The term “sea lice” is a common misnomer for the tiny, microscopic larvae of marine cnidarians. These minute organisms float freely in warm ocean waters. The resulting rash, properly known as seabather’s eruption, occurs when these larvae become trapped against a swimmer’s skin, most often beneath swimwear or hair. When subjected to pressure, friction, or exposure to fresh water, the larvae’s specialized stinging cells, called nematocysts, discharge a toxic venom. This release of venom triggers a localized allergic or inflammatory reaction in the skin, leading to the characteristic rash.
Identifying the Cause of the Rash
The initial sign of exposure may be a prickly or tingling sensation felt while still in the water or immediately upon exiting, caused by the initial activation of the larvae’s stinging cells. The visible rash typically develops a few hours after swimming, often appearing between four and twenty-four hours later.
The rash presents as intensely itchy, small red bumps, papules, or hives, and sometimes may include small blisters. A defining feature is the location of the outbreak, which is predominantly confined to areas covered by swimwear, wetsuits, or hair, where the larvae were trapped. Constitutional symptoms, such as fever, headache, chills, or nausea, can occur, especially in children or in severe cases. The rash is most common during the warmer months, generally from March through August, in tropical and subtropical coastal areas like the Caribbean, Florida, and the Gulf of Mexico.
Immediate Relief and Home Treatment
The moment a tingling sensation is felt, the immediate action should be to remove the contaminated swimwear as quickly as possible. Do not attempt to rinse the body with fresh water while the suit is still on, as the change in salinity can cause the remaining nematocysts to fire their venom. Instead, exit the water and immediately rinse the affected areas with uncontaminated salt water.
Once the suit is removed, a mild acid can be applied to the skin to help neutralize any remaining toxin. Diluted white vinegar or rubbing alcohol is commonly used for this purpose, gently applied to the affected skin. After this initial neutralization, a shower with fresh water and soap can be taken, using a gentle patting motion to dry the skin rather than rubbing, which can activate any lingering nematocysts.
Managing the resulting dermatitis focuses on reducing inflammation and controlling the intense itching. Over-the-counter hydrocortisone cream, typically a one percent concentration, can be applied to the rash to alleviate swelling and redness. Oral antihistamines, such as diphenhydramine or cetirizine, help to calm the body’s allergic response and reduce the persistent itching. Cool compresses or ice packs wrapped in a cloth can also provide localized relief from the discomfort and burning sensation.
It is generally not necessary to seek medical treatment, as the rash typically resolves within one to two weeks with home care. However, medical attention is warranted if the rash covers a large area, if you develop systemic symptoms like a high fever, severe nausea, or dizziness, or if there are signs of a secondary bacterial infection. Signs of infection include increasing pain, warmth, redness, or the presence of pus draining from the bumps. A healthcare provider may prescribe a stronger topical or oral corticosteroid for more severe reactions.
Decontaminating Swimwear and Gear
The microscopic larvae and their discharged stinging cells can remain trapped within the fibers of swimwear and gear, leading to a recurrent rash upon re-wearing the items. A simple rinse or air-drying is insufficient and risks re-exposure, as the nematocysts can reactivate when rehydrated. The most effective decontamination method involves the use of heat and chemicals to destroy the remaining organisms and venom.
All exposed items, including bathing suits, rash guards, and towels, should be washed thoroughly with detergent, preferably using the hot water setting on a washing machine. For materials sensitive to high heat, like wetsuits, soaking the items in a solution of water and vinegar before a thorough rinse and wash can help neutralize the toxins. Following the wash cycle, it is important to dry the items using a high-heat setting in a clothes dryer.
Prevention Strategies for Future Exposure
Avoiding areas where larvae are present is the most straightforward prevention strategy, which involves checking local health department advisories and beach warning flags before entering the water. Outbreaks are often localized and correlated with specific ocean currents and seasonal changes. If an outbreak is reported, it is best to postpone swimming in that area.
The choice of swimwear can also influence the risk of trapping larvae. Tighter-fitting garments are less likely to trap larvae than loose-fitting clothing, such as T-shirts worn in the water, which can gather the organisms. Immediately removing all wet swimwear and showering promptly after leaving the ocean is a simple but effective measure to minimize the time the larvae spend against the skin.
Some specialized sunscreens and barrier lotions are formulated to inhibit the discharge of nematocysts upon contact. These products work by creating a chemical barrier that prevents the stinging cells from firing when compressed against the skin. While not a guarantee against all stings, applying such a lotion to all exposed and covered skin areas before swimming can add a layer of protection against the larvae.