Harvested to near extinction in the 19th century medicinal leeches have reappeared in the wild and are simultaneously reared in laboratories as vital medical aid, writes PETER FROST.
When it comes to threatened species it is hard to get romantic about the medicinal leech (Hirudo medicinalis) — it just doesn’t have the cuddly attraction of red squirrels or wild cats.
By the beginning of the 20th century, the medicinal leech had disappeared from most of its former ranges in Europe and was declared totally extinct in the British Isles.
The decline in the species was almost certainly due to over-collecting from the wild throughout the 19th century, hence today it is a threatened and protected species in its natural habitat.
The trade in medicinal leeches was enormous. In 1833 alone, approximately 42 million leeches were imported into France for medicinal use and 30m were sent to the US from Germany every year.
In 1802 William Wordsworth had described the trade in verse. Even then the reductions in population caused concern:
He with a smile did then his words repeat
And said that gathering leeches far and wide,
He travelled, stirring thus about his feet
The waters of the pools where they abide
Once I could meet with them on every side
But they have dwindled long by slow decay,
Yet still I persevere and find them where I may.
Collecting was easy — you simply paddled in the stream and the leeches hooked on to your legs and feet to feed. Out of the water you simply peeled them off healthy and plump with your blood.
Leeches were a huge part of medicine at the time and there were few ailments for which they were not used. It was thought that the leeches could suck bad blood leaving good blood behind.
The trade in Britain and across Europe was vast but it was not sustainable and eventually there were no more. The beast was extinct and medicine moved on to less gory cures.
Then in the 1970s, medicinal leech populations started to be found again scattered across the British Isles, though not in Ireland where is it still thought to be extinct.
In 1978 a leech attached itself to a dog that had been swimming in a disused gravel pit at Dungeness in Kent. It turned out to be the previously believed extinct medicinal leech and over the next six years that same gravel pit was found to have a population of over ten thousand and more were discovered in other pits in the Dungeness area.
Strangly these were in shingle pits that had been dug and flooded only in the last 50 years. Where had the previously extinct leeches come from?
One theory suggests the leeches may have arrived from France attached to birds. France is only a few dozen miles flying distance away and leeches can attach themselves for more than an hour while feeding.
Perhaps more likely is that they were never entirely extinct in Kent — in 1985 one was found in a ditch near Dungeness on the Romney Marsh (below).
Quite possibly small, remote populations have survived throughout the period that they were thought to be extinct.
?The Romney Marsh is an expanse of approximately 27,000 hectares of low-lying land in the south-eastern corner of England. Its most prominent feature is the great shingle promontory of Dungeness that juts out into the English Channel just along the coast to the south and west of Dover. ??
Although over-collection of leeches in the 19th century has been blamed for much of its decline, it is likely that habitat loss brought about by changes in land use, draining of marshland and increased management of aquatic vegetation and water levels has also contributed.??
Medicinal leeches have been successfully bred in laboratories for some time but we know little about their actual breeding cycle in the wild.
It leaves the water to lay its eggs, which are coated in a spongy protective cocoon. The cocoons must be deposited in a place where they will not dry out nor become waterlogged and from where the young can easily reach the water after hatching.
?Recently, the leech has made a comeback in medicine and it is being laboratory-reared in large numbers for this purpose. Bloodletting has become a proven medical technique particularly valuable in plastic and reconstructive surgery.
Leech therapy is now being used to restore circulation to grafted tissues and reattached body parts — its saliva, we now know, also contains many useful medical compounds that have anaesthetic, vasodilatory, anticoagulant and clot-dissolving properties.
These developments are an excellent example of the necessity of maintaining the biological diversity of our environment. They may not be cuddly but we cannot afford to let the leech or indeed any other species face the threat of extinction.
First appeared in the Morning Star 13 May 2016.