The Host

leechThe medicinal leech is usually sold as H. medicinalis but a recent study by Dr. Mark Siddall's group revealed that the leeches commonly sold by the commercial suppliers are H. verbana. Prior to this discovery H. medicinalis was approved by the FDA as a medical device to treat venous congestion. The medicinal leech, H. verbana, is a segmented animal that is related to the earthworm. Leeches possess several evolutionary advanced features including compound eyes (one eye that is composed out of many individual eyes) and cephalization (possessing a head). Within the anterior (front) sucker are three jaws that cut the skin. During feeding, H. verbana secretes compounds with various functions including vasodilator (dilates blood vessels) and anticoagulant (prevents the blood from clotting) properties. In a single feeding event, the medicinal leech can consume over five times its body weight. The ingested blood is stored in the large crop of the digestive tract where also the extracellular symbionts are found. From the ingested blood, water and salts are removed. After such a large blood meal, the animal can go for over 6 months without feeding. Leeches are simultaneous hermaphrodites (both male and female at the same time). After an internal fertilization, the leech secretes a cocoon, that contains several individual eggs and is stripped off over the anterior end of the animal. It has been suggested that during this deposition the symbionts are transmitted from the parent to a nutritious fluid bathing the eggs in the cocoon. After the juveniles hatch from the egg, they remain for some time within the cocoon and consume the nutritious fluid, possibly acquiring the symbionts at this time. When the juvenile leeches leave the cocoon, they are fully pigmented and functional, but they cannot bite through mammalian skin. Thus it is thought that their first blood meal is from an amphibian.

The Symbionts

Investigators in the 1940s and -50s isolated pure cultures of bacteria from the digestive tract of the medicinal leech and called them endosymbionts. Initial biochemical characterization suggested that the symbionts are Aeromonas hydrophila. Since these studies the Aeromonas taxonomy has undergone many changes and, in a study using biochemical tests and 16S rRNA sequencing, we showed that the symbiont is A. veronii. These studies relied on the ability to grow the bacteria away from the leech, but most bacteria cannot be grown in the laboratory. When we used an approach that did not rely on growing the bacteria, we discovered a second bacterium living in the crop of the leech. This bacterium was most closely related to Rikenella microfusus, a member of the Bacteroidetes. Bacteroidetes are common inhabitants of vertebrate guts and cannot grow in the presence of oxygen.The early investigators proposed several functions in which the symbionts may benefit the host:

  • The symbionts may digest the ingested blood. Until very recently, no host digestive enzymes were detected and Aeromonas is well known to produce numerous extracellular hydrolytic enzymes. However, despite the ability of the symbionts to lyse erythrocytes (beta-hemolysis), the ingested blood cells are stored intact for many months, suggesting that hemolytic ability of Aeromonas is inhibited. More recently, it was shown that the host releases proteases into the intestinum of the digestive tract where the actual digestion takes place.
  • The nutritious quality of blood is poor, for example it contains very low amount of the vitamine B12. Thus it has been suggested that the symbionts serve as a source of nutrients to the host.
  • Because it is so unusual to find a single species of bacteria in the digestive tract and early in vitro results suggested that Aeromonas can inhibit the proliferation of other bacteria, it has been proposed that the symbionts prevent the growth of non-symbiotic bacteria in the crop. However, there is little experimental evidence supporting any of these beneficial functions.


Factors Contributing to the Specificity of the Symbiotic Interaction

The specificity of this symbiosis is important for the biology of the animal and for the medical application of the leech. To date our research has revealed multiple factors that contribute to the unusual simplicity.


  • The complement system of the ingested vertebrate blood remains active and kills sensitive bacteria. 
  • Leech hemocytes (macrophage-like cells) phagocytose sensitive bacteria.
  • Additional antimicrobial compounds appear to be released either by the leech or by the bacterial symbionts. But these compounds have not been identified. In addition, the native symbiotic flora may outcompete non-symbiotic bacteria for nutrients and space.

This symbiosis may be thought of as an interaction dependent upon the invertebrate host, the symbiotic bacteria and the vertebrate host that the leech feeds on.

