Possums might help to spread mosquito-borne diseases such as Ross River virus in New Zealand, according to new research.

Wellington Medical School researcher Jose Derraik, who warned this week that a mosquito-borne disease outbreak was "a matter of time", says possums in Australia carry Ross River virus and are known to be bitten by mosquitoes in this country.

New Zealand's 70 million possums are already regarded as a pest because of their impact on vegetation and on tuberculosis infection in cattle.

But Mr Derraik says in the latest issue of the Australian and NZ Journal of Public Health that perhaps the greatest threat to public health posed by the possums is that they are capable of carrying viral diseases such as Ross River, Murray Valley encephalitis virus and dengue fever.

No antibiotics have been found to fight such diseases, so people infected by them have to develop their own immunity.

This country is regarded as "virgin soil" for the diseases because only a few New Zealanders who have been infected overseas have developed immunity.

An Australian study in the same journal three years ago suggested that up to 590,000 Aucklanders could be infected if Ross River virus broke out on a similar scale to a previous "virgin-soil epidemic" in the Pacific Islands in 1989-90.

That epidemic infected 90 per cent of the population in the region of Fiji where it broke out, 69 per cent of people in the Cook Islands, 44 per cent of American Samoans and 33 per cent of New Caledonians.

Scientists believe a New Zealand outbreak would most likely occur in Auckland because it has the main international airport.

The Australian southern saltmarsh mosquito, which carries the Ross River virus, was found in the Kaipara district in 2001 and the Health Ministry is spending $40 million to eradicate it.

"We might have the pathogen of Ross River virus becoming spread among the population of possums," Mr Derraik warned.

"If an infected possum comes and hangs around in Auckland, and a mosquito bites the possum and then a person, we have all the ingredients for the disease."

Otago University Associate Professor Ted Nye - who identified an earlier Australian mosquito invader in New Zealand 40 years ago - said the virus needed temperatures above 20C for mosquitoes to spread it.

"It takes a week at the right temperature for the viral agents to build up in sufficient numbers for the mosquito to become infectious.

"I am concerned about the whole issue of global warming and changes in the distribution of vector [mosquito] species and the possible risk that this poses for an outbreak."

Health Ministry chief technical officer Sally Gilbert said last month that the ministry was monitoring Australian mosquitoes and inspecting all incoming freight containers.

But she said the ministry did not want to risk "crying wolf" by launching a major public campaign urging people to get rid of potential mosquito breeding grounds such as buckets or drains with stagnant water.

Ravenous pest

* Possums were declared a pest in 1947 and are a major threat to New Zealand native forests.

* Possums eat more than 22,000 tonnes of forest vegetation in New Zealand every night, seriously harming native bird and animal life.

* They also eat native birds' eggs and young and mature birds, invertebrates and insects.

* Possums are also known to carry disease, possibly including the Ross River virus.