Hirudo medicinalis

Medicinal leeches have been found to secrete saliva containing about 60 different proteins.[6] 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.[7] It is also thought that the saliva contains an anesthetic,[8] as leech bites are generally not painful.

Treating mastitis of a cattle with leech

A caricature of a physician prescribing leeches for a weak, bedbound woman

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 venomous and 6 non-venomous). Diseases where leech therapy was indicated include skin diseases, sciatica, and musculoskeletal pains.

Earthenware jar for holding medicinal leeches

In medieval and early modern medicine, the medicinal leech (Hirudo medicinalis and its congeners H. verbana, H. troctina, and H. 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 BC by the Greek physician Nicander in Colophon.[4] 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. The use of leeches began to become less widespread towards the end of the 19th century.[4]

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.[9]

Medicinal leech therapy (also referred to as Hirudotherapy or Hirudin therapy) made an international comeback in the 1970s in microsurgery,[10][11] used to stimulate circulation to salvage skin grafts and other tissue threatened by postoperative venous congestion,[10][12] particularly in finger reattachment and reconstructive surgery of the ear, nose, lip, and eyelid.[11][13] Other clinical applications of medicinal leech therapy include varicose veins, muscle cramps, thrombophlebitis, and osteoarthritis, among many varied conditions.[14] 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.[4] 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.[4] Leech therapy was classified by the US Food and Drug Administration as a medical device in 2004.[15]

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 synthesized 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.[16][17][18]