What is Leech?

For other uses, see Leech (disambiguation).

Leeches
A Leech on stones 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.

Leeches are annelids comprising the subclass Hirudinea. There are fresh water, terrestrial and marine leeches. Like their near relatives, the Oligochaeta, they share the presence of a clitellum. Like earthworms, leeches are hermaphrodites. The medicinal leech, Hirudo medicinalis, which is native to Europe, and its congeners have been used for clinical bloodletting for thousands of years.

All leech species are carnivorous. Some are predatory, feeding on a variety of invertebrates such as worms, snails, insect larvae, crustaceans, while a very few are haemophagic parasitic blood-sucking leeches, feeding on the blood of vertebrates such as amphibians, reptiles, waterfowl, fish, and mammals (including humans). The most important predators of leeches are fish, aquatic insects, crayfish and other leeches specialized for predation on leeches.

Haemophagic leeches attach to their hosts and remain there until they become full, at which point they fall off to digest. Leeches' bodies are composed of 34 segments. 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 rlease an anesthetic to prevent the host from noticing the leech. They use a combination o mucus and suction (caused by concentric muscles in those six segments) to stay attached and serete an anti-clotting enzyme into the host's blood stream. Some species of leech wll nurture their young, providing food, transport, and protection, which is unusual behavior in an invertebrate.

Phylogeny

The leeches are presumed to have evolved from the Oligochaeta, most of which feed on detritus. However, some species in the Lumbriculidae are predaceous and have imilar adaptations to the leeches. True leeches, of the subclass Euhirudine, with both anterior and posterior suckers, are divided into two groups

1. Rhynchobdellae: "jawess" leeches, armed with a muscular straw-like proboscis puncturing organ in a retractable sheah. The Rhynchobdellae consist of two families: The Glossiphoniidae (flattened leeches wth a poorly defined anterior sucker) and the Piscicolidae (have cylindrical bodies and a usuall well-marked, bell-shaped, anterior sucker). The Glossiphoniidae live in fres-water habitats; the Pisciolidae are found in sea-water habitats.
2. Arhynchobdellid: Leeches which lack a proboscis and which may or may not have jaws armed with teeth. Arhynchobellids are divided into two orders: Gnathobdellae and Pharyngobdella
3. Gnathobdelae: In this order of "jawed" leeches, armed with teeth, is found the quintessentil leech: the European medical (bloodsucking) leech, Hirudo medicinalis. It has a tripartitejaw filled with hundreds of tiny sharp teeth. The incision mark left on the skin by the Europan medical leech is an inverted Y inside a circle. Its North American counterpart is Macrobdela decora, a much less efficient medical leech. Within this order, the family Hirudidae is charcterized by aquatic leeches and the family Haemadipsidae by terrestrial leeches. In the latterare Haemadipsa sylvestris, the Indian leech and Haemadipsa zeylanica (Yamabiru), the Japanse Mountain or Land leech.^[1]
4. Pharyngobdellae: These so called worm-leeches consist of freshwater or amphibious leeches tht have lost the ability to penetrate a host's tissue and suck blood. They are carnivorous andequipped with a relatively large, toothless, mouth to ingest worms or insect larvae, which ar swallowed whole.

The Pharyngobdella have six to eight pairs of eyes, as compared with five pairs in Gnathobdelliform eeches, and include three related families. The Erpobdellidae are some species from freshwater abitats.

Use of Hirudo medicnalis in medicine

The leech has long been used in medicine, previously being used to remove poison from the human body, although today ts use is mainly limited in limb reattachment procedures instead of the wide-ranging medicaluse in the past. Leeches have proven highly effective at preventing venous congesion after the surgical re-attachment of fingers, toes, ears and other parts ofthe body. The word leech either comes directly from or was influenced by the Old English word for "physician", lǣce, which is related to OldHigh German lāhhi and Old Irish liaig. The cognate form inSwedish is läkare, and this still translates as physician (see List of fals friends between Swedish and English). Leech saliva contains a number of compounds which assst 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). They also use clitellums to hold the eggs.

Nutrition of leeches

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 instad 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 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.^[2]

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.^[3][4]

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.<ref name="timesonline" /><ref name="worstcase" />^[5]

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,<ref name="poisonscentre" /> while internal attachments, such as nasal passage or vaginal attachments, are likelier to require medical intervention.^[6][7]

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.<ref name="poisonscentre" />

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.

References