Dermatitis

Swimmer's itch is a skin reaction to parasite larvae present in certain freshwater bodies, especially where snails and waterfowl live. It causes itchy bumps that usually last a few days. It is not contagious. It is unpleasant but generally mild, and a doctor remains the best person to consult if in doubt.

What is it?

Dermatitis refers to skin inflammation, which can have many causes. The one discussed here has several names: swimmer's itch, cercarial dermatitis, or "duck itch" in Geneva. It is unrelated to eczema or water pollution from chemicals.

The culprit is a microscopic larva called a cercaria. Its normal host is a waterfowl, with a snail serving as an intermediate host. If it comes into contact with a swimmer, it can penetrate the skin and then quickly die. The reaction is caused by our body, not by the larva developing inside us.

Our article too many snails in my pond: are they harmful or dangerous? already covered this point. This article expands on it and explains what can be done.

Where and when it appears

Cases involve popular lakes, ponds, and freshwater bodies. The Geneva cantonal health service cites water temperatures above 20°C, the presence of aquatic plants, and ducks as favourable conditions. It adds that this is not an absolute rule, as cases also occur at temperatures below 20°C.

The ARS Normandie, for example, reported cases in a swimming pond in Normandy. The risk therefore varies depending on the location and season, and clear water is not a reliable indicator. HealthLinkBC advises checking water quality and looking for warning signs.

Recognizing the signs

HealthLinkBC describes an initial tingling sensation, followed by small red spots that enlarge within a few hours, with a rash that may appear within 12 hours. In Geneva, they are compared to insect bites. The ARS Normandie mentions spots and bumps at the entry points, and sometimes swelling, fever, or marked fatigue.

The duration varies according to sources: about 5 days according to the ARS Normandie, 5 to 6 days according to King County, 2 to 5 days for the reaction according to HealthLinkBC, with symptoms lasting up to 2 weeks. In rare cases, Geneva reports up to one month. Repeated exposure generally causes a faster and stronger reaction.

Limiting the risk

No measure is completely effective except avoiding the body of water. HealthLinkBC states this clearly, and Geneva specifies that no preventive measure or repellent cream is recognized as truly effective.

The action mentioned by almost every source is drying off thoroughly upon exiting the water. The ARS Normandie and Geneva also recommend showering. However, HealthLinkBC reminds us that showering does not remove larvae that have already entered the skin, so it is a complementary measure that should be done quickly.

Opinions differ on water-resistant sunscreen. King County indicates that it may offer some protection, but no more. Do not rely on it.

Do not feed ducks and geese: both the ARS Normandie and King County advise against it. If you manage a swimming area, report cases to your local municipality or competent local authority so they can assess the situation.

And in a garden pond?

We have found no statistical data on private ponds. The reasoning remains simple: the parasite needs both waterfowl and host snails. A pond where no one swims therefore poses little risk. However, a natural swimming area visited by ducks warrants the same precautions as a lake.

• Do not feed waterfowl or attract ducks to the swimming area. See also plants for a pond with ducks.
• Plan to enter the water via a ladder or pontoon rather than through the plants, as HealthLinkBC advises.
• Keep a towel close at hand.
• Do not try to remove all snails: they are part of the pond's balance, and sudden disruptions create other problems.

This article does not offer any products, as we do not know of any that address this issue. If you are unsure about designing your swimming area, we can discuss it.

Frequently asked questions

What is swimmer's itch?

It is an allergic skin reaction to parasite larvae present in certain freshwater bodies, called cercariae. It causes itchy red bumps.

Is swimmer's itch contagious?

No. King County and HealthLinkBC state that it is not transmitted from person to person.

How long do the bumps last?

Sources most often cite a few days, with about 5 days according to the ARS Normandie. HealthLinkBC mentions symptoms that can last up to 2 weeks. If symptoms worsen or persist, consult a doctor.

Can you catch it in a garden pond?

The parasite needs waterfowl and host snails. A pond where no one swims poses little risk. In a swimming pond visited by ducks, take the same precautions as you would at a lake.

What pond product prevents swimmer's itch?

We do not know of any and do not offer any. The sources consulted speak of prevention through actions, not through water treatment.

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Last updated: November 4, 2026.