Rabbit Virus working as expected
The rabbit calicivirus was first used as a biocontrol agent in 1997, when a Czech strain (RHDV1) was introduced illegally into New Zealand. It had a swift and large impact, with more than 70% of rabbits killed as it spread throughout the country and controlled populations in many regions. But in the 20 years since, although outbreaks continue to occur, control has declined. One reason is that young rabbits may acquire immunity from mothers or if exposed to the virus early in life.
Another cause of immunity was recently identified after the discovery of a benign calicivirus (RCV-A1) occurring naturally in wild rabbit populations in both New Zealand and Australia, which provides protection against the introduced RHDV1 strains. Work as part of an ongoing collaboration between Dr Duckworth’s team and Australian scientists revealed evidence of a very high incidence of immunity (up to 100%) to RCV-A1 in some regions .
Researchers in Australia screened a range of rabbit caliciviruses from around the world and determined that a Korean strain, RHDV1-K5, had the best chance of improving the control of rabbits. RHDV1-K5 is not expected to have the dramatic initial impact of the first release because rabbits are not a naïve population and the new strain would be competing with the existing strain. But it is hoped that RHDV1 K5 could reduce rabbit abundance by up to 40% and will provide New Zealand land managers with an improved and cost-effective tool for rabbit management, which will be integrated with follow-up rabbit management measures for better overall control.
Approval to release the RHDV1-K5 virus in New Zealand was applied for by Environment Canterbury on behalf of the Rabbit Coordination Group, which comprises regional and district councils with rabbit-prone areas, the Department of Conservation, Land Information New Zealand, and the High-Country section of Federated Farmers. Dr Duckworth assisted the group with the application and the development of a release strategy for RHDV1-K5. Key factors in the strategy included ensuring a high-quality commercial product was available, determining optimum release times when potential vector fly populations were high and the number of young rabbits (which could potentially acquire immunity) was low, selecting appropriate release sites, and using methods of bait deployment to maximise the uptake of the virus-treated baits.
Manaaki Whenua also worked with regional councils on pre- and post-virus release monitoring of rabbit numbers, and on researching the role that flies play as vectors moving the virus across the New Zealand landscape.