All you ever wanted to know about leeches, and some more

10 March 2008

“And will there be leeches…?” - a frequent question before we start some serious jungle trekking, such as trekking in the Crocker Range. My reply: “Oh, yes, don’t worry!”

Leeches are extremely intriguing animals. In fact, they are so fascinating that their conceptualisation could have flown out of the pen of a science fiction writer. Were you to read an anatomical and behavioural text on leeches without being told what the text was about you’d certainly come to the conclusion that it was the work of a fiction writer describing an alien specie. You’d probably also conclude that it was an alien species intent on human blood…!

Oh, yes, don’t worry, so if you go trekking in the Crocker Range, or in any other part of Sabah’s rainforest, you will certainly come across the occasional leech. They belong to Borneo’s wildlife we almost can guarantee you to see! As for the rest, like orang utans and elephants, you’ll need much more luck! During the wetter season of the year you might actually encounter quite a bit of leeches. They are essentially harmless, but I must admit that they are a bit of a nuisance to the trekker. There are some myths and legends around leeches that need to be demystified before each trekking, and before I share with you my passion on leeches and go into the more interesting anatomical and behavioural details here they are:

  • Leeches leave part of their head inside you once they are finished/are pulled off – wrong: leeches are not ticks and as such do not normally leave any body parts behind when removed. They also do not transmit any diseases, at least thus far no case is known.
  • Leeches wait in ambush on trees and let themselves drop onto their prey – wrong: leeches do not climb trees; in fact, they hardly can coordinate an ‘attack’ and those who attach themselves to some prey do it more or less accidentally.
  • Leeches jump – wrong: leeches cannot jump; but an excited leech can move rather fast, in a peculiar way which is described below.

Detection and Removal

Leeches do suck blood, that is not a myth! And they do attach themselves immediately at the slightest touch or when they can get within close enough range to some warm-blooded prey; then they look for a suitable site to bite. When they find humans they like to hide under the straps of your sandals or in-between your toes, or even in your boots where you don’t see them immediately; and you hardly can feel their bite. When you detect them and they are already dining on you it is ok to peel or pull them off but you’ll keep on bleeding from the bite, sometimes for hours because of the anti-coagulant the leeches use. On the other hand, if you leave the leech finish its meal and detach by itself if will ‘close’ the wound and you won’t loose an unnecessary drop of blood. You also minimise the risk of any secondary infection. Admittedly, it is not funny to walk around with a parasite who is feeding on one’s blood, even if the quantities are minimal. The best technique to remove a leech is to use your fingernails to break the seal of the anterior sucker first by pushing your fingernail along your skin and under the sucker, then repeat with the hind sucker. In order that the leech does not stick itself immediately to your fingers roll it into a ball and flick it into the jungle (not onto your trekking partners…).

If you don’t like touching or pulling on leeches, or if you can’t get a good grip on them call your guide; if he is busy with someone else’s leeches and you really, really want to get rid of the bloodsucker you might hold a flame close to it, or a lit cigarette; rub some salt on it or put a drop of medicated oil (massage oil, heat rub etc) on it; or squeeze a bit of tobacco juice on it. All of this will convince it to leave immediately but it won’t have the time to close the bite properly and you’ll keep on bleeding if the leech was already feeding on you. Besides, it might regurgitate and infect the wound in the process so the common methods above are medically inadvisable.

Leech wounds – those that the leeches have not closed themselves – tend to bleed for a long time though the blood loss is not important. You should wash the wound and use some antiseptic cream on it and apply a plaster. Healing wounds tend to itch but should be left alone. Scratching them open might cause secondary infections, and then they might become a problem!


As for preventing leeches – bad news: there is so far no commercial leech-repellent; Baygone spray or industrial strength tropical mosquito repellent only last as long as you don’t sweat or cross a river; the same applies for rubbing your legs with tobacco or stuffing your shoes with tobacco; high trekking shoes, leech socks and panty hoses only convince the leeches to crawl ever higher up until they find a juicy spot where to sink in their teeth … I personally have a theory that maybe 100% woollen knee-socks might actually and very efficiently deter them. Once I had a trekker – dead afraid of leeches and not to be convinced of their intriguing nature – who wore pure woollen socks in his trekking shoes as a habit. He also armed himself with gaiters, panty hoses and jeans and while we guides and porters all had lots of leeches during that particular trekking trip he remained absolutely free of any, and that is the only time I have observed this. Even if you wear high trekking shoes and gaiters, some leeches will eventually gain access into your shoes and maybe even bite you through your socks. But it is virtually unheard of that you don’t find at least one or two leaches in your shoes after serious trekking. So I have only one reasonable explanation for that, and I think it must have been the 100% woollen socks, that is socks that are made of a purely natural product and maybe there is some agent or property to sheep wool that deters leeches.

