Sea lice is the common name for tiny jellyfish or sea anemone larvae that get trapped under your swimsuit and sting your skin, causing an itchy, bumpy rash known medically as seabather’s eruption. The term is a bit misleading: these aren’t lice at all, and they’re essentially invisible in the water. Two species are responsible for most cases: the thimble jellyfish (Linuche unguiculata) and a sea anemone (Edwardsiella lineata). If you’ve come back from the beach with an unexplained rash under your swimsuit, this is likely what happened.
How Sea Lice Cause a Rash
The larvae of these jellyfish and anemones are barely visible to the naked eye, small enough to slip between the fibers of your swimsuit. Each larva carries nematocysts, the same microscopic stinging cells that adult jellyfish use. When the larvae get trapped between your skin and clothing, pressure from the fabric triggers these stinging cells to fire. That’s why the rash almost always appears in areas covered by your swimsuit, rash guard, or wetsuit rather than on exposed skin.
Freshwater also triggers the sting. If you step into a shower or rinse off with a garden hose while still wearing your swimsuit, the change from saltwater to freshwater causes any trapped larvae to release their stinging cells all at once. This is why many people notice the rash gets dramatically worse during or right after a post-beach shower.
What the Rash Looks and Feels Like
The rash typically appears within hours of leaving the water, though it can take up to a full day. It shows up as small, red, itchy bumps concentrated under the areas where your swimsuit sat against your skin: the waistband, chest, groin, and buttocks. In some people it looks like a cluster of tiny hives; in others it resembles a scattering of insect bites. The itching can range from mild and annoying to intense enough to disrupt sleep.
Some people, especially children, develop more than just a rash. Fever, nausea, headache, and general fatigue can accompany a more severe reaction. The rash itself usually lasts one to two weeks before fading on its own, though the itching tends to peak in the first few days.
Who Gets It Most Often
A prospective study of beachgoers identified three groups at the highest risk. Children under 16 are more likely to develop the rash and tend to have more severe reactions. Surfers are also at elevated risk, likely because they spend more time in the water and wear snug-fitting wetsuits that trap larvae effectively. The single strongest predictor, though, is having had seabather’s eruption before. People with a prior episode appear to have a sensitized immune response, making repeat reactions more likely and often more intense.
Where and When It Happens
Seabather’s eruption has been documented along Florida’s southern Atlantic coast since the early 1900s, and it occurs throughout the Caribbean, the Gulf of Mexico, and parts of the mid-Atlantic coast. The peak season runs roughly from March through August, with the worst months typically being late spring and early summer when jellyfish larvae are most abundant in warm, shallow water. Outbreaks are unpredictable. Some beaches go years or even decades without significant problems, then see a resurgence when ocean currents shift and carry larvae closer to shore.
How to Treat It
For most people, the rash resolves on its own. Over-the-counter hydrocortisone cream can reduce itching and inflammation at the sting sites. Oral antihistamines help with the itching, especially at night. Cool compresses also provide relief. For more severe reactions involving widespread rash, blistering, or systemic symptoms like fever, a doctor may prescribe a stronger topical steroid or a short course of oral steroids to bring the inflammation under control.
Prevention Tips That Actually Help
The most effective step is what you do immediately after leaving the water. Remove your swimsuit before rinsing off, not after. If possible, rinse first with clean saltwater, which won’t trigger the nematocysts the way freshwater does. Once you’ve removed your swimwear and rinsed with saltwater, a regular freshwater shower is fine.
After swimming, thoroughly wash all swimwear to remove any lingering larvae. Don’t simply hang your suit to dry and put it back on the next day. Dried larvae can still sting when the fabric gets wet again. And don’t share unwashed swimwear with anyone else.
Some beaches post purple flags or advisories when sea lice are detected in the water. Wearing less clothing in the water technically reduces the trapping effect, though that’s not always practical. Barrier creams marketed for jellyfish protection may offer some benefit, but removing your suit quickly afterward remains the most reliable prevention.
Sea Lice vs. Actual Sea Lice
To add to the confusion, “sea lice” is also the correct scientific term for a completely different organism: parasitic copepods (tiny crustaceans) that attach to fish, particularly salmon. These true sea lice, primarily a species called Lepeophtheirus salmonis, are a major problem in the fishing and aquaculture industries. They latch onto fish and feed on mucus, skin cells, and blood, progressing through multiple life stages from free-swimming larvae to adults that can move across a fish’s body and cause significant tissue damage.
These fish parasites do not affect humans. They can’t survive on human skin and pose no health risk to swimmers. If you’ve heard about sea lice in the context of salmon farming, that’s an entirely separate issue from the itchy rash beachgoers get in Florida and the Caribbean.