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Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
Annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...
a. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodies that can lengthen and contract. Both groups are
hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
s and have a
clitellum The clitellum is a thickened glandular and non-segmented section of the body wall near the head in earthworms and leeches, that secretes a viscid sac in which eggs are stored. It is located near the anterior end of the body, between the fourteent ...
, but leeches typically differ from the oligochaetes in having suckers at both ends and in having ring markings that do not correspond with their internal segmentation. The body is muscular and relatively solid, and the
coelom The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...
, the spacious body cavity found in other annelids, is reduced to small channels. The majority of leeches live in freshwater habitats, while some species can be found in terrestrial or marine environments. The best-known species, such as the medicinal leech, '' Hirudo medicinalis'', are hematophagous, attaching themselves to a host with a sucker and feeding on blood, having first secreted the peptide hirudin to prevent the blood from clotting. The jaws used to pierce the skin are replaced in other species by a proboscis which is pushed into the skin. A minority of leech species are predatory, mostly preying on small invertebrates. The eggs are enclosed in a cocoon, which in aquatic species is usually attached to an underwater surface; members of one family,
Glossiphoniidae Glossiphoniidae are a family of freshwater proboscis-bearing leeches. These leeches are generally flattened, and have a poorly defined anterior sucker. Most suck the blood of freshwater vertebrates like amphibians, crocodilians and aquatic tu ...
, exhibit parental care, the eggs being brooded by the parent. In terrestrial species, the cocoon is often concealed under a log, in a crevice or buried in damp soil. Almost seven hundred species of leech are currently recognised, of which some hundred are marine, ninety terrestrial and the remainder freshwater. Leeches have been used in medicine from ancient times until the 19th century to draw blood from patients. In modern times, leeches find medical use in treatment of joint diseases such as epicondylitis and osteoarthritis, extremity vein diseases, and in microsurgery, while hirudin is used as an
anticoagulant Anticoagulants, commonly known as blood thinners, are chemical substances that prevent or reduce coagulation of blood, prolonging the clotting time. Some of them occur naturally in blood-eating animals such as leeches and mosquitoes, where the ...
drug to treat blood-clotting disorders.


