Up to eight species of small, almost invisible box jellyfish cause Irukandji Syndrome - and there is no antivenom for their potentially fatal stings.
Dr Peter Fenner, Associate Professor at the James Cook University School of Medicine, and Mr John Hadok, of the Emergency Department at Mackay Base Hospital, have documented the first reported death from Irukandji Syndrome in the in The Medical Journal of Australia.
"More than 100 people are likely to be stung and visit hospital this summer. Tragically this year, for the first time, there were two fatalities from Irukandji Syndrome in North Queensland", said Dr Fenner.
Two middle-aged men were killed by jellyfish stings earlier this year, in separate incidents near Port Douglas and in the Whitsunday Islands.
Dr Fenner believes that the jellyfish responsible for the deaths is an undescribed species of Irukandji that lives around the outer reefs and the Whitsunday Islands of the Great Barrier Reef.
The Irukandji box jellyfish, or cubozoans, are related to the deadly large box jellyfish or stinger Chironex fleckeri, which has killed more than 60 people in Australia.
Irukandji jellyfish have nematocysts, or stinging cells, on the body as well as the tentacles. When touched, these cells fire a tiny shaft into the victim's skin, that delivers the potent venom which causes Irukandji Syndrome.
The basic symptoms develop 30 minutes after being stung. They include severe low back pain; excruciating muscle cramps in arms, legs, belly and chest; sweating, nausea, anxiety, restlessness, vomiting, and headache.
The fatal cases developed such severe hypertension, or high blood pressure, that they died from brain haemorrhages, one within 30 hours after being stung.
"The Irukandji venom causes nerve endings to release large amounts of noradrenaline, the chemical that produces the "fight-or-flight" reaction", says Dr Fenner.
"The smaller blood vessels contract, which leads to the hypertension or high blood pressure, as blood is not able to squeeze through. There is no first-aid treatment for the hypertension."
"The venom is also a direct cardiac toxin, turning the heart into a floppy bag that is not able to pump blood effectively. This leads to pulmonary oedema, or fluid building up in the lungs", he says.
Irukandji Syndrome is most prevalent between early December to mid-February, along the tropical coast from Broome in WA to Rockhampton in Queensland.
Dr Fenner cautions that the development of an effective Irukandji antivenom is not presently possible, especially when the lifecycle of the species is unknown.
Dr Jamie Seymour, Senior Lecturer at the James Cook University School of Tropical Biology in Cairns, says that the syndrome was originally attributed to one species of box jellyfish named Irukandji, or Carukia barnesi.
"In the last few years, as symptoms become better defined, it appeared that people stung in Mackay were showing different symptoms from those stung in Cairns. It led us to wonder if maybe different species are involved."
"We're able to identify cubozoans by taking skin scrapings from people who have been stung, as the nematocysts stay in the skin. Each species has nematocysts of a particular shape", said Dr Seymour.
"There are nematocysts from 7 or 8 skin scrapings that I have never seen before and cannot identify. One of these samples came from the man killed at Whitsunday Islands."
Dr Seymour and other researchers are assisting Dr Fenner by netting 1800 specimens of Carukia barnesi from around Cairns for the Australian Venom Research Unit.
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