Australian Rabbits are Doing What Comes Naturally – Again

Domesticated rabbits came to Australia with the first fleet, but it was not until 1859 when Thomas Austin released 26 on his property in Victoria that their take-over began. Little did Austin know what damage his desire for recreational hunting would cause to Australia. Over the next 70 years, rabbits became Australia's most successful feral animal. Their economic damage to agriculture through crop loss and competition with livestock was matched only by their ecological damage. They devastated huge areas of countryside destroying native vegetation and causing erosion. Their presence was also a boon to other introduced feral animals such as foxes and cats for two reasons. Small native animals no longer had cover in landscapes denuded by rabbits and they were themselves a food source for feral predators. The methods tried to control rabbits were many and varied – warren ripping, shooting, poisons, even car exhaust gases piped down into warrens. Then the search for biological control agents began. The story of myxomatosis is now folk lore in Australia, including the fact that the scientists involved injected each other with the virus to prove it was safe for humans. Initially the release of myxomatosis in 1950 was a huge success as it killed 90% of the feral rabbits that caught the disease and reduced the population from an estimated 600 million rabbits to fewer than 100 million. But within a few years, rabbits had begun to develop resistance to it. Then the virus 'fought back' and became more virulent until an equilibrium was reached. Myxomatosis still kills 40–50% of the rabbits it infects. The latest biological control attempt has been with rabbit haemorrhagic disease virus (RHDV), also known in Australia as rabbit calicivirus. RHDV, an infectious hepatitis specific to rabbits, was first described in China in 1984 where it was causing mortality of more than 95% in infected rabbits (an infected rabbit dies within 72 hours). In June 1991, the virus was brought into quarantine at the CSIRO Australian Animal Health Laboratory in Geelong for comprehensive testing. During field trials in 1995, it escaped from Wardang Island off the South Australian coast, with the help of insects. The virus then raced across the Flinders Ranges in South Australia through the arid zone, killing large numbers of rabbits as it went. However, in cool, humid parts of the country, mortality was much lower. Could there be a link between this and the fact that some wild Australian rabbits tested before the release of calicivirus had antibodies to RHDV? Those rabbits with high levels of these antibodies were discovered to have some protection to RHDV. It seemed probable that there were other viruses out there that were similar to but not identical to RHDV, but were non-pathogenic. That is, they did not kill the rabbits they infected. The possibility was that these viruses were providing rabbits with immunity to RHDV. While attempts to isolate such a virus in wild rabbits in Australia had in the past been unsuccessful, another non-lethal rabbit calicivirus (RCV) had already been identified overseas. The search for these 'other viruses' in Australia is described.