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Have Fun In Local Lakes, But Look Out For Swimmer's Itch

Courtesy Centers For Disease Control
Swimmers itch is an immune response caused by tiny, wormlike parasites with a forked tail, called schistozomes. It can be avoided by quickly toweling off or showering after a swim in a warm lake - or possibly by swimming in deeper, cooler water.

It’s the height of summer and the forecast is looking very sunny, providing great conditions for water sports on local lakes.

That also means a higher risk of swimmer's itch, which is linked to another feature of summer that many people enjoy: the presence of waterfowl.

Swimmer's itch isn’t dangerous. It’s one of those things you just really want to avoid if you can, kind of like poison ivy.

The first thing to know is it comes from a tiny wormlike parasite that doesn’t actively seek out humans. But it does have a forked tail that it uses to burrow beneath the skin of host animals. And it thrives in warmer water.

Linda Weiford, a science writer at Washington State University, says these nearly invisible critters are called schistosomes.

“They move between a tiny snail that’s found in fresh water bodies and a host – an animal host – which would usually be a duck or a goose.” 

The snail, also known as Nassarius obsoletus, lives in the mud at the bottom of lakes. Weiford says when humans swim or wade into the midst of this – usually in shallower waters – that’s when the parasites can land on people.

“Typically what happens is as they leave the snail and are looking for their bird host, a human gets in the way,” Weiford says.

“And the parasite, not realizing that it (has landed on something that’s) not a bird, enters the skin. And at that time it’s not able to survive. A human is a dead end host.”

She says the parasites die right away, so there’s no risk of disease or illness, though infections can be caused by scratching.  

If you towel off or shower when you get out of the water, you can stop the schistosomes from burrowing into your skin. 

If they do, it generally causes the red bumps and an immune response commonly known as “swimmer’s itch.” The effects can last for days or weeks.

Interestingly, birds don’t have the same response. Weiford says they don’t even seem to notice the parasites.

So remember to dry off right away after a swim in a lake. If you’re planning to stay in the water for a while, try getting out into deeper areas, where you’ll more likely be out of the path of these tiny worms looking for waterfowl.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to