Early circulation of rabbit haemorrhagic disease virus type 2 in domestic and wild lagomorphs in southern California, USA (2020-2021)

Development of a peptide-generated antibody to rabbit hemorrhagic disease virus 2 VP60 and its immunohistochemical application in natural cases

Rabbit hemorrhagic disease virus 2 (RHDV2) has spread across the United States infecting and causing death in domestic and wild rabbits. Immunohistochemistry (IHC) would be a useful tool for the detection of RHDV2 antigen in tissues as it is inexpensive and readily achievable in most diagnostic laboratories. However, there is no readily available antibody for this purpose. To fill this void, we generated an RHDV2 capsid protein VP60-specific antibody in chicken eggs and validated the antibody using formalin-fixed tissues from 5 domestic rabbits naturally infected with RHDV2. Viral antigen was detected immunohistochemically in various tissues, most prominently in hepatocytes and macrophages in liver, and in macrophages in spleen and cecal lymphoid tissue. Intravascular mononuclear cells in lung and renal tubular and biliary epithelium also were immunolabeled. Both nuclear and cytoplasmic immunolabeling were observed. This peptide-generated antibody is a potentially useful tool as an adjunct to reverse-transcription PCR or in situ hybridization for detection of RHDV2 in tissues.

Rabbit hemorrhagic disease virus 2, 2010-2023: a review of global detections and affected species

Rabbit hemorrhagic disease virus 2/genotype GI.2 (RHDV2/GI.2; Caliciviridae, Lagovirus) causes a highly contagious disease with hepatic necrosis and disseminated intravascular coagulation in several Leporidae species. RHDV2 was first detected in European rabbits (Oryctolagus cuniculus) in France in 2010 and has since spread widely. We gather here data on viral detections reported in various countries and affected species, and discuss pathology, genetic differences, and novel diagnostic aspects. RHDV2 has been detected almost globally, with cases reported in Europe, Africa, Oceania, Asia, and North America as of 2023. Since 2020, large scale outbreaks have occurred in the United States and Mexico and, at the same time, cases have been reported for the first time in previously unaffected countries, such as China, Japan, Singapore, and South Africa, among others. Detections have been notified in domestic and wild European rabbits, hares and jackrabbits (Lepus spp.), several species of cottontail and brush rabbits (Sylvilagus spp.), pygmy rabbits (Brachylagus idahoensis), and red rock rabbits (Pronolagus spp.). RHDV2 has also been detected in a few non-lagomorph species. Detection of RHDV2 causing RHD in Sylvilagus spp. and Leporidae species other than those in the genera Oryctolagus and Lepus is very novel. The global spread of this fast-evolving RNA virus into previously unexploited geographic areas increases the likelihood of host range expansion as new species are exposed; animals may also be infected by nonpathogenic caliciviruses that are disseminated by almost all species, and with which genetic recombination may occur.

Antibody response of endangered riparian brush rabbits to vaccination against rabbit hemorrhagic disease virus 2

Rabbit hemorrhagic disease virus 2 (RHDV2; Caliciviridae, Lagovirus europaeus), the cause of a highly transmissible and fatal lagomorph disease, has spread rapidly through the western United States and Mexico, resulting in substantial mortality in domestic and wild rabbits. The disease was first detected in California in May 2020, prompting an interagency/zoo/academia/nonprofit team to implement emergency conservation actions to protect endangered riparian brush rabbits (Sylvilagus bachmani riparius) from RHDV2. Prior to vaccinating wild rabbits, we conducted a vaccine safety trial by giving a single SC dose of Filavac VHD K C+V (Filavie) vaccine to 19 adult wild riparian brush rabbits captured and temporarily held in captivity. Rabbits were monitored for adverse effects, and serum was collected before vaccination, and at 7-10, 14-20, and 60 d post-vaccination. Sera were tested using an ELISA to determine antibody response and timing of seroconversion. Reverse-transcription quantitative real-time PCR (RT-qPCR) was performed on rectal swabs to evaluate infection status. No adverse effects from the vaccine were observed. Before vaccination, 18 of 19 rabbits were seronegative, and RHDV2 was not detected by RT-qPCR on any rectal swabs. After vaccination, all rabbits developed an antibody response, with titers of 1:10-1:160. Seroconversion generally occurred at 7-10 d. The duration of antibody response was ≥60 d in 12 of 13 rabbits. Sixteen animals were released and 4 were recaptured several months later, offering a glimpse into longer duration immune response. Our study has informed vaccination strategies for this species and serves as a model for protecting other vulnerable lagomorphs against RHDV2.

