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Sea Lice Rash
  • Itchy, red bumps characterize this rash for which treatment is often unnecessary.
  • Risk of exposure increases in summer months.
  • A few key tips can help reduce the risk and minimize symptoms.

With summer well on its way in North America, warmer waters along both coasts will soon swell with eager swimmers, surfers and sun worshippers.

To a lesser degree, so too will dermatologist waiting rooms.

The reason? Many beach enthusiasts will find themselves plagued by a sudden and unexpected rash.

The story they tell will often be the same—nothing seemed unusual about the water until a slight tingling sensation was felt. A few hours later, a stubborn, hives-like dermatitis had broken out.


The common diagnosis will be seabather’s eruption—colloquially known as “sea lice rash”—and it can certainly spoil the summer mood.

As millions prepare for some much-needed beach time, what exactly is sea lice rash and how concerned should swimmers be?

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What are sea lice? Confusing different creatures

The name alone is a source of worry, conjuring up exotic images of water-borne parasites and memories of having one’s hair quarantined at school.

In fact, “lice” is a misnomer here, a catch-all term for various small sea critters that has stuck, despite being incorrect.

The creature behind this rash is actually a thimble jellyfish or sea anemone in larval form.

Thimble jellyfish are found in the warmer waters of Florida and the Caribbean. Sea anemones are found in cooler waters, such as those off the coast of Long Island, NY.

What do sea lice look like?

Unfortunately, these larvae are impossible to spot—they are barely visible to the naked eye.

Larvae of the thimble jellyfish are often likened to a speck of ground pepper, while those of sea anemones are also tiny, about 2-3 mm.

While jellyfish may be seen in the water, it’s the larvae alone that are responsible for seabather’s eruption. Long after jellyfish have died off after spawning, the larvae continue to cause irritation, with no mature jellyfish around as a warning.

What causes the rash?

Depending on the season and currents, high concentrations of larvae will coalesce close to the water’s surface, where they come into contact with swimmers, surfers and snorkelers.

The larvae contain stinging cells called nematocysts. Pressure, friction or contact with fresh water will trigger them to release a toxin, causing an allergic reaction in the skin.

Bathing suits or clothing make things worse by trapping larvae against the body and provoking stings. Similarly, lying on a surfboard or towel, or sitting on a car seat in wet trunks will activate the cells. Rashes are often particularly bad around creases in the body where nematocysts have accumulated under pressure.

It’s easy to miss the stings—a weak tingling (if felt at all) may also occur after a swimmer has left the water. The organisms still discharge their toxin after death when crushed, dried out or flushed with fresh water (as when showering, for example).

What does the rash look like?

Lesions may be barely noticeable or widespread and severe. They appear as red bumps or blisters about 1mm to 1 cm in size. With time these become increasingly itchy and sometimes painful.

According to research, they usually develop within 4 to 24 hours. The severity depends on the number of stings, a person’s sensitivity and whether they have had any previous exposure to the toxin (this seems to make reactions worse).

Rash is often concentrated around areas of the body that were covered by clothing or subject to more friction—the abdomen, buttocks, chest and groin, for example.

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Are there other symptoms?

In addition to itchiness, some sufferers experience nausea, fever, chills, malaise and/or aches.

“Symptoms are usually mild and often clear by themselves,” explains Dr. Jeffrey Fromowitz, a dermatologist practicing in Boca Raton, FL, where seabather’s eruption is common. “In some cases, they can become severe, with flu-like symptoms that last for weeks.”

One study in Florida found that about 9% of subjects experienced nausea, fever or body aches. All those affected were under the age of 16. Another study noted fever in eight of its 57 cases. All were children 11 years old or younger. This suggests children and adolescents may be more susceptible to additional symptoms.

Is the rash contagious?

The sight of a bright red rash accompanied by itching in areas like the groin leads to fears of having a communicable venereal disease, or perhaps chickenpox.

Seabather’s eruption is an allergic reaction to larval toxin and is not contagious.

It’s important to remember, however, that stinging cells (nematocysts) remain on clothing and swimsuits long after use. Affected beachwear should be washed thoroughly to avoid spreading the problem in this way (more on how to do that later).

