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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 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 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.
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.
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.
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.
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.
To reduce parasite numbers in the environment:
To reduce the risk of cercariae on the skin:
Swimmer’s itch usually resolves within 1–3 weeks without treatment. However, repeated exposures can result in severe reactions.
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