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The first record of leeching comes from illustrations in ancient Egyptian tombs carved over 3000 years ago. By 100 AD, written records of leeches being used to treat a wide variety of ailments can be found in Chinese, Sanskrit, Persian, and Arabic[1]. In Ancient Greece, according to Hippocrates’ teachings, humans contained four vital bodily fluids, the ‘humors’ — blood, phlegm, black bile, and yellow bile — and diseases arose from an imbalance of these humors[2]. His theory popularised treatments such as induced vomiting and bloodletting, often with leeches, in order to rebalance the humors.

Leeching reached peak popularity in the 18th and early 19th centuries in Europe, where it was prescribed for almost every malady. Harvesting and exporting leeches was a very lucrative trade, and such was the demand for medicinal leeches that they became endangered in several countries, including France. During the 1830s, 5–6 million leeches were used annually in Paris alone, and around 35 million annually across France[2]. Fashionable ladies in this era even wore dresses embroidered with leeches[3].

However, excessive bloodletting of already ill patients inevitably did little to help their condition,simply weakening them further. In the late 1800s, as medicine slowly became more evidence-based and embraced the new sciences of pathology, physiology, and microbiology, bloodletting fell out of favour and leeches fell out of fashion.

Leeches may have been consigned to the history books forever were it not for John Berry Haycraft’s discovery of a powerful anticoagulant in leech saliva, which he named hirudin[4]. This molecule proved difficult to isolate and the structure of hirudin was not solved until 90 years later, in 1976. Hirudin is a protein that inhibits thrombin, a key component in the blood clotting process, thus acting as a strong anticoagulant[5].

Many other bioactive molecules have since been isolated from leech saliva and studied, including other anticoagulants, vasodilators, anti-inflammatories, analgesics (painkillers), and antimicrobials[5]. It is easy to see why these molecules are essential to the leech’s ectoparasitic lifestyle: anticoagulants and vasodilators allow blood to keep flowing during feeding, while analgesics and anti-inflammatories prevent the host from noticing and removing the leech. Since the 1970s, the study of these molecules and the subsequent realisation of their many potential applications has led to the re-entry of leeches into medical practice, this time in an evidence-based manner.

Today, one of the main uses of leeches is following reconstructive surgery — for example, after skin grafts or after reattaching a severed ear or fingertip. Leeches are allowed to suck blood from the patient’s wound, releasing hirudin into the blood and preventing coagulation. At the same time, secretion of vasodilators and the physical removal of blood reduces a phenomenon termed venous congestion, which occurs when blood enters the grafted tissue but cannot leave because blood vessels have not been fully repaired yet, causing damage and slowing down the healing process[6]. Leeches are also being used in the treatment of osteoarthritis, as the anti-inflammatory and analgesic compounds found in their saliva reduce both inflammation and pain in the affected joints[6].

The very properties that make leeches successful ectoparasites also make them perfectly suited for treating a number of human diseases and increasing the success of surgeries, just not for the reasons that Hippocrates believed.

References:
  1. Whitaker, I.S. et al 2004. Historical Article: Hirudo medicinalis: ancient origins of, and trends in the use of medicinal leeches throughout history. Br J Oral Maxillofac Surg. 42(2): 133–7
  2. Greenstone, G. 2010. The history of bloodletting. BCMJ. 52(1): 12–14
  3. Soth, A. 2019. Why Did the Victorians Harbor Warm Feelings for Leeches? JSTOR Daily. Accessed 13 May 2020.
  4. Senthilingam, H., Scales, H. 2014. Hirudin. Chemistry World. Accessed 13 May 2020.
  5. Sig, A.K. et al 2017. Medicinal leech therapy — an overall perspective. Integr Med Res. 6(4): 337–343
  6. Abdullah, S. et al 2012. Hirudotherapy/Leech therapy: Applications and Indications in Surgery. Arch Clin Exp Surg. 1(3): 172–180


Katarina Grobicki is a graduate student at the Department of Genetics, Cambridge.