What Are Sea Lice?

Medically Reviewed by Stephanie S. Gardner, MD on April 26, 2022
4 min read

Cooling off in the ocean on a hot day is one of life’s great pleasures. Unless you happen to get an itchy, red rash a few hours after you get out of the water.

If you were swimming in the Atlantic Ocean, the Gulf of Mexico, or the Caribbean Sea, the rash could be a condition called sea lice. While it may make you itchy and uncomfortable for a few days (or a week), there are usually no lasting effects.

You may notice a prickly or stinging sensation while you’re still in the water. But most of the time, it starts anywhere from a few minutes to several hours after you get out of the water.

A red, itchy rash will appear over parts of your body covered by your hair or bathing suit. This can include your groin, rear end, chest, abdomen, neck, or back. You may have 200 stings or more.

Other symptoms include headache, chills, fever, nausea, and abdominal cramps. It’s rare, but some people have to go to the hospital for treatment.

If you’ve had sea lice before and get stung again, you’ll likely feel it right away. Symptoms happen faster the second time around.

The rash, also called seabather’s eruption, is a form of dermatitis caused by jellyfish larvae in the water. The larvae belong to two marine species that sting: the thimble jellyfish (Linuche unguiculata) and a sea anemone (Edwardsiella lineata). They’re parasites that feed off of fish, not humans. And the word “lice” may make you think of head lice, but there’s no relation at all.

The larvae are transparent, so you can’t see them. And they’re so tiny (2 to 3 millimeters long), they get trapped in your hair, or in between your bathing suit and your skin.

When you get out, the water drains off of you, but the larvae stay behind. Your bathing suit rubs against your skin, causing the larvae to sting you and inject their toxin.

First, what not to do: Don’t jump into a shower or pool with your bathing suit on and expect relief. Fresh water will make the jellyfish start to sting again and may worsen your pain. Instead, take off your bathing suit as soon as you can and rinse your body. Wash your suit in hot water and toss it into the dryer to make sure all the larvae are gone. If you had a severe rash, you may even want to get rid of the suit.

Most cases of seabather’s eruption are easy to treat at home. You can apply treatments like hydrocortisone cream or calamine lotion to your skin a few times a day. They’ll ease the itching and pain. Antihistamines and anti-inflammatory medicines like ibuprofen may help, too. In some cases, your doctor may prescribe a steroid.

Aside from staying out of the water, there’s no guaranteed way to prevent sea lice. But there are ways to avoid the rash.

Jellyfish larvae season is generally from March through August. Their numbers are highest between April and July. Swimming in the ocean during this time doesn’t mean you’ll definitely get sea lice, though. Outbreaks come and go.

Listen to local beach reports and pay attention to flags or warning signs posted at beaches. In Florida, a common location for these jellyfish larvae, purple flags warn swimmers about the presence of stinging critters in the water.

Brush up on your knowledge of winds and currents, too. Chances of an outbreak go up where the Gulf Stream passes closest to shore. They’re also higher in places where strong easterly winds combine with shifts in currents to bring the larvae closer to shore.

Don’t wear a T-shirt in the ocean. It may trap more jellyfish larvae against your skin. Women may want to wear a two-piece bathing suit for the same reason.