Leeches: The Bad, the Ugly, and the Just Plain Horrifying

Blood-sucking parasites.

Oozing, gelatinous worms.

Horrifying, slimy masses of black flesh waiting for the next warm body.

Most of us have a particular aversion to leeches. They’re the worst parts of a mosquito crossed with the worst parts of a worm. For some, the thought of finding a leech latching onto them in the dark, wet rainforest is their own personal nightmare.

You probably expect me to try and change your mind. To tell you that of course, leeches are important to the ecosystem – or perhaps their well-documented use in modern medicine. That they are actually pretty cool animals when you get to know them. And like any creature on this Earth, they have their own significance, their own place, their own role in the great circle of life.

But honestly, they gross me out.

If you want to learn some even more horrifying facts, that might even leave you scared to go outside, then keep on reading.

A truly horrifying predicament. Image via Wikimedia Commons

Sucking, biting, and swallowing

By now, you’re probably well aware that most leeches suck blood. But not all of them use their proboscis like a straw, slurping up blood like a tired uni student does an iced coffee – or like this leech from the Amazon – although the worst part might be that it can grow to half a metre in length.

Rather than poking us, some leeches prefer to bite. The most well studied leech, Hirudo medicinalis, is one such critter. It’s been used in medicine since Ancient Egypt, and is still used in surgery, often for relieving venous congestion. This little medical helper has 3 jaws surrounded by its sucker (you know, the thing that it uses to latch onto you whilst it feasts on your bodily fluids.) The jaws gnaw at the skin to get the blood flowing, whilst it injects anti-clotting saliva into the wound. Voila! A free-flowing fountain of blood for the leech to feast on.

If you’ve ever made a leech’s acquaintance, you’ll know this fountain keeps on bleeding. And bleeding. And bleeding. Leeches keep the meal flowing by injecting anticoagulants into our blood, which stop the natural clotting process. Different species use different compounds – our friend the medical leech uses hirudin. It’s this ability to keep blood flowing which give it its medical uses!  But we’re not here for all the wonderful things leeches do, we’re here for the gross facts. So here’s another one:

Some leeches swallow their prey. Whole.

Just when you thought things couldn’t get any worse, here we are. And if you don’t believe me, take a look at this video.

This leech is likely one of the erpobdelliforms – leeches who no longer suck blood, but have upgraded from parasites to predators. With no teeth or proboscis for draining the blood of its prey, it instead uses a muscular pharynx to push food down it’s gullet. Think about that next time you’re slurping up spaghetti!


Think those long socks will protect you? Think again.

Leeches can get pretty much anywhere. This poor 13 year old boy found himself in quite a predicament, when a leech decided to make itself comfortable in his urethra. Makes you shudder, right? Two more boys also had leeches swim up into their bladders, resulting in – as you can imagine – agonising pain.

Leech infestation apparently occurs most commonly in children, in the pharynx, nose, oesophagus, and even the… well, privates. Such a case occurs to a 6 year old girl in Iran, where a leech was found hitching a ride in her vulva.

These bloodsuckers really will get anywhere. Personally, I know two people who have had leeches get behind their eyes, and one in the mouth. Apparently, the best thing to do is to just leave it – if you can stand the thought of an oozy black worm chilling in your eye socket.

I also have a phobia of needles. Figures. Image by ZaldyImg via Flickr

Needles not just for feeding

For our final twist in the horror story, some leeches engage in this really wonderful behaviour, delightfully called “traumatic insemination.”

One such leech, Helobdella stagnalis, is a perfect example. Rather than tenderly caressing its lover, the leech transfers sperm via a hypodermic injection. Spermatophores are attached to the partner, and sperm is injected through them. Did I mention leeches are hermaphroditic? This means that all leeches have both male and female reproductive organs – and the ability to forcibly inject the sperm anywhere in their poor partner’s body.

The sperm can be injected anywhere. How it gets to the ovaries is not fully understood, but this article found that spermatozoa are injected with such force that they form huge aggregations which literally force the cells apart, forming gaps in the tissue for the sperm to travel through.

Romantic, hey?


You really don’t like leeches, do you?

As an aspiring zoologist, I really shouldn’t hate any animal. But honestly, leeches test my patience with Mother Nature. I can appreciate them from afar, very far preferably – they attract me in more of a ‘morbid fascination’ kind of way. And like anything, they have their importance, and are just as fantastic as any other animal.

But keep them away from me, I beg you.


2 Responses to “Leeches: The Bad, the Ugly, and the Just Plain Horrifying”

  1. icrossing says:

    Thankyou so much!
    Really just trying to scientifically back up why leeches are awful and gross (but also kinda cool). Hope I didn’t give you too many nightmares!

  2. Emma Barbier says:

    HAHA I loved reading your article!
    I like the way you write – its very conversational and funny.

    I also like the topic you chose. Its a good idea to just ramble on about this organism you really don’t like!

    But now that I know some can measure up to half a meter and hide in uncomfortable places… I think I won’t be sleeping tonight.