Freeze-dried bunny killer

Calicivirus, also known as rabbit haemorrhagic disease virus (RHDV), inadvertently escaped during field trials in South Australia in 1995 and, after being officially released a year later, has had a significant but uneven impact on reducing rabbit numbers.

NSW Department of Primary Industries (DPI) virologist, Dr Peter Kirkland, said methods for spreading the virus have had their faults - and this has limited wider distribution of the virus.

"Until recently, the only option was to inject captive rabbits. It was a matter of 'first catch your rabbit' ..." he said.

Approval was recently given for rabbits to be infected orally, on treated carrots or grain. Although a step in the right direction, there were still significant hurdles because the viral suspension still needs to be stored at ultra-low (dry ice range) temperatures to remain viable.

Kirkland said the virus preparation has a long shelf life if stored at -80 degrees but only six months at -20, and even less when temperatures are above that.

He said that to ensure the virus reaches the end user without losing potency and with its expected shelf life, it has to be shipped frozen on dry ice.

"This means it is classed as 'dangerous goods' and complicates the logistics and costs of transportation." There can be lengthy delays or even a refusal to deliver a consignment so that sometimes the virus is inactivated during transport.

Kirkland said his laboratory at DPI's Elizabeth Macarthur Agricultural Institute would be undertaking research to produce a stock of virus in a freeze-dried form.

"It is well recognised that freeze-drying increases the stability of many virus preparations and allows for storage at either room temperature or standard refrigeration temperatures.

"The ease of shipping and longer shelf life will make it easier for the virus to be distributed and stored for release at a time when rabbits are most vulnerable."

Rabbits are Australia's most significant vertebrate pest and two biological controls have been developed to manage them. Compared to the first - myxoma virus - RHDV is a very efficient and humane method of rabbit control.

A recent study found that RHDV has had a significant effect on rabbit populations. In Victoria for instance rabbit numbers have dropped by 93 per cent since its release.

The two-year research project managed by Drs Peter Kirkland, Andrew Read and Simon Humphrys is funded by NSW DPI, the Invasive Animals Cooperative Research Centre and the Bureau of Rural Sciences through the National Feral Animal Control Program.

Source: NSW DPI