European mecicinal leech was found in Northern Europe, from Ireland in the west to the Ural Mountains in the east, and from Southern Scandinavia to the countries bordering the Mediterranean. Protected species in Lithuania, enlisted into the Red Data Book. The typical natural habitat is an eutrophic lake with a muddy substratum, littoral vegetation, and a high summer temperature. It should also be a breeding site for amphibians (frogs, toads and newts). European medicinal leeches are often reported as feeding almost exclusively on the blood of mammals (cattle, horses, deer, humans), but they will also suck the blood of fish, water birds, and especially amphibians, both the adults and their larvae. Tadpoles as well as juvenile newts are especially important for young medicinal leeches that are unable to pierce mammalian skin for the first feedings. Mature medicinal leeches leave the water to deposit their cocoons in a moist place just above the water line on the shore or bank. The spongy cocoons are laid chiefly in July and August. It is general agreed that medicinal leech take at least two years to reach the breeding stage in the wild, and slow-growing leeches may not breed until they are three or four years old. For a large part of the year when water temperatures are low, medicinal leeches are quiescent and remain buried in the mud or under submerged objects at the edge of the pond. As water temperature increases, the leeches become very responsive to water disturbance caused by a potential host, and swim towards the source of blood, responded to low-amplitude surface waves.
The European medicinal leech is one of the best-known members of the Hirudinea due to its use in phlebotomy (bloodletting). Large numbers of H. medicinalis were obtained from the wild in the 18th and early 19th centuries, and towards the end of this period, they were already scarce in many countries.
