The idea of leeches sucking your blood may be as disconcerting as having maggots eat dead flesh from a wound. The leech, however, is a fascinating creature, and although most famous for sucking the blood of its hosts.
To put leeches in context, they are a type of segmented worm – like
earthworms, although leeches tend to be more like a flattened pear shape, with the mouth at the thin end. All segmented worms are in the
phylum Annelida1,
and they are divided into three classes: the polychaetes (marine worms
such as bristleworms), the oligochaetes (earthworms and other
freshwater worms) and the hirudineans (leeches).
While broadly similar to earthworms, a true leech
2 differs by having a consistent number of segments
3,
each of which is always divided into two or more rings (annuli), and
by having a sucker at both ends of its body that it can use to move,
rather than the bristles of other annelids. Many of the aquatic leeches
can also swim in a similar manner to eels.
Leeches, like earthworms, are hermaphrodites
4,
but the reproductive systems vary between the different families of
leeches. Leeches have internal fertilisation systems: most mating is
achieved by the implantation of a ‘spermatophore’ (sperm package) into
their partner. This implantation can by done either via a protrusible
penis or by the hypodermic implantation of the spermatophore into the
body of the partner. The sperm are then transported to the eggs where
fertilisation occurs, and cocoons are formed. While many leeches leave
the cocoons to develop by themselves, in the leech family
Glossiphoniidae,
most species have the cocoons attached directly to the parent. This
allows the parent to protect and care for the young as they develop.
This includes providing food (prey) for the young leeches after they
hatch.
All leeches are carnivorous invertebrates
5,
though they are not all parasitic bloodsuckers. There are at least 700
species worldwide, filling a number of predatory niches in aquatic (or
at least moist) environments. They require dampness because their thin
skin and lack of internal organs means that they find it almost
impossible to control water loss, either by dehydration or
osmosis.
Many species of leech wait for the prey to come to them and subsist on
other invertebrates such as earthworms, insects, pond snails and
freshwater clams. These leeches are either ones that swallow their prey
whole, or ones that spear the prey with an extendable tube which is
then used to suck their victim’s juices out.
The other species of leeches comprise the bloodsucking varieties,
some still using the hollow spear-like proboscis, but most having two
or three sets of jaws with which to attach themselves to their larger
hosts as an external parasite. The jaws resemble curved saw-blades with
tiny extremely sharp teeth making either a V- or Y-shaped bite
(depending on the number of jaws).
Leech use through the ages
Leeches have been used for centuries – possibly since the Stone Age – though
modern medicine
has outgrown the early rationales for their use. One explanation for
blood-letting was that disease was caused by evil spirits, and that
these could be removed with the bad blood. There is evidence leeches
were used by the Egyptians as long ago as 1500BC. There are further
references to medicinal blood-letting in Arabic, Sanskrit, Persian,
Greek and Chinese literature. European medical thought was influenced by
Galen, physician to Roman Emperor Marcus Aurelius in the 2nd Century
AD. Galen advocated blood-letting as part of his medical theory of the
four temperaments (sanguine, phlegmatic, choleric and melancholic) that
diseases resulted from the imbalance of what
Hippocrates identified as the four humours of the body (blood, phlegm, black and yellow bile).
Most uses are medical and based on the leech’s ability to consume
blood, but one bizarre use, of questionable effectiveness, was the
Tempest Prognosticator – using the response of leeches to changes in atmospheric conditions in order to predict storms.
Leech For The Weekend Sir?
The medical use of leeches for blood-letting was prevalent in Europe
and America in the 18th to mid-19th Century, particularly in
France (millions were used each year in Paris alone) and was often associated with
Barber Surgeons. The species used for blood-letting was (and is) usually the freshwater leech
Hirudo medicinalis.
Modern Leech Use
Blood-letting fell out of favour after the 1830s when experiments
revealed that it wasn’t an effective treatment for most diseases. The
leech removed blood and left a wound that continued to bleed for some
time – this did not cure the patient. However, the 1884 discovery that
blood did not clot in leeches’ stomachs eventually led to the
identification of the effective anti-coagulant
enzyme
hirudin in the 1950s. This leech by-product has become widely used.
Leeches themselves are also still used today because of their ability to
significantly aid recovery of blood circulation following surgery.
Some surgery – for instance, the reattachment of a severed finger –
can require the blood flow to be re-established. This is done by
reconnecting the major arteries and veins. Veins, however, can be
difficult to find and connect. If not enough are reconnected, the blood
may initially enter the reattached part but not be able to exit. This
stops fresh, oxygenated blood entering and the reattachment procedure
will fail. Leeches can assist here. If applied to the damaged tissue
they can both actively suck out excess blood and also secrete in their
saliva: hirudin, which prevents coagulation; a vaso-dilator causing a
widening of blood vessels in the dermis encouraging blood flow to the
area; a form of histamine, encouraging the capillaries to become leaky;
and an anaesthetic so that the host doesn’t feel their jaws slicing
through the skin to expose the blood vessels beneath. Together these
prevent the tissue from dying off, allowing the body to re-establish
good blood flow in the damaged area in a few days.
Leech Attack!
Leeches are also familiar to people who have travelled through wet
tropical jungles and swamps and have emerged with uninvited leeches
dangling from their flesh. How did the leeches find their prey? Leeches
do have eyes (between two and ten) as well as photo-receptive cells
scattered over their bodies, but these are pretty basic and unlikely to
differentiate more than changes in intensity of light and maybe some
movement. Leeches tend to be nocturnal and shun bright light. More
unusual sense organs can allow some leeches to locate prey by detecting
vibrations (such as movement through water), by smell (following a
chemical gradient toward the prey) or even by passive sonar.
Bloodsucking leeches tend to be alerted by sensing movement and then
home in on the carbon dioxide exhaled by the prey.
One incident that may make you think twice before drinking
unfiltered water was the decimating effect leeches apparently had on
the soldiers of one of
Napoleon‘s
armies in the Syrian desert in 1799. Allegedly, the soldiers drank
water containing leeches. These attached themselves to the insides of
the soldier’s noses, mouths and throats, engorged themselves on blood,
and caused some soldiers to die of suffocation; others by loss of blood.
Although the story is anecdotal the risk is genuine, and if you can’t
remove them by gargling with
alcohol or hydrogen peroxide it may be nec
essary to puncture an engorged leech with a sharp object.
Don’t
panic. Trying to remove a leech by burning i
t with a lit
cigarette, applying salt, shampoo or
insect repellent
or just pulling at it, may cause the leech to regurgitate its meal and
leave an infected wound much worse than the one the leech alone would
have left.
Usually leeches will drop off easily by themselves when they have consumed a meal
6.
If you can’t wait, leeches can be removed using a fingernail to push
the anterior sucker (where its mouth is) aside (the one at the thin end
of the leech), and then doing the same with the posterior sucker (at
the fat end of the leech), being careful not to let it reattach itself.
If you are in good health and keep the wound clean
7
you will be unlikely to suffer any ill effects from the leech. It is
possible to be made unwell by the symbiotic bacteria in the leech’s gut:
possible complications being wound infections, diarrhoea or
septicaemia. This can be avoided by using pre-emptive
antibiotics for deliberate medicinal use of leeches.
source: www.bbc.co.uk/dna/h2g2/A4429631
