Stinging “No-See-Ums” of the Sea

Heading to a beach for Spring Break? VINEGAR (not ALCOHOL) is the ANSWER…

Have you ever jumped into the ocean and suddenly felt something stinging your arms or legs?  I’m not talking about jellyfish here, but the “no-see-ums” of the sea.

The picture above is from the Na’ Pali coastline in Hawaii. While snorkeling there, a small group of the swimmers (including myself and one daughter) were bothered enough by invisible “stings” to get back on the boat. Meanwhile the majority of snorkelers were completely fine, despite swimming right next to other people who were being stung. Back on the boat, mosquito bite-looking lesions appeared everywhere we felt stings, with some people having a great deal of surrounding redness and swelling. The red bumps continued to sting like a moderate bee sting for ten minutes or so, then simply seemed to react like a mosquito bite- more itchy than painful. Our mystery stings lasted a few days, then disappeared.

There are numerous names for rashes that crop up after swimming- sea bather’s eruption, ocean itch, and my personal favorite, sea lice, to name a few. What causes these invisible stings? Ultimately, some type of larval parasite burrows into a human instead of reaching its desired host (usually a water bird). The larvae come from corals, sea anemones and thimble jellyfish. These parasites cannot live, grow or reproduce in human skin, so they die. Unfortunately, in many but not ALL people, their presence sets off an allergic reaction– hence the red bumps and itch. With more intense allergic reactions, swimmers can develop headache, body aches, stomach aches, nausea, diarrhea and fever. Luckily the other symptoms are rare, so what most people deal with are the itchy, red bumps or blisters.

The medical name for this type of rash is cercarial dermatitis. If the stings occur on only on uncovered areas of the skin, there are typically fewer total red bumps. However, often sea bather’s dermatitis shows up beneath where a swimsuit covered the skin, because the larva can get trapped in the fabric. Swimmers can have literally hundreds of bites covering their torso.

What can you do? If you start to feel bites, remove your suit BEFORE you shower- fresh water often triggers the larvae to sting. Vinegar may neutralize the toxin and reduce additional stings- apply to your body and rinse your swimsuit in the vinegar as well.

Additional treatment is targeted at the allergic reaction. Consider taking an oral antihistamine such as name brands Benadryl, Claritin, Allegra or Zyrtec, and apply topical steroid cream or ointment on your torso or extremities (not on the face or genitals). Cool compresses may also help ease the itch or burn.

Is this rash contagious? NO- each bump is an allergic reaction to a larva burrowing into the skin, and the larva cannot be spread from one person’s body to another. Please note, however, if you borrowed a wet swimsuit from a friend, any larva trapped in the suit material could certainly sting you.

BOTTOM LINE: Itchy, burning stings or bites after swimming is a frustrating allergic reaction, but only rarely medically “scary”- simply remove your suit as soon as possible, rinse with vinegar if available, take an oral antihistamine and/or use topical steroid cream on torso or extremity lesions and your symptoms should resolve within days to a couple weeks (depending on the severity.)

9 thoughts on “Stinging “No-See-Ums” of the Sea”

  1. Thank you Dr. Grimes for sharing this experience!
    I grew up in the islands, moved away for a bit, then just moved home with my new nephews. We got back from the beach today and the little one was covered. We had no idea what was happening, as this has never happened to anyone I know.
    & I could not find any other posts/blogs mentioning this type of rash/sting. Thank you for the details and advice on recovery!

  2. Mahalo Dr. Grimes for the info. That’s exactly what I have from being in the ocean 2 days ago. I was at Nanakuli tracks beach went snorkeling felt plenty bites in the water but I couldn’t see them. I’ve been itching for the last 2 days and have bites that look like little flea bites, red in color probably cuz I’ve been scratching them. Can I use clobetasol cream? Mahalo

    • Jason, so sorry I didn’t see this when you posted! Obviously I trust you are long since healed, and your prescription steroid (clobetasol) would have been fine to use (except on the face or genitals).

  3. I just experienced this similar sting. Swimming in the ocean & felt this bee like sting on my chin. I honestly thought something split my chin open because the pain was so intense! Its been a day now & it has turned into this bump that with some water like puss. I even thought for a second maybe its a pimple? But no way, this was from the beach. I also got a headache! Never before has this happened. But glad to know more about this.

  4. A group of us were surf canoeing yesterday in Lahaina and went swimming past the reef in water that was about 50 feet deep. We all experienced tiny stings but didn’t see anything in the water. This morning we now have tiny itchy, red welts where we felt the stings. Will take some Benadryl and wash our swimming clothes in vinegar as suggested. Thanks for the information!


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