Hirudo medicinalis: The Revival of the Medicinal Leech

by Lindsey DeVault
BIOL/WATR 361, Spring 2015

Key taxa: Annelida, Clitellata, Hirudinea, Hirudinidae, Hirudo medicinalis

Hirudo medicinalis, also known as the medicinal leech, has a messy and deadly past. Over the course of history leeches have been used as a cure for just about everything, including ulcers, headaches, sexually transmitted disease, and much more. The original thought was that bad blood was causing these issues and that bloodletting, or having a leech suck blood out of a patient, was an effective remedy. As medicine advanced, the practice of bloodletting was thrown to the wayside and more effective, practical, and successful methods were adopted. However, recently leeches have seen an increase in their use during medical procedures. Their amazing blood sucking abilities are being put to use where the human body is failing. Leeches are most often being used in medicine to help where the venous system has stopped working; these situations are common when dealing with digit and skin flap reattachment. The revival of Hirudo medicinalis is mainly due to the chemical components of its saliva which can relieve pain and assist in reattaching skin flaps and digits.

Hirudo medicinalis belongs to the phylum Annelida and has several anatomical characteristics that are very distinct and distinguish leeches from other organisms in the same phylum. Leeches are classified under the Phylum Annelida and the class Clitellata. They are placed into the class Clitellata because they possess a clitellum, which is a non-segmented, thickened section located near the head of the organism (Smith, 2001). The clitellum is used during reproduction to secrete a mucus-like film around eggs. Leeches are then placed in the Order Arhynchobdellida, the Family Hirudinidae, and the Genus Hirudo. A characteristic that accompanies the order Arhynochobdellida is having 3 jaws that fall behind the mouth anatomy near their anterior end, or head. Members of the family Hirudinidae often have well-developed jaws and a short pharynx. They also have gastric cecae which aid in digestion. Another characteristic that distinguishes leeches is their diet. The sole “food” that leeches consume is blood of a host. In order to get their main source of nutrients, blood, they need to pierce the skin of their chosen host. Leeches do this with their jaws, which are full of microscopic, sharp teeth-like structures. Leeches use their muscular pharynx to suck blood from their host. Another characteristic that distinguishes Hirudinidae is the fact that they are dorsoventrally flattened and have over 20 segments (Smith 2001). Leeches typically have 32 segments each with paired nerves and a ganglion. Many can distinguish a leech because the leeches possess two obvious suckers. One sucker is posterior and the other is anterior. The anterior sucker is around the mouth and is deemed the “oral sucker.” Typically this sucker is smaller. The posterior sucker is named the “caudal sucker” and is relatively larger and more powerful. It has been discovered that a leech can hold 6-11 times their body weight in blood (Kraemer et al., 1988). This large quantity of blood allows them to live up to 2 years without feeding. While all species of leech possess two suckers, Hirudo medcinalis and H. verbena are the leeches selected for medicinal use. Leeches have relatively unstudied properties that allow them to aid in surgery or be used for other medicinal purposes. One of these properties includes their saliva and the useful chemical components found within it.

The saliva of a leech is fairly complex. The main components of leech saliva include hirudin, anesthesia, and a vasodilator, all of which are extremely helpful in modern day medicine. The saliva of the medicinal leech has been relatively unstudied, and there are some discrepancies of what is actually present. Most studies merely state that the saliva of leeches contains protein mixtures that include vasodilators, spreading agents, enzymes, and anti-coagulators (Smith, 2001). Hirudin is produced by the leech’s pharyngeal gland, which is located around the pharynx, and is said to be one of the most potent anticoagulants ever discovered (Kraemer et al., 1988). Hirudin is composed of 65 amino acids and is a chemical that is an anticoagulant. Hirudin is used to prevent coagulation of blood in the crop of the animal. Hirudin also makes the blood of the host less viscous and more easily ingested and digested. The study showed that hirudin is especially effective at combating the coagulation properties of thrombin, which is an enzyme that assists in clotting blood (Michalsen et al., 2003). Hirudin binds to thrombin and prevents it from clotting blood. After the leech has consumed it’s fill of blood it detaches itself. Normally, the place where the leech bit continues to bleed up to twelve hours. An interesting aspect of hirudin is that while it blocks thrombin and discourages clot formation, it does not disrupt the creation of clotting factors or other enzymes in the blood. This is especially useful for medicinal purposes.

Another key component for medicinal use in leech saliva is an anesthetic chemical mixture. This allows the leech to bite and continue leeching without the host ever noticing. Another component of saliva is a vasodilator. A vasodilator would increase the width of the blood vessels and allow blood to flow more quickly. This would allow the leech to be fed faster and minimize detection. Other resources have mentioned that leech saliva includes spreading agents, proteolytic enzymes, and platelet-formation inhibitors. Spreading agents increase the area that the secretions can spread (Kraemer et al., 1988). A proteolytic enzyme helps break down polypeptide chains and would most likely help the leech with digestion. Platelets are important in helping repair damaged blood vessels, platelet-formation inhibition works similarly to the anti-coagulation properties. Leeches are also known to have symbiotic bacteria living in their gut which help them digest the blood they consume. For the aforementioned reasons and many more, modern medicine has once again taken an interest in leeches, their saliva, and the impact they can have in surgery and through therapy.