Diseases of domestic rabbits by purpose; a retrospective study of 2,583 cases received at 4 diagnostic laboratories in California, USA, 2013-2022

Domestic rabbits (Oryctolagus cuniculus) are commonly kept as pets or bred for laboratory investigation, meat, fur production, or a combination of these purposes. We conducted a retrospective study to assess the prevalence of diseases in domestic rabbits according to purpose. We retrieved results of autopsies, biopsies, and cytologies from 2,583 cases received at 4 diagnostic laboratories in California from 2013-2022. Rabbits were classified as pets (2,241; 86.8%), laboratory animals (92; 3.6%), meat-production animals (60; 2.3%), or multipurpose animals (190; 7.4%). A final diagnosis was reached in 2,360 (91.4%) cases and was classified by system, etiology, and type of process. Pet rabbits had the highest median age (5.9 y; vs. 3 y, 0.67 y, and 0.25 y in meat, multipurpose, and laboratory rabbits, respectively), and most of the neoplasms were diagnosed in this group (872 of the total 896 neoplasms in the study; 97.3%), with tumors of the skin, female reproductive tract, and hematolymphoid system being the most common. Laboratory rabbits had a high prevalence of infectious enterotyphlocolitis (40 of 92; 43.4%), and ~45% of those cases were due to opportunistic colibacillosis. Infectious and parasitic pneumonias were common in meat rabbits (18 of 60; 30%); pneumonic pasteurellosis accounted for >60% of those cases. Infectious cholangiohepatitides were common in multipurpose rabbits (61 of 110; 55.5%), with rabbit hemorrhagic disease representing the most common etiology (82.4% of those cases). Our results demonstrate that purpose of use can predict prevalence of disease in rabbits submitted to diagnostic laboratories.

Practical Suggestions for Assessing Rabbit Haemorrhagic Disease Virus 2 Risk to Endangered Native Lagomorphs in North America and Southern Africa

A new form of the rabbit haemorrhagic disease virus, RHDV2, first observed in European rabbits, has spread widely among different species of hares in Europe, jackrabbits and cottontails in North America, and hares in southern Africa. However, only limited surveillance studies have been undertaken so far. It is suggested that methods developed for controlling the disease in farmed rabbits in Europe and studying the efficacy of RHDV as a biological control agent in Australia could facilitate epidemiological research on those recently affected lagomorph species. This would enable the assessment of the risk of RHDV2 to native lagomorphs, including endangered species, and the determination of the main host species of RHDV2. Because RHDV2 has not spread equally through all lagomorph species, epidemiological studies could give insights into factors important for determining host susceptibility.

Detections of Rabbit Hemorrhagic Disease Virus 2 (RHDV2) Following the 2020 Outbreak in Wild Lagomorphs across the Western United States

Rabbit hemorrhagic disease virus 2 (RHDV2) is a highly infectious, often fatal viral disease that affects both domestic and wild lagomorph species. In the United States (U.S.), the virus first was detected in wild lagomorph populations in the southwest in March 2020 and has continued to be detected in native North American lagomorph species over several years. The susceptibility of host species and exact mechanisms of environmental transmission across the U.S. landscape remain poorly understood. Our study aims to increase the understanding of RHDV2 in wild lagomorph populations by providing a history of detection. We present and summarize results from all RHDV2-suspect wild lagomorph morbidity and mortality samples submitted for diagnostic testing in the U.S. from March 2020 to March 2024. Samples were submitted from 916 wild lagomorphs across eight native North American species in 14 western states, of which 313 (34.2%) tested positive by RHDV2 RT-qPCR. Detections of RHDV2 in pygmy rabbits (Brachylagus idahoensis) and riparian brush rabbits (Sylvilagus bachmani riparius) suggest that the risk to threatened and endangered species warrants more attention. Continuing to investigate wild lagomorph morbidity and mortality events and tracking RHDV2 detections over time can help inform on disease epidemiology and wild lagomorph population trends.

Epidemiological characterization and risk assessment of rabbit haemorrhagic disease virus 2 (RHDV2/b/GI.2) in the world

A novel variant of rabbit haemorrhagic disease virus, designated RHDV2/b/GI.2, was first discovered in France in 2010. Subsequently, RHDV2 rapidly spread to Africa, North America, Australia, and Asia. RHDV2 outbreaks have resulted in significant economic losses in the global rabbit industry and disrupted the balance of natural ecosystems. Our study investigated the seasonal characteristics of RHDV2 outbreaks using seasonal indices. RHDV2 is prone to causing significant outbreaks within domestic and wild rabbit populations during the spring season and is more likely to induce outbreaks within wild rabbit populations during late autumn in the Southern Hemisphere. Furthermore, based on outbreak data for domestic and wild rabbits and environmental variables, our study established two MaxEnt models to explore the relationship between RHDV2 outbreaks and the environmental factors and conducted outbreak risk predictions for RHDV2 in global domestic and wild rabbit populations. Both models demonstrated good predictive performance, with AUC values of 0.960 and 0.974, respectively. Road density, isothermality, and population density were identified as important variables in the outbreak of RHDV2 in domestic rabbits, while road density, normalized difference vegetation index, and mean annual solar radiation were considered key variables in the outbreak of RHDV2 in wild rabbits. The environmental factors associated with RHDV2 outbreaks identified in our study and the outbreak risk prediction maps generated in our study will aid in the formulation of appropriate RHDV2 control measures to reduce the risk of morbidity in domestic and wild rabbits.

