From New World Encyclopedia

A Leech on stones
A Leech on stones
Scientific classification
Kingdom: Animalia
Phylum: Annelida
Class: Clitellata
Subclass: Hirudinea
Lamarck, 1818
Genus: Macrobdella

Arhynchobdellida or Rhynchobdellida
There is some dispute as to whether Hirudinea should be a class itself, or a subclass of the Clitellata.

Leech is the common name for any of the annelids (segmented worms) comprising the subclass (or class) Hirudinea. They typically are characterized by a small sucker on the anterior (mouth) end of the cylindrical or somewhat dorso-ventrally flattened body, and a larger sucker on the posterior end.

Leeches generally are aquatic and live in freshwater environments, but there also are terrestrial and marine species. Most leeches are predatory, feeding on a variety of invertebrates, such as worms, snails, insect larvae, and crustaceans. However, some are parasitic blood-sucking leeches, feeding on the blood of vertebrates, such as amphibians, reptiles, waterfowl, fish, and mammals (including humans), as well as mollusks. In addition, some leeches are detritivores, consuming nonliving organic material.

Hemophagic (feeding on blood) leeches attach to their hosts and remain there until they become full, at which point they fall off to digest. They all have an anterior (oral) sucker formed from the first six segments of their body, which is used to connect to a host for feeding, and can also release an anesthetic to prevent the host from noticing the leech. They use a combination of mucus and suction (caused by concentric muscles in those six segments) to stay attached and secrete an anti-clotting enzyme into the host's blood stream.

Leeches provide values to the ecosystem and to humans. Ecologically, they are important in forest and stream food chains, serving as both predator and prey according to the life cycle. Among the predators of leeches are fish, crayfish, turtles, birds, frogs, and even dragonflies and damselflies.

For humans, the hemophagic leeches can be used medically, for example, in controlling swelling, as it produces chemicals that can serve as an anesthetic and prevent blood coagulation. The medicinal leech, Hirudo medicinalis, which is native to Europe, and its congeners have been used for clinical bloodletting for thousands of years.

Some species of leech will nurture their young, providing food, transport, and protection, which is unusual behavior in an invertebrate.


Leeches belong to Annelida, a large phylum of invertebrate animals comprising the segmented worms, including the well-known earthworms.

The three major groups of annelids are the polychaetes (largely marine annelids, with over 5,500 species); the oligochaetes (earthworms and freshwater worms, with over 3,000 species); and the hirundinea (leeches, with about 500 species). However, biological classification of annelids can vary widely among taxonomists.

Some consider there to be three classes of annelids: Polychaeta, Clitellata, and Aelosomata. The Clitellata are then further divided into three or four subclasses: Oligochaeta (earthworms and freshwater worms), Hirundinea (leeches), and Branchiobdella (about 150 species of small animals that are largely parasites or commensals on crayfish), and sometimes Acanthobdellida (leech-like, temporary parasite, which is also placed in Hirundinea in some classifications). In some biological classifications, the Clitellata is considered a subphylum and the Oligochaeta, Hirudinea, and Branchiobdellida are treated as classes of this subphylusm.

Another taxonomic scheme regards two groups of polychaetes—the Archiannelida and the Myzostomaria—as classes in their own right, and recognizes four total classes: Polychaeta, Clitellata, Myzostomida, and Archiannelida. A simple classification scheme is to recognize two classes of annelids, the Polychaeta and the Clitellata, with this later group including earthworms (Oligochaeta) and leeches (Hirudinea). There have also been proposals to consider the Clitellata as part of the Polychaeta, thus making the latter term synonymous with the annelids.

The leeches (Hirudinea) are presumed to have evolved from the Oligochaeta, most of which feed on detritus. However, some oligochaete species in the Lumbriculidae are predaceous and have similar adaptations to the leeches.

True leeches, of the taxonomic group Euhirudinea, with both anterior and posterior suckers, are divided into two groups:

  1. Rhynchobdellae: "Jawless" leeches, armed with a muscular, straw-like, proboscis puncturing organ in a retractable sheath. The Rhynchobdellae consist of two families: The Glossiphoniidae (flattened leeches with a poorly defined anterior sucker) and the Piscicolidae (cylindrical bodies and a usually well-marked, bell-shaped, anterior sucker). The Glossiphoniidae live in fresh-water habitats; the Pisciolidae are found in sea-water habitats.
  2. Arhynchobdellids: Leeches that lack a proboscis and that may or may not have jaws armed with teeth. Arhynchobdellids are divided into two orders: Gnathobdellae and Pharyngobdellae
    1. Gnathobdellae: In this order of "jawed" leeches, armed with teeth, is found the quintessential leech: The European medical (bloodsucking) leech, Hirudo medicinalis. It has a tripartite jaw filled with hundreds of tiny sharp teeth. The incision mark left on the skin by the European medical leech is an inverted Y inside a circle. Its North American counterpart is Macrobdella decora, a much less efficient medical leech. Within this order, the family Hirudidae is characterized by aquatic leeches and the family Haemadipsidae by terrestrial leeches. In the latter are Haemadipsa sylvestris, the Indian leech, and Haemadipsa zeylanica (Yamabiru), the Japanese Mountain or Land leech.
    2. Pharyngobdellae: These so called worm-leeches consist of freshwater or amphibious leeches that have lost the ability to penetrate a host's tissue and suck blood. They are carnivorous and equipped with a relatively large, toothless mouth to ingest worms or insect larvae, which are swallowed whole. The Pharyngobdellae have six to eight pairs of eyes, as compared with five pairs in Gnathobdelliform leeches, and include three related families.


