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May 2005 Vol. 31, No. 5   RSS Feed for Undercurrent Issues
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They Sting, Itch, and Raise Angry Welts

— sea lice season is upon us again

from the May, 2005 issue of Undercurrent   Subscribe Now

This you knew: sea lice are not something you get from wearing your buddy’s dive hood. Nonetheless, the term “sea lice” evokes confusion when it’s used to describe the cause of a nasty, irritating, itchy rash that can appear after diving in the Caribbean or Florida waters.

In fact, the tiny culprit is not a louse at all. The aggravating rash comes compliments of stinging cells called nematocysts, most of which come from the larvae of thimble jellyfish. This confusion is compounded by the fact that the term “sea lice” is already used to describe several species of parasitic copepods in the family Caligidae.

The real culprit, thimble jellyfish larva, is too small to photograph, but even a brief encounter can yield dime-sized red welts that itch intensely for days. Serious exposure can cause nausea, vomiting, diarrhea, headache, muscle spasms, a sense of malaise, and trouble sleeping.

Subscriber Randy Harris (Trinidad, TX) describes sea lice irritation as “chiggers x 10.” Sea lice, which resemble specks of finely-ground pepper, appear in the waters off Florida, in the Gulf of Mexico, and in the wider Caribbean. The jellyfish larvae are covered with nematocytes, firing mechanisms that contain long, barbed filaments that can pierce the skin and inject a mixture of toxic substances. Firing is triggered when the larvae are disturbed either through friction, changes in osmotic pressure caused by the transition from salt to freshwater, or even the nematocyst’s drying out when a diver exits the water. The body’s reaction to the injected toxins is often immediate, but it can take up to twenty-four hours.

Sea lice can plague divers from March through September, but April through early June is the peak season. The highest concentrations usually occur between the surface and depths of 10-15 feet. Several readers suggested that women may be much more susceptible to sea lice stings than men.

Reader Alan Jenkin (Huntsville, TX) was snorkeling off Roatan in the month of April when he swam through what appeared to be a cloud of brown plankton. He soon realized he was swimming among sea lice. The resulting stings were described as “very painful for a few days.” The “brown cloud” he was swimming through was probably composed of adult thimble jellyfish, which are brown blobs about the size of a quarter that are actually less toxic than their larvae. But be aware that the larvae can be encountered even when no adults are visible.

“That night I started to develop huge bumps on my neck
that began to spread over my face and down my trunk.
There was nothing on the island to help the itching.”

Jane Gray (Charlotte, NC) was diving off Little Cayman in the month of June. “My first encounter was during a night dive. I noticed a slight stinging around my neck (I was wearing a full wetsuit). The next morning I only had a slight redness and no itching. However, during the next day’s dive, I again ran into an area with some slight stinging. That night I started to develop huge bumps on my neck that began to spread over my face and down my trunk. There was nothing on the island to help the itching, which was unbelievable.” Flying back to the States the next day, she immediately paid a call on her dermatologist, and, after skin scrapings, was told that “severe allergic reaction to jellyfish larvae was the probable culprit. Steroids and antihistamines helped, but the itching continued for a week and the red hives over my trunk and face were quite attractive!”

A few years ago, reader Joan Meskill (Seattle, WA) was literally covered with stings while diving from a Belize live-aboard in the month of April. “I was a new diver at the time. Lucky it happened at the end of the trip—the last two days. It was a long flight home, where I went immediately to the emergency room. I took a course of steroids for 5 days and had red patches on various parts of my body for months.”

How to Avoid Them

If it’s sea lice season, before diving ask the dive operator if there have been any recent encounters. Since the larvae are concentrated in shallow water, make a quick descent once you enter the water and, upon exit, make your shallow water stop around 20 feet instead of ten.

Because the larvae tend to be caught by fabric (like seining with a net), then activated by the friction between your skin and the fabric wherever it touches your body, wear a snug wetsuit or dive skin. Wearing a T-shirt or other loose-fitting garments will make matters worse, because the shirt can snare the nematocysts and rub them against your skin continuously until you take your shirt off. Also, upon exiting the water, remove your skin, wetsuit, or other clothing (to avoid having a freshwater rinse trigger the nematocyst’s firing), and then rinse yourself off immediately.

For even better protection, try a product called Sea Safe, which is formulated to prevent many jellyfish stings. Readers have reported various degrees of success with Sea Safe, but overall it appears to be an excellent preventive. Try your local dive store or buy it online at

Undercurrent readersare a resourceful bunch, and they reported many other methods for avoiding the tiny pests. Jack Hart (Conover, NC) wrote that he has almost eliminated his sea lice problems after observing his divemaster. “He came up under the boat, took his fins off, and let a bunch of air out of his regulator, which moves any larvae out to the sides, then came up and out of the water fast. I tried it and only got a couple of the lice marks on my neck; almost everyone else on the boat that week was covered in them.” One reader suggested that, “For exposed skin like the face a good application of viscous lotion or petroleum jelly can do the trick.”

And If You Do Get the Barb?

Start by immediately applying a mixture of isopropyl alcohol and vinegar. Lacking that, try pure vinegar or even Windex. Next, apply a hydrocortisone cream/lotion twice a day. Calamine lotion can also be helpful in reducing the itch. As with most allergic skin reactions, a dose of oral antihistamine (e.g., Benadryl, Claritin, Tavist) can help, but factor in how side effects like drowsiness could affect your activities.

Readers were equally creative about devising remedies and weeding out the ones that didn’t work very well. Etola Zinni (Villa Park, IL), who had a sea lice encounter on Bimini when she swam through some bands of floating seaweed to get to deep water, said she was given Right Guard (yes...spray deodorant applied directly to the itchy welts only served to burn like heck), udder balm, an aloe salve, and mouthwash. Nothing worked. At the island’s medical center, she was given a cortisone shot, cortisone pills, and cortisone cream. These did work. Etola now stocks her own dive first-aid kit with prescribed prednisone pills.

Reader Mary Chipman (Singer Island, FL), who is sealice- savvy because she lives and deals with them annually in south Florida, says “Safe Sea works as well as anything as a preventive measure. However, the best relief is Tend Skin, which is basically salicylic acid. When you put it on the sea lice sore, there is an intense burning sensation that lasts for a minute or two. Then the itching and pain goes away for hours.”

No matter what remedy you try, remember that home remedies only address mild to moderate reactions. Some reactions to the stings can be severe, and, if nothing is helping, it’s time to find a physician.

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