The term “sea lice” is commonly used to describe a skin reaction after swimming in the ocean, but the accurate name is Seabather’s Eruption. This uncomfortable condition is an inflammatory response to microscopic marine organisms, not actual lice. It frequently affects beachgoers in tropical and subtropical waters, especially during warmer months. This article clarifies the true cause of the rash and provides guidance on managing and preventing exposure.
Not Contagious: Clarifying the Source of the Stings
Seabather’s Eruption is not contagious and cannot be transmitted from person to person. The reaction stems entirely from exposure to tiny, free-floating larvae in the seawater, typically the immature stages of jellyfish (like the thimble jellyfish) or sea anemones. If multiple people develop the rash, it is because they were all exposed to the same larvae-filled water.
The injury is triggered when these larvae become trapped against the skin, usually underneath a bathing suit or in hair. The larvae possess dormant stinging cells called nematocysts. These cells release venom when subjected to mechanical pressure (like friction or rubbing) or a change in osmotic pressure, such as contact with fresh water. This explains why the rash often appears in areas covered by swimwear and why rinsing with fresh water while still wearing the suit can worsen the reaction.
The rash experienced by swimmers is an allergic reaction to the toxin released by the stinging larvae. This is an environmental exposure, not a parasitic infestation or a human-to-human infection.
Recognizing Seabather’s Eruption Symptoms
Symptoms typically begin within hours of leaving the water, though onset may be delayed up to 24 hours. The rash presents as small, red, intensely itchy bumps (papules), sometimes appearing as hives or small blisters. Lesions are almost always concentrated on skin areas covered by swimwear, such as the torso, armpits, or groin.
Some individuals, particularly children, may also experience systemic symptoms. These can include a mild fever, headache, chills, nausea, or vomiting. The intense itching causes discomfort and can lead to difficulty sleeping. The rash usually resolves naturally within several days to a week, though symptoms may occasionally persist for up to two weeks.
Immediate Relief and Medical Treatment
Treatment focuses on relieving inflammation and intense itching, and over-the-counter options are generally effective. Applying a low-potency topical hydrocortisone cream or calamine lotion to the affected areas helps reduce redness and irritation.
Oral antihistamines, such as diphenhydramine, are recommended to calm the allergic reaction and mitigate severe itching. Cool compresses or colloidal oatmeal baths provide additional soothing relief. It is important to avoid scratching the rash, as breaking the skin can lead to a secondary bacterial infection. If the rash is severe, covers a large area, or if systemic symptoms persist, consult a medical professional for prescription-strength treatments, such as oral steroids.
Preventing Exposure in the Water
The primary method of prevention is avoiding swimming in areas where outbreaks of the causative larvae are known to occur. Outbreaks often peak between late spring and early summer in affected coastal regions. If swimming is unavoidable, specific steps can minimize the risk of being stung.
Swimmers must avoid rinsing off with fresh water while still wearing their bathing suit. Immediately upon exiting the ocean, remove all swimwear and rinse the skin with ocean water, if possible, before using a freshwater shower. All contaminated clothing, including wetsuits and swimsuits, must be thoroughly machine-washed in hot water and dried to ensure any lingering stinging cells are inactive.