As annelids, leeches are triploblastic protostomes with a coelom (at least historically), closed circulatory system, and true segmentation. Protosomes are animals with [[Symmetry (biology)}#Bilateral symmetry|bilaterial symmetry]] where the first opening in development, the blastophore, becomes its mouth. Triploblastic means that they have three primary tissue areas formed during embryogenesis. A coelom is a fluid-filled body cavity. Whereas oligochaetes and polychaetes typically have spacious coeloms, in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals. Leeches' bodies are composed of a fixed number of segments, usually 34 (Myers 2001).

The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel that conveys the blood in the opposite direction. The two are connected by a vascular sinus and by lateral vessels of various kinds. The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion.

In the digestive tract, starting from the anterior sucker, is the jaw, then the pharynx, which extends to the crop, which leads to the Intestinum, where it ends at the posterior sucker. The crop is a type of stomach that works like an expandable storage compartment. The crop allows a leech to store blood up to five times its body size; because of this ability to hold blood without the blood decaying, due to bacteria living inside the crop, medicinal leeches only need to feed two times a year.

It was long thought that bacteria in the gut carried on digestion for the leech instead of endogenous enzymes, which are very low or absent in the intestine. Relatively recently it has been discovered that all leeches and leech species studied do produce endogenous intestinal exopeptidases, which can unlink free terminal-end amino acids, one amino acid monomer at a time, from a gradually unwinding and degrading protein polymer. However, unzipping of the protein can start from either the amino (tail) or carboxyl (head) terminal-end of the protein molecule. It just so happens that the leech exopeptidase (arylamidases), possibly aided by proteases from endosymbiotic bacteria in the intestine, starts from the tail or amino protein, free-end, slowly but progressively removing many hundreds of individual terminal amino acids for resynthesis into proteins that constitute the leech. Since leeches lack endopeptidases, the mechanism of protein digestion cannot follow the same sequence as it would in all other animals where exopeptidases act sequentially on peptides produced by the action of endopeptidases. Exopeptidases are especially prominent in the common North American worm-leech Erpobdella punctata. This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from Oligochaeta.

Deficiency in leeches of digestive enzymes (except exopeptidases), but more importantly deficiency of vitamins (B complex for example), is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hrudo medicinalis, these supplementary factors are produced by an obligatory symbiotic relationship with a single bacterium species, Aeromonas hydrophila, which maintains itself in pure culture by secreting an antibiotic known to medicine since the nineteenth century, well before Fleming's 1929 discovery of penicillin. Non-bloodsucking leeches such as E. punctata are host to three bacterial symbionts, Pseudomonas sp., Aeromonas sp., and Klebsiella sp. (a slime producer). The bacteria are passed from parent to offspring in the cocoon as it is formed.


Like earthworms, leeches are hermaphrodites, meaning they are organisms that have both female and male reproductive organs (ovaries and testes respectively).

Also like their near relatives, the Oligochaeta, leeches share the presence of a clitellum to hold the eggs. A clitellum is a thickened glandular section of the body wall that secretes a viscid sac in which the eggs are deposited. Once the eggs have been deposited in the sac, the clitellum slides off of the annelid's body.

The use of leeches in medicine

The leech has long been used in medicine, previously being used to remove poison from the human body or used by doctors who believed that many diseases resulted from "imbalances" in the body that could be treated by releasing blood (Longe 2006). Indeed, the word leech either comes directly from or was influenced by the Old English word for "physician," lǣce, which is related to Old High German lāhhi and Old Irish liaig. The cognate form in Swedish is läkare, and this still translates as physician.

Today, the use of leeches in bloodletting has been abandoned as a medical treatment, but it is used in a new purpose: Helping to restore blood circulation to severely injured tissue or in limb reattachment procedures (Longe 2006). Leeches have proven highly effective at preventing venous congestion after the surgical re-attachment of fingers, toes, ears, and other parts of the body and helpful in dealing with the swelling due to trauma (Longe 2006).

Two species of leeches are used therapeutically in medicine: Hirudo michaelseni and Hirudo medicinalis (Longe 2006).

