Seabather's eruption (SBE), also known as "Sea Lice," is pruritic dermatitis found in a bathing suit distribution and at sites of friction after bathing in the ocean. The eruption is caused by two saltwater species of Cnidarians: the thimble Jellyfish (Linuche unguiculata) and a sea anemone (Edwardsiella lineata). Both of these species are small enough to become entrapped underneath swimwear. Pressure, as well as exposure to freshwater, lead to the discharge of a protective organ called a nemocytes. The nemocytes is a stinging organ that releases various antigenic toxins which induce a host immune response. L. unguiculata has been most commonly reported along the southeast coast of the United States, the Gulf of Mexico and the Caribbean, but has also been reported in Brazil and Papua New Guinea. E. lineata, on the other hand, has been identified as the culprit of SBE on the east coast of the United States from the mid-Atlantic up through New York. The larval form of L. unguiculata was initially thought to the sole cause of SBE, but there is some evidence implicating other stages of the L. unguiculata life cycle as well. Typically, this eruption is seen during the spring and summer with a higher incidence of cases in May and June. Children less than 15 years old have a higher risk of SBE compared to adults. This is likely due to children spending more time in the ocean compared to adults. Surfers are also at increased risk of SBE and may develop the eruption in locations of friction such as the chest, axilla, and abdomen. Those with a history of SBE are also at increased risk of SBE and tend to have more severe presentations. 
SBE is caused by the nemocytes of L. unguiculata and E. lineata. The organisms are retained under bathing suits after leaving the water. The nemocytes of the organism are discharged by pressure between the skin and the bathing suit leading to envenomation with antigenic toxins. These toxins then trigger an immune response. 
A biopsy is not necessary for diagnosis, and there is limited data on the histologic appearance of the SBE. The histologic presentation is similar to that of an arthropod bite. There will be a superficial and deep perivascular mixed infiltrate composed of neutrophils, lymphocytes, and eosinophils.
The first symptom of SBE is pruritis or stinging noted after leaving the water. A small number of patients may notice pruritis while bathing in the ocean. Since freshwater can cause the discharge of nemocytes, patients who have showered with their swimwear may report worsening of their symptoms after showering. SBE presents as discrete erythematous, pruritic papules that may progress to vesicles or pustules. Due to intense pruritis, the lesions may be difficult to identify due to excoriations and can be impetiginized at the time of presentation. The distribution of the lesions is underneath the bathing suit with lesions more commonly located in sites of higher pressure such as the waistband. Lesions can also be found in frictional sites such as the axilla or the chest and abdomen in surfers. Patients with hairy chests may also have lesions in that area as well. The duration of the lesion is about two weeks but can last for longer than a month in some patients. Systemic symptoms are rarely associated with SBE; however, fever, malaise, nausea, vomiting, and cramps have been reported. Systemic symptoms have typically lasted less than a week. There has been one case reported of a patient with SBE having a blurry vision that resolved without side effects after one week. SBE is a clinical diagnosis, with biopsy and laboratory studies not usually indicated. If needed, titers for L. unguiculata can be positive several weeks after exposure or in patients who have been re-exposed. Recurrence of the lesions can occur if the patient re-uses the bathing suit without washing it. Therefore, the patient should be counseled to wash any swimwear before using it again. 
Seabather's eruption is diagnosed based on the morphology of lesions, the distribution of the lesions, and the history of bathing in the ocean. Laboratory studies, biopsy, or radiographic studies are not helpful in diagnosing Seabather's eruption. Titers for L. unguiculate are available, but a positive titer is only suggestive of exposure to L. unguiculate and not diagnostic for the eruption. A biopsy of SBE will present as a not specific superficial, and deep mixed perivascular infiltrate that cannot be distinguished from an arthropod bite. Patients suspected of having SBE should have a complete skin exam to identify the distribution of the lesions and should be questioned about bathing in the ocean. A correct diagnosis of SBE relies on a thorough history and physical exam.
Upon leaving the water, 5% of acetic acid can inactivate any undischarged nemocytes. However, this will not prevent the eruption. Most patients will present after they have already developed the eruption. These patients are treated symptomatically with systemic antihistamines and topical corticosteroids to decrease the inflammatory response. A second-generation antihistamine, such as fexofenadine, can be useful for pruritis during the day and diphenhydramine can be helpful at night when there is less concern for drowsiness. Topical corticosteroids are also commonly prescribed for SBE with some variation in the class of steroids based on the location of the lesions. High potency topical steroids are avoided for lesions on the axilla, groin, or face as these regions are at increased risk of side effects. Other areas are treated with a high potency topical steroid such as clobetasol. Patients should also be advised to wash their bathing suit with hot water and detergent to remove any retained nemocytes. 
The differential diagnosis includes arthropod bites, folliculitis, swimmers itch, seaweed dermatitis, diver dermatitis and allergic contact dermatitis.
Seabather's eruption is relatively common and best managed by an interprofessional team that also includes nurses and pharmacists. The treatment is supportive and most people recover.
There is no way to prevent SBE except to stay out of the water. The patient should be advised that freshwater can cause the nemocytes to discharge. After bathing in the ocean, the patient should be advised to remove their bathing suit and rinse with fresh water which may help lessen the severity of the eruption.
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