Leeches
Scientific classification
Kingdom: Animalia
Phylum: Annelida
Class: Clitellata*
Subclass: Hirudinea
Orderss
Arhynchobdellida
Rhynchobdellida
* There is some dispute as to
whether Hirudinea should be a class
itself, or a subclass of the Clitellata.

A leech is a amphibious annelid in the subclass Hirudinea. It shares with its near relatives, the Oligochaeta, 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 feed on small worms or other invertebrates. The horse leech (Haemopis sanguisuga) is known to feed on decaying flesh as well as on earthworms[1].

Use of hirudo medicinalis in medicine

The medicinal leech has a long history of use 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 reduce 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 food 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 a practise to cut 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 medicinals leech

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 stripped 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 take place. Medicinal leeches have three jaws that look like little saws, and on them are about 100 horny teeth used to incise the host.

Internal anatomy

Leeches are hermaphrodites meaning they are organism 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. Crop is a type of stomach that works like a expandable storage compartment. Crop allows leech to store blood up to five times its body size, because of these ablity 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. It is common practice for people leaving leech-infested waters to conduct an inspection of themselves to ensure 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 or match (which again causes the leech to disengage its mouthparts and flee).

Will the wound get infected? Can diseases get transmitted to me through this route? Strangely enough, there appears to be little evidence of transmission. If the wound is cleaned, there is little risk of infection. The most dangerous thing seems to be scratching using fingernails.

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