The Times, September 2002

You think of a leech with disgust: a bloodsucker, a parasite. Forties thrillers describe human blackmailers as "leeches". Someone you can't shake off at a party sticks to you "like a leech."

   But the astonishing thing about the word is that it first meant "healer", and was later applied to bloodsucking little worms because healers used these in their "leech-craft".

    The word comes, in fact. from Old English laece, "physician or exorcist", cognate with lacnian, "to heal" and Old High German lahhinon, "exorcise or heal". It went on meaning "healer" from the ninth until the seventeenth century, when prose writers began to use it mainly of an animal doctor, a "horse-leech". It was also used metaphorically in a spiritual sense: Christ and God were the leeches, the healers.

    Medical use of leechs shot up in the nineteenth century. The "Drug Museum" in New Orleans, for example, a shrine to nineteenth-century Lousiana medicine, contains hundreds of contemporary advertisements for imported "Swedish leeches". In England, by the time Wordsworth met the leech-gatherer on the moor, medical leeches were so popular they became scarce and went up wildly in price. Dorothy said in her journal that the old man they met, the original leech-gatherer, told them leeches were now so hard to find they fetched thirty shillings for a hundred, instead of two and six.

As William put it:

gathering Leeches, far and wide
He travelled; stirring thus about his feet
The waters of the Pools where they abide.
"Once I could meet with them on every side;
But they have dwindled long by slow decay;
Yet still I persevere, and find them where I may."

The gathering technique was simple:

                                             he the Pond
Stirred with his Staff, and fixedly did look
Upon the muddy water, which he conned,
As if he had been reading in a book.

   This leech, Hirudo Medicinalis, never went out of use entirely - doctors  applied them to bruises to bring down the swelling - but today it is making a starry comeback.

    One of the leech's new roles is as surgical tool in tissue grafts and re-attachment microsurgery, like sewing severed fingers back on. The surgeons' problem is circulation in the veins. Arteries are thick-walled, relatively easy to sew, and re-attach well. But veins are thin-walled and difficult to sew, especially if the tissue is damaged. Surgeons can get blood flowing in arteries but not veins. If the circulation does not work, blood going to the reattached finger becomes congested or stagnant, and the finger may be lost.

     Enter the leech.
The key thing about a leech bite is that bloodflow does not stop. The leech bite prolongs localised bleeding while their sucking gets rid of congestion. The artificial circulation they create keeps the tissue healthy and gives the graft time to re-establish its own circulation.

This happens because of an anti-coagulant in leech saliva. Several other bloodsucking creatures - ticks, vampire bats, mosquitoes and also some snakes - have worked out by evolution how to shut down the victim's clotting process while they feed on mammals, but the leech is easiest to work with. Now, leech saliva is at the biting edge of science.

Our ability to form blood clots is critical to our survival. We need to clot so we willnot bleed to death. But clotting can lead to trouble. Heart attacks are caused by blood clots in the heart, strokes by blood clots in the brain. The anti-clotting agent in leech bites has been isolated, and is now called hirudin. It is even being replicated biochemically, by DNA technology. Hirudin is currently featuring in human clinical trials for treating thrombotic disease, and may in time become an approved drug.

So the Old English phraseology was clairvoyantly accurate: leeches are indeed healers.

In the wild, however, they are not such an attractive proposition. Leeches are the wild animal you will see most often - all too often -  while walking in tropical forests of India, Nepal, Malaysia. It may not be until you feel nauseous and giddy,especially at high altitudes, that you realise you have provided them with an all-day meal.

If you stand still a moment in the jungle, particularly after it has rained (which, in moist jungle, and rainforest, is pretty often) tiny leeches swarm off the leaves and onto your shoes like scribbled spermatazoa desperate to race each other through the seams of your trainers. James Fenton's encounter with them, as described here by Redmond O'Hanlon in Into the Heart of Borneo, is typical; and his instant interpretation of what they are after is all too true.

James looked very hot. He sat on the tree trunk, held his head in his hand - and then bounded up with a yell. There was a leech on his left arm. He pulled it off with his right hand, but the leech looped over and sank its mouthparts into his palm. James began to dance, wriggling convulsively. He made a curious yelping sound. The Iban [the guides] lay down and laughed. James pulled the leech out of his right palm with his left hand. The brown-black, touch, rubbery, segmented, inch-long Common Ground Leech then twisted over and began to take a drink at the base of James's thumb.

I looked at my legs. And then I looked again. They were undulating with leeches. James's leech suddenly seemed much less of a joke. They were edging up my trousers, looping up towards my knees with alternate placements of anterior and posterior suckers. They were all over my boots, too, and three were trying to make their way in via the air -holes. There were more on the way - in fact they were moving towards us across the jungle floor from every angle, their damp brown bodies half-camouflaged against the rotting leaves. They were rearing up and sniffing at us from the trees, too, from leaves and creepers at face height.

     "Oh God", said James, "they are really pleased to see us".