Dark Banquet: Blood and the Curious Lives of Blood-Feeding Creatures

by Bill Schutt

  Rudy explained that during delicate procedures like the reattachment of an ear or finger, tiny arteries and veins are reconnected with microscopically thin sutures. With their thicker, sturdier walls, arteries are easier to work on, but the thin-walled veins that return blood to the heart are often problematic. As a result, blood pumps freely into the reattached tissue (via the surgically mended arteries) but it often pools up owing to the compromised venous flow out of the area. If not corrected, this “venous congestion” leads to blocked circulation and, ultimately, the death of the reattached tissue, which starves from the lack of oxygen and nutrients.

  “At some point, someone figured out that by placing leeches near the site of the reattached tissue you could establish a sort of artificial circulation.”

  “How’s that?”

  “The leech drains the accumulating blood so that new arterial blood can arrive on the scene. This fresh blood brings with it the ingredients necessary to repair the damaged blood vessels and stimulate the growth of new veins into the area.”

  “Got it,” I replied.

  “They might use ten to twelve leeches on a finger reattachment and several hundred after a scalp reattachment surgery.”

  Rudy explained how over a thousand leeches had been used to save the remaining portion of one Canadian man’s leg after cancer had forced surgeons to perform a partial amputation.

  “Still, it wasn’t until the 1980s that leech use progressed from being a last resort to becoming a rather standard procedure.”

  “Why the reluctance?” I asked.

  Rudy leaned across his desk and lowered his voice. “Most surgeons thought that it reflected badly on them if they needed to use leeches. Initially, it was quite a tough sell. Now we ship thousands of them each year—all over the U.S. and Canada.”

  I asked Rudy if all of the leeches he sold were headed for reattachment duty.

  “Mostly,” he replied, “but veterinarians are using them, too, for reattachments in dogs and sometimes to treat swollen ankles in horses. They’re also popular in teaching labs.” Rudy explained how leeches had relatively huge neurons (nerve cells), making it easier for students to study nervous system function at a cellular level.

  The leech-meister could tell that I was impressed but he wasn’t quite finished.

  “At Beth Israel Medical Center, studies are showing that leeches can be an effective treatment for pain and inflammation, especially for people suffering from osteoarthritis. They’re one of the first hospitals in the country to offer this type of leech therapy.”

  Rudy sat back in his chair and grinned like a proud father. “Quite an accomplishment for a creature that most people consider a repulsive, bloodsucking worm.”

  I nodded, as if I’d been a leech fan all my life. I had to admit (although not to Rudy), that even though I studied vampire bats for a living, leeches had always given me the creeps. In fact they were right up there with clowns and televangelists. Now, sitting in front of this enthusiastic cheerleader for Team Hirudo, I was starting to see his tiny co-workers in a completely different light.

  Leeches belong to the phylum Annelida (the segmented worms). The group contains around twelve thousand species and has a worldwide distribution. In addition to leeches (which belong to the class Hirudinea), annelids also include the earthworms (class Oligochaeta) and their freshwater relatives, and as well as marine wigglers like sandworms and bloodworms (class Polychaeta).

  Numbering around 650 species,*70 leeches can be found in cool freshwater environments or stagnant tropical mud wallows. About 20 percent of leech species are marine and their habitats range from shallow coastal waters to thermal vents located twenty-five hundred meters below the surface. Other leeches are completely terrestrial and they too exist in a diverse array of habitats: from tropical rain forests to the sub-Himalayan hills of northern India. There’s even an unpigmented species that inhabits a single cave in New Guinea where it feeds on the blood of bats. Although leeches are most famous (or infamous) for their bloodsucking ability, many species are predatory rather than parasitic. A few species even provide some benefit to their rather oblivious hosts.

  Annelids range in size from less than a millimeter in length to over three meters long, in the case of a giant Australian earthworm. They’re called segmented worms because their bodies are composed of ring-shaped segments (annuli) stacked one upon another (kind of like the Michelin Man—but with no arms or legs and a bit more slime).

