The Dermatologic Perils of Swimming: Seabather’s Eruption

This 10-year-old boy presented for evaluation of a rash that developed during a spring vacation on Florida’s Atlantic coast. After he had been swimming in the ocean, a pruritic, erythematous, papular rash developed on his trunk, axillae, and groin. Approximately 24 hours after the onset of the rash, he experienced malaise, chills, and a sore throat. His past medical history was unremarkable. He had been fully immunized and had had varicella infection.

This boy has seabather’s eruption, a pruritic dermatitis that occurs after exposure to seawater that contains certain species of jellyfish—particularly the larval form of the thimble jellyfish (Linuche unguiculata). The larvae are found in seawater in areas where this eruption is endemic, such as the Caribbean and the Atlantic coast of Florida, where this boy had vacationed. The jellyfish are found in these warm waters from March through August, with peak concentrations in May and June.

The first written reports of this disease came from Florida’s Delray Beach. The jellyfish larvae contain venom-filled barbs that fire their venom into the skin when the larvae become trapped under a bathing suit. Changes in osmotic pressure, such as air-drying or showering with fresh water, also trigger the barb-firing process. The venom causes a local toxic reaction and can precipitate a systemic hypersensitivity reaction. The local reaction—an erythematous, pruritic, papular dermatitis—may be especially heavy on the skin under the bathing suit and in areas covered with hair, such as the axillae.

It appears 4 to 48 hours after exposure to infested seawater. Systemic symptoms are more common in children than adults and include malaise, fever, sore throat, abdominal pain, headache, cough, and diarrhea. Both the rash and systemic symptoms generally resolve in 1 to 2 weeks. Treatment is supportive and includes antihistamines and topical corticosteroids. Systemic corticosteroids are reserved for severe cases. The best preventive measure is to avoid swimming in infested water altogether.

Those who choose to swim in potentially infested waters need to be aware that the wearing of T-shirts for ultraviolet protection increases the risk of getting seabather’s eruption. Bathers should instead use a waterproof sunblock for ultraviolet protection. While sunblock affords no protection against the jellyfish larvae, it does not increase the risk of trapping the larvae against the skin, as T-shirts do. Bathers should shower as soon as possible after swimming, without their bathing suits, which should be washed in detergent and heat-dried.