Analysis of stored rabbit blood for antibodies has shown that Australian rabbits were exposed to a non-deadly virus prior to the escape of the rabbit calicivirus from Wardang Island in 1995.

CSIRO's Dr Harvey Westbury says that this RCD relative may be acting as a natural vaccine in parts of temperate Australia.

"We believe the related virus is effectively protecting some rabbits against the usually deadly RCD.

"Refinement of existing scientific tests in the past two years has meant that we were able to go back and check stored rabbit blood samples for antibodies to RCD," says Dr Westbury.

"We looked at samples taken prior to the escape of the virus and its introduction into Australia, and found that about 50% of 392 rabbits tested had antibodies that reacted against the coat protein of RCD," he says.

"In a separate study, we collected blood from rabbits captured from a RCD-free area in central Victoria, after RCD had spread across Australia. All these rabbits had antibodies against RCD or an RCD-like virus. We found that nearly half of these rabbits were able to survive a dose of RCD that would normally have been fatal," Dr Westbury says.

Studies by CSIRO Wildlife and Ecology with the Victorian Institute of Animal Sciences and the SA Animal and Plant Control Commission also show that in the field many rabbits have antibodies to an RCD-like virus.

These antibodies can be distinguished from the typical RCD antibodies normally found if a rabbit recovers from an RCD infection. The antibodies are found more commonly and in higher concentrations in rabbits in cooler, wetter areas than in hot, dry areas, which is perhaps why RCD controlled rabbit numbers in the inland yet was less effective in temperate areas.

"Our results also confirmed that rabbits with high levels of antibodies to the RCD-like virus have a good chance of surviving RCD," explains Dr Brian Cooke of CSIRO Wildlife and Ecology. "There had been no reports of an RCD-like illness in Australia prior to RCD's release, therefore the virus that triggered the antibodies must have been a non-disease-causing but related virus."

Further studies are required to confirm whether the presence of an RCD-like virus is a contributing factor to help account for the reduced effectiveness of RCD in holding down rabbit populations in temperate areas as compared with warmer and drier parts of Australia.

This would require development of tests that easily distinguish virulent RCD from the non-deadly form of calicivirus.

Information on the RCD-like virus is important to people working on rabbit control across Australia. State rabbit control groups are considering the implications of the work to look at ways of making RCD more effective in temperate regions.