  From an evolutionary perspective, the adaptive advantage of segmentation (also referred to as metamerism) appears to be related to the fact that annelid bodies are literally divided into a series of more or less independent sections (walled off from each other by thin septa). In the distant past, these annuli may have served as an early framework for regional specialization of the animal body into a head, midsection, and tail end.

  Apparently, segmentation also allowed annelids to move more efficiently than their nonsegmented ancestors. This is because along with a metameric body plan, another adaptation evolved in this group—a body cavity known as a coelom. This fluid-filled chamber is part of the worm’s hydrostatic skeleton, so that when muscles that encircle each body segment contract, the compressed fluid within the coelom is forced toward the head end, projecting the front of the body forward. You can demonstrate this type of locomotion for yourself by squeezing one end of an elongated, water-filled balloon around the middle. Your hand represents the annelid’s circular body muscles, while the water and the expandable balloon interior act as the coelomic fluid and the coelom, respectively. In earthworms, as the body extends forward, the worm secures itself to the substrate with ventrally located pairs of microscopic, toothlike setae (or chaetae). The back end of the body is then pulled forward as longitudinal muscles that run down the length of its body are contracted. The vermiform crawling that results from this anatomical arrangement is quite a bit more efficient than the wild whipping and thrashing movements that characterize other worms like nematodes (commonly known as roundworms).

  Leeches, however, are capable of employing an alternative form of locomotion, and in these instances they resemble inchworms (which are insect larvae, not real worms). This type of locomotion is possible because of the presence of a pair of sucker disks—one located near the head end (the anterior sucker) and the other near the tip of the tail (the caudal sucker).*71

  During inchworm crawling, the leech attaches its caudal sucker to the substrate (which may be oriented horizontally or vertically). Next, the muscles encircling the body contract, extending the anterior end forward (as in vermiform locomotion). The anterior sucker then takes hold of the substrate and the caudal sucker releases its grip. Finally, the tail end of the body swings forward, planting the caudal sucker directly behind the anterior one. Inchworm crawling can be employed underwater or as aquatic leeches leave the water to lay their eggs. It provides an additional advantage by allowing the leech to move efficiently across vertical or smooth wet surfaces. Terrestrial leeches also use inchworm crawling to zero in on their warm-blooded targets. Potential meals can be tracked from a distance of about two meters away. This is done primarily through detection of vibrations produced as the prey (or host) moves through its environment as well as the carbon dioxide it exhales. Vision is also employed as photoreceptors provide the leech with information on changes in light intensity (e.g., from shadows passing nearby).

  Many other leeches are quite adept at swimming, but unlike fish, which move by undulating from side to side, leech bodies bend with an up-and-down motion that is reminiscent of dolphins and whales. Leonardo Da Vinci (1452–1519) may have been the first to study leech locomotion and he actually spent quite a bit of time getting the intricacies of their dorsoventral undulations just right.

  Unlike the parasitic torpedo attack launched against Bogart’s character in the 1951 classic The African Queen (those “leeches” were actually made out of rubber), there are aquatic leeches that gain access to their hosts via different ro
utes. Some hop aboard as their victims dip their heads for a drink. Entering through the nostrils, the leeches attach themselves to the mucous membranes that line their host’s nasal cavity. There, in the warm, humid chamber, they feed and mature, safe from detection—at least for a while.

  One famous story recounts how this type of leech attack afflicted a group of Napoleonic soldiers, crossing from Egypt to Syria in 1799. As with any army in a foreign land, obtaining water was always a major concern and things were even dicier in the days before purification techniques (like boiling or adding iodine or chlorine) could make most water at least semisafe to drink. Apparently, some of these men drank water from a lake infested with tiny larval leeches. Unbeknownst to their hosts, the creatures quickly attached themselves and began to feed. Days later the men began to take ill and medical personnel were horrified to find their patients’ noses, mouths, and throats carpeted by blood-engorged leeches. The doctors tore frantically at the vampires and it’s not hard to envision a gory scene made all the more horrible by the cries of fearful, frantic men.

