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Leeches: Expert Q&A

  • Posted 07.29.08
  • NOVA scienceNOW

On July 29, 2008, Mark Siddall answered questions about leeches large and small, the best way to remove one from your skin, whether he'd undergo leech therapy himself, and more.

Mark Siddall

Mark Siddall

Mark Siddall is Curator of Annelida (earthworms, leeches, and other segmented worms) and Protozoa at the American Museum of Natural History in New York City. Full Bio

Photo credit: Courtesy Mark Siddall

Mark Siddall

Mark Siddall is Curator of Annelida (earthworms, leeches, and other segmented worms) and Protozoa at the American Museum of Natural History in New York City. He and his labmates are interested in the taxonomy, systematics, phylogeny, and evolution of leeches and other largely neglected groups. One area of study has been the interesting symbiosis that has evolved between leeches and the microbes that live within them. To study the evolution of this partnership, Siddall has traveled to French Guyana to collect the giant Amazonian leech and to South Africa to collect the notorious "hippo leech," which feeds exclusively from the rectal tissues of its host. Other field expeditions have included "Blood Lust I" (to Madagascar and the Seychelles) and "Blood Lust II" (to Australia) as well as trips to Malaysia, Thailand, Argentina, Bolivia, Chile, and Louisiana. In addition to his curatorship, Siddall also serves as Program Director for Research Experiences for Undergraduates in evolutionary biology and systematics at the museum. He is also Treasurer of the Willi Hennig Society, a nonprofit that promotes the theory and practice of cladistics (the hierarchical classification of species based on their evolutionary ancestry), and Associate Editor of Systematics and Phylogenetics for the Journal of Parasitology. Siddall earned his bachelor's in microbiology, and his master's and Ph.D. in zoology, all from the University of Toronto.

Q: Except for wearing clothes and shoes—that is, covering my whole body—is there another way to avoid leeches getting attached to me while swimming or wading in a lake? Inge, Mississauga, Ontario, Canada

Mark Siddall: Dear Inge,
Yes. Stay very very still. They are attracted first to movement. Mind you, that's kind of hard to do while swimming or wading. But I'm pretty sure that a layer of insect repellent will probably work. We get eaten alive by mosquitoes and other bloodsucking critters in the field because we don't wear repellent. Dr. Tyson did not get any leeches on him the whole time we were in the water for the NOVA scienceNOW segment. The best we could think of was that he had put some moisturizer on earlier, so that might even work.


Q: Are there leeches where I live? Coco, 3rd Grade, New Orleans, Louisiana

Siddall: Yes! I apologize that we haven't (yet?) blogged up our expedition on the lab Web site, but in 2004 I spent about a week down in the New Orleans area looking for leeches. We had a great time that summer working at the Southeastern Louisiana University (SELU) research station at Turtle Cove near Pass Manchac and out in the Pearl River area.

The leech species you saw in the NOVA scienceNOW episode, Macrobdella decora, does not occur in your area. Instead, Macrobdella ditetra and Philobdella gracilis can be found as well as other smaller species that feed on turtles. They're just as pretty, too—bright orange bellies and a dark green back with polka dots.

Later that year I returned to do a television shoot for Discovery Channel UK, specifically because John Lydon (whom some will remember as the musician "Johnny Rotten") wanted to include leeches in his "Mega Bugs" series. As a fan of his bands, it was very cool for me to go canoeing with him and pick leeches off his legs.

I had a university student-intern working with me that summer, Anna Phillips, who was so inspired by the leeches in your area that she ended up writing her first scientific paper on them and later came to New York to do her Ph.D. degree with me.

So, in short, yes, there are leeches in southern Louisiana, and they have had a really positive impact on my own career. If you want to learn more, a great friend and mentor, Professor Bill Font at SELU, probably knows more about them than I do.



Q: Hello Mark!
I've read that leeches and maggots [which consume dead tissue and are sometimes sewn up temporarily into a person's wounds] are the only two animals approved by the U.S. Food & Drug Administration for medical use. Is this true? Also, would you yourself undergo leech and/or maggot treatment? Thanks! Lisa T. Ennis, Campobello Island, New Brunswick, Canada

Siddall: Dear Lisa,
Yes, this is true. The maggots were approved first, though. My understanding is that maggots not only get rid of damaged and dead tissue (i.e., "debrade"), but they also secrete antimicrobial compounds that inhibit bacteria and prevent infection. Apparently, during WWI it wasn't uncommon for medics to put maggots in the open wounds of badly hurt soldiers. And there's a case of someone in a car accident in whose wounds flies laid eggs while the victim was unconscious for several days, and so saved that person's life by preventing bacterial infection until the victim was found by paramedics.

