Editor's Note: The video of leeches used in surgery is a bit bloody — especially after the 2-minute mark.
Leeches get a bad rap — but they might not deserve it.
Yes, they're creepy crawly blood-suckers. And they can instill an almost primal sense of disgust and revulsion. Humphrey Bogart's character in the 1951 filmThe African Queeneven went so far as to call them "filthy little devils."
But the humble leech is making a comeback. This critter is increasingly playing a key role as a sidekick for scientists and doctors, simply by being its bloodthirsty self.
Distant cousins of the earthworm, most leech species are parasites that feed on the blood of animals and humans alike. They are often found in freshwater and navigate either by swimming or inching themselves along, using two suckers — one at each end of their body — to anchor themselves. Most species range from about a quarter inch to three inches in length.
Upon reaching an unsuspecting host, a leech will surreptitiously attach itself and begin to feed. It uses a triangular set of three teeth to cut in, and secretes a suite of chemicals to thin the blood and numb the skin so its presence goes undetected.
Some leech species can also live on land, thriving in humid environments like the forests of southern Asia. Biologistsrecently reportedthat leeches in that region can provide a valuable snapshot of which animals are present in a particular area: The parasites carry their host's blood, and DNA, within their gut after each meal.
"They find things you don't find," saidMichael Tessler, a postdoctoral scholar at theAmerican Museum of Natural Historyin New York. As an added bonus (depending on your perspective), leeches are also attracted to humans as potential blood meals, he said. Thus, the creatures do not shy away from researchers, making sample collection quick and easy.
Tessler and several colleagues gathered 750 terrestrial leeches in the genusHaemadipsafrom Cambodia, Bangladesh and southern China. Then, they analyzed the DNA from each inch-long leech's meal to identify the unwitting blood donors. With this analysis, the scientists were able to identify wild and domestic animals common in those areas, including some species of concern for conservation.
The study also revealed a few surprises. Some leech meals had come from a few bird species, and one came from a bat. "Clearly these things get around," Tessler said.
Beyond their utility in field biology, leeches have an important role to play in a surgeon's medical kit.
The association between doctors and leeches dates back to the ancient Egyptians and ancient Greeks. According to Greek philosophers, illness was the result of an imbalance in bodily fluids, or humors. They believed that applying leeches to patients would help restore a proper balance. Leeches were widely used as a cure-all for an array of ailments, especially in medieval Europe.
These practices were relegated to the status of quackery by the advent of modern medicine in the 20th century as doctors developed new, more effective treatments. Nowadays, however, medical leeches are experiencing a renaissance as their bloodsucking ability is tuned to a more scientific purpose.
Leeches come in handy during reconstructive surgeries, such as those to reattach fingers, according to Dr. Rudolf Buntic, a hand surgeon and director of microsurgery forCalifornia Pacific Medical Centerin San Francisco. During such a procedure, surgeons repair small arteries that carry blood into the severed digit. However, the tiny veins that carry blood back out may be too damaged or too small to repair, leaving blood to stagnate in the finger.
That's whenHirudo medicinaliscomes in.
"The leech acts as a vein," said Buntic.
It draws stale blood out of the reattached finger as it feeds, allowing fresh, oxygenated blood to come in. Chemicals in the leech's saliva also help prevent blood clots from forming in the damaged tissue. Doctors apply fresh leeches over the course of about ten days. This provides enough time for new, tiny veins to regrow and create channels for blood to leave the patient's finger on its own, Buntic said.
Throughout the treatment, surgeons order leeches from the pharmacy, just as they would any other medicine.
If these little guys still give you the heebie-jeebies, don't worry: They probably won't be showing up at your local drugstore anytime soon, as they're primarily used in hospitals.
You might run into them in some research labs, however.David Weisblat, a biologist at the University of California, Berkeley, has been studying leech development and evolution for more than 40 years. He's recently started a project to learn more about leech behavior and neurobiology.
This involves placing the leeches onto a checkerboard of sorts, made of sandpaper. The squares alternate between rough and smooth grains. Many of the leeches, small snail-hunters in the genusHelobdella, have a strong preference for the smoother squares, moving in strictly diagonal patterns.