Diversity and phylogeny

Some 680 species of leech have been described, of which around 100 are marine, 480 freshwater and the remainder terrestrial. Among
Euhirudinea Euhirudinea, the true leeches, are an infraclass of the Hirudinea. Taxonomy These clitellate annelids are of somewhat unclear relationships; namely the relationships of Hirudinea with oligochaetes are in need of revision. It may be that the pr ...
, the true leeches, the smallest is about long, and the largest is the giant Amazonian leech, ''
Haementeria ghilianii ''Haementeria ghilianii'', the giant Amazon leech, is one of the world's largest species of leeches. It can grow to 450 mm (17.7 in) in length and 100 mm (3.9 in) in width. As adults, these leeches are a greyish-brown colour, ...
'', which can reach . Except for Antarctica, leeches are found throughout the world but are at their most abundant in temperate lakes and ponds in the northern hemisphere. The majority of freshwater leeches are found in the shallow, vegetated areas on the edges of ponds, lakes and slow-moving streams; very few species tolerate fast-flowing water. In their preferred habitats, they may occur in very high densities; in a favourable environment with water high in organic pollutants, over 10,000 individuals were recorded per square metre (over 930 per square foot) under rocks in Illinois. Some species
aestivate Aestivation ( la, aestas (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a ...
during droughts, burying themselves in the sediment, and can lose up to 90% of their bodyweight and still survive. Among the freshwater leeches are the
Glossiphoniidae Glossiphoniidae are a family of freshwater proboscis-bearing leeches. These leeches are generally flattened, and have a poorly defined anterior sucker. Most suck the blood of freshwater vertebrates like amphibians, crocodilians and aquatic tu ...
, dorso-ventrally flattened animals mostly parasitic on vertebrates such as turtles, and unique among
annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...
s in both brooding their eggs and carrying their young on the underside of their bodies. The terrestrial
Haemadipsidae __NOTOC__ Haemadipsidae (From Greek "haima" and "dipsa" ("blood" and "thirst", respectively)) are a family of jawed leeches. They are a monophyletic group of hirudiniform proboscisless leeches. These leeches have five pairs of eyes, with the l ...
are mostly native to the tropics and subtropics, while the aquatic Hirudinidae have a wider global range; both of these feed largely on mammals, including humans. A distinctive family is the
Piscicolidae The Piscicolidae are a family of jawless leeches in the order Rhynchobdellida that are parasitic on fish. They occur in both freshwater and seawater, have cylindrical bodies, and typically have a large, bell-shaped, anterior sucker with which the ...
, marine or freshwater ectoparasites chiefly of fish, with cylindrical bodies and usually well-marked, bell-shaped, anterior suckers. Not all leeches feed on blood; the
Erpobdelliformes The Erpobdelliformes are one of the currently-accepted suborders of the proboscisless leeches (Arhynchobdellida). It includes five families: * Americobdellidae * Erpobdellidae Blanchard, 1894 * Gastrostomobdellidae Richardson, 1971 *Orobdellid ...
, freshwater or amphibious, are carnivorous and equipped with a relatively large, toothless mouth to ingest insect larvae, molluscs, and other annelid worms, which are swallowed whole. In turn, leeches are prey to fish, birds, and invertebrates. The name for the subclass, Hirudinea, comes from the Latin ''hirudo'' (
genitive In grammar, the genitive case (abbreviated ) is the grammatical case that marks a word, usually a noun, as modifying another word, also usually a noun—thus indicating an attributive relationship of one noun to the other noun. A genitive can al ...
''hirudinis''), a leech; the element ''-bdella'' found in many leech group names is from the Greek βδέλλα ''bdella'', also meaning leech. The name ''Les hirudinées'' was given by Jean-Baptiste Lamarck in 1818. Leeches were traditionally divided into two infraclasses, the
Acanthobdellidea Acanthobdellidea is an infraclass of primitive leeches; some authors place them in a separate subclass from the Hirudinea. However, the World Register of Marine Species places them within the Hirudinea, as a sister group to Euhirudinea, the true l ...
(primitive leeches) and the Euhirudinea (true leeches). The Euhirudinea are divided into the proboscis-bearing Rhynchobdellida and the rest, including some jawed species, the " Arhynchobdellida", without a proboscis. The
phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
of the leeches and their annelid relatives is based on
molecular analysis Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...
(2019) of DNA sequences. Both the former classes " Polychaeta" (bristly marine worms) and " Oligochaeta" (including the earthworms) are
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
: in each case the complete groups (
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
s) would include all the other groups shown below them in the tree. The Branchiobdellida are sister to the leech clade Hirudinida, which approximately corresponds to the traditional subclass Hirudinea. The main subdivision of leeches is into the Rhynchobdellida and the Arhynchobdellida, though the
Acanthobdella ''Acanthobdella'' is a genus of annelids The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species ...
are sister to the clade that contains these two groups.


Evolution

The most ancient annelid group consists of the free-living polychaetes that evolved in the
Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
period, being plentiful in the Burgess Shale about 500million years ago. Oligochaetes evolved from polychaetes and the leeches branched off from the oligochaetes. The oldest leech fossils are from the Jurassic period around 150million years ago, but a fossil with external ring markings found in Wisconsin in the 1980s, with what appears to be a large sucker, seems to extend the group's evolutionary history back to the
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
, some 437million years ago.


