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

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