|Scientific classification |
*There is some dispute as to
whether Hirudinea should be a class
itself, or a subclass of the Clitellata.
A leech is an annelid in the subclass Hirudinea. There are freshwater, terrestrial and marine leeches. Like their near relatives, the Oligochaeta, they share the presence of a clitellum. Many species of leech are haemophagic parasites, living on occasional meals of blood obtained by attaching themselves to fish, amphibians (frogs etc.), and mammals. The medicinal leech, Hirudo medicinalis, which is native to Europe, has been used for clinical bloodletting for thousands of years. Most leech species are predators and feed on small worms or other invertebrates.
The leeches are clitellate when breeding and are derived from some oligochaete ancestor. They attach to their hosts and remain there. Leeches' bodies are composed of 32 segments. They all have a ventral sucker formed from the last six segments of the body, and that is a combination of mucus and suction caused by concentric muscles formed from the circular muscles of the segments from which the sucker is derived. Like earthworms, leeches are hermaphrodites.
Some species of leech will nurture their young, providing food, transport, and protection, which is unusual behavior in an invertebrate. (http://www.enn.com/news/2004-08-10/s_25691.asp)
All leeches are from one of three orders:
- Rhynchobdellae (comprising leeches with an eversible proboscis)
The Rhychobdellae consist of two families: The Glossiphoniidae (flattened leeches with a poorly developed anterior sucker) and the Pisciolidae (have cylindrical bodies and usually well-marked anterior sucker). The Glossiphoniidae are in fresh-water habitat, the Pisciolidae are found in both fresh-water and sea-water habitats.
- Gnathobdellae (leeches like the medicinal leech and horse leech which have cutting pharyngeal teeth)
The Gnathobdellae has two main families. The Hirudidae includes the medicinal and horse leeches. The other family is Haemadipsidae, an tropical and subtropical group which hang themselves in wet forests and attach to passing animals.
- Pharyngobdellae (leeches have a sucking pharynx like that of ganthobdelliforms, but from which the teeth are absent)
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. Leeches were once so commonly used that doctors were popularly called leeches. In Old High German, lāhhi (etymon of leech) means "physician".
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 complex protein 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 practise in the last 25 years. The small amounts of hirundin 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 detach itself. This practise 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.
Leeches are hermaphrodites, meaning they are organisms that have both female, ovary, and male, testes, sexual reproductive organs. 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 a 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.
Questions about leech bites
- What can be done to prevent leech bites in the outdoors?
The broad conclusion seems to be: Not much. There is not much evidence in favour of any leech-repellant (unlike the strong evidence in favour of DEET against biting insects). All you can do is not expose bare skin, preferably by wearing leech socks.
Leech socks, calico oversocks worn outside the socks and trousers provide a degree of protection, since they prevent leech entry at ankle level. The pale material from which they are usually made makes it easy to spot leeches as they climb looking for the next area of bare skin on the arms or neck.
It is common practice for people leaving leech-infested waters or wet vegetation to conduct an inspection of themselves to ensure that no leeches are attached.
- What can I do once a leech bites me?
Get rid of it. Clean the wound. There is an urban legend that if the biting parts of the leech are left in the wound, healing is inhibited. There is no evidence in favour of such an assertion. Leeches can most easily be removed by sprinkling salt on them (which irritates their skin, causing them to flee) or by (carefully) burning them with a lit cigarette, heated paper clip, smoldering piece of grass or small stick or match (which again causes the leech to disengage its mouthparts and flee).
Edible lime (Chuna in India) diluted in water can be sprayed either as a deterent or onto the animal when it is attached. The solution causes the leech to disintegrate.
- Will the wound get infected? Can diseases get transmitted to me through this route?
There appears to be little evidence of transmission. Leeches are very distantly related to those organisms which carry rabies or other diseases affecting people; in addition, they need to feed so infrequently that pathogens would die before being transmitted. If the wound is cleaned, there is little risk of infection (as with any other small wound). The most dangerous thing seems to be scratching them with the fingernails. People living in leech infested areas take getting some bites as a matter of course. Although the bites are painless and generally harmless, many people are squeamish about these creatures, and the prospect of removing blood-soaked socks.
- Information on the medicinal leech (http://animaldiversity.ummz.umich.edu/accounts/hirudo/h._medicinalis)
- Detailed information on leeches (http://www.invertebrate.ws/leech)
- General information on leeches (http://www.gondar.co.uk/abe/leeches.html)
- Leech fact sheet (http://www.amonline.net.au/factsheets/leeches.htm) - a more detailed description