If you want to know how I prevent leeches from biting: I trek in flip-flops or even barefoot, and in shorts. This allows me to see leeches crawling up my legs and I can flick them off before they have a chance to bite. OK, this works with about 80% of the leeches as there are always some smart ones that hide between toes, or on the sole of my feet and since you can’t feel the bite I tend to overlook them until they are huge and ugly, gorged on my blood. At that moment I leave them to fall off by themselves so that the wound does not keep on bleeding. While trekking in single file in the jungle we also always watch the legs of the person before us and inform them of any leech so that they can be removed before they settle down for a meal.

Now as to why leeches are so truly fascinating creatures!

Leech Facts

Leeches are annelids (segmented worms) and are actually closely related to common earthworms, the ones we use for fishing. But leeches are anatomically and behaviourally rather more specialised and fascinating.

While most leeches are freshwater animals there are also many terrestrial and marine species. What you will encounter in Borneo are land leeches and they commonly live on the ground or in low foliage in humid rainforest, secondary forests, and swampy areas. During the dryer season they can be found wherever there are moistened places – along riverbanks, at water sources and small ponds. Land leeches do not enter water and cannot swim, but they can survive periods of immersion.

Belonging to segmented worms, the bodies of leeches are divided into 34 segments with powerful clinging suckers at each end. The hind sucker is very effective and when you try to remove a leech by hand you have to exercise quite some pull before it comes loose – usually the leech wins because besides the firm grip it is very slippery and can extend considerably until it slips through your fingers, literally.

The mouth is in the front (or anterior) sucker, and our leeches have usually three jaws. When you observe a recent bite you can clearly distinguish the y-shaped incision they make.

The anus of a leech is on the surface top just in front of the rear sucker.

Leeches vary in body shape. Generally their highly muscular bodies are elongated and thin, but they can become nearly round when fully gorged with blood. However, leeches are grouped according to the way the feed. The ones that interest us are the jawed leeches (or Gnatbobdellida), which have jaws armed with teeth with which they bite the host – us, for example. The blood is prevented from clotting by the production of a non-enzymatic secretion called hirudin. Land leeches commonly encountered by trekkers in the rainforests are included in this group: the tiger leech, and the common land leech.

A second group (the jawless leeches or Rhyncobdellida) insert a needle-like protrusion called a proboscis into the body of the host and secrete an enzyme, hemetin which dissolves blood clots once they have formed. Leeches which live on body fluids of worms and small freshwater snails possess such an apparatus. Not the ones we have to be worried about, but they admittedly also have an ingenious way of feeding!

A third group, (the worm leeches or Pharyngobdellida) have no jaws or teeth and swallow the prey whole. Its food consists of small invertebrates – not humans, thanks heaven!

Leeches breathe through their body wall, whereby aquatic leeches move to the surface when in water with a low oxygen levels. The body surface also contains a number of sensory organs, and together with those on the head they can detect changes in light intensity, temperature, and vibration. Leeches are also equipped with chemical receptors on their head to provide them with a sense of smell. Plus, they have eyes, sometimes only one pair but often several pairs.

As if all the above would not be sufficiently extravagant leeches are hermaphrodites and thus have both male and female sex organs. Their love act is short and probably not very exciting but surely gives the slogan ‘equality of the sexes’ a true-to-the-letter definition: when mating, two leeches intertwine and each deposits sperm in the other’s clitellar area. Rhyncobdellids have no penis but produce sharp packages of sperm which are forced through the body wall of the clitellar area. Once inside the body the sperms make their way to the ovaries where fertilisation takes place, following which the clitellum secretes a tough gelatinous cocoon rich in nutrients into which the eggs are deposited. The leech then shrugs itself free of this cocoon, sealing it in the process. Such cocoons can be buried or attached to the surrounding vegetation. The young leeches – miniature versions of their parents – emerge after several weeks to months, obviously very hungry for blood. They will start looking for a suitable prey (yes, that might be us trekking through the rainforest) and in a cycle of feeding and fasting grow to adulthood. There might be several weeks, or even months between meals, and leeches are able to withstand long periods of draught where they literally dry up. But a couple of drops of water are sufficient to awaken them to continue the cycle until they reach maturity, mate and produce the next generation. Leeches mate only once or twice during their life and then die.

While all the above makes them truly alien beings, it is the feeding you are probably interested in, because that is with which we are normally confronted: most leeches are sanguivorous, meaning that they feed as blood sucking parasites. Most have their preferred hosts but if they are not available they will look for other sources of blood. Land leeches in the Borneo rainforest can be found on bearded pigs, deer, mice and rats, squirrels or any other warm-blooded creature – we humans are not necessarily their preferred host but if we happen to walk right into the way of a hungry leech…

Some species of leeches also feed on fish, frogs, turtles or birds. And some leeches will even take a meal from other sanguivorous leeches, killing it in the process.

