Sea lice “bites” appear as small red bumps or raised papules, often clustered in areas covered by your swimsuit. The rash is intensely itchy and can include hive-like welts, making it look similar to a bad case of bug bites or contact dermatitis. Despite the name, sea lice aren’t actually lice at all, and what you’re seeing isn’t a bite. It’s a sting reaction from tiny jellyfish larvae too small to see with the naked eye.
What the Rash Looks Like
The rash from sea lice typically shows up as red, raised bumps (papules) scattered across the skin. Individual lesions range from a few millimeters to about a centimeter across. In mild cases, you might see a handful of small red dots that look like mosquito bites. In more severe reactions, the bumps can merge into larger red patches, sometimes with hive-like raised welts (urticaria) surrounding them. Some people develop tiny fluid-filled blisters within the rash.
The skin between the bumps often looks red and irritated. The rash is almost always intensely itchy, and scratching can make the bumps appear angrier and more swollen. In children, the reaction tends to be more pronounced, with larger, more widespread bumps.
Where It Shows Up on Your Body
The most distinctive feature of sea lice rash is its location: it concentrates under your swimsuit rather than on exposed skin. You’ll typically see it on your chest, abdomen, groin, and buttocks. Women often notice it under the edges of bikini tops and bottoms. Men commonly get it along the waistband area of swim trunks.
This pattern exists because of how the stinging actually happens. The larvae of a thimble jellyfish (or sometimes other jellyfish species) wash into your swimsuit while you’re in the ocean and get trapped against your skin. When you leave the water, the layer of water between your body and the suit drains away. The pressure and friction on the trapped larvae activates their stinging cells, injecting venom into your skin. That’s why the rash lines up so neatly with swimwear coverage. You may also notice stings in areas where skin folds trap the larvae, like the armpits or the crook of the elbow.
When Symptoms Appear
Some people feel a prickling or stinging sensation while still in the water, but the visible rash usually develops within a few hours of getting out of the ocean. It can take up to 24 hours for the full rash to appear. The itching and redness typically peak within the first two to three days, then gradually fade over one to two weeks. In some people, particularly those who’ve been stung before, the reaction can be more intense and last longer because the immune system mounts a stronger response on repeat exposure.
Flu-Like Symptoms in Some Cases
Most people only deal with the itchy rash. But some, especially children with a large number of stings, develop whole-body symptoms that resemble a mild flu. These can include fever, fatigue, nausea, vomiting, and abdominal cramps. These systemic symptoms generally last less than a week and resolve on their own. No cases of severe allergic reactions (anaphylaxis) have been reported from sea lice.
Sea Lice vs. Swimmer’s Itch
Sea lice and swimmer’s itch look similar but have different causes and show up in different places, both on your body and geographically. Swimmer’s itch (cercarial dermatitis) occurs in freshwater lakes and ponds and is caused by parasitic flatworm larvae released from freshwater snails. The rash from swimmer’s itch appears on exposed skin, the parts of your body not covered by a swimsuit, because the larvae penetrate bare skin directly from the water.
Sea lice rash, by contrast, is a saltwater condition caused by jellyfish larvae and clusters under swimwear. If you came out of the ocean and your rash is under your bathing suit, it’s almost certainly sea lice. If you were in a lake and the rash is on your arms and legs, swimmer’s itch is the more likely culprit. Both rashes look like red itchy bumps, but the location pattern and water type are the fastest way to tell them apart.
Where and When It Happens
Sea lice outbreaks are most common in warm ocean waters, particularly along the coasts of Florida, the Caribbean, and the Gulf of Mexico. The peak season runs from March through August, with the highest concentration of jellyfish larvae in the water during spring and early summer. Outbreaks tend to happen in waves, driven by ocean currents that push large numbers of larvae toward shore. Some beaches post warnings when sea lice are reported in the area.
How to Treat the Rash
The rash is self-limiting, meaning it will clear up on its own. To manage the itch in the meantime, over-the-counter hydrocortisone cream applied to the affected areas can reduce inflammation. Oral antihistamines help control itching, especially at night when it tends to feel worse. Cool compresses or colloidal oatmeal baths can also provide relief. Avoid scratching, which can break the skin and lead to secondary infection.
If your rash covers a large area, the itching is severe, or you develop fever and body aches, a doctor may prescribe a stronger topical steroid or a short course of oral steroids to calm the immune response.
How to Prevent It
The most effective prevention step is removing your swimsuit and showering with fresh water as soon as you leave the ocean. The key detail: take your suit off before showering. Showering while still wearing your swimsuit can actually make things worse by pushing trapped larvae harder against your skin and triggering more stings. Rinse your suit thoroughly before wearing it again, since dried larvae can retain their stinging ability.
Wearing tighter-fitting swimwear like rash guards can reduce the number of larvae that get trapped, though it won’t eliminate the risk entirely. Some evidence suggests that applying a barrier lotion before swimming may offer modest protection by making it harder for nematocysts to penetrate the skin. If you’re visiting a beach known for sea lice during peak season, checking with local lifeguards about current conditions is a practical first step.