Of all the animals that we’ve examined in the microcosmos, leeches are probably one of the few that can be used as a verb.

I don’t know what it would mean to “tardigrade” someone or something, but I do know what it means to leech off someone— to take and take from them, like a worm consuming someone’s blood. It’s funny though, isn’t it, that we use this word in such a derogatory way?

We make an insult out of an animal we nearly drove to extinction in some countries because we found them so useful, you might even ask then, are we the leech, leeching off of leeches? Before we go too deep into sympathizing with leeches, let’s be clear: leeches are very creepy for most of us anyway. James, our master of microscopes, has come across leeches several times in his pond samples.

A few years ago, he found two finger-length black leeches in his samples and decided to put them in a container with the lid completely sealed. His professor warned him that they would escape, but James figured he'd closed the lid tight enough to ensure they would stay in the container. So he left it in the bathroom and he went to bed.

The next morning, when he went to the bathroom, he saw the one thing you don’t want to see when you have a container with leeches in your home. The lid was on the floor, and the container was empty. They had somehow managed to escape, and now James was in a home with two kittens and a pair of wayward leeches.

He searched through everything and to his enormous relief, he finally managed to find the leeches behind his washing machine, covered in dust bunnies. But at least he did learn to not underestimate them. These leeches we’re looking at now are from a different genus than his runaway pair, but they also posed a challenge.

They were able to attach themselves to the glass of their fish tank with so much strength that James could not retrieve them like he normally would. And when they did detach themselves from the sides of the tank, the leeches would quickly swim down into the sediment and disappear. Eventually, James was able to get a hold of them.

But the struggles still continued. The leeches were simply not content to rest on the slides. They would escape, they would crawl, they would roll.

One even jumped down from the microscope! Now James was finally able to rein them in through a clever arrangement of slides and coverslips that let him record them. And we are both grateful for his persistence, and very glad that none of us were in the room with him and those leeches.

As we mentioned earlier, “leech” has the distinction of referring both to an animal and an insult inspired by said animal. That might not be unusual when we look at macroscopic animals, but animals of the microcosmos are of course so small that they don’t often get the opportunity to permeate our language in many forms. The use of the word “leech” to describe a person as a parasite is only a few centuries old.

But we have a long history with the word “leech,” going back to at least the year 900 CE. In Old English, “leech” could mean one of two things. One is the blood-sucking worm that we’re looking at right now.

The other meaning is “doctor”. The two definitions actually came into Old English from two different origins, but they came to be intertwined in meaning the way that leeches and doctors have been intertwined in practice. In the older understanding of medicine, built upon the concept of humors that were transported around the body through blood, blood-letting was seen as a way to restore balance.

Excess blood, it was thought, could lead to disease. And so removing that blood could be a medical necessity. Of course, you could remove the blood through surgical means.

But leeches were a nice alternative. Slower, not quite as painful. And there are some afflictions where a patient might prefer a worm to a doctor stabbing them.

The main downside was the need to prevent leeches from crawling into the patient through their mouth, nose, or anus. There are actually hundreds of leech species, but the one used for medical purposes was named Hirudo medicinalis in 1758 by Carl Linnaeus. In the wild, this worm feed on mammals, fish, and amphibians, tracking movements in the water along with heat and chemical signals to find their host.

Then, it uses its three jaws and the sharp teeth lining them to pierce the skin. The leech will usually consume anywhere from 2 to 5 times their body weight in blood, relying on a cocktail of enzymes and other molecules in their saliva to keep the blood flowing and not clotting. When they’re done, the leech might take months to digest that meal.

Around the 18th and 19th centuries, using leeches as a medical tool ballooned in popularity, thanks in part to the influence of the French physician Francois-Joseph-Victor Broussais which led to a massive trade in the worm across Europe. If you read about the history of leeching, you’ll find all sorts of terrifying statistics describing their use. For example, in 1832, France was estimated to have imported more than 50 million leeches.

Imagine transporting hundreds of thousands of leeches. Or maybe you shouldn’t imagine that. But of course, trading leeches also meant catching them, which is easy when you’re the exact host that leeches are looking for.

Leech hunters would simply use their feet as bait, extracting the worms from their own bodies to be able to send them on their way. And look, leeches weren’t some rare animal. But when people began trading them in such massive quantities, that changed.

That was a problem for trade, but also for wild leech populations. Some countries approached this problem by trying to build leech farms that could reduce the need for wild leeches. Others tried to raise prices or limit exports, sometimes even banning them.

But something else was changing as well. Scientists started doubting whether leeches were really that great of a medical tool to begin with. The most notable opponent was the French physician Pierre Charles Alexandre Louis, who argued that leeching might actually be leaving patients in worse conditions.

And this was the century where scientists began to consider an alternate explanation for disease, one built not on balances of humors, but rather on the existence of germs. And that opened up new approaches to treatment, one that did not rely on latching a blood-sucking worm onto people’s skin. So the leech trade eventually died out, but the combination of our use of leeches and the decline of their wetland habitat have left its mark on the Hirudo medicinalis population.

At first, it seemed like they may have gone extinct entirely in Western Europe, but reports of their demise seem to have been exaggerated. Not that the reality is that much better. If you look up their status in various countries in Western Europe, you’ll see every label from not threatened to rare to extinct to unknown.

But they have made a comeback in medicine. Though not to the same degree that they were used before, but Hirudo medicinalis and their close relative Hirudo verbena are used in hospitals for certain treatments, like draining the clotted blood of hematomas. And aside from their medical uses, leeches have a well-studied system of ganglia— the cluster of neurons that you can see here.

And that has made leeches a useful model for scientists to use to better understand the interplay between an animal’s nervous system and its behavior. So our history continues to be entwined with the leech. And while we started by asking whether we are really the leech here, maybe that’s too simple a question.

This is an animal whose life is intrinsically connected to ours through blood, a relationship forged through evolution and reshaped by history. So maybe the question isn’t one we can answer, but rather something open-ended: if a leech has been a worm, a parasite, a doctor, an action— what else can it be that we don’t yet know? Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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