7.T.5 RHD genetic resistance project

Rabbit haemorrhagic disease (RHD) has been highly successful in keeping rabbits low across much of Australia for the last twelve years but it is important to know if this situation will continue. From past experience with myxomatosis, it was expected that the effectiveness of RHD would eventually wane as rabbits develop genetic resistance to the disease. We are investigating this to try and discover how long this might take - important knowledge when developing plans for future rabbit control programs.

Wild rabbits from across south-eastern Australia were tested by challenge with a small quantity of RHDV (a 1:25 dilution of commercially available Czech-strain) to detect changes in case mortality rate or survival time indicative of development of genetic resistance. It is to be anticipated that populations subjected to regular severe outbreaks of RHD should show greater resistance than those subjected to very infrequent outbreaks or lower mortality.

Rabbits from nine sites across Australia have been tested and Mr Peter Elsworth (Department of Primary Industries, Queensland) is continuing the work as part of his IA CRC supported PhD studies, by testing rabbits from an additional four sites in central Australia and North Queensland.

Associated studies are being carried out in South Australia by the Animal and Plant Control Group. They aim to determine whether the calicivirus is also changing genetically, possibly co-evolving in responses to changes in rabbit resistance and assess the effectiveness of making additional releases of RHDV. In the longer term, Mr Elsworth plans to test field virus strains against the initially released Czech-strain virus to see if apparent changes in genome sequence are reflected in virulence changes. DNA samples from experimental rabbits have been sent to Nantes in France for analysis as a first step towards the identification of genes that may contribute to rabbit resistance.

Results to date

Wild rabbits from many (but not all) of the initial nine sites were found to be more resistant to oral infection with Czech-strain RHD virus than domestic rabbits used as experimental controls.

Rabbits appear to be resistant to infection but those that become infected still show high case-mortality rates (more than 90%).

Rabbits that survive experimental infection can usually be infected using a larger quantity of virus or a different challenge route.

Rabbits from different sites have different levels of resistance, possibly related to the past frequency of RHD outbreaks. Rabbits from high rainfall areas generally show lower resistance than those from dry areas where disease impact has been high.

There is good evidence that rabbits may be developing resistance to infection but the final outcome in terms of maintaining RHDV as a useful biological control agent may depend on whether or not the RHD virus is changing (i.e. co-evolving) to adjust to these changes. We cannot predict as yet whether RHD virus will remain highly infective well into the future or whether increases in rabbit resistance will eventually out-strip the ability of the virus to cause infection.

Collaborations

Australian Wool Innovation Ltd is a major sponsor of this work and a participant in the IA CRC. Other contributing participants include:

The Animal and Plant Control Group, Department of Land, Water and Biodiversity Conservation of South Australia
The Western Australian Department of Agriculture
New South Wales Department of Primary Industries
Victorian Department of Primary Industries
Queensland Department of Natural Resources and Water
IZS of Brescia, Italy has supplied monoclonal antibodies for ELISAs
INSERM, Nantes, France is analysing rabbit DNA samples.