"It's like a little pawn on the chessboard that's gone crazy," Weisblat said.
Though leeches may have an unimpressive nervous system by vertebrate standards, they are still capable of using information from their environment to make decisions, said Weisblat. And figuring out how this process works is easier in a leech than in the nervous system of mice or rats. In understanding how leeches find their way around with so few neurons, we can begin to understand how the nervous system processes and encodes information, Weisblat said.
Capturing the way leeches sense and move through their environment could also one day translate to bioengineering applications, like designing small, exploratory robotics, said Weisblat. "Leeches can go on glass surfaces, crawling with their suckers, and exploring all sorts of different ways," he said.
"It's pretty amazing, when you think about how simple they are."
Emma Hiolskiis a freelance science journalist and intern with KQED'sDeep Look video series.Her work has appeared inScience, Chemical & Engineering NewsandThe San Jose Mercury News.You can find her on Twitter @EHiolski.
For thousands of years before modern science-based medicine became the norm, bloodletting, frequently by leeches, was considered something of a medical cure-all. The treatment’s persistence was at least partially attributable to the Ancient Greek physician Hippocrates’ ‘four humours’ theory of disease, which held that illness was the result of an imbalance of the bodily fluids black bile, yellow bile, phlegm, and blood. With the rise of modern scientific medicine near the end of the 18th century, bloodletting leeches were relegated to the quack cabinet as doctors realised that the practice generally fixed very little, leaving patients weak and vulnerable from blood loss. But as this video from the science and nature documentary seriesDeep Lookshows (in occasionally graphic, ultra-HD detail that is, perhaps, not for the squeamish), medical leeches have made a surprising comeback in hospitals, especially during reconstructive surgeries. Learn more about this video at the KQED Sciencewebsite.
Leeches are making a medical comeback,at least in Russia. Seen as a cheaper option in an underfunded health system, leeches are sometimes used in place of expensive blood thinners or as a homeopathic cure for a variety of ailments. It seems backward, but leeches do have a long history in medical care dating back thousands of years.
In Europe, themedical leech’s heyday was the nineteenth century. Their use was so pervasive that France used 33 million leeches just in the year 1827. Sweden was a major European leech producer, but excessive exports coupled with a supply dip led to a severe domestic leech shortage in 1843. In the latter part of the century leeches were still widely available, often prescribed for specific conditions such as heart or liver inflammation. They were also offered over-the-counter at pharmacies.
Nineteenth century physicians got thebloodlettingidea from the widespread notion that diseases “counter- irritate” each other. For example, noting that vomiting and diarrhea often led to swift recovery from intestinal illness, they hypothesized that purging must cause some a counter-action that negates the original illness. As vomiting helpedupset stomachs, the theory went, then blood removal might help various other maladies. Over time localized leeches were seen as a superior option to cutting, especially in young or infirm patients.
Some precautions were required. The anticoagulants and vasodilators in leech saliva meant that uncontrolled bleeding was a potentially dangerous side effect. Many physicians avoided evening leeches for fear of uncontrolled bleeding all night long. Neck applications were particularly dangerous, as post-leech bleeding could be difficult to stop. Leeches were similarly not applied directly to sensitive areas such as the genitals, rectum, or tumors. Leeches applied inside the throat were attached to a string to prevent swallowing. Application to the eyelids, especially the underside, could lead to injury and was discouraged.
A leech could be induced to drop off a patient with a pinch of salt, or else would let go on its own when full. A leech could be reused by forcing it to regurgitate its blood meal, a process known as stripping.
Harvesting leeches was not for the faint of heart; many leech fishermen waded in murky waters using their own legs as bait. An even more horrifying method saw old horses driven into leech habitat, then withdrawn covered in the worms. The horses were not given sufficient recovery times between “fishing” trips and by the end of the season they were dead.
Believe it or not, medicinal leeches are still used today.They are especially helpful when veins are damagedand unable to properly drain blood from extremities—for example, in recently reattached amputations. Leeches can spread infections, so modern leeches are used only once. And just because they are large, bloodsucking, annelid worms with a checkered history in medicine, doesn’t mean leeches are always viewed with dread. Rather than being grossed out, some patients report becoming emotionally attached to their leeches.