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Context: Helicopter-based shooting has been widely used to harvest deer or control overabundant populations in Australasia, but the effectiveness and cost of this method as a deer control tool has seldom been evaluated. Aims: We evaluated the effectiveness and costs of helicopter-based shooting of fallow deer (Dama dama) and chital deer (Axis axis) in eastern Australia by quantifying (1) reductions in density, (2) the relationship between numbers killed per hour and deer density (i.e. the functional response), (3) the costs of control and (4) the effort–outcome and cost–outcome relationships. Methods: We evaluated the costs and effectiveness of 12 aerial shooting operations aiming to reduce fallow deer (n = 8) or chital deer (n = 4) population densities at nine sites in eastern Australia. Sites were characterised by fragmented woodland, and all but one operation aimed to reduce grazing competition with livestock. We used pre-control population density estimates and operational monitoring data to estimate the costs and outcomes of each operation. We combined data from all operations to estimate the relationship between shooting effort and population reduction, as well as costs associated with different levels of effort. Key results: Population reductions for operations ranged from 5% to 75% for fallow deer, and from 48% to 88% for chital deer. The greatest population reductions occurred when effort per unit area was greatest, and the largest reductions in deer density occurred when shooting was conducted in consecutive years. The functional response of hourly kills to deer density was best described by a modified Ivlev model, with the asymptotic kill rate estimated to be 50 deer per hour. There was no support for the existence of a prey refuge, that is, a threshold population density below which no deer could be shot. Helicopter charter was the primary cost of helicopter-based shooting programs, followed by labour; firearm and ammunition costs were relatively minor. Conclusions: Helicopter-based shooting can rapidly reduce deer populations over large geographic areas, but the magnitude of the reduction depends on the effort (hours of shooting) per deer per km2. Implications: Aerial shooting operations should include a pre-control population survey so that (1) measurable objectives can be established, (2) the likely level of effort and cost required for objectives to be met can be estimated and planned for, and (3) the realised population reduction can be estimated. We assessed the effectiveness and costs of helicopter-based shooting of deer at nine sites in eastern Australia. Our study demonstrated that helicopter-based shooting can rapidly reduce deer populations over large geographic areas, but that the magnitude of the reduction depends on the deer density, the hours of shooting and size of the area. The major control cost was helicopter charter. Photograph by Jordan O. Hampton. [ABSTRACT FROM AUTHOR]

Rabbit haemorrhagic disease virus (RHDV) is highly pathogenic to European rabbits. Until recently, only one serotype of RHDV was known, GI.1/RHDV. RHDV2/GI.2 is a novel virus that has rapidly spread and become the dominant pathogenic calicivirus in wild rabbits worldwide. It is speculated that RHDV2 has three competitive advantages over RHDV: (a) the ability to partially overcome immunity to other variants; (b) the ability to clinically infect young rabbits; and (c) a wider host range. These differences would be expected to influence virus transmission dynamics. We used markers of recent infection (IgM/IgA antibodies) to investigate virus transmission dynamics pre and post the arrival of RHDV2. Our data set contained over 3,900 rabbits sampled across a 7-year period at 12 Australian sites. Following the arrival of RHDV2, seasonal peaks in IgM and IgA seropositivity shifted forward one season, from winter to autumn and spring to winter, respectively. Contrary to predictions, we found only weak effects of rabbit age, seropositivity to non-pathogenic calicivirus RCV-A1 and population abundance on IgM/IgA seropositivity. Our results demonstrate that RHDV2 enters rabbit populations shortly after the commencement of annual breeding cycles. Upon entering, the population RHDV2 undergoes extensive replication in young rabbits, causing clinical disease, high virus shedding, mortality and the creation of virus-laden carcasses. This results in high virus contamination in the environment, furthering the transmission of RHDV2 and initiating outbreaks, whilst simultaneously removing the susceptible cohort required for the effective transmission of RHDV. Although RHDV may enter the population at the same time point, it is sub-clinical in young rabbits, causing minimal virus shedding and low environmental contamination. Our results demonstrate a major shift in epidemiological patterns in virus transmission, providing the first evidence that RHDV2's ability to clinically infect young rabbits is a key competitive advantage in the field. [ABSTRACT FROM AUTHOR]

