Sea Bather’s Eruption — Pediatric EM Morsels

Summer is here and along with those warm months a unique set of emergent problems that we need to consider (ex, Submersion Injuries, Vehicular Hyperthermia, Heat-Related Illness, Lawn Mower Injuries, and RMSF to name just a few) also arrives. In addition to the truly emergent and terrible conditions, there are also plenty of annoyances (ex, Summer Penile Syndrome and Gianotti-Crosti Syndrome). As if there weren’t enough rashes to puzzle, confuse, and mystify us in the emergency department, let us consider another perplexing rash that is intensely itchy with a sudden onset. Parents think it’s an allergy, poison ivy, or bug bites — but the culprit may actually be tiny marine organisms trapped under their bathing suits. While the sand from the beach may be tenacious and difficult to get rid of, there may be another sandy souvenir that is lingering and causing this discomfort. Let’s cannonball right into another scrumptious morsel about a summertime rash you wear home from the beach: Sea Bather’s Eruption

Sea Bather’s Eruption: The Basics

Sea Bather’s Eruption, or “sea lice,” is an intensely pruritic dermatitis caused by nematocyst discharge from marine cnidarians
- Most commonly: thimble jellyfish (Linuche unguiculata) and sea anemone (Edwardsiella lineata) [1]
Itchy papular eruptions that occur shortly following ocean-water exposure, classically involving areas covered by swimwear, sites of friction (waistband, groin, axillae, bikini straps), and occasionally the scalp and hair-covered body parts
Commonly affects swimmers, snorkelers, divers, surfers, and children under the age of 16 (all due to relative increased ocean exposure and more sensitive skin)
Peak season is May through July, and encountered in warm tropical waters, particularly along the southeastern US, Gulf Coast, and Caribbean [1]

Sea Bather’s Eruption: That’s Gross…

These microscopic jellyfish and sea anemone larvae are small enough to become trapped beneath swim trunks and bathing suits, and once imprisoned:

Mechanical compression from swimwear squeeze the organism
Freshwater rinsing, evaporation, and towel-drying can trigger nemocyst discharge
The larvae’s nemocysts subsequently inject antigenic toxin into the skin
The swimmer develops both immediate and delayed hypersensitivity reactions [1-2]

Sea Bather’s Eruption: Presentation

Clinically this rash looks like many of the other rashes we see in the ED
- Pruritic erythematous papules, urticarial plaques, and sometimes small vesicles
- Intense pruritis often leads to excoriation, which may obscure lesion identification and predispose to secondary impetigo [1,3]
- Rash is concentrated to groin, buttocks, trunk, axillae, and chest (particularly in surfers) [4]
- Rare, but can be associated with mild systemic symptoms including nausea and fever

Children may be disproportionately uncomfortable because of the severe itching
Interestingly, according to a recent case series, sea bather’s eruption has been shown to mimic the Koebner phenomenon likely secondary to repetitive linear scratching [1,3]

Sea Bather’s Eruption: Diagnostic Approach

Sea bather’s eruption is a clinical diagnosis, and as always, a good and thorough history gets you there:
- Sudden onset following recent travel to warm, tropical climates and ocean-water swimming
- A rash concentrated to the swimwear region that worsens after prolonged time in a wet suit, showering, or during drying off with a towel

For those of us doing biopsies in the ED… histologic findings are non-specific and closely resemble those seen with arthropod bites, demonstrating a superficial and deep perivascular mixed inflammatory infiltrate composed of neutrophils, lymphocytes, and eosinophils [1]

Almost never necessary; however, in select cases, serum titers for L unguiculata can remain positive several weeks post-exposure or following re-exposure [1,5]

Sea Bather’s Eruption: Treatment

Management is primarily symptomatic care; utilizing antihistamines and topical corticosteroids have been shown to reduce inflammatory response [6]
Topical corticosteroids are commonly prescribed, with potency tailored to the site of the lesion (avoid high-potency steroids on the face, groin, and axilla!)
In more moderate-to-severe cases complicated by suspected impetigo, treat accordingly with topical or oral antibiotics

Sea Bather’s Eruption: Prevention

Lesions generally resolve within several weeks without long-term sequelae
Recurrence can occur if contaminated swimwear is re-used [5]
Patients and families should be counseled to wash swimwear with hot water and detergent to remove residual nematocysts before subsequent use
Applying 5% acetic acid after leaving the endemic water can inactivate any undischarged nematocysts (not routinely recommended or part of common practice) [1]

Moral of the Morsel:

All Lice Are Note Equal! “Sea lice” aren’t lice at all, but are microscopic jellyfish or sea anemone larvae whose nematocysts become trapped under swimwear and discharge venom after leaving the water
Location! Location! Location! Swimwear distribution and recent warm tropical ocean water exposure is key to making this clinical diagnosis
Don’t Forget to Wash! Treat the itch and educate families on regularly washing swim suits and beachwear

References

Daniels P, Taylor A, Lum A, et al. Seabathers Eruption. [Updated 2025 Dec 13]. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482307/
Hornbeak KB, Auerbach PS. Marine Envenomation. Emerg Med Clin North Am. 2017 May;35(2):321-337. doi: 10.1016/j.emc.2016.12.004. Epub 2017 Mar 15. PMID: 28411930.
Rossetto AL, Rossetto AL, Guevara BEK, Haddad V Jr. Seabather’s eruption presents Koebner phenomenon? J Eur Acad Dermatol Venereol. 2020 Feb;34(2):e93-e95. doi: 10.1111/jdv.16008. Epub 2019 Nov 6. PMID: 31605553.
Kumar S, Hlady WG, Malecki JM. Risk factors for seabather’s eruption: a prospective cohort study. Public Health Rep. 1997 Jan-Feb;112(1):59-62. PMID: 9018290; PMCID: PMC1381840.
Sridhar SC, Deo SC. Marine and Other Aquatic Dermatoses. Indian J Dermatol. 2017 Jan-Feb;62(1):66-78. doi: 10.4103/0019-5154.198052. PMID: 28216728; PMCID: PMC5286756.
Wong DE, Meinking TL, Rosen LB, Taplin D, Hogan DJ, Burnett JW. Seabather’s eruption: clinical, histologic, and immunologic features. J Am Acad Dermatol. 1994;30(3):399-406. doi:10.1016/S0190-9622(94)70046-X

Author

Adam Brzezinski

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