Photograph by Luis Javier Sandoval, VW Pics, UIG via Getty Images
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Swarm of thimble jellyfish or thimble sea jelly, Linuche unguiculata, that produce larvae which sting and cause an itchy rash. Isla Mujeres, Mexico caribbean sea.

Photograph by Luis Javier Sandoval, VW Pics, UIG via Getty Images

What's Giving Some Beachgoers Rashes? Get the Facts.

Lifeguards reported this year’s first cases of sea lice, which aren’t lice at all.

Sea bather’s eruption is leaving some U.S. beachgoers itchy.

On Monday, lifeguards in Pensacola, Florida, raised the purple flag to warn beachgoers of dangerous wildlife in the water. Swimmers there had been leaving the shore feeling fine, only to find the next day that they had red, itchy rashes. This delayed onset gives the condition its common name: sea bather’s eruption, or sea lice.

And although there have been no reports of serious cases this year, extreme symptoms of sea bather’s eruption can include fever, nausea, and chills, usually affecting children and people with allergies.

Worried about your vacation plans? Here’s everything you need to know.

What are sea lice?

The so-called sea lice that just appeared in Florida aren’t lice at all—they seem to be the larvae of jellyfish, most likely a small species called the thimble jellyfish. They’re translucent, almost invisible to the naked eye when they’re in water, which makes them difficult to avoid.

There is a crustacean that marine biologists call sea lice, but it’s a parasite that affects fish, not people. And parasitic sea lice shouldn’t be confused with sand lice, also called sea fleas—another crustacean that lives in wet sand.

Flesh-Eating Sea Creatures are Causing Alarm August 7, 2017 - These tiny sea creatures feasting on chunks of meat are amphipod crustaceans. Commonly referred to as sea fleas, they are found in waters all around the world. They frequently scavenge fish carcasses and were recently blamed for chomping on an Australian teen's legs. While experts don't know for sure that the fleas attacked the teen, beachgoers are on full alert.

Click here to read Mysterious Flesh-Eating Sea Creature Causes Shocking Injury.

The stingers in Florida are in one of the first stages of the jellyfish life cycle. The tiny larvae are no bigger than a speck of ground pepper, and they float on the ocean’s current until settling down and growing into polyps. Then, the polyps can spawn mature medusas—the familiar umbrella-shaped jellyfish. (Read about the secret lives of jellyfish.)

Thimble jellyfish live in the waters around Mexico and the Caribbean, but their larvae ride the waves to the southeastern shores of the United States. The baby jellyfish float by every year during the summer months.

There have been instances of sea bather’s eruption in the northeastern United States, too. It doesn’t happen as frequently, and jellyfish larvae aren’t the culprits. Instead, the cause of the rashes is sea anemone larvae, which pass along the northeastern shores at different times each year, only occasionally lining up with swimming season.

What causes the rash?

The rash is the result of baby jellyfish or anemone stingers.

The larvae may be small, but they’re not defenseless. Like mature jellyfish and anemones, the larvae are covered in cells that contain toxin-filled harpoons, called nematocysts, that are ready to launch into the skin of a threat.

When the larvae find themselves in a stressful situation, like getting caught in an armpit or under a swimsuit, their harpoon-shooting cells are activated. The toxins in a particular jellyfish’s venom vary by species, but the most common ingredient is porin. Once porin enters your skin, it can open holes in the cells.

Some jellyfish, like the deadly box jellyfish, use toxins that punch holes in skin, blood, and nerve cells alike. But the rash caused by the larvae is usually just irritating. People affected by sea bather’s eruption don’t tend to notice right away, instead the rash usually appears by about 24 hours after they leave the water.

How do I avoid getting stung?

The most surefire way to avoid sea bather’s eruption is to stay out of the water. But if you’re in a swimming sort of mood, there are options to protect yourself and still enjoy your day at the beach.

First, look out for the purple flag that indicates dangerous marine life in the water. The flag might be warning against more than just stinging larvae, so it’s best to head the warning of the local lifeguards and other personnel.

Avoid swimsuits and extra clothing that might catch the stinging larvae against the skin. And when you get out of the water, don’t towel off, because that can activate stingers, according to the Auckland Regional Public Health Service. Finally, rinse both your body and your bathing gear thoroughly after being in the water

If you do get stung, the over-the-counter antihistamines and hydrocortisone creams can be used, according to Florida’s Department of Health, along with calamine lotion to treat the symptoms until they subside.