When people think of market crazes, Dutch tulips or real estate come to mind. But in the 19th century demand for Hirudo medicinalis—the European medicinal leech—nearly drove the species to extinction. Its medicinal properties were touted as a cure-all across Europe, and the animal was used to treat everything from cancer to tuberculosis to mental illness.
The coveted worm—dark brown or black in color, with a thin stripe of yellow, green, or red along its back—was popular because it supposedly had a gentle touch, yet also, importantly, a voracious appetite.
Physicians of the period often prescribed dozens of leeches to treat what ailed a patient. Someone with suspected pneumonia, for example, might have up to 80 leeches each treatment session applied across the chest. For gastritis therapy, as many as 20 to 40 leeches could be prescribed. As a result, wild Hirudo medicinalis became increasingly scarce across its range in Europe.
Victorian-era Europeans weren’t the first to look to these bloodsucking worms for succor. Leeches were used medicinally by ancient Egyptians, and later in India, Greece, and Rome. Greek physicians typically used the animals for bloodletting, to balance the humors, and also for conditions as varied as gout, fever, and hearing loss.
Leech use reached new heights in the 19th century largely because of the influence of François-Joseph-Victor Broussais, the head doctor of Val-de-Grâce in Paris. The physician declared that all ills, ranging from smallpox to cancer, were the result of inflammation, and bleeding, he said, was the cure. Bloodletting via leech became de rigueur because it was relatively safe and didn’t require any specialized skills. And leeches have natural anticoagulants in their saliva, which helps stop bleeding once they drop off a patient.
Broussais treated his own indigestion by applying dozens of leeches, and he believed leeching could have salubrious effects on animals, too: He bled his fighting cocks weekly, though the weakened birds performed poorly. Such was the demand for leeches that from 1830 to 1836 Broussais’s hospital alone used over two million of them, sometimes applying large numbers of leeches to new patients prior to any diagnosis. Other French hospitals recorded robust use during the years of peak leech popularity, too: From 1820 to 1850, some used between 5,000 and 60,000 leeches annually, according to work by Roy Sawyer, the founder of the Medical Leech Museum, in Charleston, South Carolina.
To meet demand, hospitals relied on rural workers who gathered the wild animals. Leech gatherer, unsurprisingly, was a distinctly unenviable job in the 19th century, but the work was reliable. Wading into a freshwater pond or muddy ditch to offer up his—or often her—body as bait to parasitic worms, a leech gatherer’s job was described as “employment hazardous and wearisome” by poet William Wordsworth.
Leeches were found in freshwater ponds, streams, wetlands, and ditches throughout Europe. They would sup upon the blood of many creatures: deer, horses, cattle, and humans, as well as fish, amphibians, and waterbirds.
Clamping onto their prey with three formidable jaws, each studded with about 100 teeth, leeches would often extract a tablespoon of blood before they were satiated and could then be easily detached. Repeated blood meals took a toll on beleaguered leech collectors, who endured hazards including fatigue and extreme blood loss as well as infections from organisms in the leech’s gut or transmissible diseases like syphilis. There was always a risk that the animal might regurgitate previously ingested blood.
During the Victorian era, enthusiasm for leeches spread widely across Europe and also gave rise to a leech trend glorified in European fashion and art. Leeches were embroidered on women’s dresses. Apothecaries purchased elaborate, two-foot-tall ceramic containers to prominently display and house their leeches. The need to transport leeches across vast distances for transcontinental and, eventually, transatlantic journeys also inspired innovations in leech storage.
To help meet a burgeoning American demand, in 1835 a $500 award—roughly $17,000 in modern-day dollars—was advertised for anyone who could breed European medicinal leeches in the United States, but that venture never proved successful.
The relationships between people and their parasites also gave rise to surprising long-term bonds: British Lord Chancellor Thomas Erskine, who lived from 1750 to 1823, was so grateful to two leeches that bled him when he was extremely sick that he kept the pair as companions. Storing them in a glass, he gave them fresh water daily and named them Home and Cline after two celebrated surgeons, according to Leech by University of Manchester medical historians Robert Kirk and Neil Pemberton.
Despite the popularity of the European medicinal leech, it was not an ideal product for commercialization. The species only needed a blood meal every six months and didn’t reach reproductive age for a couple years. Used leeches were often disposed of in ditches or ponds, where they could theoretically reproduce, but species overexploitation alongside draining and redevelopment of marshlands for agriculture—and the likely related losses of amphibians that the leeches relied on as food staples—fueled declines.
To help save the medicinal leech from extinction, a small number of 19th-century European governments implemented some of the first ever wildlife protections, either prohibiting leech exports or regulating leech collecting. In 1848 Russia banned taking them from May to July, the prime breeding season.
Yet these actions were not enough. By the early 1900s, the medicinal leech became endangered in many locations throughout Europe, and the animal was incorrectly believed to have disappeared from Great Britain, Germany, Sweden, and the Netherlands.
Partly because leeches could not tamp down the cholera epidemic that ravaged Europe and the United States, the animals eventually fell out of favor as a first-line medical treatment. Medical leech use endured for much more limited applications. In the early 20th century, the animals were sold in barber shops, recommended as a treatment for black eyes.
Today, the European medicinal leech is considered near threatened by the International Union for Conservation of Nature (IUCN). Its range still extends across Europe, and alongside local collecting pressures, wetland destruction, climate change, and lack of blood meals from mammals and amphibians are considered its most pressing threats. The animal’s use in modern medicine continues, particularly to assist with transplants and plastic surgery, but the animals are now often bred at laboratories in Europe and the United States.