KEY POINTS:
Avian malaria parasites found in a quarter of sparrows and blackbirds are threatening to wipe out New Zealand's vulnerable native birds.
The introduced species are left unharmed but carry the parasite, which poses a huge threat to natives such as tui and kokako with little resistance to new diseases. Two outbreaks during the past decade among zoo populations of the New Zealand dotterel and mohua (yellowhead) have shown that an outbreak could be devastating.
Landcare Research epidemiologist Dr Dan Tompkins said: "Sparrows and blackbirds don't seem to be affected at all by it. The scary thing is that they can just carry it. They've evolved with many of these strains up in the Northern Hemisphere, whereas our native birds have possibly no prior exposure. It's something new coming along that could hit them."
Avian malaria has already been a key factor in the extinction of at least 50 per cent of native birds in Hawaii, where the birdlife also evolved in relative isolation.
The first study of avian malaria in New Zealand for more than 50 years, led by Dr Tompkins, has found the disease to be much more widespread than first thought.
Although avian malaria has been in New Zealand since the 19th century - most likely through the introduction of Northern Hemisphere birds such as the blackbird - conservationists are sounding alarm bells now with the increasing spread of the mosquito that transfers the parasite from introduced birds to native species.
"The real concern is that the mosquito that is the main vector in Hawaii is moving rapidly down south through New Zealand and is increasing in abundance," Dr Tompkins said.
"So you've got these birds flapping around New Zealand happily acting as big reservoirs of infection, and then if the mosquito comes along, bites the blackbird and then bites the native bird, it can be transferring that infection across."
The introduced bird mosquito, culex quinquefasciatus, also arrived in the 19th century, but was confined to Northland and Auckland.
"Until 30 years ago, it only was found there. It's now spread as far south as Christchurch."
Dr Tompkins said the reasons were unclear, but better transportation systems were thought to play a big part in the insect's spread.
Molecular testing of more than 400 sparrows and blackbirds in various North and South Island locations found just over a quarter of birds testing positive for malarial parasites.
In Northland, Auckland, Hamilton and Gisborne, almost one in two birds had these malarial parasites, while in Dunedin, the rate was one in 20.
Dr Tompkins said the rate matched the known distribution of the carrier mosquito.
He is now involved in a large-scale project, which includes staff from ESR, Landcare Research, Massey University and Auckland Zoo, at Mokoia Island in Lake Rotorua trying to understand the disease's dynamics better. Little was known about its likely impact.
"All these different zoos, wildlife parks and breeding programmes are getting quite concerned about what this disease might do."
Disease risk
* Avian malaria has been here since the 19th century when Northern Hemisphere birds were introduced.
* It is much more widespread than first thought due to the spread of the mosquito carrier.
* Molecular testing of sparrows and blackbirds nationally found just over a quarter tested positive for malarial parasites.
* In Northland, Auckland, Hamilton and Gisborne, almost one in two birds were affected.
* In Dunedin, the rate was one in 20.
* Avian malaria has already been a key factor in the extinction of at least 50 per cent of native birds in Hawaii.
