Craig Macadam | Buglife
Medicinal leeches (Hirudo medicinalis) are intriguing invertebrates with a gruesome reputation for blood-sucking that have played an important role in medical history. Medicinal leeches can grow up to twenty centimetres (c. eight inches) in length, live for up to twenty years, and can consume ten times their own body weight in blood during a single meal. Once widespread throughout Europe, over-harvesting for use in bloodletting resulted in a catastrophic decline in Medicinal leech populations. In the British Isles, they were declared extinct and only re-discovered in the 1950s.
In November 2023, a local naturalist named Bob Merritt discovered a wild population in Dumfries and Galloway, bringing the total number of known Scottish sites to three. Now, as part of Scotland’s ‘Species on the Edge’ project, the Royal Zoological Society of Scotland (RZSS) is working with Buglife to breed Medicinal leeches at the RZSS’s Highland Wildlife Park in the Cairngorms National Park.
EPOCH spoke with Craig Macadam, Conservation Director at Buglife, about the history of the Medicinal leech, their role in medicine, and how conservation efforts are helping to preserve these wonderful creatures from extinction.
Could you start by telling us a little about medicinal leeches?
Sure. Medicinal leeches are related to worms. They differ in the way they move with a looping motion, whereas worms contract and expand as they move along. The other difference is that whilst worms consume decaying matter, leeches feed from other animals. Medicinal leeches are probably the most well-known and the most gruesome in that they feed on mammals, as well as on amphibians and other invertebrates.
Leeches appear a lot in popular culture – I think I’ve seen them in an episode of House, and in Regency period dramas. Are there any popular misconceptions about medicinal leeches we should immediately put aside?
I think the main popular misconception is that they’re going to bleed you dry; that’s the horror story. There was a sixties film called Giant Leeches, and in The African Queen, the leading actor goes into the water and comes out covered in leeches that they struggle to remove. The films present them as things that are going to kill you. They’re not. They will feed on you and either drop or swim off to digest the meal.
Most of the leeches in the UK are completely harmless – unless you’re a snail or a small invertebrate. There are two species that can feed on mammalian blood. The turtle leech is found in the UK where, as we don’t have turtles, it feeds on amphibians and sometimes on livestock if they enter the pond. The other is the medicinal leech, which feeds on mammals like cattle, sheep, or deer, and the occasional human that decides to go wild swimming.
What role do leeches play in the ecosystem?
Like many other parasites, they are a population controller: they limit the numbers of other invertebrates. It’s all about balance in ecosystems. Small leeches will be fed upon by fish or birds. There is a particular species of leech in the UK, the Duck Leech, which has adapted to this: when the duck goes under the water to feed on them, it swims up its nose and lives there feeding on the duck’s blood. I told you they were gruesome!
What role did medicinal leeches play in medical history?
As the name suggests, they have a long history in medicine. There are records from Ancient Greece and Ancient Egypt of leeches being used to treat illnesses. Doctors used to be called leeches, and the word in Gaelic for healer is ‘lighiche’, which sounds similar to ‘leech’, so there is an interesting crossover of zoology and medicine.
In the UK and Europe, the heyday of leeching was in the Victorian era. It was very popular, one of those fad fashions, I guess, all the doctors were doing it. Victorians used huge numbers of leeches. Initially, they were gathered locally, but over-exploitation by the mid-to-late 1800s led to the medicinal leech population crashing, which meant that they had to switch to imports from Europe.
Leeches from the marshes around Hamburg were collected and sent across Europe in their millions. Parisian hospitals used hundreds of thousands every year – as you can see in their recorded expenses. Leech importers and dealers could be found in all the major cities of the UK: Glasgow had three or four. You can see their adverts in directories from the turn of the twentieth century.
We had no idea there was such a trade network for leeches!
They were one of the most widely traded commodities in Europe during the mid-1800s. Leeches can survive out of the water, so to keep them alive in transit, all that was needed was to put some wet moss in the container with them. The post in the Victorian era was incredibly quick and efficient: it always amazes me when it takes me a week to get a second-class parcel here when I see correspondence between entomologists who were sharing things within days.
Are leeches used in medicine today?
Absolutely. There is a leech farm in Wales that breeds leeches for the NHS: they are used for encouraging blood flow. Leeches have a fantastic set of chemicals that are introduced when they bite you. These chemicals have been synthesised: if you see something that has got ‘hiru’ at the start of the name, it is usually related to leeches.
