Medicinal leech  (Hirudo medicinalis)

This animal has a slightly flattened cylindrical body, divided into 33 or 34 segments. The upperside is dark brown or black with six long reddish stripes, whilst the underside is speckled. There is a disc-shaped sucker at the head end. The leech is famous for sucking blood and the 'mouth' of the animal is situated within the sucker, complete with teeth. The leech also has five pairs of eyes.

The medicinal leech has been recorded in 24 countries throughout Europe as far as the Ural mountain range. It is scarce in France and Belgium, and is thought to occur in more than 20 scattered populations in the UK. These extend from Kent to Argyll and Islay in Scotland, and from Norfolk to Anglesey.

The leech is an amphibious freshwater animal, and usually found in small pools with muddy bottoms and fringed with reeds. These pools are also shared with frogs, toads or newts for at least some of the year. Leeches prefer water that is eutrophic, meaning it is high in nutrients. Where amphibians are not present in leech infested ponds, it is believed that grazing stock provide the food source.

On Romney Marsh in Kent, there are extensive populations of the medicinal leech living in the shallow parts of deep gravel pits, and in ditches that run through the grazing marshes. In Cumbria, leeches have been found in conditions that differ markedly from those in Kent. Here, it is thought that being shallower, the water warms up quickly to about 20°C, a temperature that must be attained for at least part of the year in order for leeches to survive in a pond.

Leeches feed on blood. Having attached itself to the host animal, it pierces the skin and injects an anaesthetic to hide the pain of its bite so that the host does not find the leech and remove it, and an anticoagulant chemical, which prevents the host's blood from clotting whilst the leech feeds. The length of time a leech may feed seems to vary. One surveyor, as an experiment, allowed a leech to feed on him and it fed for 83 minutes. Another, presumably less-hungry individual, fed for just 25 minutes. During a meal, it may extract 15 millilitres of blood, which can increase the size of the leech by anything up to eleven times its normal body dimensions. A leech's meal can sustain it for over six months, but it may also have to wait many months between feeds. During this time, they can digest their own body tissue to avoid starvation. Leeches find their host animals by detecting disturbance in the water, and they can prey on small creatures as well as large. A frog or a newt, for example, can die from excessive blood-loss following an attack by a leech. Leeches may also behave as predators on some species of fish such as sticklebacks, as well as on great-crested newts and marsh frogs.

Leeches are often found in the nests of birds such as moorhens, and seem to use them as shelter as well as finding a food source. Dismantling old nests can be a method of surveying for leeches. The eggs are laid in a spongy cocoon on damp ground and, at Dungeness in Kent, they are associated with the roots of willowherb. On nearby Walland Marsh, the eggs have been found in damp turf on close-grazed sheep pasture. Humans are susceptible to parasitism by leeches but, apart from feelings of disgust, most suffer no ill effects. However, medicinal leeches have been used for removing 'bad blood' from human patients for hundreds of years. Although this practice fell into abeyance by the beginning of the 20th century, leeches are once again being used to restore blood circulation following tissue grafts. Leech saliva may also prove helpful in the future, too, as it apparently contains antibiotics as well as anticoagulants that may prove useful in surgery.

Curiously, the medical use of leeches might explain their wide distribution across the country, as they are thought to have been released into ponds once they had been used for bleeding patients. It is also thought that over-collecting reduced their numbers in some areas. Over-collecting for medical purposes is unlikely to be a threat these days, given the protected nature of this species and the ability to breed leeches commercially at leech farms. Conversely, the pharmaceutical industry has funded much of the recent work under the UK Species Action Plan for this leech. In Romney Marsh perhaps the most significant issue has been the conversion of grazing marsh to arable cultivation - resulting in lowering of water levels, pollution, and fewer host species. Invasion of scrub around ponds has also been a problem.

The medicinal leech is listed in the UK Biodiversity Action Plans (UK BAP), and is included in English Nature's Species Recovery Programme. As a first step in ensuring its survival as a native species, many of the leech's known sites were designated as Sites of Special Scientific Interest (SSSIs), and a survey was commissioned to establish the species' true status.

An obvious requirement of any species of animal is a food source, and for the medicinal leech this source is other animals. The diet of leeches on Romney Marsh is not perfectly understood. Birds and frogs do seem to be major prey items, but fish have been attacked, and it is suspected that mammals are an under-recorded food item. There are several reports of them feeding on sheep. Scrub clearance around the shallow ponds on Romney Marsh has resulted in leeches taking up residence, and they have also colonised a large number of gravel pits on Dungeness originally created for bird conservation. At the site in Kent, which has the UK's largest leech population and is within the RSPB reserve at Dungeness, water birds provide the food source. At another site, Moccas Park lake in Herefordshire, grazing animals use the lake as a source of drinking water, which suits the leech very well.

See also
http://www.rmcp.co.uk/MedicinalLeech.html A study of medicinal leech (Hirudo medicinalis) populations in the Romney Marsh Natural Area, 2000; by Helen McConnell of the Romney Marsh Countryside Project.