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Volume 28 Number 3

By Frank Hubbell, DO

May 2015 – St. John, USVI

A spectacular day of snorkeling in the Caribbean Sea off the island of St. John, USVI is temporarily interrupted by an itchy, red rash on the sides and abdomen of one of the swimmers. What the heck is that all about?

She states that she did not feel any pain, stings, or itchiness while in the water. The rash came on about an hour after getting out of the ocean and seemed to coincide with jumping into the freshwater pool back at their villa to rinse off. She states that as she got out of the pool, she felt stinging sensations on her sides and abdomen. While snorkeling, she had been wearing a t-shirt over her bathing suit to protect her back from sunburn.

After a bit of research, it turns out that the rash is referred to as a seabather’s eruption or pica pica, and is caused by “sea lice.” The misnomer is that the rash is not caused by “sea lice,” which are fish ectoparasites, but instead, by the teeny, tiny larvae of the thimble jellyfish.

The Thimble Jellyfish, Linuche unguiculata, also known as a sea thimble or button jellyfish is a small, (strangely enough) thimble-sized, 13 – 20mm jellyfish. Even though it does have nematocysts, stinging cells, it is not considered a threat to humans.

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