|I recently dived at Sipadan Island, where divers are not allowed to wear gloves. At one point I accidentally brushed my hand against something that stung me. I didn’t see any fire coral or wildlife in proximity, and I didn’t experience any immediate redness or rash. One week after returning home, however, I developed a raised red rash with clustered, firm, small, round, red bumps where I had been stung. Sometimes the rash itches, and at other times it is tender to the touch, similar to a burn. It has worsened since I first noticed it two days ago; topical cortisone and tea tree oil have soothed but not diminished it. What could have caused my rash, and how should I treat it?|
It is impossible to determine precisely what stung you, but your symptoms are consistent with an injury from a sponge, jellyfish or hydroid. Hydroids may attach to any solid structure in the ocean and can be found on mooring lines and other objects.
Initial treatment of these types of stings consists of washing the area of contact with salt water, applying vinegar and treating pain with hot water (as tolerated). It is common for the rash to improve and then get worse again before it heals. At this point in your injury, apply topical steroids, and use heat to treat pain. If you see no improvement in a week or two, see a doctor; you might need oral steroids. The doctor will not need to know what organism stung you and can treat the wound like any other contact dermatitis.
— Travis Ward, EMT-P