HOLIDAYMAKERS heading to the NSW North Coast may want to pack some long-sleeve shirts and insect repellent.
They are, scientists say, among the few weapons against the Barmah Forest virus, a mysterious "emerging virus" assumed to be carried by birds and transmitted to people by mosquitoes.
There has been an outbreak of the virus on the North Coast every year for the past five years and entomologists warn it may strike Sydney's outskirts.
Unknown until 1974, the virus was first found in mosquitoes collected in the Barmah Forest, by the Murray River.
Transmitted by salt marsh mosquitoes, it can cause fever, pain in multiple joints and a rash. Patients recover, but people can feel ill for up to a month.
Baffled by the North Coast outbreaks from Tweed Heads to Port Stephens, scientists are bracing themselves for a possible sixth in the next few months.
"We don't know why this is happening," said Stephen Doggett, of Westmead Hospital's department of medical entomology.
The first outbreak was in the Northern Territory in 1992. The following year it was found in Western Australia. In 1995, 135 cases were reported along the NSW South Coast.
"We have to call this a truly emerging disease," said Dr Doggett. "There is no doubt it's a worry for the future."
Dr Doggett said that although there was no proof a sixth outbreak would occur this year, a mosquito trapping program would be implemented around Ballina, Tweed Heads, Port Macquarie and Port Stephens.
"Will this continue, or get worse? That is the question we are posing," he said.
Previous North Coast outbreaks began about January, and peaked from late March to mid-May.
Treatment for patients usually involved "pain relievers, anti- inflammatory drugs and rest".
"The only advice we can give is carry repellents containing diethyltoluamide, or picaridin. And wear loose-fitting, long- sleeve clothes and long trousers. The peak danger time is at dawn and dusk."
Because wildlife, probably birds, served as hosts for the virus, Dr Doggett doubted the disease would become a big problem in Sydney.