In 2005, the U.S. Food & Drug Administration (FDA) approved leeches for relief of venous congestion (like the case of the badly mangled hand you saw on the NOVA scienceNOW segment). Johann Friedrich Dieffenbach was using leeches for this purpose in Germany in the early 1800s. Then, in 1960, two Slovenians, Derganc and Zdravic, wrote a revolutionary paper that proved that using leeches can greatly assist in giving veins time to regrow into surgically reattached tissue.

A company in France called Ricarimpex applied to the U.S. FDA to allow Hirudo medicinalis to be imported to the U.S. for this use (though it had been going on for some time already). Interestingly, although the FDA did approve this request, we showed last year that the leeches Ricarimpex and other suppliers have been shipping are actually a different species, Hirudo verbana. It doesn't matter in terms of their efficacy. What does matter, though, is that only Hirudo medicinalis is protected in Europe, and yet Hirudo verbana is the species at risk of being overexploited.

As to the second question, yes, absolutely. Faced with the loss of a digit (any of those that I am presently using to type my reply), versus maggots or leeches placed on it to save it? Of course. In fact, I have allowed many species of leeches to feed on me just out of curiosity. (How long do they feed? How long will I bleed?) I haven't yet done this with maggots, but then I don't work on maggots.

P.S. Huntsman Marine Science Center in St. Andrews, New Brunswick was absolutely central to the work I did on marine leeches for my PhD.


Q: What is the best way to remove a leech (not being used medically) from the body? Chris Bonneu, Snohomish, Washington

Siddall: Dear Chris,
I get this question a lot. In fact, I described this in some detail for the "Worst-Case Scenarios: Travel Edition" pocket guide! The problem is not the leech, it's the bacteria in the leech's gut. Leaving aside the fact that salting a leech puts salt in the wound (generally considered a bad idea), or that burning a leech risks burning yourself (also suboptimal), salting, burning, or just plain pulling on a leech risks causing the leech to regurgitate into the wound it is feeding from.

Leeches have bacteria growing in their gut. We're not sure why, but Professor Joerg Graf at the University of Connecticut has an incredible research program investigating this symbiosis. What's unusual is not that they have culturable bacteria—even you have bacteria in your gut—but that they have only one! Dr. Graf showed more than a decade ago that the bacterium Aeromonas veronii forms a nearly exclusive relationship with European medicinal leeches, apparently killing or excluding other bacteria.

Our collaborative work has since shown that different medicinal leeches from Europe, North America, and Africa harbor just-as-specific symbionts, but of different species. For example, Aeromonas jandei is in Macrobdella decora, the leech you saw in the NOVA scienceNOW segment.

So why does this matter? Well, various Aeromonas species are awfully good at living in blood—as you might expect given that's what they're doing in the leech gut—and if they get spit up into the wound, they'll live just fine in your blood. In fact, they have a huge temperature tolerance, growing just fine from fridge temperature to body temperature.

Typically, in cases of leech therapy for venous congestion, it's important to give the patient the right antibiotic to prevent accidental infection. Knowing what bacterium is in which species could be critical in knowing what antibiotic is useful, so we're now frantically trying to understand how the diversity of leeches relates to the diversity of microbes. Most infections are local, but there's at least one case of extremely serious meningitis from leech therapy.

The same risk comes from removing a leech improperly. Thankfully, there are two proven ways to remove a leech without risk of infection. The first is to relax, watch the leech, and admire its color patterns, and its biology while it finishes its meal. This should take less than half an hour. Thirty minutes well spent contemplating the complexities of life!

