Dark Banquet - Bill Schutt [57]
Sucker attachment actually has two components: adhesion via glandular secretions and the generation of negative pressure by a circular disk of muscle that leech expert Roy T. Sawyer described as “a finely tuned suctorial device.” The inner surface of this sucker is lined with glands that secrete a mucuslike polysaccharide that aids in adhesion (in much the same manner that a wad of wet toilet paper clings to a ceiling of a dorm bathroom). In some species that prey on fishes, this adhesive substance is actually a flesh-dissolving enzyme that leaves permanent scarring at the attachment site (which is usually located near the base of a fin).
Rudy explained that none of Hirudo medicinalis’s adaptations would have elevated it much beyond the status of “rarely encountered but truly disgusting annoyance” were it not for the leech’s ability to produce one of the most potent anticoagulants known to man. Identified in the late nineteenth century, the substance (now known as hirudin) was purified in the 1950s and cloned some thirty years later. Like the clot-preventing substances found in vampire bat saliva, hirudin has helped to transform the medicinal leech and some of its relatives into superbly evolved sanguivores. When released into a bite-inflicted wound, hirudin works in combination with a painkilling anesthetic, a blood vessel–widening vasodilator, and another substance (hyaluronidase) that promotes local spreading of the leech’s potent salivary stew.
Initially, blood flows from the wound because of negative pressure produced by the anterior suction cup, but before long, suction is enhanced by wavelike peristaltic contractions of the leech’s digestive tract. The leech remains attached to its host for up to an hour, sometimes ingesting ten times its body weight in blood (medicinal leeches consume about ten milliliters of blood per meal). Once satiated, the leech (bloated like some blood-filled, organic football) releases its suction grip and falls off. Depending on the species, leeches can survive up to three years between blood meals.
For the host, even after the leech drops off, the effects of the bite are far from over. Bleeding from the wound site continues for up to ten hours, and like the aftermath of a vampire bat attack, this can leave a considerable mess. Even under controlled conditions, patients undergoing leech therapy sometimes require transfusions to replace blood lost after the leech finishes feeding.
In addition to an often-significant loss of blood, there can be additional problems for the leech-bitten. Aeromonas hydrophila is an endosymbiotic bacterium that lives in the leech’s gut. This organism can produce infection at the wound site as well as diarrhea for the human host. Because of this, leech treatment is often accompanied by a regimen of antibiotics.
What’s Aeromonas doing inside the leech?
Researchers believe that in addition to assisting in digestion, Aeromonas produces metabolic by-products that serve as vitamins and essential amino acids for the leech. Like other endosymbionts, Aeromonas gets a safe place to live, feed, and reproduce. Similarly, other species of endosymbiotic bacteria inhabit the guts of mosquitoes and vampire bats.
Often, leeches carry even nastier pathogens in their saliva—including blood-borne parasites known as trypanosomes. These protists (notable for their prominent, whiplike flagella) are responsible for several serious diseases in their human and nonhuman hosts. Thankfully, leech-to-human transmission is rarely if ever reported—probably owing to the fact that leeches do not carry the infective stages of these flagellates—which are much more likely to be found in creatures like assassin bugs (a family of insects that have evolved a long and complex relationship with trypanosomes).
“Only Hirudo medicinalis has the right combination