Recombination between non-structural and structural genes as a mechanism of selection in lagoviruses: The evolutionary dead-end of an RHDV2 isolated from European hare

The genus Lagovirus, belonging to the family Caliciviridae, emerged around the 1980s. It includes highly pathogenic species, rabbit hemorrhagic disease virus (RHDV/GI.1) and European brown hare syndrome virus (EBHSV/GII.1), which cause fatal hepatitis, and nonpathogenic viruses with enteric tropism, rabbit calicivirus (RCV/GI.3,4) and hare calicivirus (HaCV/GII.2). Lagoviruses have evolved along two independent genetic lineages: GI (RHDV and RCV) in rabbits and GII (EBHSV and HaCV) in hares. To be emphasized is that genomes of lagoviruses, like other caliciviruses, are highly conserved at RdRp-VP60 junctions, favoring intergenotypic recombination events at this point. The recombination between an RCV (genotype GI.3), donor of non-structural (NS) genes, and an unknown virus, donor of structural (S) genes, likely led to the emergence of a new lagovirus in the European rabbit, called RHDV type 2 (GI.2), identified in Europe in 2010. New RHDV2 intergenotypic recombinants isolated in rabbits in Europe and Australia originated from similar events between RHDV2 (GI.2) and RHDV (GI.1) or RCV (GI.3,4). RHDV2 (GI.2) rapidly spread worldwide, replacing RHDV and showing several lagomorph species as secondary hosts. The recombination events in RHDV2 viruses have led to a number of viruses with very different combinations of NS and S genes. Recombinant RHDV2 with NS genes from hare lineage (GII) was recently identified in the European hare. This study investigated the first RHDV2 (GI.2) identified in Italy in European hare (RHDV2_Bg12), demonstrating that it was a new virus that originated from the recombination between RHDV2, as an S-gene donor and a hare lagovirus, not yet identified but presumably nonpathogenic, as an NS gene donor. When rabbits were inoculated with RHDV2_Bg12, neither deaths nor seroconversions were recorded, demonstrating that RHDV2_Bg12 cannot infect the rabbit. Furthermore, despite intensive and continuous field surveillance, RHDV2_Bg12 has never again been identified in either hares or rabbits in Italy or elsewhere. This result showed that the host specificity of lagoviruses can depend not only on S genes, as expected until today, but potentially also on some species-specific NS gene sequences. Therefore, because RHDV2 (GI.2) infects several lagomorphs, which in turn probably harbor several specific nonpathogenic lagoviruses, the possibility of new speciation, especially in those other than rabbits, is real. RHDV2 Bg_12 demonstrated this, although the attempt apparently failed.

Sustained Impact of RHDV2 on Wild Rabbit Populations across Australia Eight Years after Its Initial Detection

Following the arrival of rabbit haemorrhagic disease virus 2 (RHDV2) in Australia, average rabbit population abundances were reduced by 60% between 2014 and 2018 based on monitoring data acquired from 18 sites across Australia. During this period, as the seropositivity to RHDV2 increased, concurrent decreases were observed in the seroprevalence of both the previously circulating RHDV1 and RCVA, a benign endemic rabbit calicivirus. However, the detection of substantial RHDV1 seropositivity in juvenile rabbits suggested that infections were continuing to occur, ruling out the rapid extinction of this variant. Here we investigate whether the co-circulation of two pathogenic RHDV variants was sustained after 2018 and whether the initially observed impact on rabbit abundance was still maintained. We monitored rabbit abundance and seropositivity to RHDV2, RHDV1 and RCVA at six of the initial eighteen sites until the summer of 2022. We observed sustained suppression of rabbit abundance at five of the six sites, with the average population reduction across all six sites being 64%. Across all sites, average RHDV2 seroprevalence remained high, reaching 60-70% in adult rabbits and 30-40% in juvenile rabbits. In contrast, average RHDV1 seroprevalence declined to <3% in adult rabbits and 5-6% in juvenile rabbits. Although seropositivity continued to be detected in a low number of juvenile rabbits, it is unlikely that RHDV1 strains now play a major role in the regulation of rabbit abundance. In contrast, RCVA seropositivity appears to be reaching an equilibrium with that of RHDV2, with RCVA seroprevalence in the preceding quarter having a strong negative effect on RHDV2 seroprevalence and vice versa, suggesting ongoing co-circulation of these variants. These findings highlight the complex interactions between different calicivirus variants in free-living rabbit populations and demonstrate the changes in interactions over the course of the RHDV2 epizootic as it has moved towards endemicity. While it is encouraging from an Australian perspective to see sustained suppression of rabbit populations in the eight years following the arrival of RHDV2, it is likely that rabbit populations will eventually recover, as has been observed with previous rabbit pathogens.

