The rabbit calicivirus "escaped" from Wardang Island, off Yorke Peninsula, in October 1995. Scientists had been testing the disease - released officially a year later - as a new biological control. The disease, which only affects rabbits, was first reported in China in 1984 and soon after in other Asian and European countries. Within a few months it had killed 64 million rabbits in Italy alone. Calicivirus now has affected rabbits in more than 40 countries on four continents. In Australia, it has reduced numbers by up to 95 per cent, allowing vegetation to regenerate.

Rabbits are building up an immunity to the deadly calicivirus and could reach plague proportions unless farmers carry out eradication programs. Scientists have warned the disease - hailed as the spearhead to win the war against rabbits when released officially in October 1996 - is about to lose its sting. They say numbers could rise unless a campaign starts soon to destroy burrows and poison rabbits missed by the virus. Already, rabbit numbers are at 20 per cent of pre-virus levels in areas of the Northern Territory where burrows have not been destroyed.

In Queensland, 67 per cent of 110 rabbits tested at one virus-release site have resistance to the disease. The Anti-Rabbit Research Foundation of Australia's chairman, Dr Rob Morrison, said yesterday a rare chance to control numbers with rabbit calicivirus (RCD) could be wasted. He said viruses tended to have a devastating initial impact, which slowly waned as animals became immune. "We can see there's a very real danger of it happening with RCD and it looks as if it's already beginning," he said. "Obviously, populations will spring up again and we'll lose the initiative."

The calicivirus has killed millions of rabbits nationally, wiping out up to 95 per cent in some areas. Rabbits cost Australia an estimated $600 million annually in damage to crops and the environment.

A virus released in the 1950's to control rabbits, Myxomatosis, started to lose its virulence within 18 months. Emeritus Professor Frank Fenner, who pioneered the introduction of myxomatosis, said there was a risk rabbits would return to plague proportions. He said nobody was certain when calicivirus would start losing its killing power. "We can't be sure whether it will happen this year but its likely that, in time, we'll lose the tremendous gains we've made," he said.