If you're in a hurry, though, the other method is simply finding the head end and pushing it sideways until it releases the suction. Careful, though: The head end is the smaller end. Most people think the business end of the leech is the fat end, but that sucker is just used for holding on (even Dr. Tyson made this mistake in the NOVA scienceNOW segment). Find the skinny end and use your finger or fingernail to push it sideways off the bite point. Once it's released, you will bleed from the wound. That's okay. The bleeding is normal and is caused by the anticoagulants the leech puts in the wound. Then, just get it to release from the fat end the same way. And for Pete's sake, put it back in the water so it can lead a normal happy leech life!


Q: Mark,
In the NOVA Web piece on the giant Amazonian leech it mentions that a symbiosis evolved between the leech and bacteria that live inside it. Each gets something out of it, and neither hurts the other. I find that fascinating. How do you think such a symbiosis came about? Is it common in the animal kingdom? Anonymous

Siddall: Dear Anonymous,
Your question concerns a line of research that I have been investigating with my good friend and colleague Dr. Susan Perkins, and that has now been taken up by my student Alejandro Oceguera-Figueroa.

It turns out that lots of blood-feeding invertebrates, including tse-tse flies, bat-flies, and lice, all harbor bacterial symbionts. They do so because although blood is nutritious in a general way, it is seriously lacking in several amino acids and several vitamins—usually those amino acids and vitamins that your own body is sequestering for itself.

Unlike the free-floating gut bacteria in medicinal leeches, the giant Amazonian leeches and all other blood-feeding leeches in a group we call Glossiphoniidae have organs whose purpose is solely to grow these symbiotic bacteria. The organs are always associated with the esophagus (throat) of the leech.

And you're right—it is fascinating. What we've been able to determine so far is that the three major groups of leeches with these systems each have completely different organs (in terms of their anatomy) as well as three completely unrelated groups of bacteria that are symbiotic in the organs.

In the North American group—species of Placobdella that feed on frogs, turtles, and us—the closest relatives of the symbiotic bacteria are nitrogen-fixing symbionts of plants! That's just weird. The South American leeches, including the giant Amazonian leech in the genus Haementeria, as well as African species of Placobdelloides (I'll expand on hippo leeches in relation to another question posed below by Dr. Denzin), have bacteria closely related to those we find in other bloodfeeding invertebrates.

So, yes, it is quite common among blood-feeding invertebrates. How it came about, I must admit, I am at a loss to explain. What's clear is that it is critical. There are several leech groups in this family that gave up blood-feeding, and each time they lost the organs and the bacteria.


Q: I was late on July 23th to watch the program on TV and missed the part about the leeches, but I searched for it in my computer. I read on the NOVA Leeches Web page that leeches are back in hospital rooms, sucking patients' wounds. Why do they want leeches at hospitals to suck patients' wounds? Ana Villarroel, Columbus, Ohio

Siddall: Dear Ana,
For starters, see my answer to Lisa Ennis's question above. But there's a much bigger story. Many people think that "leeching" is a medieval practice. In fact, it reached its height when Napoleon's surgeon (Broussais) popularized it in the mid-1800s. The historical uses of leeches for things like obesity and pneumonia are without much merit. This was pointed out in the early 20th century by P. C. A. Lewis in what may have been the first-ever clinical medicine study.

In terms of modern uses, the problem of flap or replantation surgery concerns the fact that we can reattach arteries easier than we can reattach thin-walled veins. So blood gets into reattached tissue well, but it doesn't get out. The result is swelling. Often so much swelling and clotting that the pressure can cause the (very expensively) reattached tissue to die. The problem then is how to reduce the pressure long enough for the veins to grow back. Needles used to remove blood can create deep tissue damage. A canula will drain but only if there is high pressure of blood in the tissue, and it, too, is deeply invasive.

A leech, on the other hand, actively applies its own pressure to suck the excess blood out from a relatively shallow wound, and it puts in compounds that prevent clotting. The reduced swelling allows the veins to grow back. It's about giving the tissue time, really. And it works!


Q: At the end of the show, you mentioned people not being concerned about "conserving leeches." Are leeches facing problems because of climate change or pollution? Should we be concerned about leech conservation? Barbara U., Baldwinsville, New York

Siddall: Dear Barbara,
There's a practical and a philosophical perspective on your question. Philosophically, the loss of any species risks altering the balance of an ecosystem and diminishing the richness of life on our already lonesome planet. Loss of leech species should be expected to have just as significant a perturbation to any historical balance as loss of any other species. In terms of climate change, leeches face the same problems as other species. Climate change entails shifting habitat optima. These changes will be gradual. Tropical terrestrial leeches in the jungles of southeast Asia will expand as the jungles expand. Marine leeches in the boreal or arctic realms will definitely see their ranges contract.

