The Commonwealth Scientific and lndustrial Research Organisation believes that a virus closely related to the rabbit calicivirus disease could be giving some rabbits immunity to the biological weapon trained on them.

Dr Tony Robinson, of CSIRO's Division of Wildlife and Ecology, believes that a "benign" virus similar to RCD could be protecting some rabbits in the way that cow pox protected l8th cen tury dairymaids against smallpox. If he is right, this "benign" virus could explain why RCD has failed to gain a foothold in some parts of Australia.

Dr Robinson has been testing blood samples collected from rabbits around Cooma in 1994 and 1995. The sam ples were taken before the accidental release of RCD from its island quarantine testing station off the South Australian coast in October 1995.

To his suprise, he found that 11 of the 40 samples tested positive to RCD. How ever, he said the test could not distinguish between RCD and close relatives of it.

Since no Australian rabbits had shown any clinical signs of RCD before the 1995 release, the Cooma rabbits probably had been carrying a non-lethal relative of the disease. "There's definitely no suggestion that RCD was present in Australia prior to the accidental release," he said.

He said Italian researchers, had isolated a non-lethal virus similar to RCD from samples collected there and that virus could have infected Australian rabbits. There was evidence also of a non- lethal virus circulating in other parts of Europe.

Dr Robinson is conducting further research in collaboration with NSW Agriculture to determine how widespread the pre-existing benign infection was before 1995.