National rabbit biocontrol optimisation

Community submission of rabbit samples continues to deliver valuable insights into the changing epidemiology of the various circulating virus strains in Australia. Derivatives of the original Australian RHDV2 continues to be dominant in WA, while various recombinant RHDV2 variants are dominant throughout the Eastern states.
Analysis of sequence data provided strong evidence of RHDV1 activity inhibition as a result of initial RHDV2 spread, strengthening previous evidence derived from serological data that RHDV2 suppressed the impact of RHDV1-K5 when it was released in 2017.

RHDV2 and its recombinants continue to be epidemiologically dominant in the Australian landscape, however we are seeing growing evidence of some local persistence and spread of RHDV1-K5 in Western Australia and Tasmania, and the detection of an RHDV1 field strain demonstrates that this original virus variant has not yet become completely extinct.
RHDV2 causes virus outbreaks and transmission to occur approximately one season earlier than RHDV1. The ability to infect younger rabbits may be a key competitive advantage for RHDV2 in the field, consequently altering the epidemiology of the virus in wild populations of rabbits.

While RHDV2 remains the dominant virus circulating in the field in all areas of Australia, several cases of RHDV1 K5 have been detected one location that were not associated with a targeted release. These results suggest that RHDV1 K5 may have become established in wild rabbit populations in some locations. Further monitoring will be carried out through the project to better understand if this has occurred more widely or remains the exception.
CSIRO has developed a new assay for high throughput screening of recombinant caliciviruses. This assay has now been adopted into the routine testing protocols for RabbitScan and has retrospectively been applied to all positive calicivirus samples to confirm the identity of each sample.
A pilot study investigating the suitability of using carrion feeding flies as a better broad scale tool to monitor disease activity at a landscape scale is nearing completion of its first year. Preliminary analysis of the first nine months of samples (March 2019 to January 2020) indicate that RHDV activity can indeed be detected in fly samples, but the sensitivity of this method remains unclear at this stage. The aim is to continue this study for an additional 12 months to get a better understanding of this tool to monitor RHDV activity at a wider scale.

Ongoing testing of tissue from deceased rabbits shows RHDV2 remains the dominant rabbit calicivirus circulating in Australia, with K5 detected at release sites. A new screening method is being implemented to detect recombinant viruses.
Rabbit monitoring at selected intensive sites in TAS, ACT, WA, NSW and SA is ongoing or ramping up shortly, with a minimum bi-annual sampling taking place. In addition, monitoring and sampling of the rabbit population at the Turretfield research site continues as part of this project, building on extensive knowledge of this wild rabbit population and adding these ongoing monitoring results to the long-term RHDV and myxoma disease database at this site.
A paper on optimising and validating molecular testing of carrion flies for rabbit calicivirus detection, including associated protocols was published. This provided the basis for a nationwide year-long pilot study now underway.

Intensive rabbit monitoring sites have been established with monitoring well underway at sites in South Australia and the ACT, and due to commence at other sites in Autumn 2019. Sites in South Australia are reporting the lowest abundance of rabbits in more than 20 years, indicating that circulating strains of RHDV are suppressing rabbit numbers.

The methods for sampling, processing and analysing carrion feeding flies for the presence of RHDV have been developed and validated. A national fly monitoring network at over 30 sites across Australia has been established and fortnightly sampling has commenced.

Analysis of samples from dead rabbits submitted directly to CSIRO and through RabbitScan is ongoing – over 1600 samples have been analysed since the beginning of the previous IA-CRC project RHD Boost Rollout, and ~1000 since the K5 release).