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The enigma emerged in the form of some greenish-white pus, swabbed from the wound of a patient not long after surgery. On Dec. 13th, 2012, surgeons at the University of Iowa Hospitals and Clinics had excised a tumor from his face, patching over the wound with a Post-it note of flesh from his thigh. But that flap had died, and a week later he was under the knife again, to have his wound re-mended with a strip of skin from his back.

When doctors saw blood pooling in the new graft, threatening to kill this one, too, they had the perfect treatment. Down in a hospital lab was a tank rippling with leeches. They fished some out and placed them on the man’s face.


Things got better for Christmas. For New Year’s, they got worse. The wound had started oozing pus, which smelled like a sewer and baffled the physicians. Leech guts, like ours, are crawling with bacteria, so the team had pumped the patient full of ciprofloxacin before allowing his blood to be sucked. That should have stopped any infections before they started. Then lab tests confirmed their suspicions: These bugs, called Aeromonas, were cipro-resistant.

When they searched the hospital’s records, they found another, similar case of antibiotic resistance, from July 2012, when plastic surgeons had used leeches after repairing a foot that had been crushed by a crane. There were reports from hospitals elsewhere, too: California, Missouri, even France.

An image of Aeromonas hydrophila bacteria under the microscope. W.A. Clark/CDC

Where was the resistance coming from? Five years later, a team of leech obsessives and bacteria growers have found an answer, published Tuesday in the journal mBio. Their work points to the butterfly effect of globalized medicine, with ideas and diseases ricocheting wildly through space and time, appearing, disappearing, reappearing. Just as a seemingly defunct treatment can rise from the dead, so the ghostly trace of an animal drug might resurface in humans over 4,000 miles away.


Americans, if they think about the leech at all, tend to see it as a scourge of summer camp or a horror of historical medicine. But for microsurgeons, it’s a tool for desperate times — and that’s saying something when spoken by specialists whose schedules are stacked with gruesome mishaps. Our bodies can be torn and contorted in the strangest ways, and their job is to work with what remains: Appendages severed by knives and saws, muscles and nerves ripped by the teeth of dogs, limbs reconfigured by propellers.

These repairs invariably mean dealing with damaged veins. While arteries are thick-walled — the body’s answer to the garden hose — veins are flimsier, harder to suture back into shape. “It’s just like if you were to have moistened toilet paper, one piece over the other, and you’re trying to peel the two pieces apart,” said Dr. Gregory Buncke, director of the Buncke Clinic in San Francisco. “Or like Saran wrap that’s wet and stuck to itself.”

Sometimes, after surgery, a team will notice that the harder half of that plumbing job failed. The arteries work, funneling in oxygen-rich blood, but the veins don’t, leaving the fluid no escape. The skin grows dusky, swollen, and hard. If nothing is done, the tissue will suffocate and die, ruining the graft that a surgeon just spent hours sculpting.

It’s for this scenario that American hospitals keep tanks of leeches. In Europe, they’re advertised as treatments for osteoarthritis, tendinitis, varicose veins, muscular pain, and hemorhoids. The U.S. Food and Drug Administration is less permissive: Here, a leech is regulated as a medical device, cleared only to help with venous congestion after certain surgeries.

A physician administers leeches to a patient in a lithograph from 1827. Wellcome Collection

When a nurse nestles one into a wound dressing, and tempts it with a needle prick of blood, it’s a piddling thing, no bigger than your finger. It’s been starved for months, and is wriggling with hunger, ready to feast. As it eats, it grows, the patient’s skin going from a swollen, near-death purple to a healthy-newborn pink.

The leech falls off in about 15 minutes, sausage-sized and sated, but the wound continues to bleed, still coursing with anticoagulants from the creature’s saliva. When the flow stops, four to six hours later, the nurse pops on another leech, giving veins a chance to regrow.

It wasn’t always so. As journalist John Colapinto has pointed out in The New Yorker, medicinal leech mania depleted Europe’s lakes and marshes in the 19th century, with both humans and animals wading in as bait, so that patients with everything from anemia to asthma to fevers could enjoy some beneficial blood-sucking. But the technique fell out of fashion just as germ theory was coming in. It spent some 90 years in somnolence, becoming to our minds as barbaric as lobotomy. Then, in the 1980s, it re-emerged.