One is an anaesthetic, so you don’t feel it when they bite you. The other is an anticoagulant so it can sit there draining the blood off. When they drop off, you can bleed for maybe ten hours because of the anticoagulant. There are records of women from Scotland who came to Cumbria to collect leeches: some of them were so anaemic that they were on the verge of death because they waded into little ponds to collect the leeches by letting them bite their legs. They took them off and put them into little baskets to be sent to the dealer. Of course, this led them to bleed profusely.
How has human activity impacted Medicinal leeches?
Because of over-exploitation, the Medicinal leech population crashed; in the British Isles, it was declared extinct. It was not until 1955 that they were re-discovered in a pond on the island of Islay off the West coast of Scotland, and since then, they’ve been found in about fifteen ponds around the UK. So, they are still really, really, rare.
Across Europe, the medicinal leech is threatened with extinction because of the history of over-exploitation and present-day loss of habitat. The marshes around Hamburg have all been developed or drained and made into agricultural land. In the UK, one of the biggest changes is the introduction of fish into pools and their expansion into fish ponds, which changes the whole hydrology and ecosystem.
The other big change since the Victorian era is the widespread adoption of anti-worming chemicals in livestock farming. As I mentioned, leeches are worms, so if your livestock are treated with anti-worm substances and the leeches feed on them, they will die. The rise of veterinary medicines is one of the main reasons that Medicinal leech populations have declined.
So, there are a lot of problems that the leech has had to withstand.
How were Medicinal leeches re-discovered in Scotland?
In the 1950s a leech recorder named Reynoldson was doing some survey work and stumbled upon them in a pond on Islay. It is not surprising that they were there: the name of the pond in Gaelic is ‘Loch nan Digl’, which means ‘the little lake of the leeches’. There are now three known sites in Scotland: the one in Islay, one near Oban, and a recently discovered population in Dumfries and Galloway.
We are now looking at Gaelic placenames and old maps. There are twenty-eight different sites called ‘loch of the leech’, and I’ve been particularly interested in searching for them. We also look at historical accounts: in the eighteenth and nineteenth centuries, there were big statistical accounts of Scotland written, and the ministers who made these records recorded lochs on local estates with the ‘superior kind of leech’ – which is to say the Medicinal leech. So, I’ve been going around looking at these sites.
The technique for collecting leeches is identical to that used by the Victorians: if you splash about in the water, they swim towards you, and you can scoop them up. The only difference is that today, we use waders and a pond net so that we don’t get bitten! You’ve got to keep looking down to see if you have any attached to your waders.
How has the conservation breeding programme been going, and what are the long-term goals?
I’ve been quite encouraged to see first-hand that the Medicinal leeches are quite happy to feed on the blood that we’ve been giving them. It is deer blood in sausage skins, which we have to warm up to make it more palatable for them, but they’ve been feeding directly on that, which is great.
We are now at the time of the year when we want to see them laying their eggs and creating cocoons: they lay a number of eggs and encase them in a sort of foam which hardens a bit, like the stuff you get from a DIY shop for filling holes. Hopefully we should soon get some juvenile leeches.
We have three known sites, and we are doing survey work to look for suitable places for translocation of the leeches that we are rearing. A lot of the survey work has been on Islay, where there are some promising sites, but we are looking at other Scottish islands as well.
Hopefully, conservation programmes like the one that we are doing can begin to restore medicinal leech populations.
Are there any other stories of surprise re-discoveries of British wildlife that EPOCH readers should know about?
We are actually working on another project down in Wales, which is part of the Natur am Byth! programme looking at the Scarce Yellow Sally Stonefly – another freshwater invertebrate. It only occurs in the River Dee and was thought extinct.
One chap we work with, John Davy-Bowker, had been going back there for over twenty years to search at the right time of the year for larvae and finally re-discovered them. The population was just hanging on.
Now Buglife, in partnership with Chester Zoo, has begun a conservation breeding programme. We will then repopulate other parts of the river and potentially other rivers in the area with this species to build a more resilient population. Because with all the things we throw into rivers and at wildlife, and then you put climate change on top, there really need to be sustainable and resilient populations if species are to survive.
We would like to thank Craig Macadam for his kindness and consideration in talking to EPOCH about medicinal leeches. We encourage our readers to visit the websites of the Royal Zoological Society of Scotland and that of Buglife to learn more about their ongoing conservation work.