Sea Lice: That nasty rash you get after swimming in the ocean

Sea lice, the likely reason you’re itching after swimming in the ocean

Have you ever been ocean swimming and felt like you’ve got an itchy bite? or come out in a rash after open water swimming?

The chances are it’s the result of what’s commonly referred to as ‘sea lice’.

In fact, it’s not ‘sea lice’ or ‘sea louse’ in the way fishermen might think of it. This type of sea lice/louse is a marine parasite that affect fish by feeding on the mucus, epidermal tissue, and blood of host marine fish, and actually have nothing to do with the itchy rash you’ve experienced.

Sea louse, (the marine parasite version) was also thought to be responsible for the bleeding legs of a swimmer at Brighton in Melbourne – this is not the type of sea lice that this article refers to.

What is sea lice?

A red itchy looking sea lice rach on the torso of a swimmer.
Example of sea lice rash, in this case predominately in area under bikini strap where sea lice have been trapped.

‘Sea lice’ that cause ‘itchy bites’ are tiny little jellyfish or stinger larvae that have the same stinging cells (nematocysts) as an adult jellyfish/stinger, but because they’re only small they only affect a small area.

They tend to travel in large groups or ‘blooms’ and will be prevalent in warm summer waters – some ocean swimmers suggest you’ll encounter them when there is lots of seaweed floating around.

How do sea lice cause the itch?

Sea lice are microscopic and transparent in water so you’ve got no chance of seeing them, the first you’ll know about it is when you start to itch.

You’ll get the worst of it where the larvae get trapped under your swimsuit or rashie, or even under your armpit where they feel trapped causing the stinging cells to fire.

Sea lice treatment

  1. The worst thing you can do is itch the itch, you’re best to just flush your skin in fresh water when you get out – preferably with your swimsuit removed.
  2. When drying yourself off use a patting motion rather than rubbing the affected area.
  3. For the majority of people the itch will be the worst part, and it could be delayed and last for a few days to a week after.
  4. For some people it could get worse with a severe reaction, in which case you should seek further medical assistance.
  5. Give your swimsuit a good wash (try soaking in boiling water) before you wear it again just to make sure all the cells have been washed out.
  6. If you’re looking for relief treatment try vinegar, various ‘itchy bite’/rash creams, steroid creams or antihistamines – but check with your doctor first. Other remedies include, tea-tree oil and ice packs to help cool the heat in the rash. (See comments below for reader suggestions)

This post was first published in February 2012

This post was written by Andre Slade

Andre is the owner of OceanFit with over 20 years in the swimming education, lifeguarding and coastal safety industry.

22 thoughts on “Sea Lice: That nasty rash you get after swimming in the ocean

  1. Just got a nasty surprise of sea lice stings while snorkling in Haiti. Really painful for the first 24 hours and then just really itchy. Oh yeah, and ugly looking too! My back is covered where bathing suit band was, as well as arm, waist, and neck. Happened 5 days ago and it’s becoming the gift that keeps on giving.

  2. Thanks Andre, a very informative and simplistic explanation of what sea lice are. I didn’t realise they were actually a type of jelly fish so makes sense to use vinegar on the rash. I have them and received one lot on one arm then a few days later had another swim thinking the sea water would help the rash (not realising the rash was from sea lice) then gained another rash on the other arm which looked different. I am using Urticara Urens as homoeopathic and now Apple cider vinegar. Many thanks D.

    1. Hi Barb, the rash shouldn’t really spread if you’ve washed your body in fresh water (remember to pat yourself dry, not wipe). You should also make sure you wash your swimwear before wearing again – same goes for any clothing you may have worn after your swim. If your rash gets worse you should see a doctor. I hope that helps, and you feel better soon!

      1. hi! I know you said they shouldn’t spread out, but mine has. I got the first rash six hours after coming home from the beach this past Monday, and I’ve been getting them all over my back ever since. Today, I saw another rash on my thigh, and it really creeped me out. How long will it take for them to vanish? Thank you

  3. I wore my bathing suit 1 week ago in the ocean but no bites and then again yesterday and noticed a cluster of bites where one of my bikini straps was. I did not was the bikini top inbetween uses. How long can they survive on swimwear after initial contact?

  4. My daughter contracted sea lice the other day swimming at Lake Worth beach where was a lot of sea weed, and she was red all over her body. I prepare her bath with camomile tea (i cooked around 8 bags of tea and poor it in the bath water). Then i put very greasy children’s cream on her. She is much better today.

      1. Andre, I found your explanation and advice really helpful. I now suffer from severe allergic reactions to stingers, I’m predisposed I guess, because I react also to simple mosquito bites. Thankfully, I’ve avoided bee stings. My reaction to marine stingers is almost immediate, and within half an hour is severe. The welts come up, and then blisters form (bullous] and the itch is strong. The pain from the sting is only momentary. It’s the itching and swelling that becomes a problem, and for me the blisters can last for a week or two. The scars last for a long time, but eventually disappear. I’ve tried rashies for protection, even doubling, but that doesn’t work for me, so it’s a westsuit vest now (long arms]. So far the legs haven’t been hit, but if they do, then it will be a full wetsuit just so I can keep swimming. I take an antihistamine until the swelling stops, and steroid cream till the blisters subside. Hope this information helps someone else.

  5. Hi, went swimming and next morning had these “bites” on my body. After this I’ve cleaned everything and tried not to scratch like you said but the marks keep on appearing and disappearing. Same place different places. It’s like it can’t decide where to flare up. I this normal?

  6. Our daughter has just been told she has been bitten by sea lice. we are at Umina on Holidays. She has been given antibiotics to take and we will see how it goes. She was bitten around the right ear area.I was going to attach some photos but not sure how. Thanks

    1. Hi Craig, that doesn’t sound nice for your daughter at all! It’s very interesting to hear that she was given antibiotics for it, it must have been some rash! You could send photos to info@oceanfit.com.au, we’d love to see them and we can add them to our stock photos to show others what sealice rashes can look like. All the best to your daughter. Andre

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