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The Leech: the Secret Life of a Bloodletter

Updated on July 14, 2010

The leech and medicine

The leech has been made infamous by its use in medicine as a method of bloodletting. Bloodletting was a ferociously persistent but erroneous idea that certain diseases were the result of too much blood causing an imbalance in one’s internal fluids or humors. The leech was a preferred method of draining blood. In all fairness it should be pointed out that many leech species are predators rather than parasites. However, the blood suckers did have some ideal traits for bloodletting, given their feeding method.

Leeches tend to inhabit water or particularly damp environments. They are external parasites and as such they have evolved methods of attaching themselves to their hosts without the host even knowing they are there. They attach with an interior and posterior sucker. They typically anesthetize the area where they bite. They then inject an anti-coagulant, named huridin (after their scientific classification Hirudinea). This makes blood flow readily and the leech gorges itself. From the leech’s prospective it would be ideal for it to attach on, quickly gorge itself (with blood several times its body weight) and detach without the host noticing its presence.

As their application was so painless and they drew blood so effectively they were favored blood letters. In the nineteenth century more effective remedies were developed and the leech fell out of favor until recently, where they have been found useful to deal with certain complications of surgery (when, as one might imagine, blood flow has been impeded for various reasons). Scientists are also curious about hirudin as many prevalent diseases like heart disease and strokes, are caused by blockage of blood vessels.

Leech crawling on rock
Leech crawling on rock
medical leeches on sale in Istanbul for eczemas (skin condition) and varices (distended veins)
medical leeches on sale in Istanbul for eczemas (skin condition) and varices (distended veins)

Leeches in the wild

So what happens if you encounter leeches in the wild? How might one prevent leeches from attaching and how do you get a leech off? Leeches like water and dampness. The fastest way to get a leach stuck on you is to put your exposed skin into bodies of water with leeches in them. (This is actually an effective method for harvesting them for sale.) Covering exposed skin is prudent, but like many smaller animals they have a way of getting past such things. However the individual leech should not be much of a concern, its bite is painless and is not capable of removing enough blood to be much of a threat.

Still, some care should be put into removing a leech because if it is removed improperly it may regurgitate its blood into your now open wound. The leech is attached by suckers. It doesn’t bury into the flesh at all (although it does bite), so the best method is to detach the suckers by sliding your finger nails under the leech and breaking the seal (there’s on one both ends). This leads the leech to close its jaws and it can be brushed aside. The one thing to note is that the wound, having been injected with hirudin will bleed much long than a normal wound of the same size. The wound has to be kept clean but leeches generally do not pass on infections, since they can gorge in enough blood in a single feeding to last them for months

Some fun facts

The leech is actually a worm. Like the earthworm it is an annelid, or segmented worm (34 segments to be precise)

The leech is dependent on certain forms of bacteria to digest the blood it gorges

Owing much to overharvesting (among other reasons), the European medical leech is considered threatened.


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