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Cnidaria (Coelenterates , such as Jellyfish and Sea Anemones ) Stings


Robert A. Barish

, MD, MBA, University of Illinois at Chicago;

Thomas Arnold

, MD, Department of Emergency Medicine, LSU Health Sciences Center Shreveport

Last full review/revision Apr 2020| Content last modified Apr 2020
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Cnidaria include the following:

  • Corals

  • Sea anemones

  • Jellyfish (including sea nettles)

  • Hydroids (eg, Portuguese man-of-war)

Cnidaria are responsible for more envenomations than any other marine animal. However, of the 9000 species, only about 100 are toxic to humans. The multiple, highly developed stinging units (nematocysts) on cnidaria tentacles can penetrate human skin; one tentacle may fire thousands of nematocysts into the skin on contact.

Symptoms and Signs of Cnidaria Stings

Lesions vary with the type of cnidaria. Usually, lesions initially appear as small, linear, papular eruptions that develop rapidly in one or several discontinuous lines, at times surrounded by a raised erythematous zone. Pain is immediate and may be severe; itching is common. The papules may vesiculate and proceed to pustulation, hemorrhage, and desquamation. Systemic manifestations include weakness, nausea, headache, muscle pain and spasms, lacrimation and nasal discharge, increased perspiration, changes in pulse rate, and pleuritic chest pain. Uncommonly, fatal injuries have been inflicted by the Portuguese man-of-war in North American waters and by members of the Cubomedusae order, particularly the box jellyfish (sea wasp, Chironex fleckeri), in Indo-Pacific waters.

Treatment of Cnidaria Stings

  • Removal of tentacles

  • Symptomatic treatment

  • Various rinses to treat pain and deactivate nematocysts, depending on the specific animal

Cnidaria sting treatment includes removal of adherent tentacles with a forceps (preferably) or fingers (double-gloved if possible) and liberal rinsing to remove invisible stinging cells. The type of rinse varies by the stinging organism:

  • For jellyfish stings sustained in nontropical waters and for coral stings, seawater rinse can be used.

  • For jellyfish stings sustained in tropical waters, vinegar rinse followed by seawater rinse can be used. Fresh water should not be used because it can activate undischarged nematocysts.

  • For box jellyfish stings, vinegar inhibits nematocyst firing and is used as the initial rinse if available, followed by seawater rinse. Fresh water should not be used because it can activate undischarged nematocysts.

  • For Portuguese man-of-war stings, saltwater rinse can be used. Vinegar should not be used because it can activate undischarged nematocysts.

Any difficulty breathing or alteration in level of consciousness, no matter how mild, is a medical emergency, requiring transport to a medical center and possibly injection of epinephrine.

Symptoms are treated supportively. Pain caused by most cnidaria stings is usually short-lived and can be relieved with baking soda in a 50:50 slurry applied to the skin. Soaking the affected area in a warm water bath (105-110° F) has been shown to be effective for pain relief, as have nonsteroidal anti-inflammatory drug (NSAID) analgesics. For severe pain, opioids are preferred. Painful muscle spasms may be treated with benzodiazepines. IV fluids and epinephrine can be given if shock develops. Antivenom is available for the stings of the box jellyfish C. fleckeri but not for the stings of North American species.

Seabather’s eruption

This stinging, pruritic, maculopapular rash affects swimmers in some Atlantic locales (eg, Florida, Caribbean, Long Island). It is caused by hypersensitivity to stings from the larvae of the sea anemone (eg, Edwardsiella lineate) or the thimble jellyfish (Linuche unguiculata). The rash appears where the bathing suit contacts the skin. People exposed to these larvae should shower after taking off their bathing suit. Cutaneous manifestations can be treated with hydrocortisone lotion and, if needed, an oral antihistamine. More severe reactions may require the addition of oral or IV prednisone.

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