There are different kinds of leeches. Some attach themselves to animals and fish. Some just eat the crap (literally)in the pond.
Visiting frogs, turtles, birds can bring them in.
I had the detritus - feeding leeches come into the old pond on a plant. They were only about an inch long and hung around in the filters but they were still gross.
When I enlarged the pond, everything from the old pond was cleaned and left in the sun to dry. The plants were all cleaned. I managed NOT to get the leeches in the new pond.
Now if I could just have been more careful about the d@*n snails....
sam

I had leeches in my pond that I inherited, but I didn't even know I had them until one time when I was doing a small clean-up job in early spring - when I got out of the pond I felt something on my ankle. When I looked down I saw a long red "worm", so I pulled it off and dropped it into a bottle of water until I could ID it - thing was more than 2" long when swimming in the water. Instead of saying that I found some leeches I usually say that the leeches found me :)

Im my case I think they got there by piggy-backing on a snapping turtle, but as far as I know I got rid of 'em during my big cleanup weekend (I had found 3 total, who knows how many were actually in residence). From what I've read they can become fish food - if the fish can find them, but like Sam said it's more important to find out what type of leeches you have. Until then a set of waders could be a good investment, unless you want to "bait" them out of hiding ;)

- Mike

Thanks for your responses. I think, given the description, that I have the kind that Sam mentioned. They're about an inch long and are on the decaying leaves of the plants. However, I think that I'll pass on wading in to 'bait' them out. Maybe a chicken leg might work :) I'm sure they must have come in on a plant but living in the desert I never even suspected that would happen.

Nancy

I checked with my daughter (the biologist) and she says that the detritus feeding leeches may get on you but "they won't eat you" (that doesn't really reassure me) and that they are really a good thing because they break down the dead leaves, etc so that the bacteria can work on it faster.

I know that the ones in my first pond never got on me even though I always hosed them out of the filters when I cleaned them. They didn't bother me until she told me what they were.

Kate says that for the other leeches you need to use liver and don't leave it in too long or the leeches with get full and let go.

sam

Thanks Sam, and thank your daughter for me. I have to say that the more I'm learning, the less 'tranquil' my pond is becoming :-( This has been the year that my yard has been invaded by every imaginable beastie so I'll now add leeches to the list.

Nancy

From all I've read about leeches (after a friend "discovered" a few little ones on his feet after being in the river), they are more a nuisance than anything else. They do not carry diseases, in fact, some people still use them for bloodletting of the feet (in the case of diabetes) but if you have had to pick some off after walking in a muddy area of a river/pond, it would be good to smear some antibacterial on the affected areas, as a precaution. I think of little leeches like mosquitoes that are blood-feeders, except that mosquitoes DO carry diseases!!!