Environmental Health
Surface Water (Lakes, Rivers & Streams)
Swimming in Thurston County

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Leeches are native residents in our local lakes.

Photo of 2 leechesOften called bloodsuckers, they are flattened worms, and are an important part of the natural food web in lakes. Most species of leech feed on worms, snails and insect larvae. Others species prefer vertebrate hosts such as fish, reptiles and mammals. Leeches, in turn, are a food source for vertebrates such as fish, ducks, and turtles. Leeches are typically found in shallow, protected water, among aquatic plants or under stones, logs and other debris. In Pacific Northwest lakes, when leeches attach themselves a human, it is usually because the person happened to be wading or swimming in the leechesí natural home in the shallow area along the edge of the lake.

Summertime means more leeches.

Several bathers in a lakeLeeches reproduce in the spring. The young leeches are out of their cocoons several weeks later, just in time for swimming season! While generally nocturnal creatures, leeches are attracted to water disturbance like that created by swimming and wading. Leeches prefer the shallow, protected areas of lakes. They also prefer areas with aquatic weeds, submerged branches, or other debris on which to attach themselves or to hide. So swimming in deeper waters and in areas free of plants and debris will reduce the likelihood of a leech finding you.

If you find a leech on your skin after swimming or wading, donít pull it off!

Photo of a leech on skinThe mouthparts of the leech could be left in the skin and cause infection. Using an irritant, such as salt or heat, will make the leech let go. Be sure to clean, disinfect and bandage leech bites to prevent infection as you would any other cut. A leech bite may ooze for several hours when the leech is removed. This is caused by compounds present in leech saliva that prevent blood from clotting. There may also be irritation or itching after a bite, similar to the allergic reaction some people have to mosquito bites. If the wound doesnít heal properly, contact your doctor. Leeches in our region are not known to transmit human diseases, and are generally not a public health concern.

Can you get rid of leeches in your swimming area?

There are no chemical control measures that will effectively reduce leech populations without causing harm to other beneficial aquatic animals including fish. Bait trapping can be successful in controlling leeches. Using a metal can with a plastic lid (a one pound coffee can) drilled with small holes and baited with raw meat may trap large numbers of leeches from a heavily infested area. After feeding, the leeches will have difficulty leaving the can. Destroy the contents of the can daily will help reduce the size of the leech population. Because leeches like to conceal themselves under sticks, stones and other debris, keeping swimming areas free of such material is another way to help reduce the human/leech encounter.

Resources & Other Helpful Information

  • Saltwater Beaches in Thurston County (WA Dept of Ecology)
  • County database with water quality data, continuous stream flow records, lake level data, and precipitation records, also see Annual Monitoring Reports - County reports with water quality data, continuous stream flow records, lake level data, and precipitation records.
  • Report algae blooms or swimming-related illness
  • General Questions
This page last updated: 04/05/18