The medicinal leech is one of nature’s least loved hunters. Armed with three strong interlocking jaws and with a taste for blood, they will swim hungrily towards humans, deer or cattle that wander into their ponds to bathe, fish or drink.
Yet this small predator is the focus of an unlikely reintroduction programme by conservationists working in a small laboratory deep in the Scottish Highlands, at a wildlife park best known for its polar bears, wildcats and wolves.
They want to see hundreds released into Scottish lochs and streams, in the first project to repopulate the countryside with leeches after hundreds of years of habitat loss and exploitation.
Scientists with the conservation charity Buglife and the Royal Zoological Society of Scotland have captured 14 specimen leeches from a loch near Oban on the west coast – one of only three places where they are known to survive in Scotland – and hope they will start breeding next year.
The medicinal leech, Hirudo medicinalis, is one of 37 priority species being protected as part of the “Species on the edge” conservation programme run by NatureScot, the government conservation agency and seven environment groups, involving a host of plants, butterflies, birds, bats and bugs.
The largest of 17 leech species that live in the UK, medicinal leeches were once abundant until doctors in the late 1700s latched on to their medical uses.
Before the advent of modern medicine, physicians believed bleeding with leeches could help balance the body’s “humours”, to cure illnesses and maladies. They were harvested in their millions and exported in huge quantities overseas. Women were sent bare-legged into water to act as bait, some of whom reputedly became anaemic. Pottery firms designed special tight-lidded jars to hold leeches for use in hospitals and surgeries.
Even as their exploitation by humans faded in the early 20th century, habitat loss drove down their populations to near-extinction in much of the UK. They need shallow, warmer ponds and lochs, with flowing vegetation, a good population of amphibians to feed on, and stone banks to lay their cocoons of eggs on.
NHS consultants across the UK, including hospital trusts in the Midlands, Avon, Humberside and Merseyside have reintroduced leeches for operations, as their natural anticoagulants help keep blood moving during surgery. They are using a Mediterranean variety, Hirudo verbana, farmed by a leech supplier near Swansea.
Hirudo medicinalis are legally protected under the Wildlife and Countryside Act due to their scarcity, making it unlawful to harvest them without special permission, ruling them out for medical use.
Until he found a third site near the Solway Firth in Dumfries and Galloway a few weeks ago, Craig Macadam, the conservation director of the invertebrates charity Buglife, had feared there were only two places left in Scotland where they clung on.
One was on the Hebridean island of Islay, where a small loch is being restored to help conserve them, and the second was near Oban, where their breeding population was harvested. England and Wales have about 18 isolated populations, also the focus of conservation programmes.
Macadam waded into the water to catch them, wearing anglers’ waders rather than leaving his legs bare. “You splash about and they come to you,” he said.
Medicinal leeches react that way with their prey, such as frogs and newts, waterbirds such as coots and moorhens, or deer and cows that come to the water to drink. Vibrations in the water trigger their interest.
Macadam has been researching the species for five years. On a whim, he tried searching for leech-derived placenames using the first Ordnance Survey map of Scotland and found 28 using the Gaelic for loch of the leech, Lochan nan Deala.
“If we’re reintroducing megafauna like wolves and lynx, we should be introducing their parasites as well,” he said. That is “part and parcel” of restoring and preserving healthy ecosystems. “It’s no different to doing conservation with a large cuddly animal.”
Helen Taylor, who runs the RZSS breeding project at the Highland Wildlife Park near Kingussie, acknowledges that leeches are far removed from the charismatic totems of Scottish rewilding such as golden eagles and beavers, or the wildcats also being bred for release at the park.
Now living in fish tanks in a pristine lab at the park, the leech is one four invertebrates that the RZSS is breeding there, including the pine hoverfly, the dark-bordered beauty moth and the pond mud snail.
She likens the task of preserving invertebrates to keeping a plane flying safely. Even if their exact value in a local ecosystem may be unclear, they are akin to rivets in the aircraft’s wing. Losing too many rivets, or the wrong one, can “cause the airplane to fall out of the sky”, she said.
“We’re working at both ends of the scale: we can’t do one without the other. We have to look at a whole-ecosystem approach,” Taylor said. “I would love to see a future where we can regularly see medicinal leeches in our lochs.”
Small gains
Alongside its better-known wildcat and polar bear breeding programmes, the Royal Zoological Society of Scotland is working on reintroducing far less glamorous invertebrates too. These include:
Pine hoverfly
A tiny but critically endangered insect now only seen in the Cairngorms, it is a pollinator and waste recycler: its larvae eat dead wood and can survive being frozen. RZSS specialists have bred thousands of larvae and released them in carefully selected tree stumps in the forest.
Dark-bordered beauty moths
Found in only two places in Scotland and one in England, caterpillars of this critically endangered moth were released for the first time in July after being bred at a purpose-built RZSS facility. The creature feeds on aspen suckers, a relatively rare tree often grazed by deer and livestock.
Pond mud snails
Listed as vulnerable, these aquatic snails were once found across Britain, living in ponds, ditches and marshland, hibernating in the mud when the water dries out. They help recycle organic waste and are indicators of the health and cleanliness of freshwater ecosystems.
