Hi Abigail,

Thanks for contacting the UNH Extension infoline about Leeches.  I'll share some information with you but first, the general answer is there is no safe way to get rid of Leeches in ponds and lakes.  I just learned this myself too by doing some research for you.  Unfortunately they are creepy for some folks but they are a normal part of our ecosystem.  Some will say there might be a chemical control for them but unfortunately it is not safe to the ponds ecosystem and fish, amphibians etc. can be harmed.  

Perhaps - and this may not at all be what you want to hear- you could use it as an opportunity to learn with your kids about all the many kinds of wildlife that live in the waters we like to frequent during the hot and fun days of summer.  Snails, fish, frogs and more... Could be fun...??  Well, up to you of course:)                                                        The leeches are closest to the worm family of animals and there are many different kinds, only a few of which live in our nearby waters, and only then, a few who are bloodsuckers.  They are not considered dangerous to humans and mostly feed on snails, worms and other pond life.  

However, I realize it can be alarming to come out of the water and find one on you.  Here are some safe tips on how to remove them; it is important not to leave the head behind which can happen if you just pull it off.  

"Here’s what to do if a leech attaches to you. If a leech attaches to you, it is probably because you happened to be wading or swimming in a shallow area where it was hiding. If you find a leech on your skin don’t pull it off!  The mouthparts of the leech could be left in your skin and could cause an infection. You might want to use an irritant, such as salt or heat (aim a hair dryer at it), to encourage the leech to voluntarily let go of you. After the leech is gone, clean, disinfect, and bandage the bite to prevent infection.  If the wound doesn’t heal properly, contact your doctor."  I believe the mention of an infection is simple due to any opening of the skin can invite that in.  The animals themselves don't carry anything.

This is a link to the article I got this quote from..  It has some good info on Leeches in general and how to avoid them when swimming: https://nhlakes.org/wp-content/uploads/4-Lake-Ecology-Native-Species-Leeches-NH-LAKES.pdf  and another from the US government,    https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/2020-01/bb-10.pdf5

And finally this paragraph from an article on ponds via Ohio State University, interesting because it talks about the balance of fish, especially Bluegills, which naturally help keep the leech population down : "Many a pond swimmer has come out of a pond and been taken aback with horror at the sight of leeches hanging onto their skin.  Quite frequently, that’s the end of swimming in that pond. Leeches conjure up visions of giant blood-sucking monsters or the transfer of hideous diseases to the poor victim. 

Let’s set the record straight about leeches in ponds.  Almost all ponds have leeches in them, they are part of the pond ecosystem. They attach to all kinds of pond animals, from aquatic insects to fish, and yes humans. Their abundance is largely determined by several factors. Ponds with good populations of bluegills and small bass tend to have far fewer leeches in them. Bluegills are quickly attracted to anything that wiggles along the bottom or on vegetation, and will devour a leech quite quickly. Excessive vegetation and decaying organic debris can lead to dense populations of leeches. Part of this population explosion increase is that dense vegetation inhibits the ability of bluegills to find and eat the leeches.
Do leeches transfer diseases to humans? No, I know of no instance where a leech has transferred some disease to humans.  That does not mean they cannot cause a human some discomfort.  In some folks, the attachment site can be an area of slight to intense itching. In extreme cases, the ―wound‖ can become infected and require medical attention. Like any other wound, a leech attachment site should be cleaned with a mild soap and
treated with an antibacterial, over-the-counter cream.
So what can be done with a pond full of leeches? The only approved product is copper sulfate pentahydrate which is the same copper compound used to control many nuisance algae problems. The recommended dosage to kill leeches is 5 ppm, about three times the level used to kill algae. The problem with applying 5 ppm of copper sulfate to a pond is it will kill everything else, including the fish. Thus, this strategy is only recommended only for fish-less ponds.  The best leech control strategy is to maintain good populations of largemouth bass and bluegills, whose predation on leeches should keep their populations low. Combining that with a good aquatic plant management strategy is wise, since excessive vegetation is conducive to high leech abundance. Strive to maintain plant abundance at about 10% of the pond’s surface area.

Wishing you and your kids a good and fun rest of the summer with happy memories of swimming :)

Catherine