Current Research Topics Include

  • Identification of factors that contribute to the specificity of the symbiosis.
  • Identification of bacterial genes that are required to colonize the leech.
  • Culturing the Rikenella-like symbiont.
  • The use of 16S rRNA gene sequences to identify Aeromonas species.
  • Characterize the symbiont present in the nephridia and bladders of the leech.


Modern Medicine

During the 1980's, reports were published that described the successful application of medicinal leeches to rescue surgery cases with complications. During the reattachment of severed fingers and ears, or of the detached scalp, the blood flow needs to be reestablished. This is achieved by reconnecting the major arteries and veins. In particular, the veins can be difficult to find. If not enough veins are reconnected, the blood may initially enter the reattached organ but cannot exit. This will prevent fresh, oxygenated blood from entering and, consequentially, the reattachement procedure will fail. This is where the medicinal leech comes to the rescue. The animals are applied to the tissue and they actively remove blood and secrete numerous compounds that have vasodialator, anticoagulant, and clot-dissolving properties. This prevents the tissue from dying off and allows the body to reestablish good blood flow to the reattached part.

This type of procedure is not without risk. In up to 20% of the patients receiving leech treatment after microvascular surgery, infections by Aeromonas occurred. Fortunately, these infections can usually be prevented by administering antibiotics to the patients. These infections point to the potential of this bacterium, which is cooperative in the medicinal leech, to be pathogenic in humans.

In patients with a normal immune response and good blood circulations at the site of attachment, bacterial infections do not usually occur.

Another use of leeches is to reduce the pain of arthritis in the knees. A clinical trial suggested that patients receiving leech thearpy experienced less pain than the control group.

Historical Use

Medicinal bloodletting has been practiced since the Stone Age. Evil spirits were thought to cause illness and removal of these evil spirits required blood withdrawal. Records of the medical usage of leeches dates back to the beginning of civilization. Illustrations of leech application to patients were found in Egyptian tombs dating back to 1500 B.C.. Chinese writings from the first century A.D. describe medicinal leeching. There are also references in Sanskrit, Persian, and Arabic literature. The first Western documentation of therapeutic leech use is in the poem Alexipharmaca by Nicander of Colophon (200 - 130 B.C.). Galen (129 - 189 A.D.), the personal physician to Marcus Aurelius, advanced the practice of bloodletting through the development of his humoral concept of disease. The philosophy behind this disease concept was the balance of four humors (blood, phlegm, black and yellow bile) within the human body. Disruption of this balance led to disease. However, removal of the patient's blood would correct the humoral imbalance, thus restoring good health.

The documentation of medical applications during the Renaissance is shown in numerous illustrations. One advantage of using leeches for bloodletting was that the blood could be removed in predictable quantities. During the 18th and 19th century, the usage of medicinal leeches peaked in France, and Paris alone required millions of leeches annually. The peak in use of medicinal leeches during this period was due to new theories regarding the benefits of bloodletting. In particular, Francois Broussais (1772 - 1832) proposed that all disease resulted from excess build up of blood and the alleviation of this condition required heavy leeching and starvation. As a result of the popularity of leeching, H. medicinalis became an endangered species and France had to turn to importing leeches as their own harvest was no longer adequate.

Usage of leeches was not limited to Europe. In the United States during the 19th century, leeches were also used in bloodletting. In fact, European medicinal leeches were imported because they consumed larger amounts of blood then leeches native to North America. It was proposed that American diseases were more virulent than European ones and, therefore, small bleeding, the result of using American leeches, was not effective.

After the 1830s, the practice of leeching began to decline as medical diagnostic skills improved. Physicians realized that patients who were leeched did not often recover more fully than those who were not and other treatments, including pharmaceutical and homeopathic remedies, became more popular among the public. However, the discovery that blood in the leech gut did not coagulate by John Haycraft in 1884 and the isolation of this anticoagulant, hirudin, from leech pharyngeal glands by F. Markwardt in the late 1950s ensured the medical importance of the leech.