DNA barcoding work led by Mark Siddall at the American Museum of Natural History has revealed that commercially available medicinal leeches, until now assumed to be the species Hirudo medicinalis, used around the world in research and after surgery, are actually a closely related but genetically distinct species, Hirudo verbana. Moreover, the study has shown that wild European medicinal leeches comprise at least three distinct species, not one.

After attaching its head sucker to the skin, the leech uses its three jaws with razor-sharp teeth to make a neat Y-shaped cut. Salivary ductules between the teeth secrete several pharmacologically active substances, including a local anesthetic and the potent anticoagulant hirudin.

But the discovery is not all bad news. "This raises the tantalising prospect of three times the number of anticoagulants, and three times as many biomedically important protease inhibitors as previously thought," said Dr Siddall.

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That researchers have been mistakenly using Hirudo verbana in their work for decades may call much of this research, including hundreds of scientific publications, into question and force a reconsideration of what scientists think they know about this widely studied species. And they add that Hirudo verbana should also be offered protection.

Hirudo medicinalis, described by the father of taxonomy Linnaeus in 1758, has long been considered the sole European medicinal leech. Yet, as early as 1827, at least five additional species were recognised. Now a study of their DNA has provided the first conclusive way to tell them apart.

It occurs in ponds, waters that dry up periodically, floodplain pools and small lakes. Other ecological requirements include abundant hosts (frogs, cattle and horses), silty water bottoms, dense submerged and emergent vegetation and gently sloping banks favourable for laying cocoons.

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Their analysis clearly showed that the commercial and laboratory specimens were not Hirudo medicinalis, as they were labelled, but rather Hirudo verbana.

"However, it will also require a more nuanced effort aimed at conserving these much-maligned animals, and in a manner that takes into account their impressive diversity."

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As well as the doctors who use them today in plastic surgery, numerous researchers who have studied the medicinal leech have also been using a different kind of blood sucker from the one they thought, according to a DNA study that now calls their findings into question.

Their range extends over almost the whole of Europe and into Asia as far as Kazakhstan and Uzbekistan. The preferred habitat for this species is muddy freshwater pools and ditches with plentiful weed growth in temperate climates.

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The European medicinal leech can be used for bloodletting. However, related species are usually used for medical purposes and in life sciences as model organisms.  The European medicinal leech is a classic laboratory object used for educational purposes.

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Hirudo medicinalis is distributed from Britain and southern Norway to the southern Urals and probably as far as the Altai Mountains, occupying the deciduous arboreal zone.

Because of the minuscule amounts of hirudin present in leeches, it is impractical to harvest the substance for widespread medical use. Hirudin (and related substances) are synthesised using recombinant techniques. Devices called "mechanical leeches" that dispense heparin and perform the same function as medicinal leeches have been developed, but they are not yet commercially available.[11][12][13]

Dr. Siddall and his colleagues analysed the DNA of wild leeches from across their range in Europe, as well as from samples supplied by commercial providers and university laboratories that use leeches to study the nervous system and the way genes regulate development.

Other Hirudo species sometimes used as medicinal leeches include (but are not limited to) Hirudo orientalis, Hirudo troctina, and Hirudo verbana. The Mexican medical leech is Hirudinaria manillensis, and the North American medical leech is Macrobdella decora.

Medicinal leeches are hermaphrodites that reproduce by sexual mating, laying eggs in clutches of up to 50 near (but not under) water, and in shaded, humid places.

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The first description of leech therapy, classified as blood letting, was found in the text of Sushruta Samhita, written by Sushruta in 800 B.C., who was also considered the father of plastic surgery. He described 12 types of leeches (6 poisonous and 6 non-poisonous). Diseases where leech therapy was indicated were skin diseases, sciatica, and musculoskeletal pains.

Commercially available European medicinal leeches also are used extensively by biomedical researchers studying biological processes such as blood coagulation, developmental genetics, and neurobiology.

A recorded use of leeches in medicine was also found during 200 B.C. by the Greek physician Nicander in Colophon.[2] Medical use of leeches was discussed by Avicenna in The Canon of Medicine (1020s), and by Abd-el-latif al-Baghdadi in the 12th century.[citation needed] The use of leeches began to become less widespread towards the end of the 19th century.[2]

Dr Siddall reported his discovery in the the Proceedings of the Royal Society B, with Peter Trontelj from the University of Ljublajana in Slovenia, Serge Utevsky from the V. N. Karazin Kharkiv National University in Ukraine, Tripp Macdonald of Rutgers University, and Mary Nkamany from the City University of New York.

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This carries significant regulatory implications, said the team: Hirudo verbana has not been approved by the US Food and Drug Aministration - as was thought to have occurred in 2004 - and it has no special conservation status, unlike Hirudo medicinalis, which is still afforded protection under various conservation conventions.

Medicinal leeches are any of several species of leeches, but most commonly Hirudo medicinalis, the European medicinal leech.

Studies of commercial specimens have figured prominently in the discovery and production of anticoagulants and so called protease inhibitors, some of which may have cancer-fighting properties.