The increasing problem of rabbits

Rabbit. Image - iStock.

Rabbits have been introduced to more than 800 islands around the world, including mainland New Zealand and many of its inshore and subantarctic islands. Rabbits significantly modify indigenous ecosystems and threaten the financial and ecological sustainability of agricultural and horticultural properties. Various control combinations of poisoning, trapping, dogging and shooting have successfully removed rabbits from over 18 New Zealand islands ranging in size from 1 to 3820 hectares and from many other islands worldwide. Currently Australian agencies are in the final stages of a programme to eradicate rabbits from the 12,785 ha Macquarie Island using aerial baiting, shooting, detector dogs and a biocide based on rabbit haemorrhagic disease (RHD). Broad-scale eradication on the mainland of New Zealand has never been attempted and it is not known whether the necessary conditions for successful eradication or long-term control of mainland populations can be met. For example, for successful eradication, all rabbits must be put at risk and killed faster than they can be replaced by natural births, and immigration must be prevented. Additionally, the benefits of eradication should outweigh the costs and be socially acceptable. There are many questions yet to be answered. It is clear, however, that successful and affordable control is more likely to be achieved if the effectiveness of the existing biological control agent, rabbit haemorrhagic disease (RHD) virus, can be regained and maintained.

Following its introduction to New Zealand in 1997, RHD spread rapidly, causing high mortality (often >90%) and greatly reducing the use of toxins, the costs of pest control to farmers, and the degradation of land in rabbit-prone areas. However, in most areas, secondary follow-up control was not undertaken and rabbit numbers have increased, as the effectiveness of RHD has decreased due to the high proportion of rabbits having antibodies that make them immune to circulating strains of RHD virus (RHDV). Rabbits can acquire these protective antibodies following exposure to RHDV early in life or possibly from infection with a closely-related but benign form of rabbit calicivirus (RCV) that may have been present in rabbits when they were first released in New Zealand 150 years ago. Researchers in Australia have shown that since RHD release, wild strains of RHDV in some areas have become more virulent, killing a high proportion of rabbits more quickly. They have also discovered a benign RCV, that is closely related to RHDV but is non-lethal and that infection with the benign RCV provides 30–40% protection against pathogenic RHDV strains. Janine Duckworth and her colleagues involved in the Rabbit Biocontrol Initiative project are now trying to find out whether variations in the effectiveness of RHDV throughout New Zealand are due to differences in the virulence of the virus across the country and whether any benign RCVs exist in New Zealand that may protect rabbits against RHDV infection.

Janine and members of the Rabbit Biocontrol Initiative are seeking samples of New Zealand field strains of RHDV from farmers, contractors and land users to identify RHD outbreaks. RHDV recovered from freshly-dead wild rabbits will be screened through challenge trials of naïve captive-bred rabbits to identify the most potent wild viral strains for further selection on virulence. As part of this study, the team will use molecular techniques to identify any benign RCV present and determine whether exposure to this virus protects rabbits against subsequent RHDV challenge (including the most virulent RHD strains identified in the survey). In the future, high-virulence strains of NZ-sourced RHDV will be made available to land managers to maximise the benefits of rabbit control in New Zealand and will be a key tool supporting long-term control and any attempts at regional eradication of feral rabbits.

This work is funded by the Ministry for Primary Industry’s Sustainable Farming Fund (12/055) and the Australian Invasive Animal Co-operative Research Centre.

Janine Duckworth

RHD outbreaks — virus sample collection

Samples of rabbits are sought from throughout New Zealand for genetic analysis and to identify differences between strains of RHDV that are killing rabbits in different regions of the country.
Of most interest are carcasses from 1–4 rabbits that have recently died from RHD. Fresh carcasses are best but any relatively intact carcass up to 7–10 days old is acceptable. Please label any carcasses with your contact details and the location where the rabbits were found and freeze them until Janine arranges their collection.
RHD outbreaks can be difficult to detect as rabbits often die underground and any above-ground carcasses can quickly be scavenged by hawks. Lots of hawks circling may be the indication of an RHD outbreak. The best time to look for rabbit carcasses is early in the morning before they are scavenged and the best place to look is near the entrance to burrows.

So please keep a look out for any RHD-killed rabbits and let Janine know of any active RHD outbreak, or to arrange collection of any samples, or if you have any questions.

Email: RHDBio@landcarereseach.co.nz
Tel: 03 321 9999 or 0800 743246