This week a diver approaches the DAN experts to ask about “Hydroid Sting”:

Diver question: On a recent outing with my dive club, we spent some time kneeling in the sand while practicing skills. My knee is now irritated and covered in red bumps. What happened?

DAN Answer

hydroid_sting-1This particular pattern of red bumps is most likely the result of hydroid stings. These organisms, which can look like plants, seaweed or clumps of feathers, are actually invertebrates of phylum Cnidaria. Like their relatives, fire corals and jellyfish, these animals have tiny stinging cells known as nematocysts. Hydroids are carnivores and use their nematocysts to catch passing plankton and shrimp. The stinging nematocysts may also fire into the skin of unsuspecting divers, which results in a cluster of red welts and bumps that burn and itch.

To treat these injuries, rinse the skin immediately with seawater. Try to refrain from rubbing the area, as this may induce further nematocyst discharge (this is easier said than done). Never use fresh water as it may actually cause undischarged nematocysts present on the skin to fire. Once the area is well irrigated with salt water or saline, apply white household vinegar (four to six percent acetic acid) to help neutralise remaining nematocysts. If vinegar is not available, isopropyl (rubbing) alcohol may be used. Pain can be treated with the local application of heat or hot water.

Once neutralised and cleaned, topical steroids such as hydrocortisone and antihistamines such as Benadryl (diphenhydramine) are commonly employed to reduce local skin irritation and itching. If the reaction is severe or persistent or an allergic reaction occurs, seek medication attention immediately. Reduce your risk of contacting these organisms with good buoyancy control and exposure protection such as wetsuits or dive skins.

Answer provided by DAN’s Lana Sorrell, EMT, DMT, and Nicholas Bird, M.D., MM

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