Leech

Leeches are segmented worms that belong to the phylum annelida and comprise the subclass Hirudinea. Like other oligochetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligocheates in significant ways. For example, leeches do not have bristles and the external segmentation of their bodies does not correspond with the internal segmentation of their organs. Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. They also have two suckers, with each sucker located at the end of each animal.

The majority of leeches live in freshwater environments, while some species can be found in terrestrial and marine environments as well. Most leeches are hematophagous as they are predominantly blood suckers that feed on blood from vertebrate and invertebrate animals

Leeches such as the Hirudo medicinalis, have been historically used in medicine to remove blood from patients. The practice of leeching can be traced to Ancient India and Greece and has continued well into the 18th and 19th centuries in both Europe and North America. In modern times, the practice of leeching is much rarer and has been replaced by other contemporary uses of leeches such as the reattachment of body parts as well as reconstructive and plastic surgeries.

Taxonomy and systematics
Leeches are presumed to have evolved from certain Oligochaeta, most of which feed on detritus. However, some species in the Lumbriculidae are predatory and have similar adaptations as found in leeches. As a consequence, the systematics and taxonomy of leeches is in need of review. While leeches form a clade, the remaining oligochetes are not their sister taxon but a diverse paraphyletic group containing some lineages that are closely related to leeches, and others that are far more distant.

There is some dispute as to whether Hirudinea should be a class itself, or a subclass of the Clitellata. The resolution mainly depends on the eventual fate of the oligochaetes, which as noted above do not form a natural group as traditionally circumscribed. Another possibility would be to include the leeches in the taxon Oligochaeta, which would then be ranked as a class and contain most of the clitellates. The Branchiobdellida are leechlike clitellates that were formerly included in the Hirudinea but are just really close relatives.

The more primitive Acanthobdellidea are often included with the leeches, but some authors treat them as a separate clitellate group. True leeches of the infraclass Euhirudinea have both anterior and posterior suckers. They are divided into two groups: Arhynchobdellida and Rhynchobdellida


 * Rhynchobdellida): "Jawless" leeches, armed with a muscular straw-like proboscis puncturing organ in a retractable sheath. The Rhynchobdellae consist of two families:
 * Glossiphoniidae: Flattened leeches with a poorly defined anterior sucker
 * Piscicolida: have cylindrical bodies and a usually well-marked, bell-shaped, anterior sucker. The Glossiphoniidae live in fresh-water habitats; the Pisciolidae are found in seawater habitats.

The Pharyngobdella 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.
 * Arhynchobdellida: Leeches that lack a proboscis and that may or may not have jaws armed with teeth. Arhynchobellids are divided into two orders:
 * Gnathobdela: 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 Macrobdela 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.
 * Pharyngobdella: 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.

Anatomy and Physiology
Like other annelids, the leech is a segmented animal. But, unlike other annelids, there is no correspondence between the external segmentation of a leech's body surface with the segmentation of its internal organs. The body surface of the animal can be divided into 102 annuli, whereas its internal structures are divided into 32 segments. Of the 32 segments within the body, the first four anterior segments are designated as head segments, which include an anterior brain and sucker. This is followed by 21 midbody segments. The midbody segments include 21 neuronal ganglia, two reproductive organs, and 9 pairs of testes. Finally, the last 7 segments are fused to form the animal's tail sucker as well as its posterior brain.

Reproduction and Development
Leeches are hermaphrodites, meaning each one of them has both female and male reproductive organs (ovaries and testes respectively). Leeches reproduce by reciprocal fertilization, and sperm transfer occurs during copulation. Similar to the earthworms, leeches also use a clitellum to hold their eggs and secrete the cocoon.

During reproduction, leeches utilize hyperdermic injection of their sperm. They use a spermatophore, which is a structure containing the sperm. Once next to another leech, the two will line up with their anterior side opposite the other's posterior. The leech then shoots the spermatophore into the clitellur region of the opposing leech, where its sperm will make its way to the female reproductive parts.

The embryonic development of the leech occurs as series of stages. During stage 1, the first cleavage occurs, which gives rise to an AB and a CD blastomere and is in the interphase of this cell division when a yolk-free cytoplasm called teloplasm is formed. The teloplasm is known to be a determinant for the specification of the D cell fate. In stage 3, during the second cleavage, an unequal division occurs in the CD blastomere. As a consequence, it creates a big D cell on the left and a smaller C cell to the right. This unequal division process is dependent on actinomycin, and by the end of stage 3 the AB cell divides. On stage 4 of development, the micromeres and teloblast stem cells are formed and subsequently, the D quadrant divides to form the DM and the DNOPQ teloblast precursor cells. By the end stage 6, the zygote contains a set of 25 micromeres, 3 macromeres (A, B and C) and 10 teloblasts derived from the D quadrant.

The teloblast are pairs of five different types (M, N, O, P, and Q) of embryonic stem cells that form segmented columns of cells (germinal-band) in the surface of the embryo. The M-derived cells make mesoderm and some small set of neurons, N results in neural tissues and some ventral ectoderm, Q contributes to the dorsal ectoderm and O and P in the leech are equipotent cells (same developmental potential) that produce lateral ectoderm; however the difference between the two of them is that P creates bigger batches of dorso-lateral epidermis than O. The sludgeworm Tubifex, unlike the leech, specifies the O and P lineages early in development and therefore, these two cells are not equipotent. Each segment of the body of the leech is generated from one M, O, P cell types and two N and two Q cells types.

