Size matters for deadly jellyfish

12 Aug 2012, 2:12 pm   -   Source: AAP

Two species of deadly jellyfish - one large, the other small - are confounding researchers with their potent and variable venom.

Whether big or small, there's a significant sting in the jellyfish lurking in northern Australian waters.

And studies suggest some jellyfish in Queensland and the Northern Territory produce venom that is more potent than the same species in other parts of Australia.

Emergency physician and toxicologist Dr Mark Little, from Cairns Base Hospital, said researchers had found differences in the venom of box jellyfish collected around the country.

Most deaths from box jellyfish stings - there have been about 70 since records began - have occurred in Queensland and the NT, although the species is also found in Western Australia.

And size does matter.

Dr Little said the box jellyfish become more dangerous when the bell reached a diameter of six to eight centimetres.

At this size, the number of stinging cells containing lethal venom increases.

"It looks like the size of the jellyfish matters," Dr Little told AAP.

Intriguingly, the change coincides with an alteration in the box jellyfish diet from prawns to fish, switching from devouring invertebrates to vertebrates.

The knowledge could enable researchers to pinpoint when the jellyfish, which appears to be more deadly to children, is more likely to be lethal, Dr Little said.

Although antivenom is available to treat box jellyfish stings, which result in painful welts and rashes, it may not adequately treat every patient due to the variations in venom.

But the sting of the decent-sized box jellyfish pales in comparison to the thumbnail-sized irukandji.

Victims are unmarked by this tiny jellyfish, but several hours after being stung they thrash about with severe back and kidney pain comparable to the agony of labour, Dr Little said.

Often, huge doses of morphine - between 30g to 40g - are needed to relieve the excruciating pain, he said.

The venom can lead to heart failure and bleeding in the brain, which has caused the deaths of two middle-aged male tourists in the past 10 years.

The only treatment for irukandji stings is pain relief, which is why researchers have been trying to develop a more effective therapy.

One recommended approach uses magnesium, but when Cairns researchers tested it on about 20 people and compared it in a placebo-controlled trial, the results were negative.

Dr Little, who will discuss the two jellyfish at the Australasian College for Emergency Medicine Winter Symposium in Cairns on Sunday, said the small study size or dose used may have been responsible for the lack of results.

One thing's certain - Dr Little knows which jellyfish he'd prefer to tangle with.

"I'd prefer not to get stung, but if I got stung I'd want to get stung by a box jellyfish," he said.

"It's nowhere near as painful."

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