Since leeches can ingest several times their own weight in blood at one meal they can blow up to impressive size. An average sized leech here of roughly 3cm long may become as big as your thumb when fully gorged. After the feeding the leech releases its grip, falls off and retires to a dark spot to digest its meal. Digestion is slow, enabling the leech to survive during its sometimes very long fasting periods of up to several months.

It is a myth that leeches climb trees and wait in ambush on branches overhanging trails, waiting for some suitable prey to come by and then letting themselves drop. But hungry leeches are very responsive to light and mechanical stimuli. They tend to sit along or on trails (wildlife trails and trekking trails alike) or in narrow vegetation. They may change their position frequently and explore the surroundings by head movement and body waving. They also assume an “alert posture”: they extend their body to full length and remain rigid. Researchers think that this optimises the functions of the sensory organs in the skin.

When a host approaches the leeches ‘get excited’ and start moving towards the disturbance registered by their various sensors in their typical movement: they extend the body to full length, attach themselves by the anterior sucker and pull the rear up to the head into the so-called ‘loop position’, attach themselves by the hind sucker, release the head grip and extend again to full body length, attach their front suckers, pull up the rear again and so on. If they do this on you, the best moment to flick them away without having to use any force is during the ‘loop position’. At that moment they are not yet properly attached with their hind sucker, but the front sucker is already letting go in preparation of the forward movement. Thus you can simply flick them away into the jungle, with a snap of a finger.

Moving leeches seem to move very determinedly but they actually move on a trial-and-error basis. If we happen to step very close to a leech it can more or less accidentally attach itself, and that is how they get most of their meals. If the leech has to move towards the host until the anterior sucker makes contact its chances of a meal are slim. This is the reason why, when trekking, the first two or three in the group get proportionally less leeches than those at the rear of the group. The first trekkers alert the leeches as to where about the prey is. They move towards the perceived disturbance and with any luck manage to get a ride on the last few in the group.

When a jawed leech has found a host and identified a suitable area for its meal it attaches itself firmly by both ends and gets rigid to keep in place. With its semi-circular and many toothed jaws (like tiny saws) it makes an incision in the skin and excretes a mucous from its nephropores (external openings from the kidney-like organs). This helps the sucker to adhere even better. A salivary secretion containing the anticoagulant and a histamine floods the wound and the leech relaxes its body to allow the blood to be ingested. This anticoagulant mixture allows the blood to flow and prevents clotting once inside the leech. A bacterium in the gut of the leech assists the digestion of the blood, and it has been shown that the type of bacterium varies with the type of host on which the leech feeds. The bacterium also prevents growth of other bacteria which may cause the ingested blood to putrefy.

Leeches in Medicine

For over 2000 years leeches were applied for many ailments as an adjunct to blood letting. Today there is a real clinical application in that they are of great value to plastic surgeons when venous congestion of skin and muscle flaps is a problem.

Leeches are treated in the same way as blood products and are reused only on the same patient.

Medical use of leeches also includes treatment of black eyes, and hirudin is used in the treatment of inflammation of the middle ear. Hirudin is also being developed for experimental use as a systemic anticoagulant, and may prove useful in invitro blood sampling.

  • Mann, K.H. 1962. Leeches (Hirudinea) Their structure, Physiology, Ecology and Embryology. Pergamon Press Ltd

  • Williams, W.D.Australian Freshwater Life. Globe Press

  • Sawe, R.T. Leech Biology & Behaviour (reprint). Neurobiology of the Leech, 1981. Cola Spring Harbour Laboratory

  • Seliznev, K.G. et al. Use of the medicinal Leech in the treatment of ear diseases. Relat. Spec. (Switz) 1992 54 (1) 1-4

  • Wills, M.D. et al. The Medicinal Leech: an old treatment revisited. Microsurgery (US) 1993 14 (3) 183-6

  • Richardson, L.R. Observations on the Australian Land Leech, Chtonobdella Libbata (Grube, 1866). Aust. Zoologist V. XIV (3) 1968

  • Davies, R.W., Linton, L.R., Wrona, F.J. Passive dispersal of Four Species of Freshwater Leeches (Hirudinoidea) by ducks. Freshwater Invertebrate Biol. 1982 1(4) 40-44

  • Richardson, L.R. Trypanosomes in the crop of an Haemadipsid leech. 1968 Aust. Journal of Sci. vol 30 (9)


Leech attack!

A Borneo land leech - an intriguing creature!

A leech on the prowl...

... checking the wind...

... and getting into "alert" position.

An almost fully gorged leech.

Blood loss is minimal - you simply have to believe us this!


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