In the classic film The African Queen, Humphrey Bogart noted, "If there's anything in the world I hate, it's leeches — filthy little devils!" Why has the U.S. Food and Drug Administration approved the use of these lowly and much maligned creatures?
Leeches (Hirudo medicinalis) have been used in medicine for over 2,000 years. Syrian physicians began using leeches for blood removal (called bloodletting) as early as 100 B.C. Over the years, bloodletting likely harmed more people than it helped. In fact, George Washington is said to have lost over three quarts of blood in his last days as a result of the use of leeches. Many historians credit the loss of blood as the cause of his death. The use of leeches as a way of bloodletting gave way to other methods, and the medical leech became a historical curiosity. Fortunately, blood letting by any means has also been abandoned. That is, until recently. Plastic and vascular surgeons are turning to the use of leeches with increasing frequency.
Leeches are well designed for their task, bloodletting. They have sharp teeth to cut through skin and their saliva contains both a local anesthetic and blood thinner. The bacteria in their digestive tract help to digest blood, and they are capable of drinking several times their own weight at one meal. After eating, they can survive for months without another meal.
Some interesting leech facts:
- There are over 650 different species of leech.
- The largest leech is over 18 inches long.
- The leech has 32 brains.
- The medicinal leech has three jaws and 300 teeth.
- The bite mark of a leech looks like the emblem for the German carmaker Mercedes.
- A derivative of the blood thinner in leech saliva is used to treat and prevent blood clots.
To understand why anyone would use a leech today, you need to have an understanding of the limitations of surgery. When a surgeon reattaches a severed ear, finger, lip or other body part, it is relatively easy to put large blood vessels back together. So blood flow to the body part can usually be resumed. Unfortunately, blood flow in needs to be matched by blood flow out. If blood flow out is reduced, the finger or other reattached body part will swell and can become rather painful. This is what happens if you tightly wrap an elastic band around a finger. Blood flow out requires a fine drainage system that is interrupted and really cannot be sewn back together very well. If the problem is not fixed, the reattached part can be lost. Doctors sometimes resorted to “stabbing” the reattached body part dozens of times with a needle to let the blood that was collecting drain out. This process is both painful and not very successful. In 1960, two Slovenian surgeons reported on their use of leeches to help in the attachment of a large tissue flap. Since then hundreds of patients have been helped by the use of leeches.
Leeches usually don't need much coaxing to bite and draw blood. In the hospital, a nurse cuts a hole in the center of a piece of gauze and places it where he or she wants the leech to draw blood. The leech is placed in the center and allowed to bite. The bite is painless; leech saliva may have a local anesthetic effect (leech experts don't all agree that this is real). If the leech isn’t hungry enough, the nurse may make a pinprick using a sterile needle to draw a small amount of blood. That is usually enough of a lure to coax even the most difficult leech to bite. Once the leech makes its bite, it will usually stay attached until it is full. Even after it detaches, a blood thinner in the leech's saliva provides additional oozing from the site. Leeches have a habit of moving to warm moist places when they are full, so keeping an eye on them or restricting their ability to move around is a good idea. Once they are full, they will not bite again for months. Leeches are only used once and then destroyed, usually by first putting them to sleep with alcohol and then killing them.
The medicinal leech has made its way back as a useful therapy. Maybe they aren't such "filthy little devils" after all.
Harold J. DeMonaco, M.S., is senior clinical associate in the Decision Support and Quality Management Unit at the Massachusetts General Hospital and is currently a Visiting Scholar at the MIT Sloan School of Management. He is author of over 20 publications in the pharmacy and medical literature and routinely reviews manuscript submissions for eight medical journals.