Eww, right? Except they were quite common in medicine during the 19th Century. Here's what "The Home Book of Health and Medicine" ©1834 says:
The leech is a well known species of worm that lives in water, and is applied to various parts of the body, to draw blood for the cure of disease. The medicinal leech has a flat slimy body, composed of rings, tapering towards the head; it is commonly about two inches long, about the thickness of a goose-quill; but it can lengthen and shorten itself very much. The bite of those leeches, which are found in stagnant waters and marshes, is said to cause pain and inflammation; such leeches, therefore, as well as the horse-leech, are not used, and those are preferred which are taken in the summer season, in waters having a clear sandy bottom. A leech attaches itself to any substance to which it wishes to fix, by an apparatus, constructed on the principle of a leather-sucker, which it has at both ends; the one at the head being like a horse-shoe, with a triangular mouth in the centre, and that at the other end being circular. When they fix on the body, they inflict a small wound of three little flaps, from which they suck blood until they are gorged, or till they are forced to quit their hold; this is best done by sprinkling on them a little salt.
The cases are very numerous in which leeches are useful; and in children, where it is so difficult to get blood from a vein, leeches furnish an excellent resource. Leeches are useful in the various inflammatory diseases, as ophthalmia, sore throat, rheumatism, tooth-ache, inflammation of the bowels, and uterus; in measles and scarlet fever, in hooping-cough, in head-ache, in bruises and in piles.
It is sometimes difficult to get leeches to fix; they should be kept hungry, and taken out of the water for some minutes before they are to be used, and should be dried with a soft cloth immediately before they are applied. The part should be well washed with soap and water, then with milk and water, and wetted with blood or syrup, and if there be many strong hairs, they should be shaved oft'. A large leech will draw about an ounce of blood, that is about n table-spoonful; and when they come off, the bleeding may be encouraged to a considerably greater extent, by bathing the parts with warm water, or by applying large poultices of bread and milk, or applying cupping glasses. It is sometimes difficult to stop the bleeding, and the surgeon is sent for in great alarm, especially when leeches have been applied to young children. The bleeding may generally be stop ted by proper pressure, with a little lint, or similar downy substance, for a due length of time, though this is sometimes very difficult, when there is no bone to press against; touching the wound with lunar caustic, will almost certainly succeed; but we must take care that the flowing blood do not wash the caustic down about the neighbouring parts. Sometimes the wounds made by leeches, give rise to a good deal of pain, swelling, and extensive inflammation. The best application is a cooling lotion of-sugar of lead, or diluted alcohol and water, or vinegar and water. If the pain and tension continue long, an emollient poultice of bread and milk will be useful.
Salt has been thrown on the animal to make it disgorge the blood which it has sucked, but the leech is generally killed in the experiment. A more easy way to discharge the blood, and save the animal, is to hold it in the hand, and gently squeeze it in a napkin, from the head downward; the blood flows copiously from what may appear the anus, or through the ruptured extremity of the intestinal canal, and the worm is not essentially injured.
Leeches are best kept in a bottle, half filled with pure spring or river water, covered with gauze or fine muslin. It is better not to put bran or any other substance into the water, but to.change it pretty frequently. Leeches are said to be very sensible to the electrical changes of the atmosphere.