Anatomy and physiology

Leeches show a remarkable similarity to each other in morphology, very different from typical annelids which are cylindrical with a fluid-filled space, the
coelom The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...
(body cavity). In leeches, most of the coelom is filled with botryoidal tissue, a loose connective tissue composed of clusters of cells of mesodermal origin. The remaining body cavity has been reduced to four slender longitudinal channels. Typically, the body is dorso-ventrally flattened and tapers at both ends. Longitudinal and circular muscles in the body wall are supplemented by diagonal muscles, giving the leech the ability to adopt a large range of body shapes and show great flexibility. Most leeches have a sucker at both the anterior (front) and posterior (back) ends, but some primitive leeches have a single sucker at the back. Like other annelids, the leech is a segmented animal, but unlike other annelids, the segmentation is masked by external ring markings (annulations). The number of annulations varies, both between different regions of the body and between species. In one species, the body surface is divided into 102 annuli, but the body consists of 33 segments, a number constant across all leech species. Of these segments, the first five are designated as the head and include the anterior brain, several ocelli (eyespots) dorsally and the sucker ventrally. The following 21 mid-body segments each contain a nerve
ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...
, and between them contain two reproductive organs, a single female gonopore and nine pairs of testes. The last seven segments contain the posterior brain and are fused to form the animal's tail sucker. The septa that separates the body segments in the majority of annelids have been lost in leeches, except for the primitive genus
Acanthobdella ''Acanthobdella'' is a genus of annelids The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species ...
which still have some septa. The body wall consists of a
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
, an
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
and a thick layer of fibrous
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
in which are embedded the circular muscles, the diagonal muscles and the powerful longitudinal muscles. There are also dorso-ventral muscles. In leeches the original blood vascular system has been lost and replaced by the modified coelom known as the haemocoelomic system, and the coelomic fluid, called the haemocoelomic fluid, has taken over the role as blood. The haemocoelomic channels run the full length of the body, the two main ones being on either side. Part of the lining
epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
consists of
chloragogen cells Chloragogen cells, also called ''y cells'', are star-shaped cells in annelids involved with excretory functions and intermediary metabolism. They function similarly to the liver in vertebrates. Chloragogen tissue is most extensively studied in e ...
which are used for the storage of nutrients and in excretion. There are 10 to 17 pairs of metanephridia (excretory organs) in the mid-region of the leech. From these, ducts typically lead to a urinary bladder, which empties to the outside at a nephridiopore.


Reproduction and development

Leeches are hermaphrodites, with the male reproductive organs, the testes, maturing first and the ovaries later. In hirudinids, a pair will line up with the clitellar regions in contact, with the anterior end of one leech pointing towards the posterior end of the other; this results in the male gonopore of one leech being in contact with the female gonopore of the other. The penis passes a spermatophore into the female gonopore and sperm is transferred to, and probably stored in, the vagina. Some jawless leeches (Rhynchobdellida) and proboscisless leeches (Arhynchobdellida) lack a penis, and in these, sperm is passed from one individual to another by
hypodermic injection A hypodermic needle (from Greek ὑπο- (''hypo-'' = under), and δέρμα (''derma'' = skin)), one of a category of medical tools which enter the skin, called sharps, is a very thin, hollow tube with one sharp tip. It is commonly used w ...
. The leeches intertwine and grasp each other with their suckers. A spermatophore is pushed by one through the integument of the other, usually into the clitellar region. The sperm is liberated and passes to the ovisacs, either through the coelomic channels or interstitially through specialist "target tissue" pathways. Some time after copulation, the small, relatively yolkless eggs are laid. In most species, an albumin-filled cocoon is secreted by the clitellum and receives one or more eggs as it passes over the female gonopore. In the case of the North American ''
Erpobdella punctata ''Erpobdella punctata'' is a leech in the family Erpobdellidae. It is found in freshwater streams and ponds in many parts of North America. Distribution and habitat ''Erpobdella punctata'' is a widespread species in North America. It occurs in A ...
'', the clutch size is about five eggs, and some ten cocoons are produced. Each cocoon is fixed to a submerged object, or in the case of terrestrial leeches, deposited under a stone or buried in damp soil. The cocoon of '' Hemibdella soleae'' is attached to a suitable fish
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...
. The glossiphoniids brood their eggs, either by attaching the cocoon to the substrate and covering it with their ventral surface, or by securing the cocoon to their ventral surface, and even carrying the newly hatched young to their first meal. When breeding, most marine leeches leave their hosts and become free-living in estuaries. Here they produce their cocoons, after which the adults of most species die. When the eggs hatch, the juveniles seek out potential hosts when these approach the shore. Leeches mostly have an annual or biannual life cycle.


