Medicinal leech

The medicinal leech lives in freshwater habitats from western and southern Europe to the Ural Mountains, as well as along the northeastern coastal regions of the Mediterranean Sea. It is found mainly in marshy, boggy waters and lakes with dense vegetation.

The medicinal leech is a hermaphroditic animal belonging to the phylum Annelida, within the class Clitellata. It is closely related to the earthworm, but over the course of its evolution, it has acquired several unique traits adapted to its specialised way of life.

An adult specimen is 1–2 cm wide and 10–15 cm long in a resting state, and is flattened dorsoventrally. Its body consists of 33 or 34 segments; however, secondary annulation can be observed on its surface. Its dorsal side ranges in colour from yellowish-brown to dark olive green, while its ventral side is lighter grey, often darkly marbled. The setae characteristic of annelids are secondarily absent in the medicinal leech. At both ends of its body, there are suckers, which also assist in its looping movement. The anterior sucker also has a suction function. In the pharyngeal region, there are three jaws with serrated edges. Its circulatory system is open; its body cavity has been reduced to a narrow system of channels, so-called lacunae, which replace blood vessels.

It is a parasitic ectoparasite that attaches to the external surface of its host – mainly warm-blooded mammals, including humans – and feeds on its blood.

It reproduces exclusively sexually.

Its natural population has declined significantly in recent years due to two main factors: the destruction of wetland habitats and the historical overharvesting of leeches for medical purposes. As a result, it is a protected species in many countries, and today it is bred for medical use exclusively in controlled, sterile breeding facilities.

It swims in water by undulating body movements, while on land it moves by looping locomotion, alternately attaching its two suckers. First, it anchors its posterior sucker to the surface, then its body elongates as the circular muscles in the segments contract while the longitudinal muscles relax. Next, the anterior sucker attaches to the surface, the posterior one detaches, and the body shortens as the longitudinal muscles contract. Finally, the posterior sucker is moved forward again and reattaches, so the movement is repeated cyclically. One such 'step' can move the animal forward by up to two-thirds of its body length.

The medicinal leech attaches itself to the skin using two suckers. The larger posterior sucker provides stability, while the smaller anterior sucker, which also has a sucking function, contains three saw-like jaws, each bearing approximately 60–90 tiny teeth.

The leech makes a Y-shaped wound in the skin and then begins feeding.

During feeding, the leech secretes saliva into the wound. The glands in the wall of the pharynx produce saliva that contains, among other substances, the anticoagulant hirudin, a substance that reduces the pain of the bite, and histamine-like compounds that cause vasodilation and increase local blood flow.

The leech uses its muscular suctorial pharynx to suck blood, which it then transports into a storage stomach made up of pouch-like sacs located on both sides of the midgut. Its digestive system contains bacteria that help break down nutrients and also play a role in producing certain substances, such as vitamins. During a single feeding, it can take in three to ten times its own body weight, approximately 5–15 ml of blood.

After the leech detaches, slight oozing of blood from the wound continues for a while.

Its digestion is slow, and it can withstand long periods of starvation, usually several months to a year.

The medical use of the medicinal leech is called hirudotherapy. It is used primarily in plastic and reconstructive surgery, as well as in alternative medicine. The treatment improves blood flow in the smallest blood vessels (microcirculation), for example, in reattached body parts, which can be crucial for the survival and regeneration of damaged or transplanted tissues.