Seaweed dermatitis

What is it?

Seaweed dermatitis is a skin rash caused by direct contact with a poisonous type of seaweed (alga), most commonly Lyngbya majuscula. It is different to sea bather's eruption, which is due to stings from larval forms of certain sea anemones and thimble jellyfishes, or swimmer's itch, which is due to a bite from parasitic schistosomes (flatworms).

What causes it?

There are over 3,000 species of algae, which can range in size from 1 micron to 100 meters in length. Like all plants, some can be harmful to humans. Seaweed dermatitis is caused by direct contact with Lyngbya majuscula (also known as Microcoleus lyngbyaceus). The fine, hairlike, dark-brown seaweed, commonly known as lyngbya, is distributed worldwide. Lyngbya can be found in certain tropical and sub-tropical shoreline waters but only at certain times of the year, when they can bloom. Seaweed dermatitis is due to toxins lyngbyatoxin A and debromoaplysiatoxin produced by the seaweed. The toxicity of this seaweed varies greatly depending upon region, season, and type. Not all strains of this seaweed are toxic.

What does Lyngbya look like?

Lyngbya majuscula is a blue-green alga. It usually grows in clumps, looking like dark, matted masses of hair or felt. Most often this seaweed is blackish-green or olive-green, but it also grows in shades of grey, red or yellow. The filaments can grow up to 10 centimetres long, and often become tangled with other seaweeds on reef flats, in tide pools or water as deep as 30 meters.

How does Lyngbya cause seaweed dermatitis?

While swimming, or wading in areas where the seaweed grows, small fragments of the seaweed can get caught between the swimming costume (or wet suit) and the skin. On coming out of the water, any seaweed on uncovered sites dries or is rinsed off, but any fragments caught under the swimming costume remain in moist contact with the skin. The pressure of the swimming costume on the skin then rubs the seaweed's toxin into the skin. The reaction may start a few minutes to a few hours after the victim leaves the water.

What does the rash look like?

Symptoms include itching and burning minutes or even up to 24 hours after leaving the water. A red, sometimes blistering rash occurs, sometimes in an entire swimsuit pattern. It often affects men in the scrotum and females under the breasts, but this depends on the type of swimwear used. Other symptoms include swollen eyes, irritation of the nose and throat, skin sores, headache, and fatigue. Symptoms typically last 4 to 48 hours. In more severe cases, skin sores may appear, which can last up to several weeks.

Sometimes a rash also can occur on the face and in the eyes and mouth. Some victims have swelling of eyes and mouth, but no rash.

How do you treat it?

For mild to moderate contact with the seaweed, remove your swimsuit immediately and wash skin vigorously with soap and water. Wash the swimsuit as well. The rash can be treated as a sunburn using wet towels and soothing creams (eg, calamine). A rinse with isopropyl (rubbing) alcohol may also help to decontaminate the skin. For any rash, 1 % hydrocortisone lotion should be applied twice a day. If the reaction is severe, oral steroids (prednisone) may be required.

Irrigate exposed eyes with tap water for at least 15 minutes. Any difficulty breathing may signal an allergic reaction. If the wound shows any evidence of infection, antibiotics may be needed.

How do you keep from getting it?

The only sure way to avoid seaweed dermatitis is to avoid swimming in the ocean. However, if you choose to swim in the sea, avoid waters where seaweed blooms have been reported. Health authorities generally keep a close eye on algal blooms and report them through the media. In some countries, health authorities have powers to close public beaches.

Take a shower or bathe with lots of soap and water as soon as possible. Thoroughly wash swimsuits, towels, and any associated swim gear to get rid of any attached algae.

Other information

It is also called stinging seaweed. The people in Hawaii call a variety of marine organisms stinging limu. Epidemics of this seaweed-induced rash occasionally occur in the Pacific. In Hawaii, the highest number of cases occur during the summer in windward swimming areas. Persistent trade winds blowing during these summer months may dislodge the seaweed from the bottom. Fragments then drift into swimming bays and beaches.

Other marine organisms, including tiny jellyfish (sea bather's eruption) and flatworm larvae (swimmer's itch), can cause similar-looking skin rashes. Distinguishing these from this seaweed dermatitis is often tricky.