Jellyfish stings are fairly common problems for people swimming, wading or diving in oceans. The long tentacles trailing from the jellyfish can inject venom from thousands of microscopic barbed stingers.

Most often jellyfish stings cause instant pain and inflamed marks on the skin. Some stings may cause more whole-body (systemic) illness. And in rare cases they're life-threatening.

Most jellyfish stings get better over a few days or weeks with home treatment. Severe reactions likely need emergency medical care.

Symptoms of jellyfish stings include:

• Burning, prickling, stinging pain
• Welts or tracks on the skin — a "print" of the tentacles' contact with the skin
• Itchiness (pruritus)
• Swelling
• Throbbing pain that radiates up a leg or an arm

Severe jellyfish stings can affect multiple body systems. These reactions may appear rapidly or several hours after the stings. Symptoms of severe jellyfish stings include:

• Stomach pain, nausea and vomiting
• Headache
• Muscle pain or spasms
• Faintness, dizziness or confusion
• Difficulty breathing
• Heart problems

The severity of a reaction depends on:

• The type and size of the jellyfish
• The age, size and health of the person affected, with severe reactions more likely in children
• How long the person was exposed to the stingers
• How much of the skin is affected

When to see a doctor

Seek emergency treatment if you have severe symptoms.

See your health care provider if your symptoms worsen or the wound shows symptoms of infection.

Jellyfish stings are caused by brushing against a jellyfish tentacle. Tentacles have thousands of microscopic barbed stingers. Each stinger has a tiny bulb that holds venom and a coiled, sharp-tipped tube.

When you brush against a tentacle, tiny triggers on its surface release the stingers. The tube pierces the skin and releases venom. It affects the area of contact and may enter the bloodstream.

Jellyfish that have washed up on a beach may still release venomous stingers if touched.

Types of jellyfish

Many types of jellyfish are fairly harmless to humans. Others can cause severe pain and a full-body (systemic) reaction. These jellyfish cause more-serious problems in people:

• Box jellyfish. Box jellyfish can cause intense pain and, rarely, life-threatening reactions. The more dangerous species of box jellyfish are in the warm waters of the Pacific and Indian oceans.
• Portuguese man-of-war. Also called bluebottle jellyfish, Portuguese man-of-war jellyfish live mostly in warmer seas. This type has a blue or purplish gas-filled bubble that keeps it afloat.
• Sea nettle. Common in both warm and cool seawaters.
• Lion's mane jellyfish. These are the world's largest jellyfish, with a body diameter of more than 3 feet (1 meter). They're most common in cooler, northern regions of the Pacific and Atlantic oceans.

Conditions that increase the risk of jellyfish stings:

• Swimming when jellyfish appear in large numbers (a jellyfish bloom)
• Swimming or diving in jellyfish areas without protective clothing
• Playing or sunbathing where jellyfish are washed up on the beach
• Swimming in a place known to have many jellyfish

Possible complications of a jellyfish sting include:

• Delayed skin reaction, causing blisters, rash or other irritation
• Irukandji syndrome, which causes chest and stomach pain, high blood pressure, and heart problems

The following tips can help you avoid jellyfish stings:

• Wear a protective suit. When swimming or diving in areas where jellyfish stings are possible, wear a wet suit or other protective clothing. Diving stores sell protective "skin suits" or "stinger suits" made of thin, high-tech fabric. Consider protective footwear, as stings can also occur while wading in shallow water.
• Get information about conditions. Talk to lifeguards, local residents or officials with a local health department before swimming or diving in coastal waters, especially in areas where jellyfish are common.
• Avoid water during jellyfish season. Stay out of the water in jellyfish areas when jellyfish numbers are high.

Diagnosing jellyfish stings generally doesn't require a visit to a health care provider. If you do go, your provider will likely be able to diagnose your injury by looking at it.

Your health care provider may collect samples of the stingers to help guide treatment.

Treatment for jellyfish stings includes first-aid care and medical treatment.

First-aid care

Most jellyfish stings can be treated as follows:

1. Carefully pluck visible tentacles with a fine tweezers.
2. Soak the skin in hot water. Use water that's 110 to 113 F (43 to 45 C). It should feel hot, not scalding. Keep the affected skin immersed or in a hot shower until the pain eases, which might be 20 to 45 minutes.
3. Apply 0.5% to 1% hydrocortisone cream or ointment twice a day to the affected skin.

Steps to avoid

These actions are unhelpful or unproved:

• Scraping out stingers
• Rinsing with human urine
• Rinsing with cold, fresh water
• Applying meat tenderizer
• Applying alcohol, ethanol or ammonia
• Rubbing with a towel
• Applying pressure bandages

Medical treatment

• Emergency care. Someone having a severe reaction to a jellyfish sting may need cardiopulmonary resuscitation (CPR), life support or, if the sting is from a box jellyfish, antivenom medication.
• Oral medicine. A delayed rash or other skin reaction may be treated with oral antihistamines or corticosteroids. You may also be given oral pain medicine.
• Eye flushing. A jellyfish sting on or near the eye generally requires immediate medical care to control pain and flush the eye.