The ectoderm and mesoderm of the body trunk is exclusively derived from the teloblast cells in a region called posterior progress zone. The head of the leech that come from a non-segmented region, is formed by the first set of micromeres derived from A, B, C and D cells, keeping the bilateral symmetry between the AD and BC cells.

Digestion


For leeches, the digestive system starts with the jaw, which is located ventrally on the anterior side of the body. It is attached to the pharynx, then the esophagus extending to the crop, then to the gizzard, 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; and, because the leech produces an anti-coagulant, the stored blood remains in a liquid state; because of this ability to hold blood without the blood decaying, due to bacteria living inside the crop, medicinal leeches need to feed only two times a year.

The body of predatory leeches are similar, though some may also have a protrusible proboscis, which is retracted in their mouth. Such leeches are often ambush predators, which lie in wait, and strike their prey using their proboscis in a spear-like fashion.

It was long thought that bacteria in the gut carried on digestion for the leech, instead of endogenous enzymes that are very low or absent in the intestine. It has been discovered relatively recently 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 cannot follow the same sequence as it would in all other animals in which 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 important, deficiency of vitamins, B complex for example, in leeches is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hirudo medicinalis, these supplementary factors are produced by an obligatory symbiotic relationship with two bacterial species, Aeromonas veronii and a still-uncharacterized Rikenella species. Non-bloodsucking leeches such as Erpobdella 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.

Behavior
Leeches are able to display a variety of behaviors that allow them to explore their environment and feed on their hosts. Exploratory behavior include head movements and body waving.

Feeding
Most leeches do not feed on human blood, but instead prey on small invertebrates, which they eat whole. To feed on its host, leeches use their anterior suckers to connect to a host for feeding, and also release an anesthetic to prevent the host from feeling the leech. Once attached, leeches use a combination of mucus and suction to stay attached and secrete an anti-clotting enzyme, hirudin, into the host's blood stream. Though certain species of leeches feed on blood, not all species can bite; 90% of them feed solely off decomposing bodies and open wounds of amphibians, reptiles, waterfowl, fish, and mammals (including, but not limited to, humans). A leech attaches itself when it bites, and it will stay attached until it becomes full, at which point it falls off to digest. Due to the 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, but there have been only a few reports of leeches' transmitting pathogens to humans. A study found both HIV and hepatitis B in African leeches from Cameroon.

Removal and Treatment


One recommended method of removal is using a fingernail or other flat, blunt object to break the seal of the oral sucker at the anterior 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 the sucker's seal is broken, at which point the leech will detach its jaws.

Common but medically inadvisable techniques to remove a leech are to apply a flame, a lit cigarette, salt, soap, or a chemical such as alcohol, vinegar, lemon juice, insect repellent, heat rub, or certain carbonated drinks. These will cause the leech to quickly detach, however, it will also regurgitate its stomach contents into the wound. The vomit may carry disease, and thus increase the risk of infection.

An externally attached leech will detach and fall off on its own when it is satiated on blood, which may be anywhere from 20 minutes to 2 hours or more. After feeding, the leech will detach and depart. Internal attachments, such as inside the nasal passage or vaginal attachments, are more likely to require medical intervention.

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 hirudin. Bleeding time will vary with location, and will vary from a few hours to three days. This is a function of the hirudin and other compounds that reduce the surface tension of the blood. Anti-clotting medications also affect the bleeding time. 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 around the lips or eyes, feeling faint or dizzy, and difficulty breathing.

Medicinal use of leeches
The European medical leech Hirudo medicinalis and some congeners, as well as some other species, have been used for clinical bloodletting for thousands of years. The use of leeches in medicine dates as far back as 2,500 years ago, when they were used for bloodletting in ancient India. Leech therapy is explained in ancient Ayurvedic texts. Many ancient civilizations practiced bloodletting, including Indian and Greek civilizations. In ancient Greek history, bloodletting was practiced according to the humoral theory, which proposed that, when the four humors, blood, phlegm, black and yellow bile in the human body were in balance, good health was guaranteed. An imbalance in the proportions of these humors was believed to be the cause of ill health. Records of this theory were found in the Greek philosopher Hippocrates' collection in the fifth century B.C. Bloodletting using leeches was one method used by physicians to balance the humors and to rid the body of the plethora.

The use of leeches in modern medicine made its comeback in the 1980s after years of decline, with the advent of microsurgery such as plastic and reconstructive surgeries. In operations such as these, one problem that arises is venous congestion due to inefficient venous drainage. Sometimes because of the technical difficulties in forming an anastomosis of a vein, no attempt is made to re-attach a venous supply to a flap at all. This condition is known as venous insufficiency. If this congestion is not cleared up quickly, the blood will clot, arteries that bring the tissues their necessary nourishment will become plugged and the tissues will die. To prevent this leeches are applied to a congested flap and a certain amount of excess blood is consumed before the leech falls away. The wound will also continue to bleed for a while due to the anticoagulant hirudin in the leeches' saliva. The combined effect is to reduce the swelling in the tissues and promoting healing by allowing fresh, oxygenated blood to reach the area.

The active anticoagulant principle of leech saliva, is a small protein, hirudin. Discovery and isolation of this protein led to a method of producing it by recombinant technology. Recombinant hirudin is available to physicians as an intravenous anticoagulant preparation for injection, particularly useful for patients who are allergic to or cannot tolerate heparin.