It’s official: the rabbit haemorrhagic disease virus released two years ago reduced rabbit numbers on some sites by about one third, but failed to spread further.
In March 2017, the virus, known as RHDV1-K5, was distributed on laced grain or carrots at 323 community sites across all states, at locations where local land managers or councils had agreed to abide by strict protocols and record pre- and post-release data.
Project leader Tarnya Cox said with an average of 36 per cent localised knockdown, the virus known as K5 had operated as a biocide.
“We say biocide because where we release it, it works effectively, but doesn’t appear to spread beyond that,” Dr Cox said.
Rogue virus hampers K5’s spread
Research into the reasons for K5 not spreading is continuing but one key factor was the presence of another “rogue” calicivirus strain, RHDV2, that had spread across the landscape and killed millions of rabbits well before K5 could get a foothold.
It is not known how RHDV2 entered Australia, but it has been found in many parts of the world, including New Zealand.
According to the Centre for Invasive Species Solutions (CISS), the organisation which facilitated the release of K5, RHDV2 reduced rabbit abundance by an estimated 60 per cent in New South Wales and South Australia, 66 per cent in Western Australia and 52 per cent in Victoria.
Dr Cox said the presence of this other exotic strain caused “chaos” for the K5 program.
“So the spring before we released K5, RHDV2 really started to take off and spread,” she said.
“It is now everywhere across the country.
“It took maybe 18 months for that to happen from its first detection, and it is the dominant field strain out there in most [deceased] rabbits that we receive.”
More research needed to explain patchy K5 results
More than 1,000 samples from dead wild and domestic pet rabbits have been analysed since the release of K5 at the CSIRO in Canberra.
“We have only had one confirmed case of a domestic rabbit that has died from K5 and that animal was not vaccinated,” principal research scientist Tanja Strive said.
The currently available vaccine protects against K5 and the calicivirus released in 1995, but not against the “rogue” RHDV2.
According to CISS chief executive Andreas Glanznig, there is a vaccine in development designed to cover all present caliciviruses.
Mr Glanznig said this was the first national rabbit biocontrol release in more than 20 years.
An earlier strain of calicivirus escaped quarantine on South Australia’s Wardang Island in 1995.
In addition to the release of K5 by community volunteers, there were nine government-funded release sites which were part of a National Rabbit Biocontrol Monitoring Network.
At those intensively monitored sites, the virus had an even lower kill rate, producing a “statistically insignificant” number of dead rabbits.
Dr Cox said more work needed to be done to determine why there was a discrepancy between the community and the professionally monitored sites, but the latter had to release the virus whether or not they had a large live rabbit population, whereas the community groups could drop out of the experiment if they were seeing too few rabbits.
Despite the patchy results, the program leaders stand by the decision to release K5 two years ago.
“I think in hindsight … governments made the right decision,” Mr Glanznig said.
Dr Cox said a key reason for pushing ahead was the community interest.
“If we had turned around and said ‘sorry we are not doing this now, come back in five years time’, then the opportunity to engage those people, to get them motivated about rabbit management would have been lost,” Dr Cox said.
Twenty plan for rabbit reduction
CISS is pushing ahead with what it calls a “20-year rabbit control pipeline”, based on the premise there is no single management solution.
Researchers are now looking to the next biological tool.
“At the end of the day, rabbits will develop genetic resistance so we have to be positioned on the front foot with a view to producing a new agent every eight to 10 years if we are going to keep rabbit numbers in check,” Mr Glanznig said.
While RHDV2 is still circulating through rabbit populations, there could be advantages in a formal strategic release of that strain at a time and in locations where it would have maximum impact.
Dr Cox said she was getting good feedback from land managers who were using the virus for localised control.
“All I get from people is that they release it and get all these dead rabbits later,” she said.
Watch this story on ABC TV’s Landline on Sunday at 12:30pm or on ABC iview.