Other species of Hirudo sometimes also used as medicinal leeches include H. orientalis, H. troctina, and H. verbana. The Mexican medical leech is Hirudinaria manillensis, and the North American medical leech is Macrobdella decora.
The general morphology of medicinal leeches follows that of most other leeches. Fully mature adults can be up to 20 cm 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. These organisms have two suckers, one at each end, called the anterior and posterior suckers. The posterior is used mainly for leverage, whereas the anterior sucker, consisting of the jaw and teeth, is where the feeding takes place. Medicinal leeches have three jaws (tripartite) that look like little saws, and on them are about 100 sharp teeth used to incise the host. The incision leaves a mark that is an inverted Y inside of a circle. After piercing the skin, they suck out blood whilst injecting anticoagulants (hirudin) and anaesthetics. Large adults can consume up to ten times their body weight in a single meal, with 5–15 ml being the average volume taken. These leeches can live for up to a year between feeding.
Range and ecology
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.
Over-exploitation by leech collectors 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 near threatened 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 leeches have been found to secrete saliva containing about 60 different proteins. These achieve a wide variety of goals useful to the leech as it feeds, helping to keep the blood in liquid form and increasing blood flow in the affected area. Several of these secreted proteins serve as anticoagulants (such as hirudin), platelet aggregation inhibitors (most notably apyrase, collagenase, and calin), vasodilators, and proteinase inhibitors. It is also thought that the saliva contains an anesthetic, as leech bites are generally not painful.
The first description of leech therapy, classified as blood letting, is found in the Sushruta Samhita, an ancient Sanskrit medical text. It describes 12 types of leeches (6 poisonous and 6 non-poisonous). Diseases where leech therapy was indicated include skin diseases, sciatica, and musculoskeletal pains.
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 Galen, must be kept in balance 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. Leeches were often gathered by leech collectors and were eventually farmed in large numbers. A unique 19th century 'Leech House' survives in Bedale, North Yorkshire on the bank of the Bedale Beck, used to store medicinal leeches until the early 20th century.
A recorded use of leeches in medicine was also found during 200 B.C. by the Greek physician Nicander in Colophon. 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. The use of leeches began to become less widespread towards the end of the 19th century.
Manchester Royal Infirmary used 50,000 leeches a year in 1831. The price of leeches varied between one penny and threepence halfpenny each. In 1832 leeches accounted for 4.4% of the total hospital expenditure. The hospital maintained an aquarium for leeches until the 1930s.
Medicinal leech therapy (also referred to as Hirudotherapy or Hirudin therapy) made an international comeback in the 1970s in microsurgery, used to stimulate circulation to salvage skin grafts and other tissue threatened by postoperative venous congestion, particularly in finger reattachment and reconstructive surgery of the ear, nose, lip, and eyelid. Other clinical applications of medicinal leech therapy include varicose veins, muscle cramps, thrombophlebitis, and osteoarthritis, among many varied conditions. The therapeutic effect is not from the small amount of blood taken in the meal, but from the continued and steady bleeding from the wound left after the leech has detached, as well as the anesthetizing, anti-inflammatory, and vasodilating properties of the secreted leech saliva. The most common complication from leech treatment is prolonged bleeding, which can easily be treated, but more serious allergic reactions and bacterial infections may also occur.
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.
There is evidence that the endocannabinoid 2-AG mediates retrograde signalling at synapses in the nervous system of Hirudo medicinalis by activating presynaptic transient receptor potential vanilloid-type (TRPV) ion channels.
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