The epidemiology of rabbit haemorrhagic disease, and its impact on rabbit populations, in south-western Australia

Abstract

The impact of rabbit haemorrhagic disease (RHD) on free-ranging rabbit populations, their immunological response, and the abundance of insect vectors, were monitored in detail in the southern agricultural region of Western Australia. Further, a broad-scale rabbit monitoring program was also established at nine locations across a rainfall gradient in the southern half of Western Australia to monitor the natural arrival, or controlled release, of rabbit haemorrhagic disease virus (RHDV). Changes in rabbit populations and the immune status of RHDV antibodies were monitored in these areas to enable further understanding of the epidemiology of RHD, and its impact on rabbit numbers.RHDV had the greatest impact on rabbit populations in the arid and semi-arid areas (<360 mm per annum), where rabbit numbers were reduced to, and maintained at, 10% of pre-RHD levels. Conversely, the effects of RHD on rabbit numbers in higher-rainfall areas (360–700 mm per annum) were highly variable and patchy, and in some instances RHD had little apparent impact. In higher-rainfall areas where RHD was effective, rabbit numbers were reduced by 50–78%. RHDV was first confirmed in the southern agricultural region of Western Australia in early September 1996, ~1 year after its escape from Wardang Island, South Australia. At the detailed monitoring site (485 mm rainfall per annum), rabbit numbers declined by 65% within 2 weeks of RHDV being detected. However, ~70% of the remaining rabbits had antibodies against RHDV, indicating that they had survived the disease. There was also a demographic shift towards young rabbits (<1 year old) at this time. Further, even though there was no clinical evidence of RHD recurring in this population during the 3-year study, the presence of IgM antibodies in some rabbits well after the initial epizootic suggests that low-level transient outbreaks of RHDV had occurred at this site. Although the impact of these low-level outbreaks on population dynamics were uncertain, rabbit numbers at this site had recovered to pre-RHD levels within two breeding seasons.The abundance of insect vectors on the detailed monitoring site was similar between years with and without RHD outbreaks. Thus, the failure of RHDV to develop clinical disease on this site after the initial epizootic was unlikely to have been caused by the lack of suitable transmission vectors. The apparent lack of disease recurrence at the detailed monitoring site may have been caused by the presence of a non-pathogenic form of RHDV, which seemed to impart at least some cross-immunity to RHDV in these rabbits. The presence of RHDV also caused a shift in the timing of natural epizootics of myxomatosis at this site.

Habitat factors related to wild rabbit population trends after the initial impact of rabbit haemorrhagic disease

“…Although this pattern seems clear, its absolute magnitude should be interpreted carefully, since yearly variations in rabbit abundance may partially mask long-term trends. Several studies have reported great differences in the initial impact of RHD among rabbit populations, with many of these differences being related to climatic conditions such as temperature or rainfall (Henzell et al 2002;Mutze et al 2002;Bruce et al 2004). Thus, the partial immunisation of populations against RHD using a pre-existing nonpathogenic rabbit calicivirus (RCV) in high-rainfall areas has been invoked as a putative cause of positive rabbit population trends in Australia (Cooke and Fenner 2002).…”

Biological invasions and natural colonisations: are they that different?

“…Indeed invasive species management has its roots with pathogen containment, so it is not surprising that the process of species colonisation would follow an identical framework to that for epidemiology. Epidemiological examples include the accidental emergence and subsequent spread of swine and bird flues (Neumann et al 2009), the intentional release of rabbit calicivirus in Australia (Bruce et al 2004), the unintentional spread of pathogenic chytrid fungus affecting amphibians globally (Lips et al 2006), and the recent epidemics of Ebola (Gire et al 2014).…”

Overlooking the smallest matter: viruses impact biological invasions

“…Both the more slowly evolving dsDNA myxoma virus (MV) and the faster evolving ssRNA rabbit haemorrhagic disease virus (RHDV) are both capable of causing high mortality rates in rabbits (Bruce et al . ). We anticipate that ssDNA and RNA viruses will be particularly capable of causing population collapse as they can rapidly adapt to increasing host resistance (Duffy et al .…”

“…The ongoing biological control of European rabbits (Oryctolagus cuniculus) (Linnaeus, 1758) in Australia using two different viral species provides compelling evidence that viruses can severely reduce established populations of exotic species. Both the more slowly evolving dsDNA myxoma virus (MV) and the faster evolving ssRNA rabbit haemorrhagic disease virus (RHDV) are both capable of causing high mortality rates in rabbits (Bruce et al 2004). We anticipate that ssDNA and RNA viruses will be particularly capable of causing population collapse as they can rapidly adapt to increasing host resistance (Duffy et al 2008).…”