Feeding and digestion

About three quarters of leech species are parasites that feed on the blood of a host, while the remainder are
predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
. Leeches either have a pharynx that they can protrude, commonly called a proboscis, or a pharynx that they cannot protrude, which in some groups is armed with jaws. In the proboscisless leeches, the jaws (if any) of Arhynchobdellids are at the front of the mouth, and have three blades set at an angle to each other. In feeding, these slice their way through the skin of the host, leaving a Y-shaped incision. Behind the blades is the mouth, located ventrally at the anterior end of the body. It leads successively into the pharynx, a short oesophagus, a crop (in some species), a stomach and a hindgut, which ends at an anus located just above the posterior sucker. The stomach may be a simple tube, but the crop, when present, is an enlarged part of the midgut with a number of pairs of ceca that store ingested blood. The leech secretes an anticoagulant, hirudin, in its
saliva Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ...
which prevents the blood from clotting before ingestion. A mature
medicinal leech ''Hirudo medicinalis'', the European medicinal leech, is one of several species of leeches used as "medicinal leeches". Other species of ''Hirudo'' sometimes also used as medicinal leeches include '' H. orientalis'', ''H. troctina'', and '' H. ...
may feed only twice a year, taking months to digest a blood meal. The bodies of predatory leeches are similar, though instead of a jaw many have a protrusible proboscis, which for most of the time they keep retracted into the mouth. Such leeches are often ambush predators that lie in wait until they can strike prey with the proboscises in a spear-like fashion. Predatory leeches feed on small invertebrates such as snails, earthworms and insect larvae. The prey is usually sucked in and swallowed whole. Some Rhynchobdellida however suck the soft tissues from their prey, making them intermediate between predators and blood-suckers. Blood-sucking leeches use their anterior suckers to connect to hosts for feeding. Once attached, they use a combination of mucus and suction to stay in place while they inject hirudin into the hosts' blood. In general, blood-feeding leeches are non host-specific, and do little harm to their host, dropping off after consuming a blood meal. Some marine species however remain attached until it is time to reproduce. If present in great numbers on a host, these can be debilitating, and in extreme cases, cause death. Leeches are unusual in that they do not produce certain digestive enzymes such as amylases,
lipase Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...
s or endopeptidases. A deficiency of these enzymes and of
B complex vitamins B vitamins are a class of water-soluble vitamins that play important roles in Cell (biology), cell metabolism and synthesis of red blood cells. Though these vitamins share similar names (B1, B2, B3, etc.), they are chemically distinct compounds th ...
is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In ''Hirudo medicinalis'', these supplementary factors are produced by an obligatory mutualistic relationship with the bacterial species, ''Aeromonas veronii''. Non-bloodsucking leeches, such as ''Erpobdella octoculata'', are host to morew bacterial symbionts. In addition, leeches produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the hindgut. This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from oligochaetes, and may explain why digestion in leeches is so slow.


Nervous system

A leech's nervous system is formed of a few large nerve cells; their large size makes leeches convenient as model organisms for the study of invertebrate nervous systems. The main nerve centre consists of the cerebral ganglion above the gut and another ganglion beneath it, with connecting nerves forming a ring around the pharynx a little way behind the mouth. A nerve cord runs backwards from this in the ventral coelomic channel, with 21 pairs of ganglia in segments six to 26. In segments 27 to 33, other paired ganglia fuse to form the caudal ganglion. Several sensory nerves connect directly to the cerebral ganglion; there are sensory and motor nerve cells connected to the ventral nerve cord ganglia in each segment. Leeches have between two and ten pigment spot ocelli, arranged in pairs towards the front of the body. There are also sensory papillae arranged in a lateral row in one annulation of each segment. Each papilla contains many sensory cells. Some rhynchobdellids have the ability to change colour dramatically by moving pigment in chromatophore cells; this process is under the control of the nervous system but its function is unclear as the change in hue seems unrelated to the colour of the surroundings. Leeches can detect touch, vibration, movement of nearby objects, and chemicals secreted by their hosts; freshwater leeches crawl or swim towards a potential host standing in their pond within a few seconds. Species that feed on warm-blooded hosts move towards warmer objects. Many leeches avoid light, though some blood feeders move towards light when they are ready to feed, presumably increasing the chances of finding a host.


Gas exchange

Leeches live in damp surroundings and in general respire through their body wall. The exception to this is in the Piscicolidae, where branching or leaf-like lateral outgrowths from the body wall form gills. Some rhynchobdellid leeches have an extracellular haemoglobin pigment, but this only provides for about half of the leech's oxygen transportation needs, the rest occurring by diffusion.


