How is Swimmer’s Itch Diagnosed?

Patient Presentation
A 4-year-old male came to clinic with a pruritic rash for 24 hours. He had been swimming in the local freshwater lake over the weekend. His mother had tried calamine lotion and over-the-counter strength hydrocortisone cream but said that “he just can’t stop itching.” The rash started on his legs but soon involved the area where his swimsuit had been. She denied any new soap, lotions, sunscreens, insect repellents or other new products. They were frequent users of this lake which was known to have swimmer’s itch and she said, “he just lives in the lake when we are there.” The mother said that she was also starting to itch around her ankles that morning. The past medical history was non-contributory.

The pertinent physical exam showed a healthy male who was rubbing his legs and groin. His vital signs were normal with growth parameters in the 50-90%. The rash was 2-3 mm macules with most having a papular component but no vesicles. The lesions were grouped especially in flexural areas of the ankles, knees, groin and buttocks. He had some distinct excoriations and some generalized erythema of the groups but it was difficult to tell if there was real erythema or it was secondary to rubbing. The mother’s ankles did not have any distinct lesions but did have general erythema because of her rubbing them.

The diagnosis of swimmer’s itch was made. The pediatrician recommended using an antihistamine and prescription strength topical hydrocortisone to help with the pruritis. “This usually takes a few days to go away but I think the medicines should help him be more comfortable. Swimmer’s itch often gets worse with more exposure so I would try to keep away from the lake if you can and swim elsewhere. If you go there, it helps to wash off right away and change clothes, so the bugs that cause this have less chance to get into his skin,” the pediatrician advised.

Cercarial Dermatitis (CD) is known by many names throughout the world, but is commonly known as swimmer’s itch. It is a water-borne, non-communicable infectious disease that is caused by the larval stage (cercariae) of parasitic schistosomatid flukes. The cercariae causes an allergic maculopapular skin rash in humans that is usually self-limited (usually 4-10 days) but can cause problems for up to 20 days.

CD parasites are considered an emerging disease because of the increased distribution of the problem across the globe. Different parasite species cause the problem. In a normal life cycle that occurs mainly in fresh water but also brackish water, schistosome eggs invade various species of aquatic snails that act as an intermediary hosts. Within the snails, the schistosome eggs develop into schistosome cercariae. The schistosome cercariae migrate from the snail back into the water. In the water the schistosome cercariae encounters birds or mammals which are their definitive host. The cercariae penetrate the skin of the bird or mammal and travel within the host to a definitive organ (which depends on the species) where they develop into schistosome flukes. The schistosome flukes produces eggs which leave the definitive host usually through the intestinal tract usually, but occasionally through the bladder and urinary system. The schistosome eggs then start the cycle all over.

The usual intended definitive hosts are avian, especially waterfowl. Many different varieties of aquatic snails act as the intermediary host and of the more than 100 different schistosome species, 70% can cause CD. One of the most common species which causes CD is Trichobilharzia. Humans are incompatible species and are simply affected bystanders.

CD occurs in the warm weather when snails and bathers have their height of activity. Slow moving water, water near the edge of the water body, and being in the exposed water for longer increases the risk of acquiring CD. Children, especially 5-9 year olds, who play near the water’s edge for long periods of time have increased risk. It also appears that the children’s skin is more sensitive. The risk can be decreased by swimming in places where definitive hosts are not present or are present in fewer numbers, swimming farther out from the water’s edge especially in faster moving water, not swimming for long periods of time, and washing off and changing clothes after the potential exposure. Environmental mitigation includes drug treatment of definitive hosts, drug treatment of snails or manual removal of snails from the water. Use of waders, impermeable gloves and other protective clothing is a must for some recreational or professional uses.

Learning Point
The cercariae penetrate into human skin. If it is the initial contact, the cercariae may not cause a rash but can cause allergic sensitization. With subsequent exposure, there can be a prickling feeling with entry of the cercariae and then the rash becomes extremely pruritic. With recontact, a small macular-papular rash (1-2 mm initially) centered around the entry point of the cercariae happens within 12-48 hours. The macules can remit or become larger and vesicles can form on top of the papules. There can be surrounding erythema of the rash area. Pustules can occur if there is bacterial superinfection and pigmented spots can persist after resolution of the papules. The rash usually resolves within 4-10 days but can last for up to 20 days. Acute systemic reactions such as generalized limb swelling, nausea, diarrhea and fever can occur with subsequent exposure. The diagnosis is usually clinical-based but if needed, the organism can be identified on skin biopsy. Criteria include contact with water, rash appearance within 12-48 hours of exposure, and lesions on the body only where the water was in contact. Treatment of the rash is usually with antihistamines and/or topical steroids. The differential diagnosis includes insect bites, contact dermatitis, bacterial dermatitis, and skin reactions to larval cnidarians such as sea anemones or thimble jellyfish (if in appropriate location).

Questions for Further Discussion
1. What other parasites affecting humans are water-bourne?
2. What do you recommend for summer safety?

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at, the National Guideline Clearinghouse and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for these topics: Parasitic Diseases and Water Safety (Recreational).

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

To view videos related to this topic check YouTube Videos.

Cercarial Dermatitis or Cercariosis: What’s in a Name?
Morley NJ. Trends Parasitol. 2016 Feb;32(2):92-3.

Pinto HA. Cercarial Dermatitis’ and ‘Cercariosis’: Very Broad Terms.
Trends Parasitol. 2016 May;32(5):351-2.

Horak P, Mikes L, Lichtenbergova L, Skala V, Soldanova M, Brant SV. Avian schistosomes and outbreaks of cercarial dermatitis.
Clin Microbiol Rev. 2015 Jan;28(1):165-90.

Kolarova L, Horak P, Skírnisson K, Mareckova H, Doenhoff M.
Cercarial dermatitis, a neglected allergic disease.
Clin Rev Allergy Immunol. 2013 Aug;45(1):63-74.

Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa Children’s Hospital