventually when it gets to December, January time, we actually get to about 10 degrees. So the leeches can hit their winter dormancy and then when it comes back into spring and summer, their water temperature will hit straight back up into the 25s to even sometimes 30 degrees. And that encourages their breeding process.

James - When we talk about reintroducing species into the world, you know, people often like to think of eagles or bison and maybe the less glamorous leech gets a little bit less attention. But it's no less important, is it?

Aaron - They're very important. They're very good ecological indicators. They're a great sign of a healthy ecosystem and they're animals we desperately need in our native wildlife.

James - So Naomi, tell me, when might we start putting these creatures back in UK fresh waters?

Naomi - Well, over the next couple of years we're hoping to start the wild surveys, and then after that we'll start to think about whether we can reintroduce any. So check back in with me in five years time and we'll give you an update. And obviously, you're welcome to come out and see when we've got to that point. If we are reintroducing them, that will be when we'll be doing it.