In the near term, though, medicinal leeches could face a more acute threat: extirpation from harvesting. Recently, scientists thought there was only one widely distributed medicinal leech in Europe. My colleagues Peter Trontelj and Serge Utevsky and I have shown that there are really at least three species. But only one of them is protected by the Convention on International Trade of Endangered Species (CITES). If we lose a species of leech to extinction, we also lose all of the potentially useful therapeutic agents that it produces in its salivary glands. There are a lot of people with congestive heart failure, strokes, clotting disorders, and other ailments who might have benefited from those leeches.

In my lab we are pressing ahead with innovative technology to try to rapidly characterize the salivary compounds across a wide diversity of medicinal leeches. It's laborious and expensive, but it's just the kind of work that the American Museum of Natural History thinks can flow from its long history of characterizing the world's biodiversity.


Q: Dear Dr. Siddall,
I looked at your Web site and see that you've been all over the world collecting leeches. What's the most memorable experience—both good and bad—that you ever had in the field while doing this work? What's your next expedition? Thanks! Jim

Siddall: Dear Jim,
I could tell you stories about getting a vehicle so badly stuck in the middle of nowhere during the rainy season and having to abandon it before the hippos and crocs came out after dark. I could tell you hairy stories about being caught in impenetrable clouds at 5000 m in Bolivia at night with a cliff on one side and an abyss on the other. Once a team member came down with a fever of 105° in Madagascar. There was a humorous night-collecting episode in Argentina when a Hitchcockian flock of cormorants took such exception to our presence that we had to run from them at top speed while they dive-bombed us.

We try not to wade hip-deep into waters full of nasty animals and waterborne parasites, but sometimes it's unavoidable. I have only been truly terrified in the presence of hippos and several times within just a few feet of crocodilians in Madagascar and Argentina. Don't let a hippo's incipient smile fool you; it can slice you in two with its teeth. More people die from hippo attacks than from crocs or snakes or leopards or lions. Being a field biologist entails risk. I do my best to avoid it. It's not always possible. Besides, your worst days make your best stories.

The most positive experiences come from our interactions with local people, eating what they eat, learning their languages, and watching them laugh their heads off when they figure out that we've come thousands of miles to study their leeches! Children, in particular, find our work fascinating and strange. If I had to pick one experience, it might be the day we finally found a specimen of the extremely rare Americobdella valdiviana in Chile. The worst would be the six days prior to that coming up empty-handed.

As for expeditions, we had an amazing expedition to Zambia in January, my students were in Mexico last week, and we're off to Rwanda in 2009! We've tried to be good about blogging up these "Bloodlust" expeditions, but admittedly there's a lot more we could do to consistently bring our field expeditions to people in real time.


Q: In your podcast, you talk about using the leeches to draw excess blood from damaged tissue, and you mention that this is better than needles because it doesn't cause excess tissue damage. But you also mention that the leech has three Y-shaped rows of teeth. Don't these teeth damage the tissue just as much as a needle would? Laura Adkins, Alfred, New York

Siddall: Dear Laura,
This is a very insightful question, because anything we do to intervene can itself cause damage. Leeches are by no means a panacea. It is up to a qualified physician to make this call. Inserting a needle into congested tissue has problems associated with the fact that pulling back on the syringe will draw up not just blood but also tissue into the needle. Leeches do not dig so deep—the teeth are really very, very tiny, up to a hundred on one "jaw"—and do not normally cause this kind of deep tissue damage.

Medicinal leech bites are cutaneous—they break the skin and tissues just below. Unlike the use of a canula to relieve pressure, use of leeches is not dependent on the excessive pressure in the tissue. The leeches create their own negative pressure sucking out the excess blood. A cutaneous Y-shaped leech bite will normally heal within 48 hours. If it does not, a physician should be consulted.

It should be noted, too, that millions of years of evolution have afforded leeches the ability to put anticoagulant compounds into the wound such that it keeps draining after the leech is done feeding. No syringe has ever accomplished that feat. In my lab we are trying to determine how far back in the evolution of leeches various antiplatelet and antithrombin secretions originated. The results are already quite surprising and should point to which species might provide new and interesting bioactive compounds.