Movement

Leeches move using their longitudinal and circular muscles in a modification of the locomotion by peristalsis, self-propulsion by alternately contracting and lengthening parts of the body, seen in other annelids such as earthworms. They use their posterior and anterior suckers (one on each end of the body) to enable them to progress by looping or inching along, in the manner of geometer moth caterpillars. The posterior end is attached to the substrate, and the anterior end is projected forward peristaltically by the circular muscles until it touches down, as far as it can reach, and the anterior end is attached. Then the posterior end is released, pulled forward by the longitudinal muscles, and reattached; then the anterior end is released, and the cycle repeats. Leeches explore their environment with head movements and body waving. The Hirudinidae and
Erpobdellidae Erpobdellidae is a family of leeches. It is one of the four families belonging to the suborder Erpobdelliformes of the proboscisless leeches order, Arhynchobdellida. Their members have abandoned the blood feeding habits of their ancestors and ...
can swim rapidly with up-and-down or sideways undulations of the body; the Glossiphoniidae in contrast are poor swimmers and curl up and fall to the sediment below when disturbed. File:Leech looping locomotion.jpg, alt=Sketches of the looping movement of a leech using its front and back suckers, Leeches move by looping using their front and back suckers. File:20100214 Leech climbing door at Lake Leake, Tasmania.ogv, alt=Video of looping movement, Video of looping movement


Interactions with humans


Bites

Leech bites are generally alarming rather than dangerous, though a small percentage of people have severe allergic or
anaphylactic reactions Anaphylaxis is a serious, potentially fatal allergic reaction and medical emergency that is rapid in onset and requires immediate medical attention regardless of use of emergency medication on site. It typically causes more than one of the follow ...
and require urgent medical care. Symptoms of these reactions include red blotches or an itchy rash over the body, swelling around the lips or eyes, a feeling of faintness or dizziness, and difficulty in breathing.Victorian Poisons Information Centre: Leeches
Victorian Poisons Information Centre. Retrieved 28 July 2007
An externally attached leech will detach and fall off on its own accord when it is satiated on blood, which may take from twenty minutes to a few hours; bleeding from the wound may continue for some time. Internal attachments, such as inside the nose, are more likely to require medical intervention. Bacteria, viruses, and protozoan parasites from previous blood sources can survive within a leech for months, so leeches could potentially act as vectors of pathogens. Nevertheless, only a few cases of leeches transmitting pathogens to humans have been reported. Leech saliva is commonly believed to contain anaesthetic compounds to numb the bite area, but this has never been proven. Although morphine-like substances have been found in leeches, they have been found in the neural tissues, not the salivary tissues. They are used by the leeches in modulating their own
immunocyte White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
s and not for anaesthetising bite areas on their hosts. Depending on the species and size, leech bites can be barely noticeable or they can be fairly painful.