Leech saliva contains a number of compounds that assist in its feeding. An anaesthetic limits the sensations felt by the host (and thus reduces the chance of the host trying to detach the leech). A vasodilator causes the blood vessels near the leech to become dilated, and thus provides the leech with a better supply.

Lastly, the leech saliva contains a peptide called hirudin, which is a highly effective anticoagulant. The leech needs this to prevent blood clots (which would block its feeding) from forming in the wound created by its mouthparts. These properties are difficult to achieve using other medical techniques, and it is for this reason that leeches have come back into clinical practice in the last 25 years.

The anticoagulant drug is derived from the tissues of H. medicinalis (Longe 2006). Because of the minuscule amounts of hirudin present in leeches, it is impractical to harvest the substance for widespread medical use. Hirudin (and related substances) are synthesized using recombinant techniques.

Leech bites


A Borneo leech. Note how the leech curls and fattens as it fills with blood.

The anatomy of medicinal leeches may look simple, but more details are found beyond the macro level. Externally, medicinal leeches tend to have a brown and red striped design on an olive colored background. These organisms have two suckers, one at each end, called the anterior and posterior sucker. The posterior is mainly used for leverage while the anterior sucker, consisting of the jaw and teeth, is where the feeding takes place. Medicinal leeches have three jaws—tripartite—that look like little saws, and on them are about 100 sharp teeth used to incise the host. The incision leaves a mark which is an inverted Y inside of a circle.

A leech attaches itself when it bites, and it will stay attached until it has had its fill of blood. It has been known to suck all the blood out of its host. Due to the anticoagulant hirudin that leeches secrete, bites may bleed more than a normal wound after the leech is removed. The effect of the anticoagulant will wear off several hours after the leech is removed and the wound is cleaned.

Leeches normally carry parasites in their digestive tract that cannot survive in humans and do not pose a threat. However, bacteria, viruses, and parasites from previous blood sources can survive within a leech for months, and may be retransmitted to humans. A study found both HIV and hepatitis B in African leeches from Cameroon (Nehili 1994).


One recommended method of removal of an attached leech is using a fingernail to break the seal of the oral sucker at the anterior end (the smaller, thinner end) of the leech, repeating with the posterior end, then flicking the leech away. As the fingernail is pushed along the person's skin against the leech, the suction of sucker's seal is broken, at which point the leech should detach its jaws (TO 2006; WCS 2007).

A common but medically inadvisable technique to remove a leech is to apply a flame, lit cigarette, salt, or caustic chemical such as alcohol, vinegar, lemon juice, insect repellent, heat rub, or certain carbonated drinks. These cause the leech to regurgitate its stomach contents into the wound and quickly detach. The vomit may carry disease and increases the risk of infection (TO 2006; VPIC 2007; WCS 2007).

Simply pulling a leech off by grasping it can also cause regurgitation, and adds risks of further tearing the wound, and leaving parts of the leech's jaw in the wound, which can also increase the risk of infection.

However, Longe (2006) notes that when leeches are used medically, they are commonly removed by pulling them off or loosing their attachment using cocaine, heat, or acid (Longe 2006).

An externally attached leech will detach and fall off on its own when it is satiated on blood, usually in about 20 minutes (VPIC 2007), while internal attachments, such as nasal passage or vaginal attachments, are likelier to require medical intervention (Ibrahim et al. 2003).


After removal or detachment, the wound should be cleaned with soap and water, and bandaged. Bleeding may continue for some time, due to the leech's anti-clotting enzyme. Applying pressure can reduce bleeding, although blood loss from a single bite is not dangerous. The wound normally itches as it heals, but should not be scratched as this may complicate healing and introduce other infections. An antihistamine can reduce itching, and applying a cold pack can reduce pain or swelling.

Some people suffer severe allergic or anaphylactic reactions from leech bites, and require urgent medical care. Symptoms include red blotches or an itchy rash over the body, swelling away from the bitten area (especially around the lips or eyes), feeling faint or dizzy, and difficulty breathing (VPIC 2007).


There is no guaranteed method of preventing leech bites in leech-infested areas. The most reliable method is to cover exposed skin. The effect of insect repellents is disputed, but it is generally accepted that strong (maximum strength or tropical) insect repellents do help prevent bites.

Leech socks can be helpful in preventing bites when the full body will not be at risk of contact with leeches. Leech socks are pulled over the wearer’s trousers to prevent leeches from reaching the exposed skin of the legs and attaching there or climbing towards the torso. The socks are generally a light color that also makes it easier to spot leeches climbing up from the feet and looking for skin to attach to.

There are many home remedies to help prevent leech bites. Many people have a great deal of faith in these methods, but none of them has been proven to have much or any effect. Home remedies include: A dried residue of bath soap, tobacco leaves between the toes, pastes of salt or baking soda, citrus juice, and eucalyptus oil. Diluted Calcium hydroxide may also be used as a repellent, but may be damaging or irritating to the skin.

ISBN links support NWE through referral fees


New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.