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Viruses pose 'double barrel' threat to feral rabbits in Australia

Primary Industries

FARMERS and conservation groups are optimistic a “double barrel” of viruses could have the biggest impact on Australia’s feral rabbit population in two decades.

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The calicivirus RHDV2 has been sweeping across Australia since it was first detected in the Australian Capital Territory, in the nation’s east, in May 2015.

Latest reports have suggested the virus is now present in Western Australia after crossing into New South Wales, Victoria and South Australia this year.

Meanwhile, every Australian state is preparing for the controlled release of a Korean strain of the calicivirus, RHDV1 K5 in 2017. It will be the third deadly strain of the calicivirus to sweep across Australia.

The first calicivirus, RHDV1, was released in 1996 with devastating success, particularly in arid and semi-arid areas. Although the virus is still having some effect on rabbit numbers, the population is recovering as resistance to it grows.

It is not known how RHDV2 came to be in Australia in 2015 but Livestock South Australia President Geoff Power said some areas were already noticing a lack of rabbits since the arrival of RHDV2 into the state in recent months.

He said farmers were looking forward to the release of the RHDV1 K5 strain next year.

“Hopefully it has a double barrel effect,” Power said.

“When the first calicivirus got out it just had a huge impact … it was just amazing.

“From an environmental point of view, when the numbers are down in this area, the country certainly does change for the better.”

The rabbit problem on mainland Australia dates back to the release of 24 wild rabbits for hunting purposes in Victoria in 1859.

By 1920 it is thought there were 10 billion rabbits in Australia making it the fastest spread ever recorded of a mammal species anywhere in the world.

Following the success of biological rabbit controls including the release of the myxoma virus in 1950 and the first calicivirus (RHDV) in the mid-1990s, rabbit numbers have been significantly reduced. The population is still estimated to be about 200 million, costing Australian agricultural industries more than $200 million every year.

Biosecurity SA Senior Research Officer Greg Mutze said it was so far difficult to predict the impact of RHDV2 or the effect it could have on the success of the RHDV1 K5 roll out next year.

“We have heard people report that the numbers of rabbits have dropped quite suddenly – it’s the type of reports we received when the original strain of RHDV first spread,” he said.

However, Mutze said in other areas where the new virus had been detected the rabbit numbers were yet to drop.

“We don’t know yet whether that’s because it hasn’t spread to all the rabbits yet or whether a good portion of them survived.

“We are a little bit limited at the moment because we don’t have a test to determine whether a rabbit has survived this disease.

“Most rabbits die in the warren. You get some dying above ground and of those a fair few get cleaned up very quickly by scavengers like foxes, hawks, crows or eagles so unless there’s a very high rabbit population it’s often not apparent when the virus has gone through other than the fact that you stop seeing rabbits around the place.”

While RHDV1 was particularly effective in semi-arid areas it was less deadly in coastal regions and high rainfall areas.

The Korean strain of RHDV1 was chosen for release partly because of a belief it would be more effective at reducing rabbit populations in areas of high rainfall

Rabbit Free Australia Chairman Nicholas Newland said the organisation was optimistic about the potential impact of the two new viruses.

He said the success of K5 would be determined by its effectiveness in higher rainfall and coastal areas.

“We’ve had a very wet year this year so it will be very interesting to see what happens across Australia,” Newland said.

“We see that RHDV2 presents an opportunity to use that so-called new virus in a more constructive way but it seems to us that too much effort is being made on the release of K5 when it is apparent that RHDV2 is likely to kill more rabbits.”

Rabbit Free Australia is a national organisation formed in Adelaide in 1991.

Newland said there needed to be immediate resources put into creating a test for RHDV2 so that proper monitoring and reporting mechanisms could be established before the release of RHDV1 K5.

“What’s important is that we know what’s killing what and at the moment we’re not convinced that there is enough known about RHDV2,” he said.

“At this stage it’s going to be quite hard to tell which virus is killing which rabbit without the appropriate monitoring programs in place.”

“It’s also important to take advantage of those reduced numbers rather than leave the residue behind.”

Grazing and burrowing by rabbits can cause serious erosion problems, reduce livestock feed and modify entire landscapes.

Rabbits also threaten native animal species by altering habitat, reducing native food sources and bolster other populations such as feral cats and foxes.

Arid Recovery is a research centre in Outback South Australia. Researcher Dr Hugh McGregor is using video cameras and GPS collars to monitor the behavior of feral cats as the rabbit population declines.

Dr McGregor said the reduction in rabbit numbers would likely also lead to a drop in feral cat and fox numbers, as it did when the initial calicivirus devastated the rabbit population in the 1990s.

“It’s like the calicivirus is an extremely effective cat control agent as well as a rabbit control agent,” he said.

“We’re hoping to compare the drop in cat density after the calicivirus with the increase in native animals – I’m going to suspect that they are going to start killing native animals to a greater extent but the density of cats is going to drop right off.

“Even though every cat might be eating 30 per cent more native animals, there could be a 50 – 80 per cent drop in cat density so overall I’m expecting a drop in the impact on native animals.”

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