They are the UK’s largest leech, which grows up to 20cm long and uses three sets of saw-like jaws to help suck up to ten times their body weight in blood in a single meal.
The medicinal leech (Hirudo medicinalis), driven almost to extinction in Scotland after centuries of farming for medicinal use, could make a comeback thanks to a conservation breeding project.
The Royal Zoological Society of Scotland (RZSS) is celebrating its most successful breeding season yet for the “extremely rare” species.
They have welcomed 30 hatchlings from 12 cocoons at a specially designed breeding facility for the bloodsuckers at their Highland Wildlife Park near Aviemore.
The project, now in its second year, is led by RZSS in partnership with the charity Buglife Scotland and Species on the Edge, a multi-partner programme helping vulnerable and threatened wildlife.
The project aims to reintroduce the misunderstood creatures to suitable sites across Scotland to help rebuild wild populations and reverse the species’ long-term decline.
Dr Helen Taylor, RZSS’s deputy head of conservation, said: “It is incredibly rewarding to see so many hatchlings after months of careful work by our dedicated teams.
“Medicinal leeches are often misunderstood, but they are a fascinating and beautiful species that play an important role in healthy wetland ecosystems.
“This year’s hatchlings are thriving and growing rapidly, with most having doubled or even tripled in size already.
“It is amazing to think that these tiny individuals could one day help the species recover in the wild.”
Once widespread across Scotland, medicinal leeches suffered severe population declines due to over-harvesting for use in traditional medicine, a practice that can trace its origins over 3500 years to Ancient Egypt.
For centuries, they were farmed for use in medicine, in the belief that bloodletting could cure a number of ailments.
Habitat loss, pollution and changes in land management have further reduced their numbers.
Today the species is known from only three locations in Scotland: in Argyll, Islay and in Dumfries and Galloway.
Medicinal leeches lay their eggs in sponge-like cocoons that they make themselves from their own slime.
The cocoons provide a safe place for the eggs to grow and develop before the tiny leeches start to emerge from a hole at the front.
Adults can grow up to 20cm long, and are greenish-brown with a thin red stripe and suckers, at each end — one used for leverage and the other with three sets of saw-like jaws with 100 sharp edges used to pierce the host.
After piercing the skin, they suck out blood while injecting blood thinners, with large adults able to consume up to ten times their body weight in a single meal.
The breeding programme was made possible after RZSS, Buglife Scotland and Species on the Edge identified and captured a group of the leeches in 2023.
This year’s hatchlings have now had two feeds of blood and are growing well, and will soon go into torpor — a kind of light hibernation — for the winter.
The programme aims to have enough healthy leeches to begin releasing them into the wild and start new populations in carefully selected Scottish lochs.
By breeding and releasing medicinal leeches into these sites, RZSS and its partners hope to give the “unique and ecologically valuable species” a chance to recover and reclaim its place in Scotland’s natural heritage.
Craig Macadam, Buglife Co-Leader and Director of Conservation, said: “This success demonstrates what’s possible when conservation organisations unite behind a shared vision — turning scientific expertise and dedication into real, measurable progress for endangered species.
“Medicinal leeches may seem obscure to many, but this step on their recovery in Scotland is a reminder that no species is too small, too misunderstood, or too lost to deserve our commitment to its survival.”
