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An outbreak of mosquito-borne disease such as the Australian Ross River virus will hit New Zealand "sooner rather than later", researchers warn.

Jose Derraik from the Wellington Medical School and Professor Charles Calisher from Colorado State University say mosquitoes already here will one day bite a visitor from Australia who has the virus, and set off a chain reaction that will see the disease spread through the population.

"It is simply a matter of time before an infected human is fed upon by a vector mosquito inside New Zealand and a transmission cycle is subsequently established," the researchers said in the Australian and NZ Journal of Public Health.

They urge a public education campaign to encourage New Zealanders to get rid of mosquito-breeding pockets of stagnant water.

New Zealand is virtually unique in the world, apart from Antarctica, in never having recorded a local outbreak of mosquito-borne illness.

The researchers say: "The rapid modern movement of people, the consequent routine arrival of infected humans, and [mosquitoes] already present in this country are likely to terminate this blissful state."

Any outbreak that occurs here may be serious because most New Zealanders have not developed immunity to mosquito-borne diseases.

More than 100 people enter the country from Queensland every year carrying the Ross River virus, which causes anything from mild flu-like symptoms to arthritis in all joints.

"It's endemic polyarthritis, a really nasty pain all over the joints and muscles. It can last for months," Mr Derraik said.

Two Australian mosquito species which carry the disease in their home country are already in New Zealand. The Ministry of Health is spending $40 million on a bid to eradicate one, the southern salt marsh mosquito.

Although none of the mosquitoes has spread diseases to humans here yet, they did cause an outbreak of avian pox and malaria in New Zealand dotterel chicks at the Auckland Zoo and Otorohanga Kiwi House in 1996.

That outbreak killed 10 of the 16 dotterels at the two sites.

Mr Derraik investigated the outbreak and found mosquitoes breeding in all 30 water-filled containers at the zoo, including 17 old tyres, abandoned plastic saucers, the rim of a plastic lid and a steel tub used for cooling hot metals outside a workshop.

He also found mosquitoes in drains and in plants along the zoo's "rainforest" boardwalk which can hold water at the base of the leaves. The zoo has since introduced controls to minimise the number of potential mosquito breeding places.

Originally from Brazil, Mr Derraik is now completing his doctorate in Auckland, where he lives with his New Zealand-born wife and two children.

He and Professor Calisher, a world authority on animal-borne diseases, recommend a campaign to educate New Zealanders about the need to control potential mosquito breeding sites.

They also recommend increased surveillance of shipping cargo, a training programme to help doctors identify mosquito-borne diseases and the establishment of a local laboratory that can diagnose viral infections.

Ross River virus


* Australia's leading mosquito-borne illness, infecting 3840 Australians last year.

* Symptoms range from flu-like fever, chills, sweating and headaches to pain in all joints.

* Illness from a mosquito bite can last for months.

* More than 100 people carrying Ross River virus enter NZ from Queensland each year.

* A Waikato resident was last week reported to have contracted the disease in Australia.