Q: I have a friend who once got a leech in his eye. He's a herpetologist, and it happened in Madagascar. He said that it quickly went around to the back of his eyeball and started feeding. Thank goodness, when he poured salt water in his eye, the leech came out.

I'm just wondering: Could someone swimming in a pond in New England, where small leeches can be common, get a leech in his eye? What to do then? Salt water? Call 911? Thanks. Anonymous

Siddall: Dear Anonymous,
Your friend sounds very much like my friend, Chris Raxworthy, a fellow curator at the American Museum of Natural History in New York. (Dr. Raxworthy has many more interesting stories about tropical biology and field collecting than I have.) The terrestrial leeches are found prinicipally in southeast Asia, Australia, India, and Madagascar. That distribution is highly suggestive of an origin coincident with the ancient supercontinent Gondwana, a postulate thoroughly evaluated by Dr. Liz Borda. And yes, they can wind up in some awkward places. Once I had to fish one out of a student's ear!

But, to your question. I don't think there's a lot to worry about. New England leeches are going to stick to your feet and legs. I know of no orificial leech cases in North America. This is better than I can say for Africa or Asia, where the aquatic leeches get into nostrils, ears, eyes, and some other rather awkward human orifices not suitable for comment here. (Search the medical literature for "hirudiniasis".) Dinobdella ferox (appropriately, the "terrifying ferocious" leech) actually prefers these sites for feeding. Usually salty water will get rid of them, but in some cases, such as nasal and pharyngeal, it's best done by a physician because of the risk of choking.


Q: Why doesn't a leech "bite" cause inflammation?

Have you ever removed a leech from the anus of a hippo? [Editor's note: This is a legitimate question—see Mark's bio.] Lisa Denzin, New York, New York

Siddall: Dear Lisa,
In fact, leech bites can cause inflammation, but typically this is limited to people (like me) who have grown allergic from prior exposure. I once watched my ankle swell to the size of a grapefruit over several days as a result of a feeding by Macrobdella decora.

That said, we do know that some of the bioactive compounds in leech saliva could have anti-inflammatory effects. Eglin from European leeches is a potent cathespin G inhibitor, and a mast cell chymase inhibitor. Bdellin is a plasmin inhibitor, which may inhibit various cytokines. Cytin is a chymotrypsin inhibitor. Even hirudin, by blocking the active site of thrombin, could inhibit the leukocyte recruiting effects of that molecule.

To be fair, though, very little of this has been studied in great detail. There are a couple of studies by a German group led by Dr. Dobos regarding the use of leeches in treating osteoarthritis. The results are really difficult to interpret because it is not possible to have double-blind trials; the patient always knows whether or not there's a leech on her knee.

Your hippo question was alluded to by me in the video podcast that accompanies the NOVA scienceNOW program. There is a leech, Placobdelloides jaegerskioeldi, that lives exclusively on hippo blood. As it happens, there is only one place on a hippo where the tissue is soft enough and thin enough and vascularized enough for these leeches to get a blood meal. I had traveled several times to South Africa hoping to get a specimen for our bacterial symbiont work, but reaching up a live hippo's back end is ill-advised, even if someone's distracting it from the front.

Dr. Susan Perkins and I were there in 2003, again hunting for the elusive hippo leech, when word of the "crazy scientists" got round to the Mpumalanga game wardens. One of them knew of a hippo that had to be culled because of its repeated habit of wandering into people's yards, and he found the leech for us. So the answer is yes, I have removed a leech from the anus of a hippo; someone else removed the anus from the hippo.


Q: Dr. Siddall,
I once read that leeches are "a living pharmacopeia." What does that mean exactly? I know they suck blood really well, but do they have natural drugs in their bodies that we can make use of?

Also, what's your favorite leech species and why? Thank you. Great program. Bill

Siddall: Dear Bill,
I think "living pharmacopeia" is a great way to describe leeches. A pharmacopeia is a compendium, or a list or book of drugs approved for use. Leeches certainly have a huge list of compounds in their saliva that might be useful as drugs (though, not too many are approved).

