Cercarial Dermatitis or Swimmer’s Itch – Risk and Causes

What is Cercarial Dermatitis?

Cercarial dermatitis, also known as swimmer’s itch, is an itchy rash caused by a tiny parasitic worm. It’s contracted by swimming or wading in infested fresh water lakes or ponds. The parasite’s usual hosts are waterfowl and rodents. After the parasite is excreted from the waterfowl or rodent, it then enters a snail. After further development, it leaves the snail and comes into contact with human skin. It can’t enter your bloodstream or deeper tissues, but it can cause an uncomfortable, itchy rash when it burrows into your skin.

The rash begins to itch and appears while one is still in the water. After a few hours, the itching and rash disappears. However, about 10–15 hours after the initial rash the papules and itch return. The rash appears as small, itchy red bumps that can turn into blisters. It usually clears up within a week.


Cercarial dermatitis occurs when free-swimming cercariae penetrate the skin of humans (incidental host) and cause an allergic skin reaction (types I and IV hypersensitivity). The life cycle begins as cercariae infect birds (definitive host), maturing into adult worms within blood vessels. Adult worms then produce eggs that are passed in avian feces.

The eggs hatch and liberate ciliated miracidia into the water, which infect snails (intermediate host). Miracidia mature within snails and produce free-swimming cercariae, which either reinfect birds or infect humans. Cercarie die upon penetration into human skin, thus halting the schistosome life cycle.

Causes of cercarial dermatitis

Swimmer’s itch or Cercarial Dermatitis is a disease of aquatic birds and humans are accidentally affected.

Schistosomes are parasitic flatworms with a lifecycle that involves aquatic birds (eg, ducks, geese, gulls, swans) or mammals (eg, beavers, muskrats), specific species of aquatic snails, and warm fresh or salt water.

The parasites that cause swimmer’s itch live in the blood of waterfowl and in mammals that live near ponds and lakes. Examples include:

  • Geese
  • Ducks
  • Gulls
  • Beavers
  • Muskrats

The parasite’s eggs enter the water via their hosts’ feces. Before infecting birds, other animals or people, the hatched parasites must live for a time within a type of snail. These snails live near the shoreline, which explains why infections occur most often in shallow water.

Swimmer’s itch isn’t contagious from person to person, so you don’t need to worry about catching cercarial dermatitis from someone who has this itchy rash.

Who is at Risk?

  • Anyone can get cercarial dermatitis.
  • Children are more likely to get swimmer’s itch than adults because kids spend more time in shallow waters where the parasite may be present, and are less likely to towel dry.
  • Cercarial dermatitis is not contagious; it cannot be passed from person to person.

What are the symptoms of swimmer’s itch (cercarial dermatitis)?

You can develop swimmer’s itch within minutes or days of swimming in infested water.

Symptoms of swimmer’s itch (cercarial dermatitis) include:

  • Tingling, burning or itchy skin
  • Small, reddish pimples or bumps that form a rash
  • Small blisters that form a rash

What are the complications of swimmer’s itch?

  • Episodes become more severe with repeated exposures
  • Secondary bacterial infection following scratching

How is cercarial dermatitis diagnosed?

There are several types of skin reactions similar to cercarial dermatitis, which can make it hard to tell if your condition is a cercarial dermatitis. For example, insect bites, jellyfish stings, or bacterial infections may cause similar symptoms on your skin. There are no specific tests for cercarial dermatitis. To diagnose swimmer’s itch, the doctor may ask you specific diagnostic questions to identify the allergy. Your doctor will want to know:

When you first started feeling the itchy sensation

If you’ve had a swimming session with the last 24 hours

If other people who were exposed to the water developed similar symptoms

Your doctor may also ask about your medical history, notable allergies, and whether you’re taking any special medications or supplements.

Treatment for cercarial dermatitis

The rash will go away in a few weeks to a month. Make sure to prevent infection by not scratching it. You can treat itching with any of these:

  • Cool, moist compress. This is a clean damp cloth. Use this on the area for 20 to 30 minutes, 5 to 6 times a day as needed.
  • Corticosteroid cream or ointment. You can apply this medicine several times a day on clean skin. This medicine can help reduce itching. You can put it on your skin as a cream, or take it by mouth as a pill.
  • Colloidal oatmeal bath. Soaking in water with colloidal oatmeal can help soothe itchy skin.
  • Baking soda paste. This can help relieve itching. Mix baking soda with water into a paste. Put it on your rash.
  • Other anti-itch lotion or cream. Ask your healthcare provider about other anti-itch lotion or cream that can help relieve itching. He or she may prescribe a stronger medicine if drugstore medicine isn’t helping you.

The best way to avoid getting the rash is to stay out of lakes or bodies of water known to be infested.

Lifestyle and home remedies

These tips might help reduce the itch:

  • Apply a cream or medication
  • Don’t scratch
  • Cover affected areas with a clean, wet washcloth
  • Soak in a bath sprinkled with Epsom salts, baking soda or oatmeal
  • Make a paste of baking soda and water, and then apply it to the affected areas

Prevention of cercarial dermatitis

The parasites that cause cercarial dermatitis live in the blood of waterfowl and in mammals that live near ponds and lakes. To reduce the risk of cercarial dermatitis:

  • Choose swimming spots carefully. Avoid swimming in areas where cercarial dermatitis is a known problem or signs warn of possible contamination. Also avoid swimming or wading in marshy areas where snails are commonly found.
  • Avoid the shoreline, if possible. If you’re a strong swimmer, head to deeper water for your swim. You may be more likely to develop swimmer’s itch if you spend a lot of time in warmer water near the shore.
  • Rinse after swimming. Rinse exposed skin with clean water immediately after leaving the water, then vigorously dry your skin with a towel. Launder your swimsuits often.
  • Skip the bread crumbs. Don’t feed birds on docks or near swimming areas.
  • Apply waterproof sunscreen. This has been reported to protect the skin from the parasite that causes cercarial dermatitis.

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