This piece contains frank and sometimes gruesome discussions of parasitic behaviours.
Even people who aren’t up on their medical history tend to know at least one fact: old-fashioned medical doctors used leeches. The leech is almost as iconic a symbol of antique medicine as the head lamp or the beak-masked plague doctor.
Leeches and medicine have a long history together. Medicinal leeches feature in Egyptian tomb paintings from around 1500 BCE. Nicander of Colophon (185–135 BCE) mentioned leeching in his writings and the great Greek physician philosopher Galen (129 – 216 CE) was a firm proponent of the practice. It has also been documented by ancient doctors around the world, with records in Arabic, Chinese, and Sanskrit, among others.
Leeching continued through the Middle Ages and Renaissance. It dropped off briefly before resurging in the 18th and early 19th centuries. In the 1830s, up to 6 million leeches a year were used in Paris alone, with records from 1833 showing total French usage topped 100 million leeches (some estimates put this number at a billion). Russia consumed a more conservative 30 million leeches, while Britain allegedly imported 4 million leeches in. This wasn’t particularly easy on the leeches; they nearly went extinct, were extirpated in some places, and finding leeches in ponds became a common way to supplement income. Hirudo medicalis was the most popular leech species used, but others were also employed. Hirudo decora, for example, was an American species sometimes substituted in when European leech supplies ran low.
Leeching fell out of favour as understandings of disease progressed. The miasma theory, and then eventually germ theory, made it clear that bloodletting was rarely as effective a treatment as had been previously thought. Within fifty years, leeching had dropped off precipitously.
So why leeches?
They’re certainly not the only hematophagous members of the animal kingdom (hemo/hemato = blood, phage = eat). We know of about 30,000 species that feed on blood. Bloodfeeding has cropped up independently several times throughout the animal kingdom. There are plenty of bloodsucking invertebrates, most of which feature on any ‘who’s who’ list of things you don’t want in your residence: ticks, mosquitoes, fleas, bedbugs. Don’t feel too safe just because an invertebrate is pretty, though – some members of the Calyptra genus of moth also feed on blood. On the vertebrate side, there are bloodsucking fish, most notably the lamprey. There are birds, including the oxpecker and the vampire finch. And representing the mammal side, there’s the infamous vampire bat.
Plenty of species technically eat food that includes blood, including humans. And many species will happily chug a pint or two if no other sustenance is available. But it’s not a sustainable long-term feeding strategy without some special adaptations. It turns out that blood-feeding is complicated, which is why blood-feeders are relatively rare in the world and why they are so specialized. Blood isn’t a nutritionally balanced meal and in particular, has no B vitamins. Blood-feeders often rely on bacteria in their gut for these essential nutrients. Blood is also very iron-rich. Since excess iron in the body is toxic and difficult to flush, bloodsuckers also need special adaptations to keep from poisoning themselves with the iron in their meals.
And then there are the adaptations for actually getting blood out. In this, the leech is a champion. Though leeches are such a famous parasite that the name is used as a euphemism for a moocher, not all leeches are bloodsuckers. Some are predators; some suck rotten fruit instead. Of those that are hematophagous, they divide roughly into two categories: land leeches and aquatic leeches (as the designation suggests, some leeches are terrestrial and some live in water. Bloodletting leeches are typically aquatic). Leeches are a segmented annelid (i.e. a worm) with two suckers, one anterior and one posterior. The larger posterior sucker is used for movement, while the smaller anterior sucker is for feeding. Unlike bloodsuckers like ticks, leeches don’t burrow in. They use their three jaws to slice the skin and suck the blood out.
Feeding can take up to an hour but once fed, the leech may not eat again for a year. When the leech is finished, it detaches. As with many animals, digestion is partially done through microorganisms living in the gut.
As the leech remains attached for some time, it will release a cocktail of different chemicals to make feeding easier. These include anti-inflammatories, analgesics, antimicrobials, and the most famous leech chemical, hirudin. Hirudin, discovered in 1884 by John Barry Haycraft, is an anticoagulant, which means it stops the blood from clotting. This makes it easier for the leech to feed, as well as prevents the blood from hardening in the leech’s body before it can be digested. Leech digestion is a long process!
Knowing all this about leech biology, it’s easy to see why leeching was a popular bloodletting technique. Other bloodletting methods often involved knives or other painful procedures.
The anaesthetic in leech bites means they don’t hurt. They’re also effective. While a leech only consumes about 10 – 15 ml of blood, the hirudin and other anticoagulants will keep the wound bleeding for up to two days.
Leeches had their problems, though, chiefly that they were harder to control. Several treatments have been noted for a leech that has meandered away from the treatment area and especially one that has gotten into one of the patient’s orifices (aquatic leeches tend to find it easier to feed inside their victim).
Well, thank goodness we’re in the modern era and putting leeches on patients is a thing of the past! Right?
Actually, leeches are still used in medical practice today, for a lot of the same reasons, although not the same problems. They’re still mighty efficient and mighty painless little bloodsuckers, so they’re useful to relieve venous congestion (i.e. build-up of blood) in reconstructive and plastic surgery and to promote blood flow. They’ve also been tested for relieving a host of other issues, including eczema and pain from osteoarthritis.
Of course, that doesn’t mean that you can just wander out to your local pond, scoop up a handful of leeches, and start doing surgery.
One of the risks of natural bloodsuckers is disease transmission. An animal that interacts directly with your bloodstream can easily infect you with bacteria or viruses. For example, Lime disease is spread by ticks and rabies can be spread by vampire bats.
Modern medicinal leeches are heavily regulated by Health Canada. In fact, since they secrete a substance used in medical treatment, they are technically categorized as a drug. That puts them squarely under the Food and Drugs Act and Regulations.
Medicinal leeches are raised in a controlled environment and it takes a fair bit of paperwork before they can be sold for medical purposes. It’s not just about proving that the leeches themselves are safe and effective. Manufacturers also have to prove that they are raising, processing, and even packaging them correctly. Leech application is also more refined; leeches are typically held in a syringe until they have
firmly adhered to the treatment site so they can’t wander and some physicians even suture them loosely into place.
There have been several efforts to come up with mechanical versions of the noble leech. This is not just about physical improvements, but also psychological; many patients are more comfortable hooked up to machines than covered in living bloodsuckers.
However, hirudotherapy, as it’s called, seems to be here to stay for at least a little while longer. Leeches: proof that there’s always a demand for a true expert in their field.
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