Jellyfish Sting FAQs: What You Should Know Before Entering the Water!

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Medical Author: Melissa Conrad Stoppler, MD
Medical Editor: Jay W. Marks, MD

Jellyfish stings are an all-too-common health hazard for beach vacationers. While most jellyfish stings are harmless (with a few exceptions), they can be extremely painful. The stinging sensation results when stingers at the ends of the tentacles of jellyfish and other aquatic animals come into contact with human skin, usually while wading or swimming in the ocean.

Jellyfish belong to several different classes of invertebrates:

• Hydrozoans (for example, the Portuguese man-of-war and fire corals)
• Scyphozoans, the "true" jellyfish and the most common class
• Cubozoans, for example the most toxic "box jellyfish"
• Anthrozoans (sea anemones and corals, which are related to jellyfish but are only minimally poisonous to humans)

Stings from true jellyfish (scyphozoans) are generally less toxic than those of the hydrozoans and cubozoans and usually result in injury only to the parts of the skin where contact with the tentacles occurs. The skin often displays a painful, itchy, and raised red rash that may persist for days or weeks. It is possible for an allergic reaction to occur that further increases the inflammation and severity of the rash.

The sting of the Portuguese man-of-war is more painful than that of the common jellyfish. Some people have described a sting from the Portuguese man-of-war as a feeling like being struck by a lightning bolt, and this sting has been responsible for two deaths. The Portuguese man-of-war is a large invertebrate whose tentacles can reach up to 100 feet in length. Even detached tentacles of these animals are capable of causing stings to humans for up to two weeks.

Stings from box jellyfish (cubozoans) are the most dangerous type of jellyfish sting. The box jellyfish found in Australian waters has venom so deadly that it may cause cardiovascular collapse along with respiratory and neuromuscular paralysis that can kill an adult within minutes. Poisonings by the box jellyfish of Australia require the administration of an antivenom, which reverses the effect of the poison.

If you are stung by a jellyfish, always remove any tentacles that are adherent to the skin using gloves or forceps. Application of household vinegar (5% acetic acid) will inactivate any undischarged stingers and lessen the severity of the symptoms.

Always seek emergency medical care if you are stung by a jellyfish on the face, mouth, eyes, or genital area or if you become severely ill, have difficulty breathing or swallowing, or develop severe pain following the sting. Those swimming in Australia or other areas where box jellyfish may be found should always seek emergency medical evaluation when stung by a jellyfish.