Blood clotting, while necessary, can be a serious matter if it happens in your heart, lungs, or brain. Hirudin and theromin block thrombin and prevent it from converting fibrinogen into fibrin (the stuff of clots and scabs). Calin, decorsin, ornatin, and LAPP all get in the way of platelets aggregating to each other or to torn tissue. Antistasin, therostasin, ghilanten, and guamerin each block coagulation factor Xa and typically have the ability to inhibit tumor metastases by interfering with the growth of blood vessels to the tumors. That's just for starters! There's calin, eglin, bdellin, tridegin, hemadin, detsabilase, orgelase, manillase, and hirustasin. Hementin might even break down clots after they have formed.

What's particularly exciting about this already long list is that only a few leeches have been examined in any detail. Extracting and purifying these compounds can be awfully laborious. In my lab at the Sackler Institute of Comparative Genomics, we are now taking a more direct approach by sequencing the genes that leeches are actively using in their salivary glands. The results are amazing! By doing this for a diversity of medicinal leech species from different continents, we are finding an equivalent diversity of bioactive compounds.


Q: How did you first get into studying leeches? Have they been a lifelong passion of yours? I ask because I'm one of those people who shudder when I think of them!

And what's the biggest mystery about them that you'd like to solve?

Thanks, and best regards, Patricia McCurdy, East Blue Hill, Maine

Siddall: Dear Patricia,
When I was an undergraduate student in 1988 at the University of Toronto, Dr. Sherwin Desser gave me the opportunity to do summer research with him at the Wildlife Research Station (WRS) in Algonquin Park. It was an amazing opportunity and one that changed my life. I had planned on going into medicine prior to this, but Dr. Desser handed me a project that required me to use my brain as a scientist. By the end of the summer I had solved a previously unknown lifecycle of a blood parasite of frogs.

The parasite, like malaria parasites, was vectored by a blood-feeding invertebrate. Unlike malaria parasites, the vector turned out to be leeches, not mosquitoes. I was hooked on this kind of work and spent the next six years studying blood parasites of turtles and fishes, all of which were vectored by leeches. I'll admit that at the beginning, I, too, was a little leery of leeches. However, Dr. Roy Sawyer visited the WRS that summer, showing me how to collect them and impressing on me the sheer diversity of form and pattern that leeches have.

One of the biggest mysteries about leeches that I'd like to solve is: Why is it that leeches that feed on sharks and skates do not feed on bony fish and vice versa? My guess is that they must have completely different anticoagulants in their saliva.


Q: Can you use leeches that you buy for fishing and put them on a staph infection instead of having a doctor cut it open and using antibiotics? Does this take days or weeks to make this work? Anonymous

Siddall: Dear Anonymous,
I don't think you should treat bacterial infections with leeches. These can be really quite serious and demand medical attention. The leeches typically available as bait for fishing aren't going to be much use anyway. This species, Erpobdella obscura, doesn't even feed on blood. Several times in the evolution of leeches, various lineages have given up on blood altogether. Instead they feed on oligochaetes, chironomid larvae, and other (often iron-rich) critters.


Q: Why do you think Nature made us think leeches are pests, when they are SO useful? Please tell me. Alex, Duluth, Minnesota

Siddall: Dear Alex,
I have living proof that Nature did no such thing. My five-year-old daughter loves leeches. She has been coming to my lab to play with them and on trips with me to collect them since she was just starting to walk. Not a month goes by in which she doesn't ask to go to the Museum to see the leeches. In short, I think the revulsion is learned, not instinctual.

It probably comes from the sense of violation that something is feeding on you when you didn't expect it. Thankfully, it can also be unlearned by going out there, finding leeches, and getting to see their incredible color patterns and behaviors for yourself.


Q: At what age do you recall your first encounter with a leech, and where was it? How did you react? M. Siddall, Canada

Siddall: Hi Mom!

I can't remember how old I was. I do remember Evan and I coming out of the water at Lake of Bays with leeches on us. I was pretty freaked out by the whole experience. We used to see them from time to time during the many canoe trips Dad took us on as kids. Of course, to me at the time, a leech was a leech was a leech. Funny how the leeches that freaked me out as a kid, Placobdella ornata, would later prove to be pivotal to my getting a Ph.D. and really sent me down the path of this whole career.


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