They were once used to treat everything from headaches to strangulation, and leeches are still a vital part of surgery. But how are they farmed?

Six seconds. Perhaps 10. Twelve, if it is cautious or dopey. After that, the jaws will activate, the hundreds of teeth will engage, the leech will begin to eat, and its meal is your blood. Are you wading through a tropical pond in fierce humidity? Have you returned to your guesthouse to find with horror a passenger on your leg? Possibly. But you are equally likely to be in a sterile room of a modern hospital, tended by nurses who attach these bloodsucking animals to you without a shiver. You accept them equally calmly because it has been explained to you that these leeches may save your breast, or your finger, or your ear, or your life.

Less than half a mile from the M4 motorway, in the south-west of Wales, there is a walled entrance off a road whose name I can’t pronounce, and a small sign saying Biopharm. A long and winding drive passes sheds of unclear purpose and ends in a small yard beyond an imposing cream-coloured manor house. The UK’s only leech production business looks like a health farm. Which I suppose it is. Thousands of years since leeches were first employed for medicinal purposes, and a century since “leech mania” saw blood-letting used to tackle everything from headaches to strangulation, these creatures are still used to clean wounds and improve circulation, especially after surgery.

The leech is not a slug. Nor is it a bug, reptile or insect. Sometimes it is not slimy either. The leech is an invertebrate animal belonging to the phylum Annelida, a zoological category that includes more than 15,000 species of segmented bristle worms and 650 species of leeches in the subclass Hirudinea. Not all leeches suck blood and not all bloodsucking leeches seek the blood of humans. Many have evolved to have impressively specialised food sources: one desert variety lives in camels’ noses; another feeds on bats. Some eat hamsters and frogs. The Giant Amazon leech, which can grow to 45cm long, feeds by inserting a proboscis – like a 10cm-long straw – into its prey.

The leeches that I have driven several hundred miles to encounter are freshwater, bloodsucking, multi-segmented annelid worms with 10 stomachs, 32 brains, nine pairs of testicles, and several hundred teeth that leave a distinctive bite mark. Depending on the era you live in, this resembles either a wound made by a circular saw or a Mercedes-Benz logo. Biopharm breeds Hirudo verbana and Hirudo medicinalis, both known as medicinal leeches.

Biopharm’s medicinal leeches are used throughout the world. Photograph: Alamy Stock Photo

Both varieties have two characteristics in common: they inject their host with a local anaesthetic so that their presence is rarely noticed until they have tucked in. Because of this, a leech bite will usually feel like a vague sensation, not a nip or scratch. Once their teeth are engaged, they emit the best anticoagulants known to exist, so their blood meal keeps flowing long after they have stopped feeding, often for up to 10 hours. In some surgeries that require rejoining tiny blood vessels – reattaching an amputated finger, ear, or lip, or reconstructing a breast – the blood can get stuck. A leech can make the difference between a successful reconstruction or reattachment and failure and distress. In a 2002 survey of 50 plastic surgery units in the UK, 80% had used leeches in the previous five years. The leech is in many ways a simple animal, but its anaesthetic and anticoagulant have yet to be bettered by science.

Roy Sawyer, the American zoologist who founded Biopharm, likes to call the medicinal leech a “living pharmacy”. Not only is the leech a medicinal treasure chest, but that Mercedes-Benz bite is spectacularly efficient, the tripartite shape much less damaging than a scalpel incision, which can tear surrounding tissue. Apart from the bloodsucking issue, it seems to me that the leech is one of the more polite parasites. All in all, it is an astonishing creature, but as I take my tour of Biopharm I don’t want to pick one up.

For an animal that biologists describe as rather simple, the leech needs complicated handling. Biopharm’s leech raising is done in three large rooms, each kept at a different temperature. The further in we go, the further along the path to the leech becoming a hospital device, the colder it gets. All the tanks and equipment are built to exact specifications, most of it devised by Carl Peters-Bond, who has worked here for 24 years and is showing me around today. It is the engineering and the precision that keep him at Biopharm, not the leeches. Everything here, he says with pride, is bespoke.

The first room is kept at 26C. I take a photo, although the view is just dozens of tanks draped in white muslin. Carl notices. “You can take a picture of the room, but not of the tanks.” Breeding leeches is a sensitive process of feeding and starving and warming and cooling, and leeches can be spooked even by the click of a smartphone. The tanks are where leeches are born, by the happy meeting of any two of them: leeches are hermaphrodites and very flexible. Carl lifts a corner of muslin covering a tank and picks up one. It is surprisingly beautiful, its belly striped with iridescent gold and green. Even Carl, the sober engineer, admits: “The colours are quite nice. If you see anyone else’s leeches, they’re not as nice as ours. I select them for colour.”

