Leeches play an important role in human health and ecosystems

You may feel a bit squeamish at the thought of leeches, but these wriggly little creatures are not only fascinating, they are even helpful in the world of medicine.

WHAT ARE LEECHES?
According to the Australian Museum, leeches are segmented* worms that belong to the class clitellata, like earthworms but with unique features. Bloodsucking leeches are parasites* that usually live on the exterior of their host, and their bodies can vary greatly in shape and size.

They have a small oral sucker around their mouth and a larger caudal sucker*, or muscular ring for attaching onto things, at the rear, except for marine fish parasites.

Insect enthusiast and PHD candidate at University of the Sunshine Coast Charlotte Raven said Australia has more than 100 known species of leeches and “likely 100 more awaiting discovery.”

“Leeches get a bad rap for being slimy and a bit gross, but they’re actually pretty awesome,” Ms Raven said. “For starters, not all of them suck blood. A lot of leeches just eat other small creatures like worms and snails, or help clean up dead organic matter* in the forest.”

She said that clean up crew helps to cycle nutrients and keep the soil healthy, making leeches an important part in the overall health of ecosystems*.

“Frogs, birds, fish and even other invertebrates* feed on them, so without leeches, some of our favourite critters could disappear,” she added. “Leeches are still used in medicine today to help heal injuries. So instead of being scared, think of leeches as little heroes that keep ecosystems healthy.”

WHERE DO LEECHES LIVE?
Leeches live in lots of different environments. Most live in freshwater, some live on land and others live in marine settings. Land leeches are commonly found in rainforests and moist areas, while freshwater species prefer still or slowly flowing waters.

In Australia, leeches can be found in damp areas and watercourses, except in permanently dry regions. Some even live in the ocean, feeding on marine life, not humans.

BLOODSUCKING LEECHES
The bloodsucking variety of leeches feed on a preferred host’s* blood, but can switch hosts if necessary. They might suck the blood of humans, mammals, fish, frogs, turtles, or birds. A hungry leech moves quickly to find a suitable host.

They’re classified into groups based on the body parts they use for eating:

• Jawed leeches bite the host with teeth, secreting* hirudin* to prevent blood clotting.
• Jawless leeches insert a needle-like proboscis* and secrete enzymes to dissolve clots.
• Worm leeches swallow small invertebrates whole.

“The leeches you’d most likely find on a bushwalk or in your backyard are jawed leeches,” said Ms Raven. “These leeches have sharp jaws and sometimes bite animals (including humans) to drink blood, but they aren’t dangerous and you’ll usually only notice them when they’re gone. I’ve done a lot of work in tropical rainforests and over the years have been a prized juice-box for many a leech.”

She said she usually doesn’t even notice she has been bitten until she sees a small patch of blood or a squashed leech in her boot.

“Jawless leeches, which suck blood using a straw-like mouth instead of jaws, are less common, and wormlike leeches that swallow prey whole usually live in water or very wet environments,” she said, adding it would be very exciting to come across either one.

STRANGE WAYS LEECHES ACT
Leeches move by swimming or using an “inchworm” crawling motion.

They respond to the world around them, noticing changes in light, temperature, vibration and smell.

Some can even change colour without seemingly needing to camouflage*.

LIFE CYCLE OF A LEECH
Leeches are hermaphrodites, which means they have both male and female sex organs. They mate by twisting together and swapping sperm*. They lay eggs in jelly-like cocoons, sealing it before the young hatch. Leeches die after mating once or twice.

MEDICAL USE OF LEECHES
Historically, leeches were used for bloodletting*. Today, they’re used in plastic surgery* to manage venous congestion* – which is when the veins in a reattached body part don’t work properly – and can aid in treating inflammation* and blood clotting* issues. According to the Sydney Children’s Hospitals Network, sometimes when a body part, like a finger, has become severed and sewn back on again, the blood flows into the part but not out again.

“The old blood stays trapped in the body part with no oxygen or nutrients needed for healing,” the network’s website states. “Leech saliva contains substances that stop blood from clotting and help improve blood flow, called anticoagulants*. This makes them excellent for treating venous congestion.”

Special leeches are bred for leech therapy, also called hirudotherapy. These medical-grade leeches are called hirudo medicinalis.