How long does sea bather’s eruption last?

Symptoms are short-lived—most people recover in a week.

Any further contact with the toxin could worsen and prolong the reaction. Some have experienced lesions lasting several weeks or longer.

Are there any complications or long-term effects?

“The main complication is scarring from the eruption or scarring due to scratching,” explains Dr. Caroline A. Chang, a board-certified dermatologist practicing in East Greenwich, RI.

In addition to scarring, excessive scratching can also lead to bacterial infection.

Signs of infection include increased pain, swelling, pus, redness, red streaks or warmth around the affected area. A fever is also a red flag.

When should you see a dermatologist?

“See a dermatologist for a complete evaluation if the symptoms are not resolving,” says Dr. Chang. “Prescription medicine such as topical steroids may be necessary.”

A dermatologist will be able to rule out other conditions in order to make a proper diagnosis.

How do you treat seabather’s eruption?

Certain steps can be taken immediately if you think you may have been exposed.

While the first instinct may be to wipe off with a towel or take a shower, either action could make things worse. Nematocysts are likely trapped in your bathing suit. Dousing them with fresh water or rubbing them with a towel will only cause more stings.

DermNet NZ recommends removing the bathing suit as soon as possible, then rinsing the body in uncontaminated seawater. Pat-dry rather than rub with a towel.

Once dry, dermatologists recommend applying dilute vinegar or rubbing alcohol to neutralize any toxin left on the skin.

The bathing suit and any other clothing or fabrics that were used prior to rinsing off should be thoroughly washed with detergent or vinegar, and then heat dried. One study found that merely air-drying an affected bathing suit can result in a recurrence of symptoms when later used.

Next, it’s a question of managing the discomfort until it subsides.

“Ice packs can help with the pain and itch,” recommends Dr. Fromowitz. “You can also use an OTC hydrocortisone ointment and take an antihistamine (Benadryl, Claritin, Allegra). For more severe reactions you may need to see your dermatologist for prescription topical steroids and oral prednisone.”

Unsurprisingly, the Internet has plenty of non-scientific remedies to recommend: sodium bicarbonate, sugar, urine, olive oil, and meat tenderizer to name a few. Dr. Fromowitz does not recommend any of them as they may worsen the symptoms.

Other specialists recommend bathing in a colloidal oatmeal preparation and applying calamine lotion for relief. Calamine lotion, it is cautioned, should not be used with an oral antihistamine, as there may be toxic effects (especially in children).

Anti-inflammatory drugs such as ibuprofen and aspirin can help to reduce pain and inflammation.

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How do you prevent seabather’s eruption?

Unfortunately, evading contact with jellyfish or anemone larvae is the only failsafe prevention strategy.

“Not much can be done to prevent the stings,” says Dr. Fromowitz. “Some have tried barrier creams but it seems the stinging cells penetrate through and swimmers still get the rash.”

Observe posted warnings and avoid areas where swimmers have developed symptoms.

Choice of swimwear can also reduce the risk. Avoid t-shirts or looser fitting trunks, which act as nets for the nematocysts. Women can choose a two-piece over a one-piece bathing suit to reduce the surface area that will potentially trap larvae.

Tighter fabrics also provide some defense. “A wet suit that is tightly fitting may limit exposure only to the edges of the garment,” says Dr. Fromowitz.

When is the “sea lice” season?

Research shows outbreaks in the coastal region of the United States occur intermittently between March and August, but peak from April through early July.

In the Caribbean, jellyfish breed throughout the summer, peaking in May.

Seabather’s eruption occurs in many parts of the world, so it’s best to follow local advisories.

The takeaway—keep calm and summer on

One of summer’s unpleasant possibilities, “sea lice rash” is not dangerous and should not discourage enjoyment of the sea. Prevention is your best line of defense.

If symptoms appear unusually severe or persistent, consult with a dermatologist for treatment.

» Need professional skin care advice? Use Zwivel’s online consultation tool to contact a board-certified dermatologist in your area.

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