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

Jellyfish stings vary greatly in severity. Most often they result in immediate pain and red, irritated marks on the skin. Some jellyfish stings may cause more whole-body (systemic) illness. And in rare cases jellyfish stings are life-threatening.

Most jellyfish stings get better with home treatment. Severe reactions require emergency medical care.

Common signs and symptoms of jellyfish stings include:

• Burning, prickling, stinging pain
• Red, brown or purplish tracks on the skin — a "print" of the tentacles' contact with your skin
• Itching
• 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. Signs and symptoms of severe jellyfish stings include:

• Stomach pain, nausea and vomiting
• Headache
• Muscle pain or spasms
• Weakness, drowsiness, fainting and confusion
• Difficulty breathing
• Heart problems

The severity of your reaction depends on:

• The type and size of the jellyfish
• Your age, size and health, with severe reactions more likely in children and people in poor health
• How long you were exposed to the stingers
• How much of your skin is affected

When to see a doctor

Seek emergency treatment if you have severe symptoms.

Jellyfish tentacles contain microscopic barbed stingers. Each stinger has a tiny bulb that holds venom and a coiled, sharp-tipped tube. The jellyfish uses the venom to protect itself and kill prey.

When you brush against a tentacle, tiny triggers on its surface release the stingers. The tube penetrates the skin and releases venom. It affects the immediate 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

While many types of jellyfish are relatively harmless to humans, some can cause severe pain and are more likely to cause a systemic reaction. These jellyfish cause more-serious problems in people:

• Box jellyfish. Box jellyfish can cause intense pain. Life-threatening reactions — although rare — are more common with this type. 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 on the water and acts as a sail.
• Sea nettle. Common in both warm and cool seawaters, sea nettles live along the northeast coast of the United States and are abundant in the Chesapeake Bay.
• 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 your risk of getting stung by jellyfish include:

• Swimming at times 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 hypersensitivity reaction, causing blisters, rash or other skin irritations one to two weeks after the sting
• 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 when jellyfish numbers are high.

You generally won't need to see your doctor for a jellyfish sting. If you do visit your doctor, he or she will be able to diagnose your injury by looking at it.

Sometimes treatment is based on the type of jellyfish that caused the sting. Your doctor may collect samples of the stingers.

Treatment for jellyfish includes first-aid care and medical treatment, depending on the type of jellyfish, the severity of the sting and your reaction to it.

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). If a thermometer isn't available, test the water on an uninjured person's hand or elbow — it should feel hot, not scalding. Keep the affected skin immersed or in a hot shower for 20 to 45 minutes.

Steps to avoid

These actions are unhelpful or unproved:

• Scraping out stingers
• Rinsing with seawater
• Rinsing with human urine
• Rinsing with 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, antivenin medication.
• Oral medicine. A rash or other skin reaction due to delayed hypersensitivity may be treated with oral antihistamines or corticosteroids. You may also be given oral pain medicine.
• Eye flushing. A jellyfish sting occurring on or near an eye requires immediate medical care for pain control and a good eye flushing. You will likely be seen by a doctor specializing in eye care (ophthalmologist).