In 1816 Dr. James Rawlins Johnson published his Treatise on the Medicinal Leech. Besides the aforementioned methods of leech use, he studied the leech itself with exacting care. He tested to see if they were cannibals (they were); he froze them with or without salt to see if they would die (snow plus salt was worse). He even had leech fights between burly horse leeches and medicinal leeches (the horse leeches won). He also tortured them with carbonic acid, mercury, gas pumps, and olive oil, and was surprised to find that the leech “is very tenacious of life.” (The authors stopped reading after they reached this choice sentence: “Hermaphroditic impregnation may occur in a single leech.” The visual is…never mind. Just don’t.)…

There was also the matter of recycling. Leeches weren’t always tossed after a feeding. They could be reused up to fifty times if the animals were “encouraged” to disgorge: just dab a little salt on their mouths (which is really starting to sound like the leech equivalent of dabbing hydrochloric acid on a human). The leeches would then vomit like Ozzy Osbourne in his heyday. The cost savings were considerable. Other doctors would drop the engorged leeches into vinegar (a whole bath of acid!) and they’d perk up. Or so we imagine. In this manner, they could be used twice a week for up to three years.

Quackery: A Brief History of the Worst Ways to Cure Everything by Lydia Kang MD and Nate Pedersen.