The return to the American hospital wasn’t without roadblocks. Buncke still remembers the clear, mucus-like pus that oozed from his leech therapy patients, before he started specifically targeting Aeromonas with cipro. But in combination with the antibiotic, leeches have become a standard, if only occasionally used, part of the post-operative arsenal. Two manufacturers are licensed to sell leeches for therapy to distributors in the U.S.: Rimcarimpex SAS, in France, estimates that it ships some 80,000 stateside a year, while Biopharm Leeches Ltd., in Wales, puts its figure at a little below 20,000.

As with most medical supplies, producing leeches is an obsessive affair. Carl Peters-Bond, biology manager at Biopharm, speaks of them in parental tones — but with more knowledge than any father should have about his children. He knows that you can get them to copulate by creating an artificial summer, with warm water and strong light. He knows the resulting cocoon will resemble the sponge on a radio mic, but yellow instead of black. He knows their favorite sort of farm-animal blood is cow.

To move a leech, he says, one has to hold it between your fingers like a pair of chopsticks. “I have caught up to 1,300 leeches in 10 minutes,” he went on. “I used to be able to do that when I was younger, but I got repetitive strain in my leech-catching fingers so I can only do 600 or so.”

He’s even tasted them, when a chef was researching a Transylvanian recipe for leeches fed on goose blood, with garlic and butter. “I won’t do it again,” he said. “It tasted like it smelled: very tangy. It’s probably the foulest thing you’ve ever had — and my mother can’t cook.”

Tubes historically used to apply leeches to patients. Wellcome Collection

But even someone like Peters-Bond didn’t have the expertise that the doctors in Iowa needed to solve their pus mystery of 2013. They’d tested their tank water, and found no cipro-resistant Aeromonas.

“We had a problem,” recalled Dr. Bradley Ford, director of microbiology at the University of Iowa Hospitals and Clinics. “The Aeromonas, the only place they could be was inside the leech. And we didn’t have a way to look into that.”

What they needed was someone, somewhere, who was intimately familiar with the bugs in a leech’s gut.

Joerg Graf was living in California, studying the cooperation of bioluminescent bacteria and the Hawaiian bobtail squid, when a visiting scientist suggested he might want to look inside leeches. They had only one genus of bacteria in their gut, the visitor said, so they’d be an elegant example of animal-microbe symbiosis.

Graf moved to Switzerland to pursue the idea, and soon discovered it wasn’t true: There were, in fact, two. Graf kept studying them anyway, and when the Iowans found him, he was in a lab full of leeches and their flora at the University of Connecticut.

He’d noticed in his own experiments that some antibiotics had stopped working on Aeromonas, and reports were now cropping up of resistant infections in leech therapy patients elsewhere too, so he enlisted Lidia Beka, a Ph.D. student, to trace the trouble back to its roots.

For that, she needed samples. The lab already had some of its own, stored in a freezer that functioned as a kind of Historical Museum of Leech Gut Isolates. That collection was about to expand. They sent out emails, and bacteria came back in reply — little tubes of them, isolated from swabs of pus or surgical instruments in Iowa, Missouri, California, and France.

Other bugs were to be harvested from new orders of leeches, including one sent from Ricarimpex and passed on, unopened, by their American middlemen, a company called Leeches U.S.A. Once they arrived, Beka set about extracting their microbiomes. She tied a lasso around each parasite’s neck, so it wouldn’t vomit up its gut contents, and then dipped it in ethanol, an anesthetic and leech-skin sanitizer at the same time. Now, knocked out, it was ready to be pinned down through the suckers, slit down the front, and suctioned for its gastric juices.

University of Connecticut graduate student Lidia Beka with a collection of leeches. UConn Photo

With her collection complete, Beka sent her different bacterial strains off for sequencing. Peering into all of those different microbial genomes, isolated and preserved like fossils from different eras of leech-ordering, they could see a distinct historical pattern. “In 2013 and 2014, in four different shipments, 95 percent of the strains had this resistance marker,” said Graf, lead author of the mBio paper. “We go back to 2011 and 2009, it’s less than 1 percent.”

Where there’s resistance, there might be antibiotic. Sure enough, when chemical test results came back from the leeches ordered in 2013 and 2014, their gut juices showed traces of cipro. But they also contained some enrofloxacin, a veterinary drug, which becomes cipro when broken down by the body.

These drugs were only present in tiny amounts, though, and Beka wasn’t sure if it would be enough to push bacteria so strongly towards resistance. So she organized a kind of bacterial Olympic games. After feeding leeches two strains of bacteria at once, she would then see how the bugs fared in lab dishes with varying amounts of antibiotics. As predicted, when no drug was present, the susceptible bacteria won out. But once the concentration of antimicrobials approached the level they’d found inside the leeches, the resistant microbes took over.

