Swimmer’s Itch (Cercarial Dermatitis)

Have you been affected by swimmer’s itch? Head to www.swimmersitch.ca to report your infection and support freshwater research in Alberta!


What is swimmer’s itch?


Swimmer’s itch (also known as duck itch, lake itch, or cercarial dermatitis), is a temporary skin rash that some people develop after swimming in water infested with microscopic parasites called schistosomes. These parasites normally live in the bloodstream of certain birds and mammals, as well as in the bodies of some species of water snails. Infected snails will release a free-swimming larval form of the parasite into the water, and if they come into contact with a swimmer, they will try to burrow into that person’s skin. Fortunately, humans are not suitable hosts for the parasite and the larvae die shortly after penetrating the outer layer of skin. This causes an allergic reaction to occur which leads to the formation of an itchy bump. Swimmer’s itch is found throughout the world, in both freshwater and salt water, and is more frequent during the summer months.


Although uncomfortable, swimmer’s itch is not considered dangerous and is usually short lived- the reaction typically clears up on its own within a week or two. In the meantime, you can control the itching with over-the-counter or prescription medication.

How does the water become infested with these parasites?


The adult parasite lives in the blood of infected birds such as geese, ducks, gulls, swans, and certain mammals like muskrats and raccoons. The adult parasite will produce eggs that are then passed in the feces of the infected birds or mammals (1).

If the eggs land in or are washed into the water, the eggs will hatch, releasing small, free-swimming larvae called miracidia (2). These larvae swim in the water in search of certain species of aquatic snails.

If the miracidia find one of these snails, they will infect the snail, multiply, and undergo further development. Under optimal conditions, infected snails will then release a different type of microscopic larvae, known as cercariae (3), into the water. These free-swimming larvae will then search out a bird or mammal host to infect in order to complete their lifecycle (4).


magnified image of a cercariae under the microscope


Each dot represents one cercariae released from the snail in the top left

Swimmer’s itch occurs when these cercariae encounter a human (4), and accidentally penetrate the human skin instead of their natural hosts such as waterfowl and aquatic mammals. Because humans are not a proper host, the parasites cannot develop, and quickly die. This results in an allergic reaction and the development of an itchy rash on the skin.

When and where are these parasites found?

The parasites responsible for swimmer’s itch can be found in many lakes, ponds, and reservoirs throughout Alberta. Most cases are usually reported during the warm summer months (i.e. June-September) when beach use is at its heaviest. The highest incidence of infection appears to be late in the day, after a period of sustained wind.


The larvae are most likely to be present in shallow water near the shoreline and close to aquatic vegetation (i.e. the same areas snails are often found). The larvae tend to concentrate near the surface of the water where they have the greatest chance of coming into contact with birds and mammals.

Note: Wind can push the tiny parasites to all edges of the lake, often some distance from the snails the larvae came from.

Health effects associated with swimmer’s itch

What are the Symptoms?

Symptoms can range from mild skin irritation to a severe, itchy red rash depending on how sensitive an individual is to the parasites and how many larvae actually get into the skin. The reaction, which looks like reddish pimples or blisters, may appear on exposed skin (i.e. skin not covered by a swimsuit, wetsuit, etc.) within minutes to days after swimming or wading in infested water.

swimmers-itch-pictures   photo (2)

Typically, as you begin to dry off and the larvae start to burrow into the skin, you may notice a tingling sensation develop on exposed parts of the body. Small, pin sized red spots/pimples will then develop within 12 hours anywhere the larvae have penetrated the skin. Eventually, the tingling sensation disappears, and the red spots enlarge, resulting in small itchy blisters, similar to an insect bite. This may form a larger, itchy rash which can persist for days to weeks, but should clear up on its own. Although it might be difficult, it is important not to scratch the itchy area too much as this may result in secondary bacterial infections.


The degree of discomfort will vary for each person depending on the sensitivity of the individual to the parasite, the severity of the infestation, and prior exposure. Because swimmer’s itch is caused by an allergic reaction to the parasites, the more often you swim or wade in infested water, the more intense and immediate the symptoms will be.


Important Note: Be aware that swimmer’s itch is not the only rash that may occur after swimming in fresh or salt water.

See: cyanobacteria (blue-green algae)

How long do the symptoms last?

Since the larvae die shortly after entering the skin, the infection is considered self-limiting, lasting on average 2 to 5 days. However, the symptoms associated with infection (i.e. the burning or itchy feeling) can persist for up to 2 weeks.

Who is at risk of getting swimmer’s itch?

Anyone who swims or wades in infested water may be at risk of getting swimmer’s itch, and the more time you spend in that water, the higher your risk becomes.

