Challenges in the rabbit haemorrhagic disease 2 (RHDV2) molecular diagnosis of vaccinated rabbits

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.

Diagnosis of a Rabbit Hemorrhagic Disease Virus 2 (RHDV2) and the Humoral Immune Protection Effect of VP60 Vaccine

Rabbit hemorrhagic disease (RHD) is known as rabbit plague and hemorrhagic pneumonia. It is an acute, septic, and highly fatal infectious disease caused by the Lagovirus rabbit hemorrhagic disease virus (RHDV) in the family Caliciviridae that infects wild and domestic rabbits and hares (lagomorphs). At present, RHDV2 has caused huge economic losses to the commercial rabbit trade and led to a decline in the number of wild lagomorphs worldwide. We performed a necropsy and pathological observations on five dead rabbits on a rabbit farm in Tai'an, China. The results were highly similar to the clinical and pathological changes of typical RHD. RHDV2 strain was isolated and identified by RT-PCR, and partial gene sequencing and genetic evolution analysis were carried out. There were significant differences in genetic characteristics and antigenicity between RHDV2 and classical RHDV strain, and the vaccine prepared with the RHDV strain cannot effectively prevent rabbit infection with RHDV2. Therefore, we evaluated the protective efficacy of a novel rabbit hemorrhagic virus baculovirus vector inactivated vaccine (VP60) in clinical application by animal regression experiment. The result showed that VP60 could effectively induce humoral immunity in rabbits. The vaccine itself had no significant effect on the health status of rabbits. This study suggested that the clinical application of VP60 may provide new ideas for preventing the spread of RHD2.

Down the Rabbit Hole: Domestic Rabbit Owners' Perceptions of Rabbit Hemorrhagic Disease Virus 2

Rabbit hemorrhagic disease virus 2 (RHDV2) is a highly contagious pathogen that infects wild and domestic rabbits and hares (lagomorphs). Globally, RHDV2 has resulted in substantial economic losses for commercial rabbit trade and caused wild lagomorph population declines. Previous research on RHDV2 suggests that human-mediated movement of rabbits may contribute to the spread of RHDV2. We conducted the first survey of individuals who own or interact with domestic rabbits to identify their rabbit husbandry behaviors and knowledge of, and concerns about, RHDV2. In 2021, we surveyed 1807 rabbit owners, breeders, and rescue staff in the USA. Respondents had a high level of knowledge about RHDV2. Respondents believed RHDV2 posed a high risk to rabbit-related activities and were concerned about its economic and ecological impacts. Nearly half of respondents always kept their rabbits indoors, but 10.7% of respondents allowed their rabbits outside frequently on properties used by wild lagomorphs. Respondents with five rabbits or less were generally willing to vaccinate their rabbits, but respondents with larger herds argued that vaccines were cost prohibitive. Given respondents' concerns about RHDV2, communication about disease prevention should highlight the adverse ecological and economic consequences of RHDV2.

Age and Infectious Dose Significantly Affect Disease Progression after RHDV2 Infection in Naïve Domestic Rabbits

Rabbit haemorrhagic disease virus 2 (RHDV2 or GI.2, referring to any virus with lagovirus GI.2 structural genes) is a recently emerged calicivirus that causes generalised hepatic necrosis and disseminated intravascular coagulation leading to death in susceptible lagomorphs (rabbits and hares). Previous studies investigating the virulence of RHDV2 have reported conflicting results, with case fatality rates ranging from 0% to 100% even within a single study. Lagoviruses are of particular importance in Australia and New Zealand where they are used as biocontrol agents to manage wild rabbit populations, which threaten over 300 native species and result in economic impacts in excess of $200 million AUD annually to Australian agricultural industries. It is critically important that any pest control method is both highly effective (i.e., virulent, in the context of viral biocontrols) and has minimal animal welfare impacts. To determine whether RHDV2 might be a suitable candidate biocontrol agent, we investigated the virulence and disease progression of a naturally occurring Australian recombinant RHDV2 in both 5-week-old and 11-week-old New Zealand White laboratory rabbits after either high or low dose oral infection. Objective measures of disease progression were recorded through continuous body temperature monitoring collars, continuous activity monitors, and twice daily observations. We observed a 100% case fatality rate in both infected kittens and adult rabbits after either high dose or low dose infection. Clinical signs of disease, such as pyrexia, weight loss, and reduced activity, were evident in the late stages of infection. Clinical disease, i.e., welfare impacts, were limited to the period after the onset of pyrexia, lasting on average 12 h and increasing in severity as disease progressed. These findings confirm the high virulence of this RHDV2 variant in naïve rabbits. While age and infectious dose significantly affected disease progression, the case fatality rate was consistently 100% under all conditions tested.

