European Medicinal Leech: Features, Biology, and Medical Use
An Intriguing and Useful Organism
The European medicinal leech (Hirudo medicinalis) is an intriguing organism. It’s related to earthworms and has a segmented body. Leeches are classified as either parasites or predators. The parasites include the medicinal leech. They feed on liquid blood, which they obtain from their host with the aid of a chemical in their saliva called hirudin. The substance acts as an anticoagulant. The blood-sucking habit of the medicinal leech has been used to help certain medical problems, as its name suggests. Scientists have recently sequenced the animal’s genome, which might lead to some interesting and hopefully useful discoveries.
Several species are referred to as "medicinal leeches". In this article, I use the term to refer to the European medicinal leech, or Hirudo medicinalis. The species is used in medicine in Europe and in other parts of world. The International Union for Conservation of Nature (IUCN) classifies the wild population of the species as Near Threatened.
External Features of Leeches
Leeches belong to the phylum Annelida, like earthworms. The body of a leech is long and worm-like or short and wide, depending on the species. The members of the phylum Annelida are segmented on the outside of their body and on the inside.
According to the latest analysis, leeches have 34 internal segments. The animals have a larger number of external segments than internal ones. In some leeches the external segments are obvious while in others they are barely visible.
Like earthworms, leeches are classified in the class Clitellata within the phylum Annelida because part of their body is surrounded by a thickened area called a clitellum. Unlike the case in earthworms, a leech's clitellum is noticeable only during the reproduction period.
The clitellum produces a ring of secreted material that surrounds the body. The ring picks up eggs and sperm from openings in the animal as it moves forward towards the leech's head. It eventually slides off the head and forms a cocoon. The process is shown in the video below. The youngsters develop inside the cocoon.
Leeches are usually classified in the subclass Hirudinea within the class Clitellata. Some sources refer to the Hirudinea as a class instead of a subclass.
Habitat, Suckers, and Movement
Many leeches live in fresh water, but some species live in the ocean or on land. Aquatic leeches can swim through the water by moving their body in an undulating style. On land, the animals often inhabit moist areas in forests. The land animals don't seem to enter water deliberately, but many can survive a short period of immersion in water.
According to the Australian Museum, some terrestrial species burrow into the soil when the environment is dry. Their body dries out and becomes rigid. If the soil is moistened, the animals quickly revive.
A leech has a small sucker surrounding its mouth at its anterior (front) end and a larger sucker at the posterior (rear) end. The suckers are visible only in certain situations. The anterior sucker attaches the leech to its victim. The posterior sucker also attaches the leech to the victim and in addition provides leverage during movement over a solid surface. Leeches often use a looping or "inch worm" style of movement when they are approaching a suitable host. The animals have muscular bodies.
Nervous System and Senses
The nervous system of a leech is based on neurons, or nerve cells. A neuron consists of a cell body containing the nucleus. The cell body has extensions called dendrites and another extension called an axon, as shown in the illustration above. The nerve impulse travels from the dendrites to the cell body and then along the axon to another neuron.
A leech is sometimes said to have a brain in its head, but the structure has a different composition from the brain of more advanced animals. The head region of a leech contains two connected ganglia. A ganglion contains the cell bodies of multiple neurons.
Axons from the leech's head extend along the animal's body as a nerve and meet another ganglion, which is known as a segmental ganglion. This in turn extends its axons along the body as a nerve until another segmental ganglion is reached. The process repeats along the length of the animal. Closely aligned ganglia are located at the end of a leech, which are known as the tail ganglia or the posterior brain. Branches leave the main nerve cord and go to various parts of the body.
The head of a leech has eyespots or ocelli that can detect light but can't form an image. The animal is able to detect vibrations, temperature, and the presence of certain chemicals via sense organs on its surface.
Though some websites say that leeches have 32 brains, the "brains" are really ganglia and are much simpler than the brains of many more advanced animals.
