Hirudo medicinalis, the European medicinal leech, is one of several species of leeches used as "medicinal leeches".
Other species of Hirudo sometimes also used as medicinal leeches include H. orientalis, H. troctina, and H. verbana. The Asian medicinal leech includes Hirudinaria manillensis, and the North American medicinal leech is Macrobdella decora.
Medicinal leech populations were severely reduced in many countries during the 19th century due to the high demand in medical contexts, and remain endangered in many countries today.
Morphology
The general morphology of medicinal leeches follows that of most other leeches. Fully mature adults can be up to 20 centimeters 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 resemble saws, on which are approximately 100 sharp edges 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 while injecting anticoagulants (hirudin). 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 feedings.
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. A study done in Poland found that medicinal leeches sometimes breed inside the nests of large aquatic birds, noting that conservation efforts directed at bird habitats may also indirectly help preserve dwindling leech populations.
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 through drainage has also contributed to their decline. Another factors include the replacement of horses - medicinal leeches' preferred host species - by motor vehicles and mechanical farming equipment, and the 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, located at Lydd, England, 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. The species is protected under Appendix II of CITES meaning international trade (including in parts and derivatives) is regulated by the CITES permitting system.
Medical use
Beneficial secretions
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.
Historically
The first recorded use of leech therapy was 3,500 years ago in Ancient Egypt. The next recorded uses of leeches in medicine come in the last few centuries BCE, by the Greek physician Nicander in Colophon and in the ancient Sanskrit text Sushruta Samhita. Leech therapy is mentioned a few hundred years later in Shennong Ben Cao Jing, a 3rd-century CE book of traditional Chinese medicine.
Medical use of leeches was discussed by Avicenna in The Canon of Medicine (1020s), and by Abd al-Latif al-Baghdadi in the 12th century.
These sources indicated leech therapy for a wide variety of ailments, including edema,. "blood stasis", and skin diseases.