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Swimmer's itch

Authors: A/Prof Marius Rademaker, Dermatologist, Hamilton, New Zealand, 2004. Updated: Hana Numan, Medical Writer, New Zealand. Copy edited by Gus Mitchell. September 2021.


Swimmer's itch — codes and concepts
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What is swimmer’s itch?

Swimmer’s itch is an itchy skin rash caused by an allergic reaction to the larval forms of parasitic flatworms released from aquatic snails into fresh and salt water lakes, ponds, and lagoons.

Swimmer’s itch is technically called cercarial dermatitis, as it is caused by cercariae, the immature larvae of schistosomes. Many countries have their own local name(s) for swimmer’s itch including ‘duck itch’ (New Zealand), ‘duckworms’ or ‘duck fleas’ (US), ‘hoi con’ (Thailand), ‘kobanyo’ or ‘kubure’ (Japan), ‘sawah’ (Malaysia), and ‘rice paddy itch’ or ‘clam diggers itch’.

Swimmer’s itch

Who gets swimmer’s itch?

Swimmer’s itch can affect anyone swimming in waters with infested snails. Young children are particularly susceptible as they tend to remain in shallow warm water where the snails are found in the highest numbers. However, it is only after repeated exposure to cercariae that the allergy develops and a rash eventuates.

What causes swimmer’s itch?

Swimmer’s itch 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 adult worm lives in a bird/animal, and the eggs are excreted via the intestines in the faeces. An egg hatches in the water to become a free-swimming miracidium in search of a specific snail species. The miracidium enters the snail, elongates into a sporocyst, then matures into a cercaria which burrows out of the snail into the water. Cercariae may penetrate the skin or are ingested by the aquatic bird/animal, develop into an adult worm, and the cycle starts again. Cercariae may accidentally attach to human skin, penetrate the skin, die, and cause a local allergic reaction.

What are the clinical features of swimmer’s itch? 

Swimmer’s itch presents within hours of exposure after the film of water has dried on the skin. Initial symptoms are itch or a tingling sensation which settles quickly, leaving tiny red spots where skin penetration by the cercariae has occurred. Intense itch develops over a number of hours and the red spots can enlarge to form papules and, occasionally, hives. Blisters may develop over the next 24 to 48 hours.

The clue to diagnosis is that the rash is usually limited to areas of the skin exposed directly to the water, so skin covered by swimwear is typically protected.

Swimmer's itch

Credit: Images are from Tracz ES, Al-Jubury A, Buchmann K, Bygum A. Outbreak of swimmer's itch in Denmark. Acta Derm Venereol. 2019;99(12):1116–20.

What are the complications of swimmer’s itch?

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

How is swimmer’s itch diagnosed?

Swimmer’s itch is suspected clinically based on the characteristics of the rash following bathing in infested waters.

Skin biopsies are not diagnostic but may be performed to exclude other causes of an itchy, blistering, or urticarial rash.

There is no widely available blood test to confirm an allergy to cercariae.

What is the differential diagnosis for swimmer’s itch?

What is the treatment for swimmer’s itch?

Preventative measures

To reduce parasite numbers in the environment:

  • Avoid bird-feeding where people swim
  • Feed birds with a drug to treat the parasite
  • Reduce vegetation in high-risk areas to make the environment less favourable for the water snails
  • Use of chemical molluscicides such as copper sulphate or copper carbonate in small lakes to kill potentially infested snails.

To reduce the risk of cercariae on the skin:

  • Immediate towelling after exiting the water to reduce skin penetration of the parasites
  • Avoid swimming in shallow warm waters.

Symptomatic measures for itch and rash

What is the outcome for swimmer’s itch?

Swimmer’s itch usually resolves within 1–3 weeks without treatment. However, repeated exposures can result in severe reactions.

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Bibliography

  • Gordy MA, Cobb TP, Hanington PC. Swimmer's itch in Canada: a look at the past and a survey of the present to plan for the future. Environ Health. 2018;17(1):73. doi:10.1186/s12940-018-0417-7. Journal
  • Tracz ES, Al-Jubury A, Buchmann K, Bygum A. Outbreak of swimmer's itch in Denmark. Acta Derm Venereol. 2019;99(12):1116–20. doi:10.2340/00015555-3309. Journal
  • Żbikowska E, Marszewska A. Thermal preferences of bird schistosome snail hosts increase the risk of swimmer's itch. J Therm Biol. 2018;78:22–6. doi:10.1016/j.jtherbio.2018.08.023. PubMed

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