Seabather's eruption is a pruritic dermatitis caused by a hypersensitivity reaction to the immature nematocysts of larval-stage Thimble Jellyfish (Linuche unguiculata), sea anemones (Edwardsiella lineata) and other larval cnidarians.
It should not be confused with Swimmer's itch. However, the term "sea lice" is also sometimes used to describe Seabather's eruption.
This reaction occurs mainly in the tropics and in Florida, due to the proximity of the Gulf Stream to the U.S. state. It has been identified in other locations as well.
At certain times of the year this can be a problem in some areas of the Bahamas particularly around New Providence.
Cases have been found in Bermuda in 2011, 2014 and 2015 with Paradise Lakes proving particularly bad. In 2015 many felt the effects after swimming in Mangrove Bay and Harrington Sound.
Seabather's eruption is also an issue in various parts in Anilao and Mindoro in the Philippines. This typically occurs within 30 ft. below surface, just above the coral reefs. Snorkelers are most prone because of their proximity to the surface. Scuba divers encounter less of this, and usually in shore dives. The occurrence of stinging is highly probable whenever plankton is abundant.
Occurrences in Thailand indicate that the eruption isn't an isolated issue for Florida. As recently as October 2007, individual reports indicate that cnidarian larvae capable of causing Seabather's eruption are present in the Gulf of Siam along the western coast area near Hua Hin. In January 2014, reports of larvae and Seabather's eruption occurred around Koh Phi Phi and Ko Yao Noi. An outbreak in Koh Phi Phi occurred in March, 2016.
Cases were first identified in Brazil in 2001.
Swimmers at Auckland's east coast beaches and in the rest of the Hauraki Gulf can suffer seabather's eruption, typically during summer.
The reaction is identified by severe itching around small red papules 1mm to 1.5 cm in size located on areas of skin that were covered by water-permeable clothing or hair during ocean swimming. Initial swimmer exposure to the free-floating larvae produces no effects, as each organism possesses only a single undeveloped nematocyst which is inactive while suspended in sea water. However, due to their microscopic size and sticky bodies, large concentrations of larvae can become trapped in minute gaps between skin and clothing or hair. Once the swimmer leaves the ocean, the organisms stuck against the skin die and automatically discharge their nematocysts when crushed, dried out, or exposed to fresh water. This is why symptoms usually do not appear until the swimmer dries themselves in the sun or takes a freshwater shower without first removing the affected clothing.
Treatment is symptomatic.
Treatment does not require a doctor's attention unless the case is severe, with most affected using a topical anti-itch cream (diphenhydramine) and a cortisone solution (hydrocortisone). Do not scratch the area, and avoid any clothing that may irritate the affected area; scratching will result in localized swelling and intense itching.
Upon exiting the water, prompt removal of swim clothing (while it is still wet) followed by a warm sea-water shower largely negates the risk of Seabather's eruption even in endemic areas. A hot freshwater shower with soap (paying particular attention to the hair and areas covered by the suit) is a somewhat less-effective alternative if uncontaminated seawater is unavailable. The contaminated swimsuit should be machine washed with laundry soap and dried in warm air.
Animals can be affected as well, and a cortisone solution for humans can be used on dogs.
Symptoms generally arise later after one takes a shower. It is unusual to notice the eruptions immediately. Symptoms can last from a few days up to two weeks, the shorter time being the norm.