The largest of the leeches found in the UK and the only one known to feed on human blood. However so much more defines this species and there are many fascinating things to know about this marvellous creature.
Medicinal leech have a similar morphology to that of other leeches, with a cylindrical, dorsoventrally flattened body divided into segments. They are the biggest leech in the UK by quite a margin with adults growing up to 20cm. They are dark brown to black in colour, with thin green, yellow and red stripes down their dorsal side, forming much intricate patterning than on first sight. They have two suckers, a posterior sucker, used as leverage to move, and an anterior sucker, which contains the jaws and is used for feeding. Medicinal Leech have three jaws (tripartite) with around 100 teeth that are used to attach to the host.
Unlike common perception, they do not feed predominantly on mammals. Throughout the year they will feed on the blood of a range of prey depending on the seasons and availability of prey. This includes amphibians, fish, birds and mammals. Amphibians are often their favoured meal, especially during the amphibian breeding season in the spring. On many smaller species they become more of a predator than parasite, causing fatal blood loss.
Little is known about the exact requirements of medicinal leech habitat, as they are now restricted to a handful of sites and there has been little research into their ecology. Similarities can be drawn from the current habitats they are found and historic observations. They appear to favour shallow ponds and ditches with fluctuating water levels that dry down during the summer months. Shallow water helps to ensure water temperatures remain relatively high during the spring and summer, which is particular important for breeding. All sites where they are present are regularly grazed. Grazing provides both food and controls vegetation growth – ensuring ponds do not become too overgrown. However grazing must not be too intense as they require dense stands of long leaved marginal vegetation to create cocoons in which to lay their eggs. They are an opportunistic feeder and can go many months between feeds. However they do require habitat which supports a range of prey, ensuring they have a succession of hosts through the seasons.
They reproduce sexually, creating cocoons from long leaved marginal vegetation about clutches of up to 50 eggs. These cocoons are places in shaded humid spots near the water, but not underwater.
Although medicinal leeches are successfully bred in laboratories, little is known about their requirements for breeding in the wild. They leave the water to lay their eggs, creating spongy protective cocoons from marginal vegetation. These cocoons are left in humid places, which will not become waterlogged or dry out, and where the young can easily reach the water upon hatching.
Distribution and threats
Once a widespread species they have dramatically declined over the past centuries. They fully are protected by law under the Wildlife and Countryside Act (1981). It is against the law to harm this species or damage their habitat in anyway.
Initial population decline was as a results of over exploitation for medicinal use. There were few aliments they were not prescribed for. The belief was that leeches were able to remove the ‘bad’ blood that caused sickness. This trade hit its peak in the middle of the nineteenth century. Each year tens of millions of leeches were exported to Europe and America. The European leech was favoured over its American counterpart (Hirudo decora), due to the larger amounts of blood it was able to consume.
It is likely that this trade has masked other factors that have contributed to medicinal leech decline. Other principle reasons behind decline have been from changes in land use, wetland drainage, loss of grazing and regulation of water levels. The introduction of worming treatments, such as Ivermectin in the late 70’s and early 80’s, is thought to be another key contributor. As a blood parasite medicinal leech are vulnerable to these chemicals through livestock grazing at their ponds. The full extent of the impact of Ivermectin and over worming treatments is not known, but it is likely to have caused mass fatalities.