You're at the beach, the water is perfect, and you feel a tiny prickle under your swimsuit. You ignore it. Then another one. By the time you get back to the hotel, your skin is screaming. It’s not actually lice. Not the kind that jump around on kids' heads in elementary school, anyway. We’re talking about "thimble jellyfish" larvae, specifically Linuche unguiculata. Most people call them sea lice because, honestly, the name just stuck, but they are actually microscopic jellyfish babies packed with stinging cells called nematocysts. If you've ever wondered how to treat sea lice without losing your mind to the itch, you need to understand that you aren't dealing with a bite. You’re dealing with a thousand tiny venom injections trapped against your skin.
It's miserable.
The rash, often called "seabather's eruption," shows up exactly where your swimwear was clinging to you. Why? Because the fabric acts like a sieve, trapping the larvae against your body. When you walk out of the ocean, the pressure of the suit—or even rinsing off with fresh water while still wearing the suit—triggers those little stingers. They freak out and fire.
The immediate "don't do this" list
Seriously, don't jump straight into a fresh-water shower with your swimsuit on. That is the single biggest mistake people make. Fresh water changes the osmotic pressure around the stinging cells, causing any larvae still stuck in the fabric to fire their venom all at once. You'll go from a mild tingle to a full-blown inflammatory nightmare in seconds.
Instead, get that suit off. Fast.
If you are still at the beach and realize you’ve been hit, the best thing you can do is rinse with salt water—the same water you were just swimming in. It won't trigger the cells. Once the suit is off, you can use a tool like a credit card or a tongue depressor to gently scrape the skin. This helps remove any remaining larvae or stinging cells that haven't fired yet. Some people swear by rinsing with white vinegar. While vinegar is the gold standard for some jellyfish stings, the Florida Department of Health notes that for sea lice specifically, it’s a bit of a toss-up. It might help, or it might just irritate the skin more.
💡 You might also like: Why the Long Head of the Tricep is the Secret to Huge Arms
How to treat sea lice once the rash hits
Once the red bumps appear, you’re looking at an immune response. Your body is reacting to the toxin. Most cases are mild, but "mild" is a relative term when you can't stop scratching.
Hydrocortisone cream (1%) is your first line of defense. Apply it thin and often. It helps dampen the inflammatory cascade. If the itch is keeping you up at night—which it usually does—you’re going to want an oral antihistamine. Benadryl (diphenhydramine) is the classic choice because it knocks you out while it works, but non-drowsy options like Claritin or Zyrtec are better for daytime.
Actually, many dermatologists, including those who see this frequently in South Florida or the Caribbean, suggest a "double-barrel" approach. Use the cream for the localized spots and the pills for the systemic itch.
- Colloidal oatmeal baths: Like Aveeno. It sounds old-fashioned, but it coats the skin and really takes the edge off.
- Calamine lotion: It’s messy and pink, but the cooling effect is legit.
- Ice packs: Heat is your enemy here. Heat dilates blood vessels and can actually make the itching feel more intense. Cold constricts them.
If you start running a fever or notice the bumps are turning into blisters with pus, stop the home treatment. That's a sign of a secondary bacterial infection—usually from scratching with dirty fingernails—or a severe allergic reaction. At that point, you need a doctor for real-deal prescription steroid creams or even a round of oral prednisone.
Why your swimsuit is the enemy
You have to realize that the larvae are tiny. Smaller than a grain of pepper. They get caught in the weave of the fabric.
📖 Related: Why the Dead Bug Exercise Ball Routine is the Best Core Workout You Aren't Doing Right
If you just wash your suit in the sink and hang it to dry, you’re asking for a round two. Those stinging cells can stay active for a surprisingly long time, even after the larvae are dead. There are documented cases of people getting "re-stung" weeks later by putting on a swimsuit that wasn't properly cleaned.
Basically, you need to nuke it. Wash your swimwear in hot water with plenty of detergent and then throw it in the dryer on high heat. This breaks down the proteins in the venom and ensures any lingering nematocysts are neutralized. Honestly, if it's a cheap suit and the rash was bad enough, some people just toss the suit in the trash to be safe. I don't blame them.
Real-world prevention (because the itch isn't worth it)
The best way to figure out how to treat sea lice is to not get them in the first place. Check the beach flags. In places like Florida or Mexico, lifeguards will often fly a purple flag for "dangerous marine life." Most people think that means sharks or man-o-wars, but often it’s a warning for sea lice during their peak season, which is usually between March and August.
There is also a product called Safe Sea. It’s a specialized sunscreen that contains ingredients designed to deactivate the stinging mechanism of jellyfish and sea lice. It’s not 100% foolproof, but many frequent divers and long-distance swimmers swear by it. It makes the skin "slippery" to the larvae so they can't get a grip.
Also, consider what you wear.
👉 See also: Why Raw Milk Is Bad: What Enthusiasts Often Ignore About The Science
Loose-fitting swimsuits or "board shorts" provide more "trapping" area than a tight Speedo or a bikini. If you’re diving or snorkeling in infested waters, a wetsuit or a "stinger suit" (a thin lycra suit) can help, but you have to make sure the cuffs at the wrists and ankles are tight. Otherwise, the larvae get inside the suit, and then they’re trapped against your entire body. That leads to the "full-body" rash, which is a level of discomfort I wouldn't wish on my worst enemy.
The biological weirdness of sea lice
It’s kind of fascinating if it wasn't so painful. These larvae are just trying to grow up into thimble jellyfish. They aren't trying to attack humans. We are just large, salty obstacles that get in their way. The stinging is an automatic reflex.
What’s wild is that not everyone reacts the same way. You can have two people swimming in the exact same spot; one comes out covered in welts, and the other is fine. It’s a matter of individual sensitivity to the toxin. Some people's immune systems just flip out more than others. If you have a history of bad reactions to bee stings or other jellyfish, you’re likely to have a harder time with sea lice.
Moving forward: Your action plan
If you are currently scratching while reading this, here is exactly what you should do right now:
- Strip down. Get that swimsuit off and keep it away from your skin.
- Rinse with white vinegar if you have it, but don't scrub. Scrubbing just pushes the stingers deeper.
- Take a cool shower. Skip the loofah. Use a mild, fragrance-free soap.
- Apply 1% hydrocortisone. Do this three to four times a day.
- Get some antihistamines. If it's night, go for the Benadryl. If it's morning, grab some Cetirizine (Zyrtec).
- Launder everything. Put your swimsuit, towels, and even your beach cover-up through a hot wash and a high-heat dry cycle.
- Monitor for infection. If the redness starts spreading in streaks or you feel "flu-ish," get to an urgent care.
The rash usually clears up on its own in about 2 to 4 days, but for some people, it can linger for a couple of weeks. Just remember: don't scratch. Easier said than done, I know. But scratching breaks the skin and leads to scars and infections that last way longer than the initial sting. Stay cool, stay medicated, and maybe stick to the hotel pool for the next day or two.