The following writeup may be disturbing to those with particularly delicate constitutions, since it's about letting slimy things suck out your blood. Well, put down the tea and feather duster and grow a pair, because it's a fascinating subject.

Leeches in history

Bloodletting is a part of medical practice dating to ancient times; it was attested in the ancient Greek medical texts compiled by Hippocrates. The Greek conception of health was based around the four humors: phlegm, black bile, yellow bile, and blood, each associated with one of the four elements. Imbalances of the humors - an overabundance or deficiency in one of them - was considered the origin of disease, and so bloodletting was a logical treatment when a patient had too much blood. It was mainly accomplished in two ways - either by cutting into a vein and allowing blood to escape, or by applying leeches, which took blood at a slow, predictable rate, and fell off when full.

Leeches were used for bloodletting in classical Greece and Rome and they continued to be a central part of European medical practice throughout the ages. Indeed, the word "leech" comes from an Old English word for physician. Outside Europe, illustrations of leech application on the walls of the pyramids of Egypt dating from 1500 BCE demonstrate its popularity. A first century CE medical text from China describes the use of leeches; other references are found in Sanskrit, Persian, and Arabian literature.

Bloodletting, both through incisions into the veins and through leeching, remained popular in Europe through the Middle Ages, and by the Renaissance, bloodletters practiced in public baths. With the dawn of science, leeches gained further popularity. In the first half of the 19th century, the scientific journals were full of articles extolling their virtues. Trade in medicinal leeches was becoming a major industry; they were so heavily harvested (by people who bravely waded into swamps and allowed them to attach to their legs) that they became nearly extinct in Europe, and were imported heavily during this period.

The use of leeches was often quite gruesome: a medical text from 1634 describes the application of leeches to "the gums, lips, nose, fingers" and even "the mouth of the womb". They would be applied to the part of the body that was infected: the interior of the nostrils for some conditions, and even to the tonsils in cases of tonsilitis. Dozens would be applied, and stored in special jars when they were full until they were ready to be used again. Sometimes a patient would be bled continuously for days at a time.

By the mid 19th century, the fad for leeching had begun to fade as it became clearer that in many cases, overuse of them was hastening deaths rather than preventing them. An 1854 paper on the self-limiting nature of disease hurt leeches' popularity, as doctors began to recognize that mere coincidence had made leaches seem effective in many cases. Leeches then became identified with superstitions of earlier times and eventually they were used very little, if at all.

Modern use

In the 1980s, reports began to surface of a renewed use of leeches in medicine. While they are no longer used for quite as many purposes as previously, they are still extremely useful for one particular circumstance. Their ability to draw off blood in a slow, controlled fashion is invaluable when for some reason there is inadequate drainage of blood from a site. If the arteries are working normally but the veins are not, blood can pool, preventing new blood from entering - eventually leading to tissue death. Leeches can be used to draw off blood, functioning almost like an extra vein to ensure that normal circulation can happen.

This is very useful in reattachment of severed body parts and in plastic surgeries, as it's easier to attach arteries than the much smaller veins. Leeches may be used for several days, permitting new veins to establish themselves (it takes around five or six days) so that normal circulation can develop. More conventional approaches to bloodletting in these cases are not successful - slicing open the tissue and even treatment with anticoagulants doesn't work. So leeches are now available for medical purposes. They're relatively cheap and after they feed for twenty to thirty minutes, blood continues to ooze out for up to a day.

Minor complications occur with their use, though. For one, once the leeches are full they tend to ooze off the patient and hide in dark corners to frighten other patients. Sometimes they even hide in patients' orifices if they're not monitored fairly carefully. Also, many patients are reluctant to use them. Surprisingly, though, once the treatment is underway patients don't seem to mind as much. Not only are they the only alternative to loss of a digit or a part of the body in some cases, but the bites are painless and patients begin to enjoy their slimy new friends. Sometimes they even give them names. A leech can be reused after several days (being stored in the refrigerator in the meantime - they enter hibernation when chilled).

How they work

The medical leech, Hirudo medicinalis, secretes several chemicals that make it useful. It produces hirudin, a small protein that acts as an anticoagulant by blocking the thrombin protein that is required in clotting. It produces anaesthetics that make the bite painless, as well as vasodilators, which promote circulation. The leech's gut is colonized with only one bacterium, Aeromonas veronii, which has a particular symbiotic relationship with leeches. The bacterium aids the leech in digesting blood and produces antibiotics that play a role in preventing infections. Occasionally, patients become infected with A. veronii, though this is rare in patients with normal immune systems as long as arterial bloodflow is sufficient. Prophylactic antibiotics are generally used to ensure that infection doesn't occur.

The hirudin present in leech saliva means that a bite will ooze for several hours to a day after the leech is removed, which means that a single leech can eventually remove 150ml of blood from the application site. Hirudin is so useful that it has been produced by genetically engineered bacteria. Hirudin is still under study, but one day it may end up being more useful than heparin, another common anticoagulant which is used in the treatment of heart attacks. However, despite the artificial production of hirudin, leeches are here to stay at least for the time being in their particular niche use. Pinprick injections of anticoagulants to mimic leech bites don't seem to have the same effect in draining blood. Devices have been marketed as "artificial leeches" but none work as well as the real critters. Leeches seem to perform a service that modern medicine has not replaced.

Biology of Hirudo medicinalis

Medicinal leeches are one of 650 species of leech. They belong to phylum Annelida, the segmented worms. Medicinal leeches have 33 or 34 segments, with brown to black upper surfaces with six reddish-brown stripes running lengthwise. The bottom is speckled, and each end has a sucker used to attach to food sources. The forward one surrounds the mouth which has three jaws and many tiny teeth. They also have five pairs of eyes.

They are hermaphrodites - each leech has male and female gonads, and they reproduce on land. They move on land by inching like inchworms, while moving in water with a waving motion. They normally hang out on the shoreline, hidden under large objects and watching for large mammals through a reflexive response to shadows. They'll go as long as six months between feedings, which is convenient for storing them for medical use. They're naturally found in small, muddy ponds in much of western and southern Europe.

Other uses for leeches

There is very preliminary evidence from a medical study published in the Annals of Internal Medicine compared the effectiveness of leeches to painkilling gel as a treatment for osteoarthritis pain in the knee. Leeches seem to reduce pain for at least a week, which is better than any conventional treatment. It was a small study - 24 patients got leeched, compared to 27 in the painkiller group, and of course it couldn't be be conducted blindly. Nevertheless, this suggests that leeches still have new secrets for medicine.