Death in four RHDV2-vaccinated pet rabbits due to rabbit haemorrhagic disease virus 2 (RHDV2)

OBJECTIVES

To create awareness of single RHDV2 infections and cases of death despite immunisation with RHDV2-specific vaccine.

MATERIALS AND METHODS

Retrospective case series of four companion rabbits. Patient signalment, vaccination history, pathology and molecular examinations were obtained from the medical records.

RESULTS

The animals died peracutely or with nonspecific symptoms like apathy and inappetence. Pathological examination indicated and molecular biological findings confirmed RHDV2 infection in four animals. Several partner animals died at the same time under the same circumstances.

CLINICAL SIGNIFICANCE

This is the first report of fatalities in RHDV2-vaccinated companion rabbits due to rabbit haemorrhagic disease virus 2 infection with documented case and vaccination history. Veterinarians should be aware of possible single fatal RHDV2 infections despite vaccination, should initiate the clarification of suspected cases and inform vaccine manufacturers and competent authorities.

Spillover events of rabbit haemorrhagic disease virus 2 (recombinant GI.4P-GI.2) from Lagomorpha to Eurasian badger

Rabbit haemorrhagic disease (RHD) is a major threat to domestic and wild European rabbits. Presently, in Europe, the disease is caused mainly by Rabbit haemorrhagic disease virus 2 (RHDV2/b or Lagovirus europaeus GI.2), the origin of which is still unclear, as no RHDV2 reservoir hosts were identified. After the RHDV2 emergence in 2010, viral RNA was detected in a few rodent species. Furthermore, RHDV2 was found to cause disease in some hare species resembling the disease in rabbits, evidencing the ability of the virus to cross the species barrier. In this study, through molecular, histopathologic, antigenic and morphological evidences, we demonstrate the presence and replication of RHDV2 in Eurasian badgers (Meles meles) found dead in the district of Santarém, Portugal, between March 2017 and January 2020. In these animals, we further classify the RHDV2 as a Lagovirus europaeus recombinant GI.4P-GI.2. Our results indicate that Meles meles is susceptible to RHDV2, developing systemic infection, and excreting the virus in the faeces. Given the high viral loads seen in several organs and matrices, we believe that transmission to the wild rabbit is likely. Furthermore, transmission electron microscopy data show the presence of calicivirus compatible virions in the nucleus of hepatocytes, which constitutes a paradigm shift for caliciviruses' replication cycle.

A Potential Atypical Case of Rabbit Haemorrhagic Disease in a Dwarf Rabbit

Simple Summary

We report an unusual clinical case in a pet rabbit vaccinated against rabbit haemorrhagic disease virus (RHDV, GI.1), that developed a prolonged hepatic disease, and was diagnosed RHDV2 (GI.2) positive post-mortem. This finding is a warning to all veterinarians that rabbit haemorrhagic disease should also be considered for differential diagnosis despite the history of RHDV vaccination and the need to update vaccination programs against the current RHDV2 circulating strains.

