• 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.