Leech
- For other uses, see Leech (disambiguation).
| Leeches | |
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| A Leech on stones | |
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| Subclass: | Hirudinea Lamarck, 1818
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Arhynchobdellida or Rhynchobdellida | |
Leeches are annelids comprising the subclass Hirudinea. There are freshwater, 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, fishes, and mammals (including humans). The most important predators on leeches are fishes, aquatic insects, crayfishes 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 are known to release an anaesthetic to remain unnoticed by the host. 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.
Some species of leech will nurture their young, providing food, transport, and protection, which is unusual behavior in an invertebrate.
Origin
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 similar adaptations to the leeches.
Taxonomy
True leeches, of the subclass Euhirudinea, with both anterior and posterior suckers, are divided into two groups:
- Rhynchobdellae: "jawless" leeches, armed with a muscular straw-like proboscis puncturing organ in a retractable sheath. The Rhychobdellae consist of two families: The Glossiphoniidae (flattened leeches with a poorly defined anterior sucker of the butt) 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.
- Arhychobdellids: Leeches which lack a proboscis and which may or may not have jaws armed with teeth. Arhychobdellids are divided into two orders: Gnathobdellae and Pharyngobdellae
- 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: the insignia of Mercedes Benz. Within this order, the family Hirudidae is characterized by aquatic leeches and the family Haemadipsidae by terrestrial leeches.
- 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.
Use of Hirudo medicinalis in medicine
The leech has long been used in medicine, although today its use is mainly limited to limb reattachment procedures instead of the wide-ranging medical use of the past. 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.
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. The small amounts of hirudin present in leeches makes it unsuitable to be harvested for more general medical use, so hirudin (or related chemicals) have been synthesised using recombinant-DNA technology.
Bdellatomy is the practice of cutting the leech open slightly while it is sucking blood to let the blood in it out, so, thinking that it is not full yet, the leech continues to bite instead of detaching itself. This practice was first recorded in 1868 by Daily News.
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 that look like little saws, and on them are about 100 sharp teeth used to incise the host.
Reproduction
Leeches are hermaphrodites, meaning they are organisms that have both female, ovary, and male, testes, sexual reproductive organs.
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 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 our 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.
The effects of leech bites
A Leech attaches itself when it bites, and it will stay attached until it has had its fill of blood. 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. The amount of blood loss from a leech bite is more of a nuisance than a danger for humans.
Leech bites tend to itch while healing, but this is normal. Itching is not reason for alarm, although the wound should be watched to ensure it is healing and to make sure there are no signs of infection.
Leeches do carry parasites in their digestive tract, but these cannot survive in humans and do not pose a threat. If the wound is cleaned, there is no more risk of infection than any other small wound. The wound should not be scratched as this may complicate healing and introduce other infections.
Removal and Treatment
Many people believe that it is best not to remove a leech but to wait for the leech to have its fill and detach itself. This does prevent the risk of additional damage or complications from removal, but concerns about leaving portions of the leeches sucker attached to the wound are exaggerated. It is generally considered preferable to remove the leech promptly.
Leeches should not be removed by burning or applying harsh agents such as salt. This may damage the bite further and cause additional pain. Most importantly, damaging the leech may cause it to regurgitate stomach bacteria that may lead to infection of the wound.
--Basic Instructions for Proper Removal and Treatment--
- Do not attempt to crush or burn the leech. Do not squeeze the leech and attempt to pull it off.
- Find the oral sucker. This is the sucker at the small end of the leech.
- Place your fingernail next to the sucker. Push towards the bite. Attempt to push the sucker to the side, away from the skin.
- Once the oral sucker is detached, quickly detach the sucker at the other end of the leech (the large end of the leech). This can be done by flicking the leech or by pushing the under the leech and up against the sucker.
- Clean the wound to prevent infection, to remove detached sucker parts, and to remove the anticoagulant that the leech has secreted.
- The wound may bleed for several hours but then should begin to heal normally.
- Keep the wound clean and dry.
- Allergic reactions can occur to leech bites. If there are signs of a severe reaction, seek medical help.
- Monitor the wound for infection, and contact a doctor if the wound becomes infected.
An alternate removal procedure is to apply a heat rub. Leeches have an adverse reaction to the menthol contained in heat rubs and will detach quickly upon contact with the substance. A smoker can also burn the leech with a lit cigar or cigarette to make it cringe and fall off (a feature introduced in the game Metal Gear Solid 3).
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, 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
- Sawyer, Roy T. 1986. Leech Biology and Behaviour. Vol 1-2. Clarendon Press, Oxford
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