There was still a chance that some other antibiotic contamination had occurred elsewhere. So Graf’s team did what disease detectives often do: They built a family tree.

If the bacteria swabbed from a patient in California, say, were genetically different from the bugs in Iowa, which were in turn distinct from the bugs that Beka had recently suctioned out of new leeches, then the resistance might not have traveled inside the animals. Their pedigree told a different story. Bacteria collected from operating rooms in France, Iowa, and California all bore a striking family resemblance. And a Los Angeles patient’s bug sat incongruously next to a French leech bug, like long lost brothers. “They’re nearly identical,” said Beka.

On a recent day in Graf’s lab, Beka began to pull off the cloth that had been secured over a jug of some 30 leeches. They hadn’t been fed in a year, and as soon as there was an opening, their segmented bodies began arcing up into the air, like mini-cobras rearing to make a kill. Beka hit them back into the water — gingerly, with a single finger — but they kept coming, a swaying mass of them, all probing the air for a whiff of food.

When she started this project, she hadn’t been afraid of leeches. Her fear arose only when she was visiting her family in Tirana, Albania, where leeches are sold in beat-up plastic water bottles at the market, so you can pick up a remedy for your swollen ankle while out buying fruit. Beka asked for 10, hoping to bring them back to the lab.

Pharmacy leech jar from England, 1830-1870. Science Museum, London, via Wellcome Images

“I remember I was standing in my grandfather’s kitchen in Albania, and I lost one, down the drain of his kitchen sink,” she said. “It just gave me nightmares.”

Graf interjected. “There’s a horror movie … a swim team that uses steroids and then the leeches get exposed to the steroids and then increase in size. My lab watched it once for Halloween.”

“It clearly affected me,” said Beka.

Graf started to laugh. But the film, as ridiculous as it was, wasn’t all that different from his latest paper — except it was the bacteria inside, not the leeches themselves, that had become stronger in response to some careless overflow of drug.

How the overflow had taken place still wasn’t exactly clear. “In Europe, there was the fear, after bovine spongiform encephalopathy … that it was dangerous to give cattle blood to leeches, so in Germany they switched to using horse blood, and in France they probably switched to poultry blood. And enrofloxacin is approved for veterinary use in poultry,” said Graf, who is a consultant for a German leech farm, which is not FDA-cleared to sell leeches for therapy in the U.S.

Brigitte Latrille, a former fencing champion and the director of Rimcarimpex, the French supplier, told STAT in an email that no agency has ever contacted her about switching away from cow blood, but that her leeches have indeed been fed chicken blood since 2001. She added that during the period of Graf’s study, the company was not aware of any issues with the leeches distributed in the U.S. In 2017, her team began specifically testing every lot of their parasite-devices for drug-resistant bacteria before shipping them out, and two years before that, she said, “we began to progressively and proactively introduce additional controls to ensure safe use of our leeches, particularly since antibiotic resistance in healthcare facilities has become a growing concern worldwide.”

In the grand scheme of things, this tale of Aeromonas is low on the list of microscopic worries. Dr. Paul Walker, the surgical resident who saw the first patient in Iowa, has since moved on to Loma Linda University Health, in California, and he hasn’t stopped using leeches. He’s just switched away from cipro, and the infections have gone away.

If you’re trying to pinpoint how antibiotics we’ve released into our environment end up affecting us, said Ramanan Laxminarayan, director of the Washington-based Center for Disease Dynamics, Economics & Policy, “this is one millionth of the answer. There are a million of these pathways that come back to bite us.”

Often, resistant bacteria hitch less esoteric rides, huddling in the lungs of tourists, clinging to food as it zings around the globe. Those are much likelier sources of serious infection, but our nightmares lead us astray. Instead of worrying about the contaminations of our own creation or the microbes lurking unseen, we worry about the leech.

“We forget … that bacteria get everywhere,” said Dr. Mark Holmes, who has spent 15 years researching antimicrobial resistance at the University of Cambridge in the U.K. “When they are present on a farm, yeah they’re getting away from the farm in medicinal leeches, but people on the farm will physically carry those bacteria into the community, the waste material from the farm will spread it into the environment. We’ve got to admire how nature has enabled bacteria to survive.”

This story has been updated to incorporate new comments from Brigitte Latrille, at Ricarimpex. 

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