Pigeon-Lake,-ABChildren are usually more affected by swimmer’s itch than adults. This is because children tend to have more sensitive skin, are more likely to swim and play in areas with high concentrations of larvae (i.e. shallow water, close to the shore), and are less likely to dry themselves off with a towel immediately after leaving the water.

Is swimmer’s itch dangerous?

No. The rash and itch are mostly just irritating, especially for younger children. Normally there is no serious risk of danger associated with infection because the parasite cannot enter the blood and the rash does not spread from the initial points of contact with the larvae. The infection is also self-limiting, which means it will usually clear up on its own in time.

It should be noted, however, that there is a risk of developing a secondary bacterial infection in the skin. Scratching the area too much may result in the rash becoming infected; if this occurs, you should contact your health care provider right away.

Call your doctor if you still have the rash after two weeks or if you have signs of infection, such as:

  • Increased pain, swelling, warmth, or redness
  • Red streaks leading from the area
  • Pus draining from the area
  • A fever

Can swimmer’s itch be spread from person-to-person?

No. Swimmer’s itch is not contagious which means it cannot be spread from one person to another.

What can I do if a rash develops?

There are a number of non-prescription options you can take at home, which may reduce some of the itchiness associated with swimmer’s itch. Common treatments include:

  • Applying anti-itch lotion (e.g. Calamine lotion) on the affected area; these can be purchased without a prescription from your local grocery or drugstore, and help to sooth some of the itch.
  • Applying a cool compress on the affected skin
  • Taking shallow, lukewarm baths, with 3 tablespoons of baking soda added to the water or applying a baking soda paste to the rash (which is made by stirring water into baking soda until it reaches a paste like consistency)
  • Bathe in Epsom salts
  • Soak in colloidal oatmeal baths
  • Taking antihistamines

Remember to avoid scratching as much as possible, as this may cause the rash to become infected. If itching is severe, your health care provider may suggest prescription strength lotions or creams to lessen the symptoms.

Call your doctor if you still have a rash after two weeks or if you have signs of a bacterial skin infection.

Recreational Water Use

Once an outbreak of swimmer’s itch has occurred in the water, will the lake always be unsafe?

No. Many factors must be present for swimmer’s itch to become a problem in water. Since these factors can change over time (sometimes within one season), swimmer’s itch will not always be a problem. However, there is no way to know how long water will be unsafe. Larvae generally survive 24 hours in the water once released from the snail, and an infected snail will continue to shed cercariae throughout the remainder of its life. For future snails to become infected, migratory birds or mammals in the area must also be infected so the lifecycle can continue.

images (1) snail_great_pond_lymnaea_stagnalis_09-06-11_1

Is it safe to swim in an outdoor swimming pool?

Yes. As long as the swimming pool is well maintained and chlorinated, there is no risk of swimmer’s itch. The appropriate snails must be present for swimmers itch to occur.


How can I avoid swimmer’s itch?

Unfortunately, there is no sure way to avoid swimmer’s itch entirely, unless you avoid lakes, ponds, and beaches all together. There is however certain precautions you can take to reduce the risk, including:

  • Avoiding areas known to be infested:
    • Speak with other visitors in the area, local health officials, or parks representatives about water conditions before going into the water.
    • You can also check for warning signs at public beaches, lakes and picnic areas, however, not all beaches will have signage for swimmer’s itch. Or check online, there are various websites including swimmersitch.ca which posts recent reports of swimmer’s itch for various lakes across Canada
    • Avoid areas with dense aquatic vegetation, as this is where snails are most likely to be found
    • Try to enter the water from a dock or boat, rather than wading in from the beach, since the larvae tend to concentrate near the shoreline.


  • Toweling down:
    • Drying yourself off with a towel as soon as you come out of the water may remove the parasites before they can penetrate the skin.
  • Showering:
    • Taking a shower immediately after leaving the water will not remove any larvae that have already burrowed into the skin, but it may limit the severity of the infection by washing away any parasites still on the surface.
  • Applying waterproof sunscreen:
    • It has been suggested that applying waterproof sunscreen before entering the water may help to reduce the number of larvae that penetrate the skin.

Can Swimmer’s itch be controlled at the Source?

It was commonly believed that chemical treatment of snail populations would break the life cycle of the parasite associated with swimmer’s itch. However, such actions can result in ecological damage and are generally not permitted in public water. Furthermore, swimmer’s itch is caused by the free swimming larval stage, which can be blown or carried many hundreds of meters, perhaps kilometers, from the snail source, which makes chemical control of limited value.

Where to get more information on swimmer’s itch:


  • This site provides a brief overview of the health risks associated with swimmer’s itch.


  • This site provides information about what swimmer’s itch is and where it has been found in Canada. There is also a section where you can report your itch.


  • This site describes in detail what causes swimmer’s itch and answers common questions relating to the health effects of swimmer’s itch.

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