Irukandji syndrome is a medical condition induced by the sting of Carukia barnesi, the Irukandji jellyfish, and other cubozoans. Irukandjiis the name of an aboriginal tribe that once lived in the Palm Cove, north of Cairns, in Far north Queensland.

Stings were quite frequent in the area around where the Irukandji lived.  So in 1952 Hugo Flecker named Irukandji syndrome to describe the set of severe, dramatic systemic symptoms that occur some 30 minutes after the sting: severe prostration typified by backache, muscle pains, chest and abdominal pains, headache, nausea, vomiting and restlessness, together with localised piloerection (hair standing on end), localised sweating, high heart rate and high blood pressure. Only in 1964 Dr Jack Barnes confirmed the cause of the syndrome to be due to the small box-shaped Irukandji jellyfish. In order 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 symptoms. All ended up in hospital. The tiny jellyfish was later called Carukia barnesi.

Recent research now suggests this syndrome is caused by at least five or six small carybdeids similar to C. barnesi and two larger carybdeid jellyfish species (bell diameter about 60–70 mm at maturity). Carukia barnesi, the first jellyfish shown to cause Irukandji syndrome, is a member of the class Cubozoa, genus  Carukia, family Carybdeidae. The Carukia is a small clear jellyfish, measuring 20 mm in diameter and 25 mm in the depth of its bell. There are four tentacles (they appear like a string of pearls), one from each corner, which contain a large number of stinging cells called nematocysts. The tentacles may extend from 5 cm to 1 metre in length, depending on the degree of contraction. The bell of the Irukandji (Carukia) also contain nematocysts. Carukia barnesi have stinging cells on both body and tentacles.

Each of the numerous nematocysts are like tiny spring loaded hypodermic needles. Contact with the skin causes the nematocyst to discharge and inject a tiny amount of a very potent venom. The more contact, the greater the amount of venom injected. The Irukandji is rarely seen before one his stung (or after) because of their small size (just like the end of a thumb) and the near transparency of their body (almost invisible in water).

The Carukia is an open water creature and found in deeper waters of the reef and the islands, at depths up to 10-20 metres but prevailing currents and winds can sweep the Carukia inshore causing a relative mass stinging of bathers. So the jellyfish are found offshore as well as along coastal beaches. Irukandji stings have been confirmed in shallow water as little as 30 mm. deep where they become concentrated at the waters edge. The number of jellyfish in inshore waters can vary between years, probably due to changing weather conditions.

Apart from the tropical waters of the east and west coasts of Australia, reports of Irukandji syndrome have come from various locations around the world (Hawaii, Florida, French West Indies, Bonaire, the Caribbean, Timor Leste and Papua New Guinea, Indonesia, Fiji) but are mainly restricted to areas lying between the Tropic of Capricorn and the Tropic of Cancer. But the distribution could be much wider.

Carukia barnesi has been shown to cause illness ranging from local symptoms to severe Irukandji syndrome with cardiac dysfunction. The sting itself is often barely noticed, is only moderately painful or irritating in marked contrast to the Chironex with its immediate excruciating pain.

The sting increases in intensity for a few minutes and then decreases over the next half hour. A red coloured 5–7 cm reaction surrounds the area of contact within five minutes

Small papules  (pimples) appear like a cluster or line of dots which mirror the position of singing  cells on the animals. They reach their maximum in about 20 minutes, before subsiding.  But a complex of typical systemic symptoms begins after a latency of about 30 minutes (range 5 to 120 minutes).

The symptoms include an array of systemic symptoms such as severe generalized pain, headache, severe abdominal cramps (91%), back pain, limb or joint pain, excruciating muscular cramping in all four limbs, the abdomen and chest, nausea and vomiting, profuse sweating and anxiety, restlessness, palpitations, tachycardia, a rise in cardiac troponin levels, epistaxis, stertorous breathing and erythematous flushing of the face, neck and anterior chest, a feeling of impending doom.

Hypertension, pulmonary oedema, delayed toxic heart  dilatation and heart failure that could be fatal if not treated. A few develop painful neurasthenic burning pain in both lower limbs or in the jaw, priapism or acute angioneurotic oedema within minutes of the initial sting, often accompanied by an audible wheeze.

A clinically distinctive feature of CNH is the pain associated with the skin lesion. In contrast, cutaneous tumors, such as Basal Cell Carcinoma, Keratoacanthoma, Squamous Cell Carcinoma are usually painless, even when ulcerated. . But often, biopsy is necessary if patients with CNH have chronic solar damage and a history of skin cancer.

The majority of people are able to be discharged in 24 hours when symptoms have settled. A syndrome of malaise and muscle aches may take up more than a week to resolve completely. Victims with an underlying heart condition or the elderly would be at significant risk of death or major morbidity from the Irukandji syndrome. The Irukandji syndrome has been misdiagnosed as a heart attack and as decompression illness.

The severe hypertension sometimes cause a fatal brain haemorrhage. Victims frequently require hospitalisation for management of severe pain and high blood pressure. A sting by Carukia barnesi does not necessarily result in Irukandji syndrome. Factors affecting venom load, such as thickness of the keratinised skin, presence of hair, length of tentacle involved, duration and pressure of the contact between tentacle and skin have been proposed by other authors.

The exact mechanism of action of Irukandji venom is unknown. It has been suggested that catecholamine excess (increase in circulating noradrenaline and adrenaline) may be an underlying mechanism in severe Irukandji syndrome. First aid consists of immediate flushing the area with vinegar to inactivate stinging cells present on the skin and to prevent further envenomation.

The nematocysts of the Irukandji (Carukia) are inactivated by vinegar. The effects of vinegar in treating an Irukandji envenomation may be limited due to the delay in recognising the sting. Once the symptoms are obvious it appears there are few if any active nematocysts left on the skin. However, vinegar would seem to be a reasonable first aid measure if available, especially if commenced early. There is no antivenom; treatment is largely supportive. So, because it’s impossible to eradicate jellyfish from the water, we have to learn to live with these animals, heeding informations on their presence on the area where we are going to swim.