But in the wild the leech, a relative of the earthworm, is globally threatened and in Britain there are only a few small, widely scattered populations. The species is one of 116 animals and plants for which rescue plans have been proposed by a joint government and wildlife charities committee.
Several of these populations are in lochs in Argyll. When surveyors from Scottish Natural Heritage, a government wildlife conservation arm, went to look for the leech last year at eight sites where it had been previously recorded, they only found it in two. "Every species has a right to exist in its natural habitat," says Martin Gaywood, who looks after the leech for SNH. "The medicinal leech is rather a charismatic species - people are both repulsed and intrigued by them."
The leech is also found on the island of Anglesey, Cumbria, and a few sites in southern England. Its stronghold is on the shingle spit of Dungeness, but the total UK population only amounts to a few thousand.
The species needs warm (and therefore shallow), nutrient rich and fairly still waters with abundant water plants. The two-inch long adults are hermaphrodites and lay egg-containing cocoons in the late summer. In the following spring tadpoles are an important food source for the young leeches.
The medicinal leech is much larger than all but one of the other dozen species found in Britain, and it also has the most catholic diet - it will attach itself to mammals (including us), birds, amphibians and fish. It swims towards a source of movement in the water, inserts its sucker, injects a little anti-coagulant and drinks deep, taking up to five times its own body weight in blood. Once full it drops off and lies low, spending the next few weeks or even months digesting the meal.
Collection for medicinal purposes may have been an important bygone reason for its decline in Britain and elsewhere (although this may also be how it arrived here from Europe in the first place).
The rescue plan calls for nationwide survey to pinpoint its remaining haunts by the year 2000, with safeguards for all of these. Ponds should be dug near some of these water bodies to provide extra habitat. The maximum annual cost of this programme is put at pounds 17,000.