What does a jellyfish sting look like?

Red welts, lines of dots or a rash where the tentacles touched. Box jellyfish leave long whip-like welts with a ladder pattern, bluebottles leave chains of small beads, and mild species leave itchy red spots or bumps.

How long does a jellyfish sting last?

A mild sting fades within hours to a day or two. A severe box jellyfish sting can take weeks to heal. Sometimes the rash and itch return one to two weeks later.

What should I put on a jellyfish sting?

First aid comes first: vinegar for a box jellyfish or unknown sting, hot water for a bluebottle. After that, a cold pack; hydrocortisone cream and antihistamines on a pharmacist’s or doctor’s advice. Never alcohol or urine.

Do jellyfish stings leave scars?

Mild stings usually do not. Box jellyfish stings can leave dark or pale lines for months and sometimes scars. Protect the marks from the sun.

What should I do if I get stung by a jellyfish in Thailand?

Leave the water, alert a lifeguard, pour vinegar for at least 30 seconds if you do not know the species, and remove the tentacles. Call 1669 for breathing problems, severe pain or a stung child.

What a jellyfish sting looks like

A jellyfish sting shows as red welts, lines of dots or a rash where tentacles touched the skin, with burning pain and itch. Long whip-like welts with a ladder pattern point to a box jellyfish, a chain of small red beads to a bluebottle. With severe pain, a stung child or difficulty breathing, call 1669.

The mark on the skin often tells you which jellyfish it was — and that changes the first aid. If you are not sure, treat it as a box jellyfish sting, the most dangerous kind in Thailand.

Portuguese man o’ war sting marks on an arm and shoulder — thin, winding red lines — Portuguese man o’ war: thin, winding red lines where the tentacles touched the skin. © Txo, CC0 1.0 (Wikimedia Commons; modified: cropped and resized)

Man o’ war sting on a child’s forearm — a line of small red beads along the arm — A mild man o’ war sting: a chain of small red “beads” along the tentacle track. © Alewaliliha, CC BY-SA 3.0 (Wikimedia Commons; modified: cropped and resized)

Jellyfish sting rash on the side of the torso — clusters of small pink bumps — A mild jellyfish sting: an itchy rash of small pink bumps. © Jmarchn, CC BY-SA 3.0 (Wikimedia Commons; modified: cropped and resized)

Sting marks by type

Box jellyfish: long red or purple welts that look like whip lashes and often cross each other. They may show a ladder-like pattern across the lines. The pain is severe and immediate, and blisters can form within hours.

Bluebottle (Portuguese man o’ war): thin lines of small red or whitish beads and blotches, like a string of pearls. The track is usually shorter and finer than a box jellyfish’s, and the burning pain is milder.

Mild species: redness, small red spots or an itchy, hive-like rash. It often fades within a few hours.

How it feels and how long it lasts

Expect a sharp, burning pain at first, followed by itching. Mild stings fade within hours, bluebottle marks usually within a day or two, and box jellyfish welts can last for days or longer.

Delayed reactions happen: itching and a rash can flare up at the same spot one to two weeks later. After a severe sting, dark or pale marks may stay for weeks or months, and scarring is possible.

Call 1669 or go to hospital if

There is difficulty breathing, chest pain, weakness, confusion or collapse.

A large area is stung (for example, more than half an arm or leg), or the face, eyes or genitals; or the person stung is a child.

Severe back or belly pain, heavy sweating, vomiting, anxiety or a racing heart start 20–60 minutes after the sting. This is Irukandji syndrome from small box jellyfish, and the skin mark can be barely visible.

There are signs of allergy: hives all over the body, swelling of the lips or face, wheezing. If the person stops breathing, start CPR.

What to do right away

Get out of the water and call for help. For a box jellyfish or an unknown sting, pour vinegar over the skin for at least 30 seconds, then gently lift off any tentacles with tweezers or a gloved hand.

If it is definitely a bluebottle, skip the vinegar: rinse with seawater, remove the tentacles and soak the area in hot water at about 45 °C for 20–30 minutes (hot but not scalding). No hot water? Use a cold pack wrapped in cloth.

Do not rub the skin or scrub it with sand, and do not use fresh water, alcohol or urine. Do not apply a pressure bandage or tourniquet.

Skin care afterwards

Once first aid is done, a cold pack wrapped in cloth eases pain and itching. Antihistamine tablets and hydrocortisone cream can help — ask a pharmacist or doctor; both are sold in Thai pharmacies.

Do not scratch or pop blisters, and keep the marks out of the sun. See a doctor if after 2–3 days the redness spreads, the area feels hot, there is pus or a fever — these are signs of infection.

Sting or allergic reaction?

A normal sting stays where the tentacles touched. An allergic reaction spreads beyond it: hives over the body, swelling of the face, lips or tongue, wheezing, dizziness. This is anaphylaxis — call 1669 at once. If the person carries an adrenaline auto-injector, it should be used.