Australia's biocontrol programs using Myxoma virus in 1950 and the Rabbit Calicivirus in 1995 have been extremely successful in drastically reducing pest rabbit numbers in Australia. However, there is no status quo in rabbit biocontrol: the virus and rabbits constantly co-evolve, and changes in virus virulence and innate resistance in the host leads to rabbit numbers increasing yet again. We are continuing research to find new strategies that will help maintain benefits of biocontrol in the future.

The challenge

A growing problem

By 1950 rabbit numbers in Australia reached 600 million.

Rabbits were introduced to Australia in 1859 by a wealthy Victorian grazier keen on the sport of hunting. After a fire destroyed the enclosures, rabbits started their campaign to conquer Australia. Within 70 years they spread to 70 per cent of Australia's landmass, the fastest known invasion by a mammal anywhere in the world. The damage caused by rabbits ranged from massive grazing pressure and competition with livestock to spreading weeds and accelerating erosion, and many farms were abandoned during the peak of the rabbit plaque.

Even today, with rabbit numbers still at a fraction of pre-Myxomatosis levels, rabbits cause an estimated AUD$200 million per year economic damage, in addition to countless detrimental effects on biodiversity. Due to their selective grazing habits, less than one rabbit per hectare can completely prevent the regeneration of certain palatable native tree and shrub species.

Our response

Biocontrol with viruses

CSIRO's predecessor, CSIR, carried out initial trials that ultimately resulted in the successful release of the Myxomatosis for the biological control of rabbits in 1950. As a result there was a dramatic reduction of Australia's rabbit population and AUD$68 million was recovered for our wool and meat industries within two years.

It was the world's first successful biological control program of a mammalian pest, taming a scourge that had threatened Australian agriculture and environment. However, by the late 1950s, host-pathogen co-evolution led to a less severe form of the disease, and rabbit numbers increased again, although not to pre-1950 levels.

In 1984 a new emerging disease of rabbits was described, Rabbit Haemorrhagic Disease Virus (RHDV), a calicivirus. The virus was brought to Australia in 1991 and was extensively assessed for its suitability as Australia’s second rabbit biocontrol agent. After its escape from quarantine in 1995, it was officially released a year later. RHDV again reduced rabbit numbers to very low levels, with greatest impact in arid zones and lesser impact in high rainfall areas.

Further, scientists observed that RHDV was not as effective in cool, high rainfall areas and they suspected that a non-lethal calicivirus within the rabbits, closely related to RHDV, was to blame. This virus was identified in 2009, and it was shown that it can indeed partially and transiently protect rabbits from the lethal calicivirus RHDV, and thereby likely contributes to the lack of biocontrol success in the more temperate areas where the benign virus is present.

RHDV kept rabbit numbers low for over a decade and, in contrast to Myxomavirus, a reduction in virulence has so far not been observed. However, evidence for developing resistance in some Australian wild rabbit populations has now been described, and rabbit numbers are again on the rise.

The results

An ongoing battle

Dr Tanja Strive from CSIRO's Biosecurity Flagship and the Invasive Animals CRC has found that some rabbits in cool, high-rainfall areas carry a benign virus that gives them immunity to calicivirus.

The interaction between the rabbit host and its viruses forms a complex dynamic equilibrium, fine tuned to enable optimal spread of the virus without completely eliminating the host population, in a variety of environments. The examples of Myxoma virus and RHDV show that following the initial impact of a new biocontrol agent, rabbit numbers will likely bounce back again in the medium term.

This highlights the need to not solely rely on biological control to manage pest rabbits, but to always combine it with conventional control methods.

Nevertheless, biological control is by far the most cost effective large-scale control option, and keeping rabbit numbers low is essential for Australia's biodiversity and rural industries. Current research is therefore aiming at ways to help the virus stay ahead in the co-evolutionary arms race with its rabbit host, to protect the gains made by the past successful biocontrol initiatives and to keep rabbit numbers below the damage threshold.

New and more deadly strains of RHDV are currently being studied in the effort to stop rabbits from further damaging Australia's agricultural and natural environment.

A collaborative research project run by the Invasive Animals Cooperative Research Centre, led by the NSW Department of Primary Industries has evaluated overseas strains of RHDV for their ability to supplement Australia's existing biocontrol toolbox.

Subject to release approval, the project could deliver benefits of up to a billion dollars to the Australian economy over the next 15 years.

A second line of research is currently developing a platform technology to accelerate and direct the natural evolution of the virus. The ultimate goal of this non-GMO approach is to repeatedly select tailored virus strains for subsequent virus releases, giving the virus the cutting edge to stay ahead in the co-evolutionary arms race with its host.

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