Cercariae – worms in lakes spoil the pleasure of swimming

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Written By Rivera Claudia

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A day of swimming in a lake can also lead to sleepless nights – especially if there are cercariae, worm larvae, in the lake. When the water temperature in a lake exceeds 20 degrees, they multiply and look for a host to reproduce.

If you’ve been chosen as a host, it can cause unbearable itching, so much so that bed rest is no longer necessary. If you experience symptoms such as itching, red spots, and mosquito-bite-like welts and papules after swimming in the lake, this could be a sign of cercaria infection. A dermatologist will diagnose what is known as bath dermatitis.

What are cercariae?

Cercariae are microscopic larvae of worms – the so-called trematodes. The parasites swim freely in warm water – preferably in the flatter peripheral areas, as temperatures there reach a comfortable 20 degrees or more.

In fact, cercariae are specialized for aquatic birds. They pierce the skin to develop further. Their eggs are then excreted by the birds in their feces. These baby parasites seek out a freshwater snail as an intermediate host to mature there and start the cycle all over again. They swarm and look for a new host—at best, a duck or other aquatic bird; if things go badly, a swimming human.

Are cercariae dangerous?

Yes, but only for waterfowl. For humans, an infestation is usually just unpleasant and annoying. Because our immune system does a great job – the larvae die immediately under the skin. But this causes itching and hives.

How long have you been suffering from the effects of bath dermatitis?

When the symptoms of bath dermatitis—that is, cercariae infestation—reduce varies from person to person, depending on the person’s sensitivity. It can take anywhere from a few hours to ten to 20 days. If the itching becomes unbearable, buy soothing ointments from the drugstore. If in doubt, consult a dermatologist before scratching the areas, as this can lead to inflammation.

Can cercariae infestation be prevented?

It is best to find out in advance whether a cercariae infestation has already been reported in your favorite lake. And then it only helps if you want to be on the safe side: avoid swimming or at least avoid the shore area and shallow areas. Since cercariae tend to stay in the warmer coastal areas, you can go a little further in a canoe before getting into the water. Basically, it is an individual decision whether to go swimming despite the cercariae. Some people are disgusted by the idea, but the parasites are not dangerous.

Can you reduce your risk of infection?

If you don’t want worms to stop you from taking a shower, here’s a tip: rub yourself thoroughly with a towel after bathing. This means that the larvae can come off again, can’t penetrate your skin and can’t trigger an allergic reaction.

Is the water quality of the affected lakes poor?

An infestation by cercariae says nothing about the microbiological quality of the lake water. After European Environment Agency Water Quality Report (European Environment Agency EEA) (external link), more than 90 percent of German bathing waters have good to excellent water quality. Details at Federal Environment Agency in German (external link).

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