Leeches have seen revitalization in their use in medicine. They have been used effectively and efficiently in surgery, and there is evidence for their usefulness in relieving pain, joint stiffness, and improving joint function. Leeches first began being used in medicine to remove copious amounts of blood for various ailments. This method quickly proved to be ineffective, and leeches faded from medicinal use. Now, medical teams and hospitals are using leeches to assist in reattachment of skin flaps, grafts, and severed digits. When a piece of skin or digit is severed or when skin grafts are performed doctors can often successfully reattach the severed object. The problem arises when the blood vessels have been damaged and cannot repair quick enough to effectively remove blood from the damaged area of the body. This is called venous congestion. Skin grafts do not have their own blood source or blood vessels, which can result in necrosis, or death, of the graft (Click here for more information).

The use of leeches in surgery and post-surgery began in the 1960s (Kraemer et al., 1988). Often leeches are used as a last resort to help with skin grafts as well. In one study there were several cases where the individual had previously had skin removed due to sarcoma and received a skin graft (Utley et al.,1998). The grafts were normally done on the face or neck. The skin flaps of these particular individuals were not receiving blood well and the cells were beginning to die. The doctors chose to use leeches to assist with the venous congestion. Since the blood was not being removed via the veins, it was accumulating in the region of the skin flap causing the tissues to die. In all cases mentioned in the study, all skin flaps were saved from necrosis via the use of leeches. Leeches are commonly used for skin grafts and skin flaps, but they can also be used when ears or other digits have been severed as well. They are most often called in when the damaged area has a complex and dense network of blood vessels that has obtained severe damage. The blood vessels cannot be immediately repaired when the skin or digit is reattached, so leeches assist in making sure blood is not pooling. The leeches are useful not only because they can suck rather large quantities of pooling blood out of a congested area, but also because the chemical in their saliva allow them to do so in a quick and relatively painless manner.

Another study documented the effectiveness of leeches used to help relieve patient’s pain with osteoarthritis. Leeching was compared to a topical cream and was rated more effective at increasing joint function, reducing joint stiffness, and decreasing joint pain (Michalsen et al., 2003). Relief from the chronic pain was felt immediately and up to a month later (Lauche et al., 2014). Some patients did report itching at the sight of leech application. The use of leeches seemed far superior and the treatment cost about $70, which is fairly inexpensive when compared to prescribed treatment.

Three possible reasons were listed for why leeches seemed to have a superior effect at increasing joint function and reducing pain and stiffness in the joint. The first is that the chemical components, such as the vasodilators, anesthetic chemicals, and other inhibitors are working within the body and its tissues. The second is that leeches could have antinociceptive effects. Since pain is caused by nociceptive activation, this could be a key factor in relieving pain. Thirdly, leeches could be rated superiorly to topical treatment because of the placebo effect. Perhaps the patients subconsciously or consciously believe the leeches should be more effective and thus report they are more effective than they are in reality. This study suggested that leeches be compared to more invasive means of restoring joints of those suffering from osteoarthritis (Michalsen et al., 2003). Hirudin has also been extracted and isolated and used in creams and other topical treatments.

With any medical operation, there are often complications and the use of leeches is no exception. Often hours after removal of the leech there is continual bleeding. This is said to be caused by, in part, the vasodilator in the leech’s saliva. Other complications from leech usage in medicine result from the symbiotic bacteria living in the leech’s gut, Aeromonas hydrophilia. This bacteria helps digest the blood consumed. However if it gets into the host’s bloodstream, problems can arise (Kraemer et al., 1988). This scenario is most likely if someone were to remove the leech and squeeze the body of the leech in the process. This squeezing causes the leech to vomit the contents of its stomach and gut, often back into the wound. Leeches can also carry viruses in their gastrointestinal tract for up to 120 days. There have been cases of people becoming resistant to hirudin because they are repeatedly exposed to the leech and its saliva (Kraemer et al., 1988). There have also been cases where leeches have wondered away from their designated area and gone missing, although this is infrequent. It is also to be noted that leech therapy should never replace modern medicine, but rather serve alongside the best most effective modern measures in order to ensure the best results.

Leeches continue to play an intriguing role in modern medicine. Leeches possess interesting and relatively unstudied chemical components in their saliva that prove to be extremely useful in modern medicine. Their saliva consists of an anesthetic chemical, anticoagulants, and vasodilators, as well as, some other lesser known chemical mixtures. The use of leeches and their saliva are being further investigated to see how they can further benefit skin graft, flap, and digit reattachment. They are used with discretion and while they cannot undo extreme damage, they have proved useful in saving and rescuing digits and skin flaps from complete loss, as well as relieving those suffering from chronic osteoarthritis pain.

References Cited

• Kraemer, B., K. Korber, T. Aquino &A. Engleman. 1988. Use of leeches in plastic and reconstructive surgery: A review. Journal of Reconstructive Microsurgery 4: 381-86.
• Lauche, R., H. Cramer, J. Langhorst & G. Dobos. 2014. A systematic review and meta-analysis of medical leech therapy for osteoarthritis of the knee. The Clinical Journal of Pain 30: 63-72.
• Michalsen, A., S. Klotz, R. Ludtke, S. Moebus, G. Spahn & G.J. Dobos. 2003. Effectiveness of leech therapy in osteoarthritis of the knee. Annals of Internal Medicine 139: 724-29.
• Smith, D.G. 2001. Pennak's Freshwater Invertebrates of the United States: Porifera to Crustacea. New York: J. Wiley.
• Utley, D.S., R.J. Koch & R.L. Goode. 1998. The gailing glap in gacial plastic and reconstructive surgery: Role of the medicinal leech. The Laryngoscope 108: 1129-135.