The medicinal leech is one of nature’s most underappreciated hunters. Armed with three powerful interlocking jaws and with a taste for blood, they swim eagerly toward humans, deer, or livestock that wander into their ponds to bathe, fish, or drink.
Yet this small predator is the subject of an unlikely reintroduction program led by ecologists working in a small laboratory in the heart of the Scottish Highlands, in a wildlife park best known for its polar bears, wild cats 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 from conservation charity Buglife and the Royal Zoological Society of Scotland captured 14 leech specimens in a loch near Oban on the west coast – one of only three places where they survive in Scotland – and hope they will start breeding next year. .
The medicinal leech, Medicinal Hirudois one of the 37 priority species protected under the “Species on the margins” conservation program run by NatureScot, the government conservation agency and seven environmental groups, involving a multitude of plants, butterflies, birds, bats and insects.
The largest of 17 leech species living in the UK, medicinal leeches were once abundant until doctors in the late 1700s. hooked on their medical uses.
Before the advent of modern medicine, doctors believed that bleeding with leeches could help balance the body’s “humours”, to cure illnesses. They were harvested by the millions and exported in large quantities abroad. Women were sent bare-legged into the water to serve as bait, and some of them became anemic. Pottery companies designed special jars with tight-fitting lids to hold leeches for hospitals and doctors’ offices.
Although their exploitation by humans declined in the early 20th century, habitat loss brought their populations to the brink of extinction across much of the UK. They need shallow, warmer ponds and lakes with free-flowing vegetation, a good population of amphibians to feed on, and stone banks on which to lay their egg cocoons.
NHS consultants across the UK, including hospitals in the Midlands, Avon, Humberside and Merseyside, have reintroduced leeches for operations, as their natural anticoagulants help keep the blood flowing during surgery. They use a Mediterranean variety, Verbena Hirudo, bred by a leech supplier near Swansea.
Medicinal Hirudo are legally protected by the Wildlife and Countryside Act due to their rarity, which makes it illegal to harvest them without special permission, excluding them for medicinal purposes.
Until finding a third site near the Solway Firth in Dumfries and Galloway a few weeks ago, Craig Macadam, director of conservation at invertebrate charity Buglife, feared only two places in Scotland where they clung.
One was on the island of Islay in the Hebrides, where a small lake is being restored to help with their conservation, and the second was near Oban, where their breeding population was harvested. England and Wales have around 18 isolated populations, which are also the subject of conservation programmes.
Macadam waded into the water to catch them, wearing fisherman’s waders rather than leaving his legs bare. “You splash around and they come to you,” he said.
Medicinal leeches react this way with their prey, such as frogs and newts, water birds like coots and moorhens, or deer and cows that come to drink from the water. Vibrations in the water trigger their interest.
Macadam has been studying the species for five years. On a whim he tried to look up place names derived from leeches using the first Ordnance Survey map of Scotland and found 28 using the Gaelic for leech loch, Lochan nan Deala.
“If we reintroduce megafauna like wolves and lynx, we should also introduce their parasites,” he said. This is “integral” to restoring and preserving healthy ecosystems. “It’s no different than doing conservation with a big stuffed animal.”
Helen Taylor, who runs the RZSS breeding project at the Highland Wildlife Park near Kingussie, acknowledges that leeches are a far cry from the charismatic totems of Scottish rewilding such as golden eagles and beavers, or wild cats also bred for release in the park.
Now living in aquariums in a pristine laboratory at the park, the leech is one of four invertebrates that the RZSS breeds there, including the pine hoverfly, the dark-bordered moth and the pond mud snail.
She compares the task of preserving invertebrates to keeping a plane safely in flight. Although their exact value in a local ecosystem is unclear, they are akin to rivets in the wing of an airplane. Losing too many rivets, or one bad rivet, can “knock the plane out of the sky,” she said.
“We work at both ends of the scale: we can’t do one without the other. We need to look at a whole ecosystem approach,” Taylor said. “I would like to see a future where we can regularly see medicinal leeches in our lochs.”
Small gains
Alongside its more well-known wildcat and polar bear breeding programmes, the Royal Zoological Society of Scotland is also working to re-introduce much less glamorous invertebrates. 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 frost. The RZSS specialists have raised thousands of larvae and released them into carefully selected tree stumps in the forest.
Beauty butterflies with dark edges
Found in just two locations in Scotland and one in England, the caterpillars of this endangered butterfly were released for the first time in July after being reared in a specially built RZSS facility. The creature feeds on aspen suckers, a relatively rare tree often grazed by deer and livestock.
Pond Mud Snails
Classified as vulnerable, these aquatic snails They were once found throughout Britain, living in ponds, ditches and marshes, hibernating in the mud when the water dried up. They help recycle organic waste and are indicators of the health and cleanliness of freshwater ecosystems.