Abstract

Rabbit haemorrhagic disease (RHD) is a highly contagious infectious disease of European wild and domestic rabbits. Rabbit haemorrhagic disease virus (RHDV, GI.1) emerged in 1986 in Europe, rapidly spreading all over the world. Several genotypes of RHDV have been recognised over time, but in 2010, a new virus (RHDV2/RHDVb, GI.2) emerged and progressively replaced the previous RHDV strains, due to the lack of cross-immunity conferred between RHDV and RHDV2. RHDV2 has a high mutation rate, similarly to the other calivirus and recombines with strains of RHDV and non-pathogenic calicivirus (GI.4), ensuring the continuous emergence of new field strains. Although this poses a threat to the already endangered European rabbit species, the available vaccines against RHDV2 and the compliance of biosafety measures seem to be controlling the infection in the rabbit industry Pet rabbits, especially when kept indoor, are considered at lower risk of infections, although RHDV2 and myxoma virus (MYXV) constitute a permanent threat due to transmission via insects. Vaccination against these viruses is therefore recommended every 6 months (myxomatosis) or annually (rabbit haemorrhagic disease). The combined immunization for myxomatosis and RHDV through a commercially available bivalent vaccine with RHDV antigen has been extensively used (Nobivac^® Myxo-RHD, MSD, Kenilworth, NJ, USA). This vaccine however does not confer proper protection against the RHDV2, thus the need for a rabbit clinical vaccination protocol update. Here we report a clinical case of hepatitis and alteration of coagulation in a pet rabbit that had been vaccinated with the commercially available bivalent vaccine against RHDV and tested positive to RHDV2 after death. The animal developed a prolonged and atypical disease, compatible with RHD. The virus was identified to be an RHDV2 recombinant strain, with the structural backbone of RHDV2 (GI.2) and the non-structural genes of non-pathogenic-A1 strains (RCV-A1, GI.4). Although confirmation of the etiological agent was only made after death, the clinical signs and analytic data were very suggestive of RHD.

RHDV2 epidemic in UK pet rabbits. Part 2: PCR results and correlation with vaccination status

Objective

To report PCR results and vaccination status of rabbits with rabbit haemorrhagic disease following an investigation into sudden or unexpected death.

Materials and Methods

PCR testing for RHDV2 and RHDV1 was performed on rabbit liver samples at two laboratories. Laboratory A reported results as positive or negative; Laboratory B reported results quantitatively as RNA copies per mg liver, categorised as negative, inconclusive or positive. The vaccination status of rabbits with both histopathological features of rabbit haemorrhagic disease and positive PCR test results were collated.

Results

PCR results matched histopathological findings in 188 of 195 (96%) cases. Seven individuals showed equivocal results, all of which had histopathological features of RHD but three tested PCR‐negative and four results conflicted between laboratories. RHDV2 was the serotype detected in all PCR‐positive cases. Histological features of rabbit haemorrhagic disease and PCR test results were positive in 125 rabbits; 51 unvaccinated, 56 in‐date with Nobivac Myxo‐RHD and 13 vaccinated against RHDV2 – although nine of these were vaccinated within 10 days of death.

Clinical Significance

PCR testing complements histopathology in cases of sudden death in rabbits by confirming the diagnosis and identifying virus serotype, but there can be false negatives. Although RHDV2 is currently prevalent in UK pet rabbits, vaccination against both RHDV1 and RHDV2 is recommended. Failures of RHDV2 vaccine are infrequent.

Emerging Infectious Diseases of Rabbits

Recently, multiple infectious organisms have been identified as the cause of emerging diseases in lagomorphs. The most important of these emerging diseases is rabbit hemorrhagic disease virus (RHDV) type 2, a new variant with differences in pathogenicity to classical RHDV. Hepatitis E is considered an emerging zoonotic infectious disease, with widespread prevalence in many different rabbit populations. Mycobacteriosis has been recently reported in other captive domestic rabbit populations. This article provides a recent review of the published literature on emerging infectious diseases in rabbits, including farmed, laboratory, and pet rabbits, some of which have zoonotic potential.

Retrospective serological analysis reveals presence of the emerging lagovirus RHDV2 in Australia in wild rabbits at least five months prior to its first detection

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.

Rabbit haemorrhagic disease: experimental study of a recent highly pathogenic GI.2/RHDV2/b strain and evaluation of vaccine efficacy

