Sea lice and swimmer's itch: what's actually biting you, and how to stop it

Quick answer: what is that itchy rash after swimming?

If it appeared under your swimsuit after swimming in warm salt water, it is most likely 'seabather's eruption' — commonly called sea lice — caused by tiny jellyfish or sea-anemone larvae trapped against your skin by the swimsuit, which then sting. If it appeared on exposed skin after fresh or brackish water, it is more likely 'swimmer's itch' (cercarial dermatitis), a reaction to a microscopic parasite of birds and snails that burrows into skin and dies (it cannot infect humans). Neither is caused by real lice, both are harmless, and both are self-limiting.

So the location and water type tell you which: under-the-swimsuit rash in the sea = sea lice; exposed-skin rash in fresh/brackish water = swimmer's itch. Either way, the response is to soothe the itch and wait — they clear on their own.

Sea lice (seabather's eruption)

'Sea lice' is a misleading nickname — there are no lice involved. The cause is the microscopic larvae of certain jellyfish and sea anemones, which get caught between your skin and your swimsuit (or in hair, or under a wetsuit). When you leave the water and the fabric presses them against you — or when you shower in fresh water while still wearing the suit — the larvae's stinging cells fire, producing an itchy, bumpy red rash exactly in the covered areas. It typically appears within hours, itches for several days, and then fades without lasting harm.

The tell is the pattern: the rash maps to where your swimsuit was, because that is where the larvae were trapped. This is why it is often mistaken for an allergy to the suit — but it is the larvae, and it is harmless and temporary.

• Cause: tiny jellyfish/anemone larvae trapped under the swimsuit.
• Rash appears in covered areas, within hours, itching for days.
• Harmless and self-limiting; not an allergy to the swimsuit itself.

Swimmer's itch (cercarial dermatitis)

Swimmer's itch is a different rash with a different cause. It comes from cercariae — the larval stage of certain flatworm parasites (schistosomes) that live in freshwater snails and infect water birds. These larvae can burrow into human skin by mistake, where they immediately die (they cannot survive in or infect people), but the body's reaction to them causes an itchy rash. It appears on exposed skin (the opposite pattern to sea lice), often in shallow, warm fresh or brackish water near shore where snails and birds are present, and clears over days.

So swimmer's itch is essentially an allergic-type reaction to a harmless-to-humans parasite that dies in your skin. Unpleasant and itchy, but not dangerous and not contagious — and the exposed-skin, freshwater pattern distinguishes it from sea lice.

How to treat and soothe the itch

Both rashes are treated the same way: soothe the itch and let it resolve. Over-the-counter antihistamines (oral or cream), hydrocortisone cream, calamine lotion, and cool compresses all help the itching; avoid scratching, which can break the skin and cause infection. The rash generally settles within a few days to a couple of weeks. For sea lice, do not shower in fresh water while still wearing the contaminated swimsuit, as this can trigger more larvae to sting — remove the suit first. Wash swimwear thoroughly (hot wash and dry) to remove any remaining larvae.

So the management is simple symptomatic relief plus patience. If the rash is severe, widely spread, blistering, or accompanied by fever or feeling unwell (rare), seek medical advice, but the overwhelming majority resolve with home soothing and time.

How to prevent both

Prevention differs slightly. For sea lice: remove your swimsuit and shower (ideally with the suit off) promptly after swimming, rinse and dry off, and heed local warnings when seabather's eruption is reported (it comes in seasonal outbreaks). A tighter or less absorbent suit traps fewer larvae; some people react less in rash guards. For swimmer's itch: avoid shallow, warm, snail-and-bird-frequented freshwater margins, towel off briskly immediately after leaving the water (which can knock off larvae before they burrow), and avoid attracting ducks to swimming areas.

So the shared prevention theme is: get out, get the suit off, rinse and towel down promptly. Doing that reduces both rashes, since it removes the trapped larvae (sea lice) or dislodges them before they penetrate (swimmer's itch), before they can cause much reaction.

When it's something else

Not every post-swim rash is sea lice or swimmer's itch. Jellyfish stings leave linear welts where a tentacle touched; heat rash appears in sweaty covered areas without water exposure; sun allergy (polymorphic light eruption) follows sun rather than water; and contact dermatitis can come from the swimsuit material or sunscreen. If a rash is very painful (not just itchy), shows linear sting marks, blisters severely, spreads with fever, or does not improve over a week or two, consider these alternatives and seek medical advice.

So use the pattern and context to distinguish the common, harmless swimmer's rashes from the less common ones that need attention. Sea lice and swimmer's itch are itchy nuisances that pass; a persistent, painful, or systemic reaction is a reason to get it looked at.

A note: general information, not medical advice

This guide is general safety and wellbeing information for beachgoers, not medical advice. Reactions to stings, infections and heat vary between people, and severe or worsening symptoms — spreading pain, difficulty breathing, high fever, or signs of a serious allergic reaction — need urgent professional medical care.

If you are unsure, seek advice from a lifeguard, pharmacist or doctor, and call your local emergency number for anything severe. When in doubt, get it checked.