When swimmers refer to “sea lice,” they are describing a common, intensely itchy skin condition known by medical professionals as seabather’s eruption. This reaction occurs after swimming in ocean water where microscopic marine life is present. While the name suggests a parasitic infestation, this is a misnomer, and the condition is not caused by actual lice. The resulting rash, while uncomfortable and sometimes severe, is typically a localized reaction that resolves on its own.
Understanding What Causes Sea Lice Rash
The rash is not caused by true sea lice, which are small crustaceans that parasitize fish. Instead, seabather’s eruption is caused by the larvae of various jellyfish and sea anemones, primarily the larvae of the thimble jellyfish (Linuche unguiculata) in warm waters like Florida and the Caribbean. These larvae are nearly invisible, often measuring less than half a millimeter. They float freely in the water column, often near the surface, and become trapped in the mesh of a swimmer’s bathing suit or clothing.
Once trapped, the larvae’s stinging structures, called nematocysts, are triggered to discharge their toxins into the skin. This stinging action is provoked by specific mechanical or chemical stimuli. Pressure and friction, such as toweling off or the movement of a tight swimsuit against the skin, are common triggers. Exposure to fresh water, such as showering while still wearing the contaminated suit, can also cause the nematocysts to fire, intensifying the reaction.
Recognizing the Signs of Exposure
Symptoms of seabather’s eruption typically begin hours after leaving the water, though some people report a prickling sensation while still swimming. The most characteristic sign is the appearance of small, red, intensely itchy bumps known as papules, which may sometimes develop into blisters. The distribution of the rash is highly specific, concentrated in areas covered by swimwear, such as the waistline, groin, armpits, and under the breasts.
The severity of the reaction varies considerably depending on the number of larvae trapped and the individual’s immune response. Intense itching and a burning sensation are almost universal, often persisting for several days. In some cases, particularly in children, the systemic absorption of the toxins can lead to more generalized symptoms. These may include a mild fever, chills, headache, nausea, or a general feeling of fatigue.
The rash usually lasts from two to four days, though it can persist for up to two weeks in more severe cases or if continually irritated. Because the larvae can also become entangled in body hair, rashes may occasionally appear on the neck or chest, even if those areas were not covered by fabric. The persistent itching can lead to scratching, which introduces the risk of secondary bacterial skin infection if the skin barrier is broken.
Treating the Rash
Managing seabather’s eruption focuses on reducing the allergic reaction, alleviating the intense itching, and preventing further stinging. The immediate priority upon suspecting exposure is to remove the contaminated swimwear and rinse the body with ocean water, if possible, before using any fresh water. If available, applying diluted vinegar or rubbing alcohol to the skin before a freshwater rinse may help neutralize any unfired nematocysts still on the skin.
For at-home treatment, over-the-counter antihistamines like diphenhydramine can help alleviate the systemic allergic response and reduce the itching. Topical treatments are also effective, including calamine lotion or a low-dose hydrocortisone cream applied directly to the rash. Applying a cold compress or a cloth-covered ice pack to the affected areas for short periods can help reduce localized inflammation and provide temporary relief from the burning sensation.
It is important to avoid vigorous scrubbing or scratching, as this can worsen the rash and potentially lead to infection. Medical attention should be sought if the rash is widespread, if signs of infection develop—such as pus, increased warmth, or red streaks—or if systemic symptoms like a high fever or vomiting are severe. In persistent or severe cases, a healthcare provider may prescribe a stronger topical or oral corticosteroid to control the inflammation.
How to Avoid Sea Lice
Prevention starts with checking local beach conditions before swimming, especially during peak seasons (spring through summer). If local advisories or warnings about “sea lice” are posted, it is best to avoid entering the water entirely. If you choose to swim, opt for tight-fitting swimwear, as loose clothing like t-shirts or one-piece suits can trap more larvae against the skin.
After swimming in potentially affected waters, the most important step is to exit the water and immediately remove the swimwear without rinsing it in fresh water while still wearing it. The suit should be thoroughly washed in hot water and detergent before being worn again, as the stinging cells can remain viable in dry fabric. Rinsing the skin with salt water immediately after leaving the ocean can dislodge any remaining larvae before they are triggered.
Some swimmers use topical barrier creams or sunscreens that are advertised to deter stinging marine life, which may offer a slight layer of physical protection. Upon removing the suit, shower with soap and fresh water to remove any residual larvae from the skin, and then gently pat the skin dry with a towel instead of rubbing, which can trigger any remaining nematocysts.