The jellyfish tentacles can extend for several feet and are lined with venom-filled cells (nematocysts). One tentacle may fire thousands of nematocysts into the skin on contact. On contact, each cell fires a barbed thread that penetrates the victim's skin. Jellyfish belong to several different classes of invertebrates:
- Hydrozoans (for example, the Portuguese man-of-war and fire corals)
- Scyphozoans, the "true" jellyfish and most common type
- Cubozoans, for example the most toxic "box jellyfish"
- Anthrozoans (sea anemones and corals, which are related to jellyfish but only minimally poisonous to humans)
Symptoms of a jellyfish sting can vary depending upon the type and location of the sting. The pain can be severe, particularly in the first hours after an attack, and itching is common. There may be weakness, nausea, headache, muscle pain and spasms, lacrimation (tearing) and nasal discharge, increased perspiration, changes in pulse rate, and chest pain. Welting may persist for weeks at the site and scarring may remain.
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 occurred. The sting of the Portuguese man-of-war is more painful than that of the common jellyfish, and this sting has been responsible for two reported deaths. Even detached tentacles of these animals are capable of causing stings to humans for up to two weeks. Allergic reactions to jellyfish stings sometimes occur that further increase the inflammation and severity of the wounds.
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. Those swimming in Australia or other areas where box jellyfish may be found should always seek emergency medical evaluation when stung by a jellyfish.
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. Over-the-counter pain relievers should be started immediately for minor stings. Narcotics may be needed for severe pain. Serious stings may require oxygen or cardiorespiratory assistance. IV fluids and epinephrine may be needed if shock develops. Always seek emergency medical care if you are stung by a jellyfish and you become severely ill, have difficulty breathing or swallowing, or develop severe pain following the sting.