The evolution of white-tailed jackrabbit camouflage in response to past and future seasonal climates

The genetic basis of adaptive traits has rarely been used to predict future vulnerability of populations to climate change. We show that light versus dark seasonal pelage in white-tailed jackrabbits (Lepus townsendii) tracks snow cover and is primarily determined by genetic variation at endothelin receptor type B (EDNRB), corin serine peptidase (CORIN), and agouti signaling protein (ASIP). Winter color variation was associated with deeply divergent alleles at these genes, reflecting selection on both ancestral and introgressed variation. Forecasted reductions in snow cover are likely to induce widespread camouflage mismatch. However, simulated populations with variation for darker winter pelage are predicted to adapt rapidly, providing a trait-based genetic framework to facilitate evolutionary rescue. These discoveries demonstrate how the genetic basis of climate change adaptation can inform conservation.

Case series: Four fatal rabbit hemorrhagic disease virus infections in urban pet rabbits

Four pet rabbits (Oryctolagus cuniculus cuniculus) diagnosed with a fatal infection by rabbit hemorrhagic disease virus (RHDV GI.2) were identified in the same week and further investigated. All animals lived in an urban environment (Lisbon, Portugal), were between 8 months and 2 years old and none had been vaccinated against RHDV2 (GI.2). Three animals arrived at the clinic and died shortly afterward and it was only possible to collect material for RT-qPCR (RHDV) test. These rabbits tested positive for RHDV2, with high viral loads. In the fourth case, additional clinical and post-mortem gross and histological evaluations were performed. This 8 month old intact female indoor pet rabbit was presented with apathy, tachypnea and tachycardia. Radiographic projections revealed no clinical revealed no clinical abnormalities. Serum biochemistry revealed a significant increase in AST and ALT with a small hypoglycemia. Abdominal ultrasound revealed an acute hepatitis. Despite hospitalization support, after 30 h of admission, the rabbit lost consciousness and developed anorexia and pyrexia in the last minutes before death. Post-mortem analysis and molecular testing by RT-qPCR, confirmed the diagnosis of RHDV2 (GI.2) infection also with high viral load. In conclusion, this paper reports a case series that demonstrates the severe infectious ability and the high mortality associated with RHDV even in rabbits from urban environments. Furthermore, it highlights the importance of always considering rabbit hemorrhagic disease (RHD) as a differential diagnosis in pet rabbits with non-specific clinical signs, and should warn veterinarians that pet rabbits living indoors can also be infected with a fatal outcome.

Rabbit Haemorrhagic Disease Virus 2 (RHDV2; GI.2) in Ireland Focusing on Wild Irish Hares (Lepus timidus hibernicus): An Overview of the First Outbreaks and Contextual Review

Rabbit haemorrhagic disease virus 2 (RHDV2; GI.2) is a pathogenic lagovirus that emerged in 2010, and which now has a global distribution. Outbreaks have been associated with local population declines in several lagomorph species, due to rabbit haemorrhagic disease (RHD)-associated mortality raising concerns for its potential negative impact on threatened or vulnerable wild populations. The Irish hare (Lepus timidus hibernicus) is endemic to Ireland, and is of conservation interest. The first cases of RHDV2 in Ireland were reported in domestic rabbits (Oryctolagus cuniculus) in 2016, soon followed by the first known case in a wild rabbit also in 2016, from a population reported to be experiencing high fatalities. During summer 2019, outbreaks in wild rabbits were confirmed in several locations throughout Ireland. Six cases of RHDV2 in wild hares were confirmed between July and November 2019, at four locations. Overall, 27 cases in wildlife were confirmed in 2019 on the island of Ireland, with a predominantly southern distribution. Passive surveillance suggests that the Irish hare is susceptible to lethal RHDV2 infection, and that spillover infection to hares is geographically widespread in eastern areas of Ireland at least, but there is a paucity of data on epidemiology and population impacts. A literature review on RHD impact in closely related Lepus species suggests that intraspecific transmission, spillover transmission, and variable mortality occur in hares, but there is variability in reported resistance to severe disease and mortality amongst species. Several key questions on the impact of the pathogen in Irish hares remain. Surveillance activities throughout the island of Ireland will be important in understanding the spread of infection in this novel host.