The Digestive Tract and Digestion
The digestive tract of a leech travels from the mouth to the anus. It consists of a pharynx, an esophagus, a crop, a stomach, an intestine, and a rectum.
The crop is an elongated structure with ten chambers, which are positioned one after the other along the digestive tract. Each chamber has a sac on either side called a cecum. The crop and its ceca act as a storage site for blood. The blood may be stored there for months. If a blood-sucking leech gets a good meal, it may not have to eat again for a long time.
The crop leads to a small stomach, which digests the blood. The digested material is absorbed by the lining of the intestine. The undigested remains of the food are sent to the rectum and then out of the body via the anus, which is located on the dorsal (upper) surface of the animal.
Circulation, Respiration, and Excretion
A leech has a closed circulatory system. This means that the blood (or the haemocoelomic fluid as it's sometimes called) is always within vessels. A dorsal vessel travels above the digestive tract and a ventral vessel travel below the tract. They are connected by lateral vessels that are contractile and act as hearts. The fluid flows forward in the dorsal vessel and backward in the ventral one. Smaller vessels are connected to the main ones.
In the medicinal leech, gas exchange during respiration occurs through the body surface. Oxygen enters the body and carbon dioxide leaves it. The animal doesn't have lungs or gills.
The leech has pairs of nephridia along its body. A nephridium is an excretory organ that removes wastes from body fluids. The wastes are release into the outer environment through nephridiopores.
Leeches are hermaphrodites, which means that they produce both eggs and sperm. They mate with another leech in order to exchange sperm instead of undergoing self-fertilization.
The testes are arranged in pairs along the middle section of the body. The number of pairs depends on the species. The sperm from the testes is released into a tubular system that leads to an opening on the ventral or lower surface of the body. A leech has only one pair of ovaries. These are located just in front of the first pair of testes and produce the eggs. Like the sperm, the eggs reach an opening on the ventral surface of the body via a tubular system.
The released sperm and eggs are picked up by the moving ring of material produced by the clitellum. The juveniles that develop from the fertilized eggs resemble the adults in appearance except with respect to their size.
The leech scientist in the video below knows what he is doing with respect to the animal that he is using. Knowledge is very important if anyone deliberately allows a leech to bite them, as described in the "Warning" box below.
The term "medicinal leech" often refers to Hirudo medicinalis, especially in Europe. Other leech species are sometimes referred to as medicinal leeches, however. The "product" listed and approved for medical use by the FDA (Food and Drug Administration) in the United States is the Hirudo medicinalis species.
The European medicinal leech has a thick and segmented body. The upper surface is mostly greenish brown to dark brown but is decorated with green, yellow, orange, or red stripes or blotches. The undersurface is usually lighter in color and may be green, orange, or yellow. The animal sometimes appears to be entirely dark brown or even black. When examined closely under the right lighting conditions, other colors may appear and the leech may look quite attractive.
Most leeches suck blood to obtain nutrients, but some eat animals or decaying plant material instead of blood.
Life of Hirudo medicinalis
Hirudo medicinalis lives in Europe and parts of Asia. It's reared in medical labs in other parts of the world as well as in its native habitat. In nature, it's found in freshwater habitats such as ponds, shallow lakes, and marshes. Its diet isn't restricted to mammalian blood. It will feed on mammals that enter the wetland, but it also feeds on blood from other animals, including frogs, turtles, and fish.
The animal has a tripartite (three-part) jaw, which is said to cut human skin with little pain. The leech drops off its victim's body once it's full. Often, the bite seems to do little harm, especially if the animal that's bitten is large. If the leech transmits an infection or if multiple leaches attack the victim, however, the effects might be serious.
Medicinal leeches must be obtained, applied, and monitored by qualified health practitioners (or qualified scientists in certain situations). Any time an animal bites a human, infection is possible. In addition, uncontrolled blood loss is dangerous. Specialists know how to reduce the possibility that an infection or other harmful effects will appear when a medicinal leech is used.