HOW TO PREVENT GETTING LEECHES ON YOU
If you are worried about leeches feeding on you during your next bushwalk or camping trip, there’s a simple way you can avoid them sucking your blood, said entomologist* and author of Eyes on Flies, Dr Bry the Fly Guy.

“Wear long pants, long sleeves and tuck your pants into your socks and your shirt into your pants,” Dr Bry said. “You don’t want to have an opening, because what will happen is you’ll walk through the bush and they’ll jump on your foot and wriggle up.

“So, you want to seal yourself up from them.”

He said insect repellent also helped to keep leeches away.

Leech bites, while causing oozing and possible infection, don’t cause a dangerous amount of blood loss.

Leeches in Australia generally don’t spread diseases to humans, although allergic reactions to their bites have been reported.

HOW TO REMOVE A LEECH
If you do get a leech on you, avoid putting salt or insect repellent on the leech as this can cause the leech to vomit into your wound and increase the risk of an infection. You should also avoid pulling the leech off.

One option is to leave the leech on you until it finishes eating and falls off naturally.

But if you don’t want to wait, Healthline advises you should find the head and mouth of the leech. Its head is usually narrower than the rest of its body and the narrowest part that is attached to your skin is most likely its mouth.

Then you should pull your skin tight where the leech is attached and gently slide your fingernail or a bank card under the leech’s mouth to separate it from your skin.

As soon as the leech has detached, flick it away so it doesn’t bite you again.

Make sure you clean the wound with disinfectant and bandage it up to stop the bleeding and keep it clean.

POLL

GLOSSARY

• segmented: made up of a series of similar parts
• parasites: organisms that live off other organisms (hosts) at their expense
• caudal sucker: a muscular ring on the rear or the leech used for suctioning onto the host so that the leech can firmly stay attached while feeding
• organic matter: decomposing plants and animal matter
• ecosystems: the way plants and animals interact together in an environment
• invertebrates: animals that don’t have backbones
• host: animals that parasites feed on
• secreting: producing and oozing out
• hirudin: a peptide in the saliva of leeches that stops blood from clotting
• proboscis: a long, sucking mouthpart
• camouflage: blend into its surroundings
• sperm: male reproductive cells
• bloodletting: removing blood for medical reasons
• plastic surgery: repairing or reattaching parts of the body that have been damaged or detached from injury
• venous congestion: when the blood gets stuck in a reattached body part and doesn’t flow out of it through the veins to be cleaned by the heart
• inflammation: pain and swelling caused by infection
• blood clotting: when we are cut our blood forms clots to stop us bleeding which forms a scab and prevents excessive blood loss while protecting against infection
• anticoagulants: substances that prevent against blood clots from forming
• entomologist: an insect expert

EXTRA READING
Aussie bloodsucker series: lice
Aussie bloodsucker series: fleas
Aussie bloodsucker series: ticks

QUICK QUIZ
1. How many known species of leeches are there in Australia?
2. What are leeches used for in modern medicine?
3. Why are leeches important for the health of ecosystems?
4. How can you avoid getting leeches on you?
5. How do worm leeches eat?

LISTEN TO THIS STORY

CLASSROOM ACTIVITIES
1. Let’s love leeches!
Design a poster, meme or logo. Your purpose is to make everyone forget the yucky things about leeches and love them.

Time: allow at least 45 minutes to complete this activity
Curriculum Links: English, Science, Visual Communication Design

2. Extension
Do you know what bloodletting was used for in medicine and why? Use your research skills to find out. Use the information that you have found to write paragraphs explaining what this shows you about how and why medicine has changed over time.

Time: allow at least 45 minutes to complete this activity
Curriculum Links: English, Science

VCOP ACTIVITY
Wow word recycle
There are plenty of wow words (ambitious pieces of vocabulary) being used in the article. Some are in the glossary, but there might be extra ones from the article that you think are exceptional as well.

Identify all the words in the article that you think are not common words, and particularly good choices for the writer to have chosen.

Select three words you have highlighted to recycle into your own sentences.

If any of the words you identified are not in the glossary, write up your own glossary for them.

Extension
Find a bland sentence from the article to up-level. Can you add more detail and description? Can you replace any base words with more specific synonyms?

Down-level for a younger audience. Find a sentence in the article that is high level. Now rewrite it for a younger audience so they can understand the words without using the glossary.