The phylum Annelida is subdivided into three classes: Polychaeta ("bristle worms", principally marine animals), Oligochaeta (e.g. earthworms) and Hirudinida ("leeches"). Among the latter there is a subclass of Hirudinea (the true leeches) with 12 families. They include both terrestrial, freshwater and saltwater species. There are approximately 650 species, but not all of these constitute a problem for humans. Terrestrial (semiterrestrial is a better term) and amphibious species are common in Southeast Asia, the islands in the Pacific Ocean, India and South America. Aquatic species occur worldwide. They are seldom found in low-calcium water. They are good swimmers. Usually victims are people visiting marshy areas or walking in or near slow-moving small brooks or streams.

Until recently, no gut proteolytic enzyme had been identified in leeches. Then how can a leech digest its blood meal? The digestion process appears to be helped or even carried out by symbiotic bacteria. Investigators in the 1940s and -50s isolated pure cultures of bacteria from the digestive tract of the medicinal leech. While most digestive tracts are colonized by complex mixture of microbes, in the digestive tract of the medicinal leech, only one symbiont is found. Initial biochemical characterization suggested that the symbiont was Aeromonas hydrophila. Since these studies the Aeromonas taxonomy has undergone many changes. Biochemical tests and 16S rRNA sequencing showed that the symbiont is A. veronii biovar sobria. This bacterium has been implicated in wound infections, diarrhoea and septicemia. These infections usually can be prevented and treated by the administration of antibiotics. In patients with a normal immune response and sufficient blood flow to the site of attachment, the therapeutic application of the medicinal leech is considered to be safe. Leeches have been increasingly used in microsurgery without widely reported infection problems. The continued bleeding following leech therapy rinses out the wound and may be an important factor diminishing the probablility of wound infection in the patient.

Investigators proposed several functions in which the symbionts may benefit the host.

• The symbionts may digest the ingested blood. Aeromonas is well known to produce numerous extracellular enzymes. However, despite the ability of the symbionts to lyse erythrocytes (beta-haemolysis), the ingested blood cells are stored intact for many months, suggesting that haemolytic ability of Aeromonas is inhibited. More recently, it was shown that the host releases proteases into the lumen of the digestive tract where the actual digestion takes place.
• The nutritious quality of blood is poor, e.g. it contains very low amount of vitamin B12. It is possible that the symbionts serve as a source of nutrients to the host.
• Because it is so unusual to find a single species of bacteria in the digestive tract and early in vitro results suggested that Aeromonas can inhibit the proliferation of other bacteria, it has been proposed that the symbionts prevent the growth of non-symbiotic bacteria in the crop. However, there is little experimental evidence supporting any of these beneficial functions.

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How does the leech acquire its symbiont? Leeches are simultaneous hermaphrodites (both male and female at the same time), but they do not self-fertilise. After internal fertilization, the leech secretes a cocoon, that contains several individual eggs. It is likely that during this deposition the symbionts are transmitted from the parent to a nutritious fluid bathing the eggs in the cocoon. After the juveniles hatch from the egg, they remain for some time within the cocoon and consume the nutritious fluid, possibly acquiring the symbionts at this time. When the juvenile leeches leave the cocoon, they are functional, but they cannot bite yet through mammalian skin. It is thought that their first blood meal is from an amphibian.

Hirudo medicinalis or the medicinal leech is still used for medical purposes. At present its use in reconstructive microsurgery is studied, for example for decongestion, anticoagulation and vasodilatation in a newly re-attached finger or ear. Hirudin is a 65-amino acid polypeptide, a direct thrombin inhibitor which is not dependent upon antithrombin III or other plasma proteins. [Heparin is antithrombin III-dependent]. The genetic-recombinant hirudin analogues desirudin and lepirudin (HBW-023) are polypeptide-anticoagulantia which are being studied for possible medical applications, such as heparin-associated thrombocytopenia, deep venous thrombosis, thrombo-embolic disorders, myocardial infarction, angina pectoris and as adjuncts in angioplasty procedures. Bivalirudin is another hirudin-analogue under investigation.

Hirudin is commonly used as an anticoagulant for the treatment of thrombosis in patients with heparin-induced thrombocytopenia. Hirudin treatment should be monitored with the PTT. The usual therapeutic dose of hirudin in patients with normal kidney function is 0.4 mg/kg intravenous bolus followed by 0.15 mg/kg/hour continuous intravenous infusion. The dose has to be significantly reduced when the creatinine is >1.6 mg/dL.

Aquatic species can attach to the conjunctiva, nose, nasopharynx, vagina and urethra. When they attach themselves to the epiglottis, trachea or bronchi, serious complications are likely. Internal bleeding, haemophthysis, chronic headache, dysphagia and hoarseness occur. As a rule, the leech itself does not transmit any pathogens, but wounds can become secondarily infected. Aeromonas infections can occur, but are rare. Following repeated bites, hypersensitivity can occur. Removal of a leech can be facilitated by applying a little alcohol or vinegar. If necessary, a burning cigarette may be held near the parasite. No attempt should be made to remove the animals rapidly because the jaws can remain behind. The leeches can be loosened by local application of cocaine or lidocaine. They are removed carefully with a forceps, using a laryngoscope or endoscope. After wound cleaning, local pressure should be applied to stem the bleeding. The bleeding tendency can persist for many hours, sometimes even up to 2 days. This illustrates the power of the animal’s anticoagulantia. For prevention, protective clothing should be worn. A repellent, such as dimethyl phthalate or dibutyl phthalate, may be applied.