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). These 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 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, GI symptoms and fever. Primarily what is seen, however, are itchy red bumps or blisters.
The medical name for this type of rash is cercarial dermatitis. If the stings occur on exposed areas, 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 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? Absolutely not- 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. 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.)