Education And Debate

Lesson of the Week: Seabather's eruption—a case of Caribbean itch

BMJ 1996; 312 doi: http://dx.doi.org/10.1136/bmj.312.7036.957 (Published 13 April 1996) Cite this as: BMJ 1996;312:957
  1. Ruth M MacSween, registrara,
  2. Hywel C Williams, senior lecturera
  1. a Dermatology Department, Queen's Medical Centre, Nottingham NG7 2UH
  1. Correspondence to: Dr MacSween.
  • Accepted 29 February 1996

Seabather's eruption is an intensely pruritic dermatitis that occurs after exposure to seawater and affects the areas of the body covered by swimwear. It has probably existed for centuries, but formal reports of it are relatively recent.1 With the increasing popularity of foreign travel, seabather's eruption is likely to be seen more often in patients returning from tropical climes. Larvae of species belonging to the phylum Cnidaria—which comprises jellyfish, corals, sea anemones, and hydra—have been implicated as the cause of this condition.2 3 4 The members of this phylum share a similar stinging mechanism, which is mediated by subcellular organelles called nematocysts. We report two recent cases of seabather's eruption caused by the larvae of thimble jellyfish (Linuche unguiculata).

Seabather's eruption should be suspected if there is a history of exposure to seawater followed by the appearance of a papular, pruritic rash affecting the areas covered by the swimsuit

Case reports

In May a 31 year old accountant and her husband travelled to Cancun, on the Caribbean coast of Mexico, for their honeymoon. On the last day of the trip they took a final swim in the ocean, and within minutes of entering the water they became aware of a prickling sensation on their skin. Over the next 24-48 hours each developed a florid, pruritic rash. Examination of our patient revealed a widespread eruption of discrete, erythematous papules that were virtually confined to the area covered by her onepiece swimsuit (fig 1, left). The papules were particularly numerous over the breasts and where the side seams of the costume had lain. She was systemically unwell with a mild fever of 37.5°C. Her husband had an identical rash over his genitals and buttocks, again limited to the area covered by his swimming trunks.

Fig 1Fig 1Fig 1
Fig 1

Left: Seabather's eruption: discrete, inflammatory papules with distribution that corresponds to the “tan line,” showing that they are confined to the area covered by a one piece swimsuit. Middle: Adult form of jellyfish Linuche unguiculata (courtesy of Professor Terri Meinking, University of Miami). Right: Larva of Linuche unguiculata that has been pressed under a cover slip to imitate conditions under a swimsuit. Around it are several nematocysts that have fired their stinging thread-like tubules (courtesy of Professor Terri Meinking, University of Miami)

A biopsy of one of the lesions showed upper dermal oedema, lymphangiectasia, and a severe perivascular inflammatory infiltrate. The infiltrate was predominantly lymphocytic with one or two eosinophils, which would be compatible with a sting reaction. Other sections of the biopsy specimen revealed a possible entry point, with a localised area of inflammation overlain by a patch of parakeratosis. No foreign material was identified in the sections. A serum sample was sent to a reference laboratory for enzyme linked immunosorbent assays (ELISA) against a battery of jellyfish antigens. The patient was found to have an antibody titre of 1:3600 to antigen of Linuche unguiculata, making this the most likely cause of her seabather's eruption. Other antibody titres were low.

Both patients were given topical 0.05% clobetasol propionate for symptomatic relief. As our patient's rash covered a larger area and she had systemic symptoms, she was given a two week reducing course of oral steroids, starting at 40 mg daily. This relieved the symptoms, but the rash took four weeks to fade and left behind some faint atrophic scars at eight weeks follow up.

Discussion

The key to the diagnosis of seabather's eruption is history of exposure to seawater followed by the appearance of typical skin lesions. It is caused by the stings of cnidarian larvae, in this case the larvae of L unguiculata (fig 1, middle). These jellyfish breed in Caribbean waters from March until September, with a peak breeding season in May. Clouds of larvae can easily be washed inshore by high tides or strong winds. The larvae, at about 0.5 mm in size, are just small enough to pass through the weave of most swimwear and become trapped against the skin. Any external pressure on the larvae, or changes in osmotic pressure caused by evaporation of seawater or by the bather showering in fresh water, causes the larval stinging mechanism to fire (fig 1, right).

Jellyfish larvae, like other members of the phylum Cnidaria, sting by means of nematocysts. These tiny, spherical structures contain a highly folded, eversible tubule. An internal hydrostatic pressure of 150 atmospheres is capable of everting the tubule, complete with toxin, with accelerations of up to 40000 g, one of the fastest events in biology.5 The tubule is hollow with internalised barbs that become externalised during eversion and inject the venom into the dermis. Jellyfish venoms are a complex mixture of enzymes and peptides that are both toxic and antigenic to humans.6 Such dermatotoxic processes can be difficult to distinguish clinically. The venoms stimulate the humoral immune system, and high antibody titres may be detected in patients with a history of exposure to jellyfish or their larvae.7 Cell mediated immunity may also play a part, and there has been a recent case report of a patient with positive patch tests to antigen of Olindas sambaquiensis.8

For the popular coastal resorts of the Caribbean, seabather's eruption poses a public health hazard that occurs at the peak of the tourist season. Avoiding all activities in the sea from March until August is unlikely to be an acceptable preventive measure for tourists. Warnings on the beaches during outbreaks may be helpful. The overall surface area of swimsuits, which trap stinging larvae against the skin, will influence the area of skin affected. However, the potential advantages of scantier swimwear during ocean activities must be weighed against potential disadvantages such as increased exposure to ultraviolet.

I thank Professor Joseph Burnett and Helene Rubinstein, University of Maryland School of Medicine, for their help with the ELISA test; and Professor Terri Meinking, Department of Dermatology and Cutaneous Surgery, University of Miami School of Medicine, for her helpful comments and kind permission to use figures 2 and 3.

Footnotes

  • Funding None.

  • Conflict of interest None.

References

  1. 1.
  2. 2.
  3. 3.
  4. 4.
  5. 5.
  6. 6.
  7. 7.
  8. 8.