Swimming Illnesses and Risks

There are risks of disease transmission when swimming in lakes and streams. Here are steps you can take to reduce the risk. 

  • Shower before you swim, if possible.
  • Do not swim, or allow family members to swim, if currently ill with vomiting or diarrhea, or have been ill within the last two days.
  • Keep children who are not toilet trained out of the water.
  • Do not allow children with dirty diapers in the water.
  • Do not swim if you have a skin infection.
  • Keep pets off the beach and out of the swimming areas.

Keep informed and practice these risk-reduction techniques, then you can relax and enjoy the summer at your favorite swimming beach. Please share this with others, so they, too, can enjoy their swimming experience.

Swimmer's Itch, Leeches, and Weird Things in the Water

Swimmer's Itch

  • Where do the parasites come from?

    The adult parasite exists in the blood of infected water fowl, such as ducks, geese, swans and gulls and in aquatic animals such as beaver and muskrat. The eggs produced by the adult parasite develop in the intestinal tract of its host and are excreted in the feces of the bird or animal into the water. These eggs can hatch in the water, releasing small free-swimming larvae which swim in search of its second host, a certain species of aquatic snail. The larvae infect the snail, multiply and develop into a different type of larvae. This larval form then swims about searching for a suitable host (bird or aquatic animal) to continue the life cycle. There is no way to know how long the water may be unsafe. Larvae generally survive for 24 hours once they are released from the snail. However, an infected snail will continue to produce cercariae throughout the remainder of its life.
  • Who is at risk of Swimmer's Itch?

    Anyone who swims or wades in infested waters may be at risk. The larvae are more likely to be present in shallow water by the shoreline, therefore children are most often affected because they tend to swim, play, and wade in shallow water more than adults.


bryozoa_sm.jpg      freshwater_bryozoan.jpg

  • What are they?

    While these strange, jelly-like, blobs pose no threat to lake users, they may cause a fright if you are unfamiliar with them. Bryozoans are aquatic animals with a name that literally means "moss animals".

    Bryozoa are colonies of tiny colonial animals called zoids and appear as gelatinous globs up to the size of a football. Most species are marine animals, however there is one class, Phylactolaemata, that live exclusively in freshwater. They are often attached to submerged surfaces such as tree branches, roots, rocks, pilings, docks, etc. Sometimes, a clump that has broken loose can be found free-floating or washed up near the shoreline.

  • Water Quality Indicator

    Bryozoans filter water for their food like sponges and feed on small micro-organisms such as diatoms and other unicellular algae. Each zoid in the colony has whorls of delicate feeding tentacles swaying slowly in the water catching food. Some freshwater varieties are thought to be useful indicators of water quality. It is said that they like water that is eutrophic, which means very productive lakes with lots of food.

  • Bryozoa Reproduction

    Bryozoans reproduce by more than one method. When you look at them, there is a large number of tiny black dots visible imbedded in the jelly. Each of these tiny black dots becomes a statoblast. A statoblast is a cyst, similar to egg or spore. At the end of summer, as the lake water cools, the bryozoan dies. The jelly dissolves and releases the statoblasts; when released they look like small, dark brown disks with radiating barbed spikes.  


  • Leeches are native residents in our local lakes.

    Often 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 leeches’ natural home in the shallow area along the edge of the lake.
  • Summertime means more leeches.

    Leeches 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, don't pull it off!

    The 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 doesn’t heal properly, contact your doctor. Leeches in our region are not known to transmit human diseases, and are generally not a public health concern.


  • Swimmer's Itch Fact Sheet (CDC)

  • Annual Monitoring Reports
    These reports are prepared by Thurston County Public Health and Social Services Environmental Health and Thurston County Community Planning and Economic Development Water Planning. The reports are made in cooperation with the cities of Olympia, Lacey, and Tumwater, and the Washington State Department of Ecology.

Contact Us

​Social Media

Follow us on social media