Our fear of leeches all in the mind

TREKKING in the jungles of Borneo has its privations.

It’s hot and humid, rain buckets down in sudden storms and there are mosquitoes, some of which carry malaria. Snakes are hard to spot. They can be coiled in branches just above your head and it’s no use talking loudly to warn them off; serpents are deaf. Ants, 3cm long, deliver a sting the pain of which is said to last 24 hours.

On a recent trip, however, I was surprised to find that most people’s jungle nightmares involved none of these bogeymen. The demon of the rainforest, the creature which really spooks visitors from the West, is a relative of the humble earthworm, the leech. Having no limbs of their own, leeches resort to hitch-hiking to get around. They hang on to things using a sucker on the head and another at the tip of the tail. Brown leeches, one of two types found in Borneo, live in muddy puddles. When a person approaches, the brown leech grabs onto a boot and climbs aboard.

The leech’s ability to penetrate garments is legendary. It can negotiate several layers of clothing without even tickling the unsuspecting victim. Such feats are all the more remarkable since clothed people didn’t visit jungles until comparatively recently. These skills were honed using furry mammals as hosts. Sliding beneath a sock, the leech wriggles its way up the leg under the trousers in the hot sweaty darkness. Tiger leeches have a similar modus operandi but wait for their victims on leaves or branches. Striped red and yellow, they can latch onto your head or neck.

Reaching a bare patch of its victim’s skin, the leech sinks its teeth into the flesh, administering a local anaesthetic as it does so. You can feel the bite of a tiger leech very faintly. The brown leech’s incision can’t be felt at all. A substance known as hirudin, is also injected. It prevents clotting. The blood flows into the parasite like fuel from a petrol pump. Having gorged itself on the meal of a lifetime and swollen to perhaps to ten times its normal weight, the bloated worm disengages quietly and falls to the forest floor. It won’t feed again for months. A slight bleed follows the intruder’s departure but soon the only evidence of the blood theft is a reddish scab on itchy skin. The vampires don’t carry infection; victims are not at risk. The horrors of leech encounters are entirely psychological. There’s a consolation, however; your blood-donation helps the eco-system.

The best defence against the little Dracula is a pair of ‘leech socks’. The protective wall of thick synthetic material extends from toes to thigh and is laced tightly at the top. Despite the precautions, some enterprising leeches manage to storm the battlements. Undressing in the evening, the hapless trekker is likely to find a wriggling bloodsucker or two, with head still embedded in a thigh or buttock. Applying a lighted match, local people told me, makes the creatures vomit into the wound, causing infection. Instead, a blade is slid along the skin surface, decapitating the parasite before it can puke up its stomach contents. The severed head is then scratched out.

But leeches have their good side; they make enthusiastic medical practitioners. Illness, according to the Roman physician Galen, resulted from an imbalance in bodily ‘humours’. ‘Bleeding’ would help restore equilibrium, so blood was drawn from the hapless patient and leeches, the ideal painless extractors, were employed to do this. Blood-letting was a useless, and potentially dangerous, treatment. The practice, however, continued well into the 19th Century.

Now leeches are being called on again. They are not being asked to draw blood but to use their anti-coagulant skills in delicate micro-surgery. The hirundin they produce stimulates blood flow and, by mopping up the fluid, a leech can relieve pressure when little pools of blood form.

Leeches used to be common in Ireland. They liked horse blood but with the demise of horse-power, their numbers fell. Swans, which I catch for ringing, sometimes have leeches on their nostrils or on the flesh around the eyes, the only soft tissues available on the bird. About 2cm long, they are a fraction of the size of their Bornean cousins.