Medicinal leech

Overview
Medicinal leeches are any of a group of several species of leeches but most commonly Hirudo medicinalis or the European Medical Leech. General morphology follows that of most other leeches. Fully mature adults can be up to 20cm in length and are green, brown or greenish brown with a darker tone on the dorsal side and a lighter ventral side, the dorsal side also has a thin red stripe. Their range extends over almost the whole of Europe and into Asia as far as Kazahkstan and Uzbekistan. The preferred habitat for this species is muddy freshwater pools and ditches with plentiful weed growth. They feed by attaching to vertebrates via the suckers at either end, piercing the skin and injecting anti-coagulants (Hirudin) and anaesthetics before sucking out blood. Large adults can consume up to 15 grams of blood in a single meal. Medicinal leeches are hermaphrodites and reproduce by sexual mating, laying eggs in clutches of up to 50 near (but not under) water, and in shaded, humid places.

Range & Distribution
Over-exploitation in the 19th Century has left only scattered populations and reduction in natural habitat though drainage has also contributed to their decline. Another factor has been the replacement of horses in farming (horses were medicinal leeches' preferred food source) and provision of artificial water supplies for cattle. As a result this species is now considered vulnerable by the IUCN and European Medicinal Leeches are legally protected through nearly all of their natural range. They are particularly sparsely distributed in France and Belgium, and in the UK there may be as few as 20 remaining isolated populations (all widely scattered), the largest (at Lydd) is estimated to contain several thousand individuals, 12 of these areas have been designated Sites of Special Scientific Interest. There are small transplanted populations in several countries outside their natural range including the USA.

Medicinal Use
In medieval and early modern medicine, the medicinal leech (Hirudo medicinalis and its congeners Hirudo verbana, Hirudo troctina and Hirudo orientalis) was used to remove blood from a patient as part of a process to "balance" the "humors" that, according to Hippocrates, must be kept in balance in order for the human body to function properly. (The four humors of ancient medical philosophy were blood, phlegm, black bile, and yellow bile.) Any sickness that caused the subject's skin to become red (e.g. fever and inflammation), so the theory went, must have arisen from too much blood in the body. Similarly, any person whose behavior was strident and "sanguine" was thought to be suffering from an excess of blood.

Modern medicine again has a use for medicinal leeches. They provide an effective means to reduce blood coagulation, relieve pressure from pooling blood, especially after plastic surgery, and stimulate circulation in reattachment operations for organs with critical blood flow, such as eye lids, fingers, and ears.

Devices called "mechanical leeches" have been developed which dispense herapin and perform the same function as medicinal leeches, but they are not yet commercially available.