In 2010, a variant of the rabbit haemorrhagic disease virus (RHDV) belonging to a new GI.2 genotype was identified in France and rapidly spread worldwide. Due to antigenic difference, new vaccines including G1.2 strains have been developed to confer adequate protection. An increase in the pathogenicity of the circulating strains was recently reported. The objective of this experimental study was to characterise the infection with a highly pathogenic GI.2/RHDV2/b isolate (2017) and assess the efficacy of Filavac VHD K C+V vaccine (Filavie) against this strain. Four and 10-wk-old specific pathogen-free rabbits were inoculated with a recommended dose of vaccine. After 7 d, controls and vaccinated rabbits were challenged and clinically monitored for 14 d. All animals were necropsied and blood, organs and urine were sampled for quantitative reverse transcription polymerase chain reaction (RT-qPCR) analysis. In adult groups, regular nasal and rectal swabbing were performed, and faeces were collected after death to monitor RNA shedding. In control groups, the challenge strain induced acute RHD between 31 and 72 h post-inoculation, with a mortality rate of 100% for kits and 89% for adult rabbits. Except for a shorter mean time to death in kits, similar clinical signs and lesions were observed between age groups. The vaccination significantly prevented all mortality, clinical signs, detection of viral RNA in serum and gross lesions in kits and adult rabbits. In adult groups, we also demonstrated that vaccine significantly protected from detectable RNA shedding via naso-conjunctival and rectal routes. Two weeks after challenge, RNA copies were not detected by PCR in the liver, spleen, lungs, kidneys, faeces and urine of vaccinated adult rabbits. The findings for kits were similar, except that very low levels of RNA were present in the liver and spleen of a few rabbits. These data show that immunisation prevented any significant viral multiplication and/or allowed a rapid clearance. We concluded that, despite the quick evolution of GI.2/RHDV2/b strains, the protection conferred by the vaccine remains adequate. In the context of coexistence of both GI.1 and GI.2 genotypes in some countries, with the circulation of multiples recombinant viruses, the vaccination should be based on the association of strains from both genotypes.

Detection and characterisation of rabbit haemorrhagic disease virus strains circulating in Egypt

Rabbit haemorrhagic disease (RHD) is a highly virulent viral disease of Oryctolagus cuniculus which threatens the rabbit population in Egypt and worldwide. The etiological agent is the rabbit haemorrhagic disease virus (RHDV), a member of the family Caliciviridae. The aim of this study was to identify the possible evolutionary changes of the currently circulating RHDV in Egypt. Twenty suspected cases were collected from outbreaks that occurred in non-vaccinated rabbit flocks during 2015 and 2016. Liver homogenate samples were investigated for detection and identification of circulating RHDV using haemagglutination (HA) and reverse transcriptase polymerase chain reaction. Further characterisation of selected five viral strains was performed by nucleotide sequencing of VP60 gene. All twenty tested samples were haemagglutinin positive and VP60 gene was amplified. Based on nucleotide sequence analysis, four isolates were identified as classical RHDV strains, while one isolate was assigned as RHDVa variant strain but with the same HA pattern. The new RHDVb variant was not identified.

Progression of rabbit haemorrhagic disease virus 2 upon vaccination in an industrial rabbitry: a laboratorial approach

<p>Rabbit haemorrhagic disease virus 2 (RHDV2) emerged recently in several European countries, leading to extensive economic losses in the industry. In response to this new infection, specific inactivated vaccines were developed in Europe and full and rapid setup of protective immunity induced by vaccination was reported. However, data on the efficacy of these vaccines in an ongoing-infection scenario is unavailable. In this study we investigated an infected RHDV2 indoor industrial meat rabbitry, where fatalities continued to occur after the implementation of the RHDV2 vaccination, introduced to control the disease. The aim of this study was to understand if these mortalities were RHDV2-related, to discover if the dead animals showed any common features such as age or time distance from vaccination, and to identify the source of the outbreak. Anatomo-pathological analysis of vaccinated animals with the virus showed lesions compatible with systemic haemorrhagic disease and RHDV2-RNA was detected in 85.7% of the animals tested. Sequencing of the <em>vp60</em> gene amplified from liver samples led to the recognition of RHDV2 field strains demonstrating that after the implementation of vaccination, RHDV2 continued to circulate in the premises and to cause sporadic deaths. A nearby, semi-intensive, RHDV2 infected farm belonging to the same owner was identified as the most probable source of the virus. The main risk factors for virus introduction in these two industries were identified. Despite the virus being able to infect a few of the vaccinated rabbits, the significant decrease in mortality rate observed in vaccinated adult rabbits clearly reflects the efficacy of the vaccination. Nonetheless, the time taken to control the infection also highlights the importance of RHDV2 vaccination prior to the first contact with the virus, highly recommendable in endemic areas, to mitigate the infection’s impact on the industry.</p>