zoology biology benno meyer rochow science blog blood leech leeches

Blood Suckers

The good, the bad, and the evil

On an expedition to the Onabasulu cannibals of the Southern Highlands in the mountainous interior of Papua New Guinea near Mount Bosavi with my companion Tom Ernst, at that time a PhD-student and later university professor in the USA, we were constantly pestered, beleaguered one can almost say, by blood-sucking terrestrial leeches. They were sitting on slippery stones, clinging to wet foliage, attached to any vantage point on logs or trees that allowed them to wave their menacing front ends around in circles, ready to leech on to their victim as soon as they sensed our approach. These annelid animals are so adept at taking meals from the occasional passers-by that in spite of our utmost vigil, we once counted 42 individual bloodsuckers all simultaneously taking blood meals from Tom’s covered legs.

Being highly motivated and no bigger than a matchstick prior to a “meal”, these bloodthirsty annelids find the tiniest holes in the garment and even squeeze through the holes of the boot laces in order to reach their goal. They may have waited for many months for this chance and do not want to miss it. Before cutting a hole with its concentric ring of tiny oral teeth into the skin of its warm-blooded host, a leech will use a mild anaesthetic from its salivary glands to make the procedure as painless as possible. The sucking is then aided by a powerful pumping mechanism as well as the injection of an anticoagulant substance, called “hirudinin” or for short “hirudin” into the wound. Blood-sucking ticks, incidentally, also use anti-clotting chemicals to prevent the blood from stopping to flow. In order to “harvest” as much protein from the blood as possible, the leech begins to separate the solid constituents of the blood, i.e. the blood cells, from the water component of the blood liquid, i.e., the blood plasma. It achieves this separation while still sucking and by excreting the unwanted and non-nutritious watery fraction of the blood through pores in its body, giving the leech the appearance of intense sweating at work. Once the leech is satiated and twice as big and a great deal heavier as before the meal, it drops off and finds a peaceful place to rest and digest. The wound it leaves behind, however, keeps bleeding for many hours after the assault.

The fact that the ingested blood can be stored up to a year in the leech’s body without putrefaction has puzzled (and of course interested) scientists for a long time, but apparently symbiotic pseudomonad protozoans gobble up Staphylococcus and other harmful bacteria. As the blood sucking leeches (and by no means all species of leeches are blood-suckers, for many are harmless, do not suck blood or lead a parasite’s life and feed on earthworms) can remove appreciable quantities of blood from under the skin painlessly and are even known to help in cases of osteoarthritis, they have for a long time played a role in treating human illnesses. Given the fact that they do not transmit diseases when in the hand of the professional, physicians have nowadays re-discovered the usefulness of the medicinal leech Hirudo medicinalis in the removal of swellings and blood effusions in awkward places.

The days, however, when doctors were attacking all sorts of illnesses with this “living syringe” are not likely to ever come back: it is estimated that in 1846 in France alone 20-30 million medicinal blood sucking leeches assisted doctors and were used on patients, while London surgeons at around the same time imported 7 million of these animals annually. And today? The big question is whether Brexit will affect leech imports into the UK, right?

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

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