  As in other similarly large groups of animals, there is a wide degree of variation in leech diets. About three-fourths of all known species are bloodsucking parasites, feeding primarily on the blood of vertebrates (including sharks, bony fishes, frogs, turtles, snakes, crocodiles, birds, and mammals). Parasitic leeches do not generally specialize on any one particular prey. For example, the medicinal leech Hirudo medicinalis usually feeds on frogs, but it will readily take human blood. And while we’re on the topic, it should also be noted that dietary relationships between leeches and other organisms aren’t all stacked in the leech’s favor. Leeches are commonly fed upon by fish, birds, salamanders, snakes, and even other leeches.

  According to Dr. Mark Siddall, a leech expert and curator at the American Museum of Natural History, the first leeches were freshwater blood feeders, but alternative feeding modes have evolved at least six times in various leech groups. Several of these lineages have become predators, feeding solely on invertebrates (like their earthworm cousins, snails, or even other leeches). Unlike parasitic leeches, which can survive for extended periods of time without feeding, predaceous leeches feed frequently (generally, every one to three days). Another difference is that the predators digest their food rapidly, while their parasitic relatives are able to hold blood within their guts in an undigested state for up to several months. Predatory species are also equipped with a highly mobile, hoselike proboscis that is first employed to probe the potential prey. Once this tactile reconnaissance is complete, the leech inserts the proboscis into the flesh of its soon-to-be meal and proceeds to suction feed for up to several hours—vacuuming out virtually all of the prey’s soft-bodied interior. Often, the predator is joined by others of its kind, and the scene comes to resemble a Chinese buffet table attacked by senior citizens. In each instance, the ravenous feeding frenzy leaves behind only carnage.

  Some leeches are neither parasites nor predators. Among these are several members of the family Branchiobdellidae. These species are remarkable for the manner in which they partition their highly specific habitat—the shells of the freshwater crayfish. Although the hard outer covering of these minilobsters might seem an unlikely place for leeches to hang out, there can be up to seven species living on a single individual. In what stands as an extreme example of microhabitat partitioning, different leech species inhabit different regions of the crayfish’s body. For example, one type of leech lives attached to the crayfish’s antennae, while another lives out its life adhering to the pincer-tipped legs (the chelipeds). Unlike parasitic leeches that feed on the blood of their hosts, the behavior of these branchiobdellids is more comparable to the previously mentioned endosymbiotic bacteria living in the guts of mammals like cows. Although not as important as bacterial endosymbionts are to their hosts, these ectosymbionts provide a service to the crayfish by grazing on the micofauna (algae, diatoms, and bacteria) that attach to its body.

  This leech/crayfish association is not always positive for the host since some branchiobdellids don’t provide the aforementioned cleaning services. A few species cause no harm—they’re merely opportunists, consuming scraps of organic matter scattered about by the messy crayfish as it shreds its food into bitesized bits. Another branchiobdellid, however, is parasitic. It lives within the crayfish’s gill chamber where it feeds on the gill filaments and blood of the same freshwater crayfish whose bodies are so precisely partitioned by its nonparasitic cousins. Even worse, however, are the instances in which crayfish are infested by lethally high numbers of leeches, which can sometimes cover their bodies like living carpets, ultimately killing them.*72

  Early use of leeches by man reflected the importance of bloodletting as a therapeutic tool. Leeches also gave practitioners an alternative when “breathing a vein” wasn’t appropriate. For example, leeches could be applied to parts of the body that were difficult or impossible to bleed by lancing or other means. Inflamed tonsils might call for leeches to be attached to the back of a patient’s throat or the bloodsuckers might be employed to drain persistent hemorrhoids. Leeches were applied to the scrotum to treat the swollen testicles that resulted from gonorrhea and they were also commonly used to treat maladies of the female reproductive system. Additionally, leeches were the preferred method of bleeding women and children “who required a gentle withdrawal of blood.”

  In what is arguably the strangest use of leeches on record, the sixteenth-century French historian Pierre de Brantôme recounted how leeches were inserted into the vaginas of women on their wedding nights so that they could “seem like the virgins and maidens they used to be…so as the gallant husband who comes on his wedding night to assault them, bursts their bulb from where the blood flows.”

