Tiny jellyfish cause a rash of misery on eastern beaches

The tiny jellyfish are invisible in the ocean. Photo / Supplied

Tiny jellyfish are behind a rash of painful attacks on beachgoers in Auckland, health authorities say.

Swimmer's rash - often falsely attributed to sea lice - has become a common sight as La Nina's warm currents encourage the spread of microscopic jellyfish on eastern beaches.

With most of Auckland's coastal waters now up to 22C, regional health authorities have warned swimmers to take care at Takapuna, Cheltenham, Narrow Neck and Red Beach.

Swimmers at Waiheke Island and Orewa Beach have also been affected.

St John medical director Tony Smith said the jellyfish were about the size of a grain of pepper and caused incredibly irritable sores.

"It is the itchiest you will be in your life. You become blotchy, and even with calamine lotion you'll still find yourself scratching away."

The tiny animal gets into swimsuits and then gets trapped in the fabric's fibres. The stings, which come after swimmers have left the water, appear as little red bumps, often hundreds to thousands of them close together.

Towelling down can cause the jellyfish to sting, because they release stinging cells when put under pressure.

Orewa lifeguard Alistair Dickson said the rash problems became more acute a week ago, with nearly every swimmer marked by the jellyfish.

"It comes and goes every summer but in the last week I've got it every time I've gone swimming."

Dr Smith said it was impossible to number cases because people did not usually seek hospital treatment.

But some Auckland pharmacies reported up to 10 individuals a day seeking antihistamines or topical cream for jellyfish stings.

Pharmacist Rina Kim from Unichem pharmacy in Oneroa on Waiheke said she had seen about 30 cases of swimmer's rash a week.

"It's the worst I've seen it. It's mostly been children, and all of them have been marked under their togs or wetsuits."

Children are more commonly affected than adults as their skin is softer.

The larvae are usually found in warm, still water in sheltered areas. La Nina's northwesterly breezes mean the jellyfish have been blown into Auckland's eastern beaches.

Swimmers are far less likely to be stung by the larvae at beaches with heavy surf, such as Piha.

Medical officer of Health Simon Baker said the only sure-fire way to avoid the rash was not to swim at affected beaches.

But swimmers could lessen their chances of rash by not wearing large, baggy clothing, and by removing togs on leaving the water.

MICROSCOPIC ATTACKERS
* About a millimetre wide, and invisible in the ocean.
* Also known as hydromedusae, they are a cousin-species of jellyfish, with tiny tail-like projections.
* Deliver a stinging toxin through the tail, which is designed to immobilise their zooplankton prey.
* They tend to sting under clothing, unlike sea lice, which sting exposed areas.
* The stinging cells (called nemocysts) are also released when the jellyfish are trapped or put under pressure.
* Only avoided by not swimming at affected beaches.
* If a rash develops, have a shower, wash togs, and get into cool clothing. Calamine lotion can help, or mild steroid creams if the rash is persistent.