Hirudo medicinalis and Hirudinaria manillensis Feeding Physiology and Medical Use
by Zach Kleemann
BIOL/WATER 361, Fall 2013
Freshwater leeches (Hirudinea) occur in a variety of habitats, from marshy wetlands, to large lakes and rivers, where they feed on everything from decaying organic matter to bodily fluids, particularly blood. The species that often receive the most attention are those generally categorized as ectoparasites, which detach from their host after feeding and are still considered free-living organisms. Some of these blood-eating species, most notably the medicinal leech, Hirudo medicinalis, and more recently the Malaysian leech, H. manillensis, have been studied extensively because of the physiological adaptations that they exhibit for feeding. In a natural setting, these leeches feed very sporadically and must be able to ingest large quantities of blood in one feeding event; in one study H. medicinalis were observed to consume almost nine times their initial weight in blood (Dickinson & Lent, 1984). This need to gorge themselves whenever an opportunity is presented, however, presents some challenges for these leeches. First and foremost, blood is intended to stay within an organism, and when a wound allows it to leave the body, there are clotting factors that quickly stop the loss of blood. In addition, other animals will likely attempt to rid themselves of attached leeches if they are noticed, and opening up a wound from which to feed is likely to cause some sense of pain in the host. To combat these challenges, sanguivorous leeches have developed a suite of proteins in their saliva, which act as anticoagulants, anesthetics, and antibiotics. These proteins, resulting from the infrequent feeding behavior of leeches, have been translated into several different uses in the world of medicine.
In order to utilize these proteins in a meaningful way, we must first understand the circumstances under which they developed and how they actually function at the organismal level. As described by Dickinson & Lent (1984) there is an entire series of events that leads up to a leech encountering a host and potentially a feeding event; this includes stimulation by, and reaction to, waves and water movement, changes in light patterns, temperature of an object or organism, and chemical signals. The leech will position itself near the surface of the water to maximize its ability to detect waves, and after detecting changes in water movement and sometimes light levels, the leech will begin swimming towards the source of disturbance. After swimming to an object, it can determine if and where a bite should be made based on temperature. H. medicinalis is noted to be thermotactic when selecting a location to bite, actively seeking temperatures typical of warm-blooded animals (Dickinson & Lent, 1984). Once attached, an average leech feeding event lasts about 25 minutes, and is facilitated by repeated waves of muscle movement through the body, known as peristalsis (Lent et al., 1988). This movement acts not only to continuously draw blood into the crop and gut of the leech, but also to facilitate even and complete filling of all the lateral chambers of the gut, which maximizes feeding efficiency.
Consequently, the leech needs a mechanism to keep the host’s blood from clotting for a relatively long period of time, which is where its salivary proteins come into play. Saliva is secreted directly into the wound, delivering these anticoagulant and anesthetic compounds straight to their site of action, which allows the leech to continue feeding until it can no longer ingest any more blood. The main anticoagulant components identified in leech saliva are proteins known as hirudin (from H. medicinalis) and bufredin (from H. manillensis), but many other compounds with similar and different functions have also been identified (Ghawi et al., 2012). The action of these salivary proteins lasts long after feeding has ceased; while the blood is stored and digested in the gut, often for several months, the anticoagulant and antimicrobial compounds keep the blood in a liquid form while also protecting the leech and its food supply from pathogens (Ghawi et al., 2012). Interestingly, Ghawi et al. (2012) also noted that the strength of leech saliva extract varied by season, and was more potent and active in leeches collected during the dry season. Perhaps this coincides with the maximum likelihood of encountering the chance for a feeding event; if water is low or scarce, animals will be forced to visit larger permanent bodies of water, where leeches are more likely to reside.
