Mike Jeffries, Northumbria University, Newcastle

As we tidy away the Dracula capes and glow-in-the-dark plastic fangs for another
winter, one notorious blood sucker has had a particularly good year.

For the first time the medicinal leech Hirudo medicinalis has been bred in captivity at London zoo, part of a longer-term project to help this fascinating if unloved creature. Once widespread in the UK, the medicinal leech is now rare due mainly to habitat loss and historic collecting for medical use.

In Ireland, the leech was driven to extinction in the 19th century. Their vulnerability is recognised in international conventions and biodiversity audits.

You may have heard about leeches being used indiscriminately to treat all kinds of illnesses, from cancer to mental illness in the 19th century. The discrediting of general “bleeding” as a treatment for everything from hysteria to syphilis in the 19th century largely put an end to this history.

However, in 1884 leech saliva was identified as an anticoagulant, called hirodine.

Surgeons still use leeches to improve the success rates of surgery, such as when reattaching severed fingers as their saliva prevents post-surgery blood clotting inside veins.

The leeches’ medical comeback

The medicinal leech was once widespread in Britain and Ireland. Their popular use in medicine resulted in a lucrative international market in the 18th and 19th centuries. They seem to have been a bit of a luxury treatment, too expensive for the poor. The use of leeches to treat all sorts of ailments goes back to at least 1500BC, appearing in Egyptian tomb decorations.

Leeches were imported in Britain and Ireland from Europe, Russia and Africa, but some of these leeches weren’t what they seemed. For example, the less effective leech, Hirudo decora, was passed off as the more effective medicinal leech. H decora, which was imported, does not bite as deep and so was less effective at draining blood.

Declining leech numbers and increasing prices led to attempts at leech farming, which seem to have failed although commercial stock are farmed today and at least one Welsh leech merchant has survived since 1812 and still supplies the NHS and vets.

At the same time as their population declined so did their medical use in Britain, fading away in the early 20th century due to cost, rarity and changing medical treatments.

Leeches made a medical comeback as “hirudotherapy”, starting in the 1960s and widely used since the 1980s. Researchers have found over 100 substances in leech saliva useful to medicine, such as anti-inflammatory agents, antibiotics and pain relievers.

The main focus though is plastic surgery, where tissues are being reconnected, such as skin grafts or detached ears, noses (and other bodily extremities prone to variations in blood flow).

Sometimes in reconstructive surgery, blood pools in the veins and can’t flow back to the heart and lungs, which can cause tissue death. That’s when leeches would be applied straight to the wound site. When they bite, their saliva transfers anticoagulants into the patient’s bloodstream, which helps increase blood flow.

The comeback is the result of the leech’s remarkable ability to take a blood meal without the blood coagulating inside it, which would be fatal to the leech.

Other chemicals increase blood flow to the bite area and the person bitten often does not notice as the leech releases anti-inflammatory and pain-killing substances. Leech saliva also has anti-microbial properties, which protect the wound it leaves from infection.

Hopefully I have convinced you it is a good thing that 40 baby medicinal leeches were born this year at London zoo, a modest number but a breakthrough given these are from one of the remaining native populations in the UK. The baby leeches offer hope of boosting fragmented wild populations.

As a slithery, blood sucking creature of pools and wetlands, leeches can be a hard sell to the public. And humans have long distrusted the mud and ooze of its wetland home.

There was even a 1959 black-and-white horror film, called Attack of the Giant Leeches, playing on our fear of them.

But I would argue leeches have a sense of mystery and excitement.

I suspect anyone who has ever run a pond dip (trawling a pond with a small net) with children will have heard a similar shriek of delight as a leech is spotted in the catch, although these are more likely to be one of the 16 other freshwater leeches that live in the UK that the medicinal leech. And our aversion to them is misplaced as most of the 600 species worldwide are not bloodsuckers, at least not the blood of humans.

These fascinating creatures have tiny sensory structures all over their body called sensilla that are thought to help them detect water motion and light. Some leeches, including medicinal leeches, have as many as five pairs of eyes on their head although some have just one pair.

They are more caring than most invertebrate parents and some pass nutrients to their young across their bodies a bit like a placenta. Some even starve themselves to feed their babies when food is hard to come by.

So, remember that appearances can be deceiving and leeches are our friends. Humans have a complicated history with leeches but it looks like they are here to stay – if we let them.

Mike Jeffries, Associate Professor, Ecology, Northumbria University, Newcastle

This article is republished from The Conversation under a Creative Commons license. Read the original article.