Medicinal leeches

According to the theory of evolution, leeches are among the oldest groups of animals on Earth. Their evolutionary ancestors are thought to have emerged several hundred million years ago, and fossil finds indicate that their basic body plan did not change significantly over very long geological periods. Because they lack a hard skeleton, leech fossils are relatively rare.

Today around 650 species of leeches are known, but only a small proportion of them are used in hirudotherapy. In Europe, the species most commonly used are Hirudo medicinalis, Hirudo verbana and Hirudo orientalis.

Medicinal leeches belong to the annelids. They have a front and a rear sucker, which they use for movement and feeding. In the front part of the body is a mouth with three jaws, which create the characteristic three-pronged bite. On the head they have several light receptors, and across the body numerous sensory cells, with which they detect movement, vibrations and other stimuli from their surroundings.

Leeches are hermaphrodites, which means that each individual has both male and female reproductive organs. They reach sexual maturity in a little over a year, and young leeches develop from the cocoons they secrete. Their lifespan can exceed twenty years in favourable conditions.

In the wild they are most often found in calm freshwater environments, such as marshes, ponds and slow-flowing waters. When they sense a suitable host, they approach it and feed for only a short time, after which they can digest the meal for several months. Thanks to their exceptionally efficient metabolism, they can survive for a very long period without a new meal.

Leeches are excellent swimmers and surprisingly agile on land as well. They are capable of considerable changes in body shape, which allows them to pass easily through narrow openings. Anyone who has ever fed or reared them knows that they are true masters of escape.

Due to the drainage of wetlands, changes in agriculture and intensive exploitation in the past, the natural populations of medicinal leeches have declined sharply in many parts of Europe. For this reason, leeches from specialised breeding centres are now used almost exclusively for therapeutic purposes, while many natural populations are protected.