Leech

Leeches
A Leech on stones
Scientific classification
Kingdom: Animalia Phylum: Annelida Class: Clitellata Subclass: Hirudinea Lamarck, 1818 Genus: Macrobdella
Orders
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 slightly dorso-ventrally flattened body and a larger sucker on the posterior end.

Leeches generally are aquatic and living 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.

food chains. The most important predators of leeches are fish, aquatic insects, crayfish and other leeches specialized for predation on leeches.

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.

medicinal: 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.

Overview

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 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 (have 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 which lack a proboscis and which 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.

1. 1. 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. The Erpobdellidae are some species from freshwater habitats.

Anatomy

Annelids are triploblastic protostomes with a coelom (at least historically), closed circulatory system, and true segmentation. Protosomes are animals with 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. Leeches' bodies are composed of a fixed number of segments, usually 34 segments (Myers 2001).

Polychaeta: "A variety of marine worms" plate from Das Meer by M. J. Schleiden (1804–1881)

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; archiannelids may lack the coelom entirely. The coelom is divided into a sequence of compartments by walls called septa. In the most general forms, each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms, they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.

Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (internal fold of the intestine or intestine inner wall), to increase surface area, along much of its length.

Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers.

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 systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.

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. Growth in many groups occurs by replication of individual segmental units; in others, the number of segments is fixed in early development.

Use of Hirudo medicinalis in medicine

The leech has long been used in medicine, previously being used to remove poison from the human body, although today its use is mainly limited in limb reattachment procedures instead of the wide-ranging medical use in the past. Leeches have proven highly effective at preventing venous congestion after the surgical re-attachment of fingers, toes, ears and other parts of the body. 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 (see List of false friends between Swedish and English).

Leech saliva contains a number of compounds which 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 provide 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.

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 synthesised using recombinant techniques.

The anatomy of medicinal leeches

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.

Reproduction

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

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 earthworm's (annelid's) body. Like earthworms, leeches are hermaphrodites.

Nutrition of leeches

Typical leech found in Malaysian jungle.

Starting from the anterior sucker is the jaw, 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 can not 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 of digestive enzymes (except exopeptidases) but more importantly deficiency of vitamins, B complex for example, in leeches 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 19th 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.

Leech bites

Effects

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

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 an 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 which 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.

Removal

One recommended method of removal 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.

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.

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.

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

Treatment

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.

Prevention

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 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.

References

ISBN links support NWE through referral fees

• Longe, J. L. 2006. The Gale Encyclopedia of Medicine. Detroit: Thomson Gale. ISBN 1414403682

http://animaldiversity.ummz.umich.edu/site/accounts/information/Hirudinea.html Myers, P. 2001. "Hirudinea" (On-line), Animal Diversity Web. Accessed October 06, 2007 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Hirudinea.html.

• Sawyer, Roy T. 1986. Leech Biology and Behaviour. Vol 1-2. Clarendon Press, Oxford

See also

• Tempest Prognosticator – the application of leeches in a barometer

External links

Wikispecies has information related to:

Hirudinea

Wikibooks Dichotomous Key has more about this subject:

Hirudinea

• Leech fact sheet, Australian Museum
• North American leeches
• How to remove a leech
• Biotherapy with leeches and maggots

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