The menu at Biopharm is always black pudding. In the two years it takes to raise a European leech for medicinal use, it is fed sheep’s blood served in sausage casing once every six months. Biopharm used to feed its residents with cow’s blood, which was more successful. The leeches ate it more readily, and one cow held the blood volume of 10 sheep. But BSE has ruled out cow blood, for leeches and humans.

A medicinal leech in a tank. Photograph: Alamy Stock Photo

Carl points out an immobile leech on the bottom of the tank. “That’s what they do in the wild. When they feed, because they have a huge reserve of blood, they’ll bury themselves in the mud or moss.” He describes the leech as a sort of oil tanker: all its reproductive organs are on the front where the cab would be. “The central organs are on its side. It’s got two hearts, one on each side. The bulk of it is storage.” A fed leech can swell to up to five times its body weight. A small leech can expand eightfold.

Carl sticks his finger in the water and a leech immediately appears. “He’s sniffing around now.” Actually, it is more of a tasting: Carl thinks they sense the sugars and oils in the skin. He picks one up, but isn’t bitten. “I’m not very attractive to leeches.” A bigger problem is leeches biting each other. They can digest at different rates. ‘“Maybe one leech has shrunk down to 300mg and it’s in a tank with a leech that is three or four grams.” That is a recipe for murder: a big hungry leech will eat from a small hungry leech, and sometimes the biting can get fatal. The best method for peace among leeches is to adjust the temperature so they are half asleep and half awake. The safest leech is a spaced-out leech.

Biopharm also experiments with tank size to give leeches the optimal amount of exercise. Carl is tank builder, leech grower, and personal trainer: leeches have to be exercised twice a day. It’s not complicated, as training programmes go. “I’ll go and pick one up and put it at the other end of the tank.” It will swim, and it can lose weight quite quickly. Sometimes it gets more exercise than Carl bargained for. Their most annoying talent, he says, is for escape, even from Biopharm’s tanks. He has often arrived home to find some attached to his ankles. “I’m usually surprised if I don’t find 10 leeches in the footwell of my car. They stick to your shoe and then they dry out.” He says this, and I look at my feet.

When leeches swim, they travel fast and beautifully. On land, they move by suction: they suck with the front sucker, then the rear, and that is their locomotion: it is an efficient but not elegant movement. (It is nothing like earthworm locomotion, which is done by peristalsis-style burrowing, in waves.) But in water, they are different. They are sinuous. As Robert Kirk and Neil Pemberton write in their fine book Leech: “By flattening and manipulating their bodies into wavelike patterns, leeches are capable of swimming at speed and with an elegance few other creatures can rival.”

It doesn’t matter how good a swimmer a Biopharm leech is. It will be packaged in gel and sent to a hospital pharmacy, and sooner or later – its work done – it will be killed. In 2004, the US Food and Drug Administration gave Hirudo medicinalis an unclassified status as a marketable medical device. Single-use only: all leeches employed in hospital settings must be exterminated with alcohol solution once they have fed and dropped off.

This seems ungrateful, but a filled leech is a biohazard. Leeches can transfer blood from one person to another. “They’re worse than that,” says Carl. “They’re a needle that can walk.” Biopharm sells a special euthanasia kit called Nosda to dispatch the leeches humanely. This includes the alcohol required, various pots and, with misplaced kindness, “leech-friendly forceps”.

The leeches in the cold room are almost hospital-ready. They have had four feeds in their lifetime and been starved for six months. If he is lucky, Carl says he can get a leech from birth to a hospital pharmacy in two years. But usually it’s about three. The starving is because a hungry leech, when applied to a human, is an efficient leech.

We are not allowed into the final room, as it is bathed in UV light to make the leech as sterile as possible. Nor do we see the packing: leeches make their onward journey in a proprietary polymer gel. There is skulduggery in leeching: when I ask Carl if there is any corporate spying, he won’t answer, except to say: “We don’t need to. No one has a yield like ours.”

Ninety per cent of the leeches born at Biopharm grow up to be walking needles. It helps that they are flexible, with a tolerance of temperatures from -5C to 40C. If it is hotter, they travel with ice chips. They have to arrive in good order: they have work to do.

This is an edited extract from Rose George’s Nine Pints: A Journey Through the Mysterious, Miraculous World of Blood, to be published by Portobello Books on 25 October, price £14.99. To order a copy for £12.89, go to guardianbookshop.com or call 0330 333 6846. Free UK p&p on orders of more than £10, online only. Phone orders min p&p of £1.99.