In human culture

The leech appears in Proverbs30:15 as an archetype of insatiable greed. More generally, a leech is a persistent social parasite or sycophant. The medicinal leech ''Hirudo medicinalis'', and some other species, have been used for clinical bloodletting for at least 2,500 years:
Ayurvedic Ayurveda () is an alternative medicine system with historical roots in the Indian subcontinent. The theory and practice of Ayurveda is pseudoscientific. Ayurveda is heavily practiced in India and Nepal, where around 80% of the population rep ...
texts describe their use for bloodletting in ancient India. In ancient Greece, bloodletting was practised according to the
theory of humours Humorism, the humoral theory, or humoralism, was a system of medicine detailing a supposed makeup and workings of the human body, adopted by Ancient Greek and Roman physicians and philosophers. Humorism began to fall out of favor in the 1850s ...
found in the Hippocratic Corpus of the fifth centuryBC, which maintained that health depended on a balance of the four humours: blood, phlegm, black bile and yellow bile. Bloodletting using leeches enabled physicians to restore balance if they considered blood was present in excess. Pliny the Elder reported in his '' Natural History'' that the
horse leech ''Haemopis sanguisuga'' is a species of freshwater leech in the family Haemopidae. It is commonly called the horse-leech, but that is due to the similarity of its appearance to the leech ''Limnatis nilotica'', which sometimes enters the nasal c ...
could drive elephants mad by climbing up inside their trunks to drink blood. Pliny also noted the medicinal use of leeches in ancient Rome, stating that they were often used for gout, and that patients became addicted to the treatment. In
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
, ''lǣce'' was the name for a physician as well as for the animal, though the words had different origins, and ''lǣcecraft'', leechcraft, was the art of healing. File:Leech Finders Costume of Yorkshire 1814.jpg , alt=Print from Costumes of Yorkshire 1814 showing three women searching for leeches in a pond, "Leech finders" from ''
The Costume of Yorkshire ''The Costume of Yorkshire'' is an 1814 book by George Walker illustrating the various styles of dress worn by people of differing traditional professions in the county of Yorkshire in the 19th century. Author George Walker was born in 1781 nea ...
'' by George Walker, 1814, engraved by Robert Havell File:Pharmacy leech jar, England, 1830-1870 Wellcome L0058506.jpg, alt=Photograph of white and blue ceramic English pharmacy leech jar with airholes in the lid, Pharmacy leech jar with airholes in the lid. England, 1830–1870. File:Three leeches in the role of physicians attend a grasshopper in the role of the patient and announce a course of bloodletting Wellcome V0011722.jpg, alt=Coloured lithograph cartoon showing three leech doctors treating a grasshopper patient, Three leech doctors decide on bloodletting for their
grasshopper Grasshoppers are a group of insects belonging to the suborder Caelifera. They are among what is possibly the most ancient living group of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago. Grasshopp ...
patient. Lithograph by F.-J.-V. Broussais from a cartoon by J. J. Grandville,
William Wordsworth's 1802 poem " Resolution and Independence" describes one of the last of the leech-gatherers, people who travelled Britain catching leeches from the wild, and causing a sharp decline in their abundance, though they remain numerous in Romney Marsh. By 1863, British hospitals had switched to imported leeches, some seven million being imported to hospitals in London that year. In the nineteenth century, demand for leeches was sufficient for hirudiculture, the farming of leeches, to become commercially viable. Leech usage declined with the demise of humoral theory, but made a small-scale comeback in the 1980s after years of decline, with the advent of microsurgery, where venous congestion can arise due to inefficient venous drainage. Leeches can reduce swelling in the tissues and promote healing, helping in particular to restore circulation after microsurgery to reattach body parts. Other clinical applications include varicose veins, muscle cramps, thrombophlebitis, and joint diseases such as epicondylitis and osteoarthritis. Leech secretions contain several bioactive substances with anti-inflammatory,
anticoagulant Anticoagulants, commonly known as blood thinners, are chemical substances that prevent or reduce coagulation of blood, prolonging the clotting time. Some of them occur naturally in blood-eating animals such as leeches and mosquitoes, where the ...
and antimicrobial effects. One active component of leech saliva is a small protein, hirudin. It is widely used as an anticoagulant drug to treat blood-clotting disorders, and manufactured by recombinant DNA technology. In 2012 and 2018, Ida Schnell and colleagues trialled the use of ''
Haemadipsa ''Haemadipsa'' is a genus of leeches, with members commonly known as ''jawed land leeches''. These annelids are known from subtropical and tropical regions around the Indian and Pacific Ocean. Well-known ''Haemadipsa'' are for example the Indian L ...
'' leeches to gather data on the biodiversity of their
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
ian hosts in the tropical rainforest of Vietnam, where it is hard to obtain reliable data on rare and
cryptic Cryptic may refer to: In science: * Cryptic species complex, a group of species that are very difficult to distinguish from one another * Crypsis, the ability of animals to blend in to avoid observation * Cryptic era, earliest period of the Earth ...
mammals. They showed that mammal
mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...
, amplified by the polymerase chain reaction, can be identified from a leech's blood meal for at least four months after feeding. They detected Annamite striped rabbit, small-toothed ferret-badger, Truong Son muntjac, and serow in this way.


Water pollution

Exposure to synthetic estrogen as used in contraceptive medicines, which may enter freshwater ecosystems from municipal wastewater, can affect leeches' reproductive systems. Although not as sensitive to these compounds as fish, leeches showed physiological changes after exposure, including longer sperm sacs and vaginal bulbs, and decreased
epididymis The epididymis (; plural: epididymides or ) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the ...
weight.


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References


General bibliography

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External links

* * * {{featured article Articles containing video clips Clitellata Extant Silurian first appearances