Uses of Leeches in Medicine
Medicinal leeches were once a popular medical treatment. They fell out of favor for a while but have now become popular again for specific conditions. They must be applied by a medical practitioner who understands how to deal with the animals and how to help and protect the patient. Medicinal leeches are bred in medical laboratories under specific and usually sterile conditions.
The saliva of a medicinal leech contains a chemical called hirudin, which acts as an anticoagulant. Hirudin is a protein (or a peptide) made of 75 amino acids joined together. Peptides are shorter chains of amino acids than proteins, but the point where a peptide is said to become a protein varies.
Hirudin is a potent anticoagulant. Leech saliva contains other anticoagulants. According to the press release from the researchers studying the genome of the medicinal leech, the saliva of Hirudo medicinalis and the closely related Hirudo verbana "contains the strongest blood thinners known to medicine". It's often claimed that leech saliva also contains an anesthetic, which prevents a victim from knowing that it's been bitten, but some researchers doubt whether this anesthetic actually exists.
Leech therapy is used in some cases of reconstructive surgery, such as finger reattachment and skin grafts. It's beneficial in circumstances in which blood flow through transplanted tissues is required and in which blot clots would be harmful. The leeches are allowed to suck blood for a limited amount of time. Somewhat sadly after their service to humanity, they are then killed. A leech that has fed on one human's blood mustn't be used to treat someone else due to the possibility of disease transmission. Doctors sometimes administer an antibiotic with a leech treatment in case bacteria already present in the animal hurt the patient.
Incredibly, the leech uses 15 different proteins known to negatively affect the blood-clotting mechanism in vertebrates, and 17 other proteins that are likely also part of the same anti-clotting process— Sebastan Kvist, Royal Ontario Museum, via ScienceDaily
The Sequenced Genome of Hirudo medicinalis
A group of researchers coordinated by the Royal Ontario Museum in Canada has recently sequenced the genome of the medicinal leach. The genome is the complete genetic information (or set of genes) in an organism. Genes control many of the characteristics of an organism. A gene is a section of a DNA or deoxyribonucleic acid molecule that codes for a particular protein. Cells make the proteins coded for in their genes, depending on certain factors.
The researchers hope that their analysis will enable scientists to better understand the medicinal leech's abilities and potential uses and perhaps help physicians to decide when leech therapy is appropriate.
The research could also be useful in another way. Hirudin is available as an isolated protein. Other proteins in the leech's saliva may also be helpful in dealing with human health. If we know which sections of a leech's DNA code for these chemicals, we may be able to add the relevant genes to bacteria. Bacteria are sometimes able to make the proteins coded for in the DNA that is added to them by scientists. Some species of bacteria are able to reproduce frequently under suitable conditions, which might enable a large population with a beneficial gene to be produced.
Therapy From Nature
Nature has provided us with some useful ways to treat certain health problems, though some methods may need to be modified before they are used. It's interesting that an old treatment like leech therapy is being used again.
Hirudin may not be the only medically useful chemical in medicinal leech saliva. The saliva contains multiple chemicals and appears to be beneficial for additional human health problems. People with certain pre-existing health conditions shouldn't be treated by leeches, however. A doctor's advice is very important for anyone considering leech therapy.
- Information about the subclass Hirudinea from the Encyclopedia Britannica
- Facts about the Hirudinea from the Natural History Museum in the UK
- Information about leeches from the Australian Museum
- Freshwater leech facts from the Mountain Lake Biological Station, University of Virginia
- European medicinal leech entry from the IUCN Red List
- The European medicinal leech Hirudo medicinalis: L.: Morphology and occurrence of an endangered species by U. Kutschera and Joy Elliott from the Zoosystematics and Evolution journal
- A visit to a medicinal leech farm from The Guardian newspaper (The title of the article indicates that the leech has ten stomachs, but these are actually the crop chambers.)
- Medicinal leech product classification from the U.S. Food and Drug Administration
- Genome of Hirudo medicinalis sequenced from the ScienceDaily news service
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Linda Crampton