Two Views of Leeches, A Century Apart

Two genome sequences of the European medicinal leech Hirudo medicinalis have just been revealed in a pair of papers, and the unexpected complexity may translate into new anti-coagulant drugs.

Some quick leech factoids: Of the 650 species, about 20 percent live in the ocean, where they feed on fish. The longest leech known extends 18 inches. A leech has 32 distinct brains and the genome extends about 230 million base pairs of DNA. Leeches belong to the same phylum as the earthworms, Annelida.

A Long History

The animals practice “hematophagy” – literally “eating blood.” A leech will gorge itself to five times its weight until, satiated, it drops off its victim. The jaws are strong enough to penetrate a hippo’s hide.

Leeches have been used medicinally in bloodletting for thousands of years.

Before the modern medical era, leech saliva drained a person of blood so that the four “humors” – blood, phlegm, black bile and yellow bile – could rebalances themselves, correcting a dubious disease-induced upheaval. Some reports trace the first recorded use to about 1000 B.C., in India.

More recently, in the 1700s, barbers, who performed medical procedures, slapped the animals onto selected portions of their customers’ anatomy to cure various ills: to the nose to stop nosebleeds, to the anus and rectum to relieve gastrointestinal distress, to the vagina to stimulate menstruation, to the big toe to treat gout, and even to the tonsils.

Today, the major anti-coagulant in leech spit, hirudin, has a variety of uses, from reattaching fingers to removing blood building up after surgery. It is used to alleviate clotting that occurs when the platelet count falls after using heparin, to treat hematomas in the skin, blast away varicose veins, help skin grafts settle in, and to promote blood vessel healing.

Some uses are more dubious.

The actress Demi Moore, for example, told David Letterman when she was on his show at age 45 that she underwent leech sucking (after bathing in turpentine) to detox and appear young enough for then-husband 30-year-old Ashton Kutcher. Her hirudotherapy was apparently unsuccessful because the marriage didn’t last.

A Century-Old View

Before reading the new papers, published in Scientific Reports from Sebastian Kvist at the Royal Ontario Museum and colleagues and from Russian researchers in BMC Genomics, I went to my shelf of old medical books. I found this wonderful description, in a tome called The New Illustrated NATURAL HISTORY, by the Rev. J. G. Wood, MA, FLS. It’s from 1920 – a century ago:

“The common Leech is almost as familiar as the earthworm, and is one of a genus which furnishes the blood-sucking creatures which are so largely used in surgery. It belongs to a large group of Annelida which have no projecting bristles to help them onward and are therefore forced to proceed in a different manner. Leeches are wonderfully adapted for the purpose to which they are applied, their mouths being supplied with sharp teeth to cut the vessels, and with a sucker-like disc, so that the blood can be drawn from its natural channels; while their digestive organs are little more than a series of sacs in which an enormous quantity of blood can be received and retained.

Everyone who had had practical experience of Leeches, whether personally a sufferer or from seeing them applied to others, must have noticed the curious triangular wound which is made by the teeth. If the mouth of a Leech be examined, it will be seen to have three sets of minute sand saw-like teeth, mounted on as many projections, which are set in the form of a triangle. The wound made by this apparatus is rather painful at the time, and is apt to be troublesome in healing, especially in the case of very thin-skinned persons, requiring the application of strong pressure and even the use of some powerful caustic. At one meal the Leech will imbibe so large a quantity of blood that it will need no more food for a year, being able to digest by very slow degrees the enormous meal which it has taken. It is a very remarkable fact, that the blood remains within the Leech in a perfectly unchanged state – as fresh, as red, and as liquid as when it was first drawn – and even after the lapse of many months is found to have undergone no alteration.”

Despite this scary description, leech spit contains an anesthetic, so if you don’t mind the creatures feasting on your skin, it probably won’t hurt.

The Biology of Leech Spit

Hirudin, a 65 amino-acid-long peptide discovered and named in 1884, was isolated in the 1950s and its structure determined in 1976. Natural hirudin is a mixture of variations of the peptide, but a pure form became available using recombinant DNA technology in 1999, marketed as Lepirudin.

Hirudin acts at the final step of the blood clotting pathway, blocking the enzyme thrombin, which breaks down fibrinogen into the thready fibrin that knits a clot.

The genome of Hirudo medicinalis revealed a richness of variations on the blood thinner theme – not just hirudin. The researchers were surprised.

“Incredibly, the leech uses 15 different proteins known to negatively affect the blood-clotting mechanism in vertebrates, and 17 other proteins that are likely also part of the same anti-clotting process. This is far more than we anticipated, and the insights generated by this research will allow medical professionals to better understand how and when to use leeches in their practice,” said Kvist, an evolutionary biologist.

The contents of the leech spit cocktail have intriguing names: eglin C, destabilase I, ghilanten, guamerin, cystatin, ficolin, manillase, bdellin, piguamerin, antistasin, bdellastasin, among them.

“It is astonishing that the most utilized, well-known and famous leech species in the world had not been investigated at this level. This research provides critical insight into the evolution of bloodfeeding in leeches and will play an important role in future research,” said senior author Mark Siddall, Curator of Annelida and Protozoa at the American Museum of Natural History in New York City.

I now have a new respect for these creatures that I’d previously only thought about as creepy pests in a lake at Girl Scout camp.