The European rabbit (Oryctolagus cuniculus) is one of the most devastating invasive species in Australia. Since the 1950s, myxoma virus (MYXV) and rabbit haemorrhagic disease virus (RHDV) have been used to manage overabundant rabbit populations. Resistance to MYXV was observed within a few years of the release. More recently, resistance to lethal RHDV infection has also been reported, undermining the efficiency of landscape‐scale rabbit control. Previous studies suggest that genetic resistance to lethal RHDV infection may differ locally between populations, yet the mechanisms of genetic resistance remain poorly understood. Here, we used genotyping by sequencing (GBS) data representing a reduced representation of the genome, to investigate Australian rabbit populations. Our aims were to understand the relationship between populations and identify possible genomic signatures of selection for RHDV resistance. One population we investigated had previously been reported to show levels of resistance to lethal RHDV infection. This population was compared to three other populations with lower or no previously reported RHDV resistance. We identified a set of novel candidate genes that could be involved in host–pathogen interactions such as virus binding and infection processes. These genes did not overlap with previous studies on RHDV resistance carried out in different rabbit populations, suggesting that multiple mechanisms are feasible. These findings provide useful insights into the different potential mechanisms of genetic resistance to RHDV virus which will inform future functional studies in this area. [ABSTRACT FROM AUTHOR]

The lagovirus rabbit haemorrhagic disease virus (RHDV) has been circulating in Australia since the mid‐1990s when it was released to control overabundant rabbit populations. In recent years, the viral diversity of different RHDVs in Australia has increased, and currently four different types of RHDV are known to be circulating. To allow for ongoing epidemiological studies and impact assessments of these viruses on Australian wild rabbit populations, it is essential that serological tools are updated. To this end, reference sera were produced against all four virulent RHDVs (RHDV, RHDV2 and two different strains of RHDVa) known to be present in Australia and tested in a series of available immunological assays originally developed for the prototype RHDV, to assess patterns of cross‐reactivity and the usefulness of these assays to detect lagovirus antibodies, either in a generic or specific manner. Enzyme‐linked immunosorbent assays (ELISAs) developed to detect antibody isotypes IgM, IgA and IgG were sufficiently cross‐reactive to detect antibodies raised against all four virulent lagoviruses. For the more specific detection of antibodies to the antigenically more different RHDV2, a competition ELISA was adapted using RHDV2‐specific monoclonal antibodies in combination with Australian viral antigen. Archival serum banks from a long‐term rabbit monitoring site where rabbits were sampled quarterly over a period of 6 years were re‐screened using this assay and revealed serological evidence for the arrival of RHDV2 in this population at least 5 months prior to its initial detection in Australia in a dead rabbit in May 2015. The serological methods and reference reagents described here will provide valuable tools to study presence, prevalence and impact of RHDV2 on Australian rabbit populations; however, the discrimination of different antigenic variants of RHDVs as well as mixed infections at the serological level remains challenging. [ABSTRACT FROM AUTHOR]

Rabbit hemorrhagic disease virus (RHDV) was released in Australia as a biocontrol agent for wild European rabbits ( Oryctolagus cuniculus) in 1995-96; however, its effects were variable across Australia with the greatest population reductions seen in lower annual rainfall areas (<400 mm). There is speculation that the reduced effectiveness observed at higher annual rainfall sites is at least partially due to the presence of a nonpathogenic calicivirus (RCV-A1). The RCV-A1 is related to RHDV and confers partial and transient protection against lethal RHDV infection in laboratory tests. What is not well understood is where, how, and to what degree RCV-A1 impedes the effect of RHDV-mediated rabbit control under field conditions. We investigated seven wild rabbit populations across six states and territories representing different seasonal rainfall zones across Australia, four times during 2011-12, to investigate if the presence and prevalence of RCV-A1 coincided with a change in RHDV immunity status within these populations. Besides serology, tissue samples from both trapped and shot rabbits were collected for virus detection by reverse transcription PCR. Overall, 52% ( n=258) of the total samples ( n=496) tested positive for RHDV antibodies and 42% ( n=208) positive for RCV-A1 antibodies; 30% ( n=150) of the sera contained antibodies to both viruses. The proportion of rabbits with RHDV antibodies increased significantly at sites where RCV-A1 antibodies were present ( χ21, α=0.1, P<0.001). Evidence that preinfection of RCV-A1 may lead to a higher proportion of sampled rabbits with antibodies to both viruses was found at only one site. [ABSTRACT FROM AUTHOR]