Irukandji syndrome is a condition induced by venomization by the sting of Carukia barnesi, a species of Irukandji jellyfish, and certain other box jellyfish. The condition is rarely fatal, but if immediate medical action is not taken, within only 20 minutes, the victim could go into cardiac arrest and die. The syndrome was given its name in 1952 by Hugo Flecker, after the Aboriginal Irukandji people who live in Palm Cove, north of Cairns, Australia, where stings are common.
In 1964, Jack Barnes confirmed the cause of the syndrome was a sting from a small box jellyfish: the Irukandji jellyfish, which can fire venom-filled stingers out of its body and into passing victims. To prove that the jellyfish was the cause of the syndrome, he captured one and deliberately stung himself, his son, and a local lifeguard, and observed the resulting symptoms.  Other cubozoans possibly can cause Irukandji syndrome, but only seven jellyfish are positively identified (Carukia barnesi, Alatina cf. mordens, Carybdea alata, Malo maximus, Malo kingi, Carybdea xaymacana, an as-yet unnamed "fire jelly", and another unnamed species).
When properly treated, a single sting is normally not fatal; however, two people in Australia are believed to have died from Irukandji stings, which has greatly increased public awareness of Irukandji syndrome. It is unknown how many other deaths from Irukandji syndrome have been wrongly attributed to other causes. The exact mechanism of action of the venom is unknown, but catecholamine excess may be an underlying mechanism in severe cases. Animal studies appear to confirm a relationship between envenoming and an increase in circulating noradrenaline and adrenaline.
Most stings occur during the summer wet season in October-May in North Queensland, with different seasonal patterns elsewhere. Because the jellyfish is very small, and the venom is only injected through the tips of the nematocysts (the cnidocysts) rather than the entire lengths, the sting may barely be noticed at first. It has been described as feeling like little more than a mosquito bite. The symptoms, however, gradually become apparent and then more and more intense in the following five to 120 minutes (30 minutes on average). Irukandji syndrome includes an array of systemic symptoms, including severe headache, backache, muscle pains, chest and abdominal pain, nausea and vomiting, sweating, anxiety, hypertension, tachycardia and pulmonary edema. One unusual symptom associated with the syndrome is a feeling of "impending doom". Patients have been reported as being so certain they are going to die, they beg their doctors to kill them to get it over with. Symptoms generally abate in four to 30 hours, but may take up to two weeks to resolve completely.
Similar to other box jellyfish, first aid consists of flushing the area with vinegar to neutralize the tentacle stinging apparatus. As no antivenom is available, treatment is largely supportive, with analgesia being the mainstay of management. Antihistamines may be of benefit for pain relief, but most cases require intravenous opioid analgesia. Fentanyl or morphine are usually chosen. Pethidine, (meperidine, brand name Demerol in the US) should be avoided, as large doses are often required for pain relief and in this situation significant adverse effects from the pethidine metabolite norpethidine may occur.
Magnesium sulfate has been proposed as a treatment for Irukandji syndrome after being apparently successfully used in one case. Early evidence suggested a benefit; however, according to a later report, a series of three patients failed to show any improvement with magnesium; the author reiterated the experimental status of this treatment.
Reports of Irukandji syndrome have come from Australia, Hawaii, Florida, the French West Indies, Bonaire, the Caribbean, Timor Leste and Papua New Guinea. Cubozoan species other than Carukia barnesi are presumed to be responsible for envenomations outside Australia.
The severity of the pain from an Irukandji jellyfish sting is apparent in the 2005 Discovery Channel documentary Killer Jellyfish about Carukia barnesi, when two Australian researchers (Jamie Seymour and Teresa Carrette) are stung. Even under the "maximum dose of morphine", Carrette remarked she "wished she could rip her skin off", and is later seen writhing uncontrollably from the pain while lying on her hospital bed. In one scene, her feet are shown contorting and digging into the bed, and she is wiggling her toes and kicking her feet around. When the camera moves back, she is seen rubbing her face, her body is contorting in agony, and her legs are rapidly sliding and kicking around on the bed. Seymour, at his worst, is also seen writhing in pain, curled up in a ball and barely able to speak. He said he wished that he was stung by Chironex fleckeri, instead, since "the pain goes away in 20 minutes or you die".
Another recent program aired on the Discovery Channel, Stings, Fangs and Spines, featured a 20-minute spot on Irukandji syndrome. In the segment, a young Australian woman was stung and developed a severe case. In a testament to the severity of pain involved, a re-enactment (featuring the actual victim portraying herself) shows her screaming and violently thrashing around on the hospital bed in an almost convulsive state, for the bulk of the segment. She later commented that this unbearable pain lasted for hours, and added, "I didn't think it was possible for anyone to endure that level of pain without turning into a vegetable."
On the television program Super Animal, a woman compared her pain from childbirth to her experience with Irukandji syndrome: "It's like when you're in labor, having a baby, and you've reached the peak of a contraction—that absolute peak—and you feel like you just can't do it anymore. That's the minimum that [Irukandji] pain is at, and it just builds from there."
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