In previous centuries, medicinal leeches were used to treat a wide variety of ailments thought to be caused by an excess amount of blood or tainted blood; these claims had no basis in biology or science and simply did not work, and for a period the use of leeches in western medicine had all but disappeared. However, in 1894 Haycraft demonstrated that leeches contained some sort of anticoagulant chemical, and many other experiments have been conducted since which have identified very specifically the types of proteins found in leech saliva as well as the compounds that they affect. Now that these special salivary proteins are better understood and possible to isolate or synthesize, they have seen an increase in use in various parts of the medical field. This means that they could have potential applications for everything from skin grafts to body part reattachment to plastic surgery. There has also been a push in recent years for more natural medicines and medical treatments, as opposed to strictly synthetic pharmaceuticals, and the various benefits and effects of leech saliva extracts have been a possible alternative. In some cases, leeches may still be applied to a patient live and active in cases where blood may be pooling or not draining from an area, and can aid in restoring blood flow and circulation in these tissues (Wright & Finical, 2000). H. medicinalis has also been used across a wide variety of plastic and reconstructive surgery procedures, and is suggested or proven to help recovery in cases of transplanted skin and other tissues, as well as reattached body parts from fingers to ears (Whitaker et al., 2003).
Perhaps the leading area of research and innovation today is the production of hirudin-like compounds, both through direct synthesis and purification of leech saliva extracts. In addition to these purely synthetic and purely leech-based preparations of anticoagulant proteins, there is a third preparation that falls between these two as a sort of semi-synthetic option. Amplification of the DNA responsible for these proteins can be performed, the amplified genes can be inserted into bacteria and the resulting proteins harvested, in much the same way as insulin is produced. At this point in time, however, purification requires a lengthy and relatively inefficient chromatography process to achieve a usable product, and some bacterial colonies with recombinant DNA from leeches have shown disappointing yields of the proteins (Seong et al., 1997). Because of the amounts of time and effort required for procuring high quality proteins from the natural source, as well as their low yield, attention has turned again to improving the production of recombinant proteins. According to Markwardt (2002), hirudin is the most effective known natural anticoagulant that works on a particularly important blood clotting factor known as thrombin, which has led to a focused effort on the synthesis of recombinant hirudin. This has in turn resulted in an increase in availability of these recombinant hirudins, compared to low availability of those extracted directly from leeches.
There are perhaps a couple of drawbacks to using medicinal leeches, both from a health concern standpoint as well as a social opinion or attitude stance. The more serious of these two problems stems from the use of live animals in a medical setting, and the fact that live animals carry the inherent risk of containing their own parasites or pathogens. In fact, Whitlock et al. (1983) were able to reliably culture Aeromonas hydrophila, a bacteria capable of causing gastroenteritis, cellulitis, and myonecrosis in humans, from the leeches they studied. The other issue with medicinal leeches, which is minor by comparison, is the negative stigma associated with leeches themselves; many people simply do not want to be bitten and fed upon by slimy aquatic organisms, even in the name of medicine. In both of these cases, the problem can be reduced or eliminated with the use of recombinant hirudins, so that there is no physical contact between leeches and patients.
Nevertheless, these medicinal leech proteins are promising, and some European health and beauty products are already utilizing these versatile compounds for their anticoagulant, anti-inflammatory, and pain-reducing qualities (Wright & Finical, 2000). Another less studied aspect of some of the constituents of leech saliva is their broad antibacterial properties, particularly against E. coli, though these components appear to be a relatively small fraction of the total active proteins found in leeches (Abdualkader et al., 2011). This area is still lacking in diversity of information available, however, and could use more research and more diversity of taxa included.
Medicinal leeches like H. medicinalis and H. manillensis have evolved with a rather unique feeding mechanism and pattern, which resulted in the biosynthesis of special proteins like hirudin and bufredin. Because of these qualities, leeches have been used in medicine for centuries, but only relatively recently have their scientific properties begun to be researched and understood. Even since these initial discoveries, most of the work done on medicinal leeches has centered around testing extracts and their effects, which opens the door for a great amount of further research on these proteins at the molecular level. As a result of considering and understanding these intriguing properties of the proteins found in leech saliva, scientists and doctors have taken great steps towards better and more natural treatment of many common problems, both surgical and topical.
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