What we've done in the past is either:
- take them to our local hospital or GP clinic for safe disposal
- put them in a coke/cordial/juice bottle etc and screw the lid shut and place in garbage. Then no one can get a needle stick injury from it.
BTW I am not sure whether or not you can get infections from a needle used by a horse, but if someone gets a needle stick injury, how are they to know that it was used for a horse, not a human? They will be subjected to blood tests and immunisations and a lot of worry if they got a needle stick injury from a discarded horse needle.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare