Part of the show Science Question and Answer - New Horizons Mission
Jordan in the USA asked:
Can I have a leech for a pet? I am allowed to buy one? If so, where can I buy one?
I've picked some up by wading in streams occasionally. If you wade around then they should find you! Medical companies do actually supply and you can use them for medical purposes. In the old days people used to blood let because they thought that it was good for people. But in fact leeches are very useful because they have venom that stops clotting. It's called hirudin and it's a tiny protein. When they bite you they inject it and it keeps the blood from clotting so they can keep drinking. Surgeons love them because if you have a bit of plastic surgery when a part of your body has been severed such as a finger or piece of ear, it's very easy to reattach things like arteries because they're quite large and chunky. But the veins are quite floppy, difficult to identify and it's very difficult to reconnect them, so blood can get into your tissue quite easily but not back out again. This means that the tissue can end up starving because the circulation can't go all the way through. So what scientists and doctors have found is that if they attach leeches around the site where you want to stitch something back together, you can keep pulling to blood out. As long as you keep a nice flow of blood coming in, then the tissue revitalises and the vessels grow back together. When the leech gets full, it just drops off. There are companies that will supply medicinal leeches but they're usually used by doctors, not members of the general public.