Twelve Things You Probably Didn't Want to Know About Leech Therapy

By Amy D'Orazio, author of Done For The Best

The nineteenth century was what is known as the golden age of leech therapy. Although the development and acceptance of germ theory—the idea that diseases were caused by microscopic pathogens—was right around the proverbial corner (1830s or so), at the turn of the century most physicians had been educated in humoral theory and believed disease was caused by an imbalance in one or more of the four humors.

Humoral theory was first proposed in the era of Hippocrates, over two millennia prior to the regency era! The body, it was believed, was comprised of four humors— blood, phlegm, black bile, yellow bile—and each of these was related to a particular organ: blood to brain, phlegm to lungs, black bile to spleen, and yellow bile to gall bladder. Blood was considered the dominant humor and thus were imbalances in the blood the most critical determinant of illnesses. Thus did blood-letting become an vitally important tool in medical treatment. There were various methods used for a reduction in blood, from simple venesection by a scalpel or lancet, to leeches.

Here are 12 things to know about leech therapy, some of which might surprise you.

1. Most species of leech will not ingest human blood. The one that will is hirudo medicinalis and its use in the early 19th century was so popular and so widespread that it believed to be extinct for almost a century.

2. Hirudo medicinalis has three jaws with approximately 100 sharp teeth on each outer rim. The saliva carried by the leech has both an anesthetic (to make the bite painless) and an anti-coagulant (to keep the blood flowing). Treatment was thus not painful, and complications were minimal. In some cases there was scarring and thus were private body areas/mucus membranes preferred.

3. Leeches could be ‘prescribed’ by physicians or people could undertake the use of them of their own accord. Some people kept leeches in the house but they were also available for purchase by many apothecaries. Most physicians would charge a shilling per leech but they did not apply them theirselves—that was a job for an assisting nurse, a maid, or a brave family member.

4. The leech would be applied based on where they thought the issue was centered, and no place was off limits. Nostrils, breast, private body cavities, the temples…even the larynx or inside of the mouth—if that was where the problem was believed to be residing, that was where the leeches were applied. For cases of generalized inflammation, leeches would be applied to the torso between the breasts and navel.

5. Dozens of leeches might be applied based on the treating physician’s or apothecary’s discretion. Twenty or so per session was not uncommon and a patient might have several sessions over many days; in one documented case a man suffering from gonorrhea had 130 leeches applied to his swollen testicle over the course of 4 days.

6. Each leech ingests approximately its own bodyweight in blood, or about 5-15 mL (between a teaspoon or a tablespoon). To put that in context, a course of 20 leeches would thus drain a person of anywhere from 100-300 mL of blood. During a typical modern blood donation, 470 mL or a pint is given (an average adult has about 4500-5500 mL of blood).

7. Once applied, the leech generally took about 30 minutes to several hours to complete its feed. The leech/leeches would fall off once sated and the therapy would thus be complete. Leech therapy was usually performed along with other ‘treatments’ such as emetics, purgatives, or enemas, to really get those humors balanced out.

8. You would not wish to pull off a leech manually as it might kill or harm the leech for future use. Leeches were re-used but unfortunately the excessively slow digestion of the animal means that once it has fed, it does not need to be re-fed for 9 month-1 year. Because they were so expensive, and decreasing in number through the years, people came up with ways to be able to re-use them more quickly, such as placing the engorged leech on salt which made it vomit out the ingested blood.

9. The harvesting of leeches became a big business throughout the late Georgian and early regency eras. It was done mostly by women, but men might do it too, and involved simply walking through whatever body of water had them, and allowing them to attach themselves to your legs. They would then be removed with a little salt and placed in jars. France developed leech ‘farms’ and became a significant exporter of medicinal leeches after the Napoleonic wars ended.

10. In the 1970s, it was recognized that leeches could be of use post-surgically in some cases where venous congestion might be a problem. For example, in the case of limb re-attachments, leftover blood within the limb could cause death of the tissue or gangrene. Leeches are useful in such cases to remove the blood and allow the limb tissue to heal naturally. Leeches have also been used in cases of priapism secondary to the use of erectile dysfunction treatment. In 2004, leeches became classified in the United States as medical devices.

11. Aggressive bloodletting by leeches might have contributed to the death of America’s first president, George Washington who awoke one night with a putrid throat and inflammation of the epiglottis. He was bled vigorously over the course of eight hours, leading to a loss of 40% of his total blood volume, which is fatal although having his throat close and generalized shock also contributed to his death.

12. Resolution and Independence by William Wordsworth (published in 1807) includes several stanzas inspired by his encounter with a leech gathered near his home in the Lake District in autumn 1800.

He told, that to these waters he had come

To gather leeches, being old and poor:

Employment hazardous and wearisome!

And he had many hardships to endure:

From pond to pond he roamed, from moor to moor;

Housing, with God's good help, by choice or chance;

And in this way he gained an honest maintenance.

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Sources and References:

Codell-Carter, K. Leechcraft in nineteenth century British medicine. Journal of the Royal Society of Medicine 94:38-52 (2001)

Greenstone, G. The history of bloodletting. British Columbia Medical Journal vol. 52, No. 1, January February 2010

National Constitution Center Staff. The mysterious death of George Washington. December 2023

O'Dempsey T. Leeches--the good, the bad and the wiggly. Paediatrics and International Children’s Health. 2012 Nov; 32 Suppl 2:S16-20

Valauri FA. The use of medicinal leeches in microsurgery. Blood Coagulation and Fibrinolysis. 1991 Feb;2(1):185-7