How did I get leeches in my pond?

I have a pond attached to my well. It is spring fed and constantly overflows so we dug a hole and the water fills up in the hole.We have no fish and the dogs drink out of it.The kids played in it last summer and it was fine. This year we had snails,little black triangular shaped. Now I was walking around in it cleaning out debris and two slug type things attached to my feet. I am uming they are leeches.How do I get rid of them and how did they get there anyway?The birds love the pond too and I don't want to hurt them.I added bleach to kill the algae, will this kill the leeches too?