  According to Brantôme, battering this bogus maidenhead (or having it battered for you) invariably led to an annelid-assisted version of postcoital bliss: “And both are bloody and (there is) a great joy for both and in this way the honor of the citadel is safe.”

  Right.

  Medicinal leech use reached its zenith in Europe in the first half of the nineteenth century, where Napoléon’s chief surgeon, François-Joseph Broussais, ascribed to the idea that all diseases were a result of too much blood (Galen’s plethoras again—fifteen hundred years later). As a result, Broussais prescribed leeches (and the always-popular “starvation”) in much the same way that a physician today might recommend aspirin and bed rest. Given Broussais’ tremendous influence on European medicine, the use of leeches exploded in the 1830s, with over forty-one million used in 1833. French troops were bled for every conceivable ailment. Some of them were treated with as many as fifty leeches at a time—so many, in fact, that they were said to be wearing glistening “coats of mail.” Fashion-conscious ladies of the time even wore dresses “à la Broussais,” decorating them with imitation leeches. Leech use was so heavy that the medicinal leech was driven to the point of extinction (and it remains endangered today). Eventually, they had to be imported from places like Asia.*73

  Given the degree of crushing poverty that existed at the time, leech collection became a steady, if not particularly pleasant, way to generate income. Nearly all that was required to start a dynamic career harvesting Hirudo was access to a lake, pond, or swamp that had leeches living in it. Leech collectors simply rolled up their pants (or skirts) and waded into the nearest leech-infested body of water. Then they stood around (swamps suddenly became the place to hang out) until a hungry leech or two swam by and decided to latch on to a leg or foot. Once the parasite had secured itself, the “lucky” collector would gently pull the leech off and place it into a basket. Presumably, those folks with more time on the job were able to remove the leech after it had attached itself but before it had initiated a bite. For many, it appears that collecting leeches in this manner was far from a pleasant way to make a living. As described by the Reverend J. G. Wood in 1885:

  The Leech-gatherers take them in various ways. The simplest and most suc
cessful method is to wade into the water and pick off the leeches as fast as they settle on the bare legs. This plan, however, is by no means calculated to improve the health of the Leech-gatherer, who becomes thin, pale, and almost specter-like, from the constant drain of blood, and seems to be a fit companion for the old worn-out horses and cattle that are occasionally driven into the Leech-ponds in order to feed these blood thirsty annelids.

  “Today, leeches are raised in tanks partially filled with distilled water and refrigerated,” Rudy said.*74 “They’re hermaphrodites, so everybody gets pregnant. After Hirudo mates, it crawls out onto land to lay its cocoons—which look like foamy little footballs. The babies hatch from the cocoons in around three weeks.”

  “Do you raise your own leeches here?” I asked.

  “No. We get them as adults from a company called Ricarimpex. They’ve been in business since 1845.”

  “Yikes,” I said, impressed that any company could stick around that long—no less one that sold leeches and whose name seemed to have been thought up during a night of drunken Scrabble. I even briefly thought about suggesting a motto (“With a name like Ricarimpex you know our product sucks!”), but I held back.

  “There’s another company, Biopharm, that was started by Roy Sawyer in South Wales.”

  “Biopharm? Cool name,” I said, a bit too quickly and Rudy shot me a puzzled look.

  Back in the early nineteenth century, several companies, including Ricarimpex, sprang up in the marshy regions near Bordeaux, France. (The former company name was actually Ricard-Debest-Bechade.) These leech suppliers flourished because of the sudden demand that resulted from Dr. Broussais’ “if it’s sick, stick a leech on it” brand of medicine. Leeches were now required in vast numbers and the old capture technique of “wade in and wait” was soon replaced by leech husbandry and the establishment of permanent breeding pools. New roads and rail links meant that leeches could be sent farther and farther afield. The practice of leeching spread to the United States, although American species were found to be deficient because of their relatively puny size.