Swimmer itch (or swimmer's itch) is a patchy, red rash that develops when the skin comes into contact with fresh or salt water contaminated with parasites. It often appears within 48 hours of exposure to the water and may last up to a week. It is also referred to by several other names, including “duckworms,” “duck fleas,” “rice paddy itch” or “clam digger's itch.”

Although there are several parasites that can cause reactions after swimming, swimmer's itch usually refers to infestation with trematode parasites. These parasites are found in infected snails, which shed larvae that burrow into certain animals, such as waterfowl, muskrats and beavers, and live in their bloodstream. The host animals shed a different form of larvae in their feces. If these larvae land in water, they swim until they find a suitable snail and infect that snail, then the life cycle starts over again. Humans are not suitable host animals, so the larvae die after they burrow into human skin. This causes the allergic reaction that is known as swimmer's itch.

Swimmer itch typically produces a mild reaction in which the skin becomes red and raised. Itchy areas, sometimes resembling small pimples or blisters, appear. The skin may also tingle or burn. In most cases, only skin not covered by swimsuits, wet suits or waders is affected.

Symptoms associated with swimmer itch normally disappear without treatment. In some cases, patients may use creams and ointments to treat symptoms associated with this condition.

Swimmer itch (or swimmer's itch) is a patchy, red skin rash that causes itching on the parts of the body that have been in the water. Also known as cercarial dermatitis, it often appears within 48 hours of exposure to the water and may last up to a week. Symptoms normally disappear without treatment. It is also referred to by several other names, including “duckworms,” “duck fleas,” “rice paddy itch” or “clam digger's itch.”

Swimmer itch can result from contact with fresh or salt water. It is caused by the presence of trematode parasites. Trematode parasites are released into the water by both snails and certain birds and mammals, which serve as host animals. Trematode larvae cannot live in the human bloodstream, so the parasites die soon after burrowing into human skin. Nevertheless, they may cause an allergic reaction, which results in swimmer's itch. The condition is not contagious.

Swimmer itch is found throughout the world, and is more common during the summer months because of three reasons:

• Water warms to a temperature conducive for snails to grow and reproduce rapidly.
  
  
• Migrating birds infected with the trematode parasite return from their winter habitats to their summer homes. In addition, other aquatic birds return to full activity.
  
  
• Swimmers and bathers are present in the waters in greater numbers than at any other time of the year.

Children are at especially high risk for swimmer itch. The parasite larvae that spread the disease are most often found near the shoreline in shallow water, where children tend to swim. In addition, children are less likely to towel dry themselves after leaving the water.

In tropical countries, parasites that cause swimmer itch sometimes cause severe disease when found in contaminated water. These parasites are not found in the United States.

Not all rashes that result after swimming are swimmer itch. Many other types of rashes can result from contact with fresh or salt water.  

There are many kinds of waterborne parasites that might cause reactions. These include jellyfish blastocytes, which cause a condition known as "sea lice" and mature jellyfish. However, swimmer's itch usually refers to a reaction caused by trematode parasites, which can be found in fresh and salt water. These parasites are found in infected snails. In certain conditions – chiefly on warm, sunny days on calm, freshwater bodies such as lakes, but also in ocean water – the water warms to a temperature conducive for snails to grow and reproduce rapidly.

As this occurs, the snails release a type of larvae parasite known as cercariae into the water. These parasites find their way into the blood of aquatic and migrating birds (e.g., ducks, geese, gulls, swans) and mammals (e.g., muskrats and beavers), where the adult parasites live out their life cycle.

Once the birds and mammals become infected, the eggs of the parasite are passed into the water through the animals’ feces. The eggs contaminate the water when they hatch, releasing free-swimming larvae. The larvae search for a species of aquatic snail and infect the snail, and the life cycle begins over again. 

People who swim in this water may come into contact with the parasites. Soon after the parasites enter the superficial layers of human skin, they die, causing an allergic skin reaction known as swimmer itch.

The conditions that create swimmer itch are not always present. Once released into the water, larvae tend to live for about 24 hours without a host before dying. However, infected snails continue to produce the cercariae larvae throughout their lifetimes.

In some cases, exposure to algae, chemical pollutants or sewage may cause a form of dermatitis that is virtually indistinguishable from swimmer itch.

Swimmer itch produces a mild reaction in which the skin becomes red and raised. Itchy areas, sometimes resembling small pimples or blisters, appear. The skin may also tingle or burn. In most cases, only areas of skin not covered by swimsuits, wet suits or waders are affected.

Symptoms of burning, tingling or itching can appear within minutes to days of exposure, while pimples develop about 12 hours later and eventually turn into small blisters. Symptoms tend to disappear within a few days to a week of exposure. Patients who scratch excessively may cause a secondary bacterial infection.

The reaction associated with swimmer itch is allergic in nature. As a result, any subsequent exposures to swimmer itch can cause more significant symptoms than the initial case. Skin reaction may be more severe and persistent, including blistering of the affected skin. Symptoms may also develop more quickly with subsequent exposures.

Swimmer itch can be difficult to diagnose. A physician will perform a complete physical examination and compile a thorough medical history. However, skin biopsies and blood tests are not helpful in diagnosing the condition. Swimmer itch often resembles other skin conditions, including:

• Impetigo (bacterial infection of the skin)
  
• Chickenpox
  
• Poison ivy rash
  
• Herpes simplex viruses

A physician is most likely to diagnose swimmer itch based on a patient history of swimming in an area where swimmer itch commonly occurs, with symptoms following shortly after.

In most cases, swimmer itch does not require medical treatment. The condition usually resolves on its own over time. However, over-the-counter anti-itch creams or ointments may be used to treat the symptoms of itch or irritation. Other methods used to treat symptoms include:

• Oral antihistamines
  
• Corticosteroid creams
  
• Cool compresses
  
• Bathing in Epsom salts or baking soda
  
• Soaking in oatmeal baths

Prescription-strength lotions and creams and oral corticosteroids also are available to treat more severe cases of swimmer itch. Patients should not use any over-the-counter or prescription medication without first consulting a physician.

Although the itching associated with swimmer itch can be severe, affected individuals should avoid scratching. Scratching can cause abrasions and lead to infection. Patients who develop secondary bacterial infections from scratching can be treated with antibiotics.  

There are several steps that people can take to reduce their risk of developing swimmer itch. These include:

• Avoid swimming in bodies of water that have been identified as problem spots for swimmer itch. Avoiding the water is the most effective prevention method. Some areas post signs on the shoreline warning of unsafe water. Individuals who choose to enter the water should swim from a raft or boat farther out from the shore when possible. Since parasite larvae are especially likely to be present in shallow water by the shoreline, those who swim at a distance far from the shore are less likely to come into contact with the larvae.
  
  
• Do not swim or wade in marshy areas where snails are often found
  
  
• Rinse exposed skin with fresh water after swimming or wading. Vigorously dry the skin with a towel.
  
  
• Do not attract birds to areas – such as by feeding them – where people are known to swim.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to swimmer itch:

1. Do my symptoms suggest swimmer itch?
   
   
2. What may have caused me to develop swimmer itch?
   
   
3. Is swimmer itch dangerous to my overall health?
   
   
4. What are my treatment options?
   
   
5. When can I expect my symptoms to subside?
   
   
6. Am I more likely to develop swimmer itch at certain times of year?
   
   
7. How can I prevent swimmer itch?
   
   
8. Will my symptoms be the same if I develop swimmer itch again in the future?