Recombination at the emergence of the pathogenic rabbit haemorrhagic disease virus Lagovirus europaeus/GI.2

Comment on Shah et al. Genetic Characteristics and Phylogeographic Dynamics of Lagoviruses, 1988-2021. Viruses 2023, 15, 815

Shah and colleagues [...].

Characterisation of Lagovirus europaeus GI-RHDVs (Rabbit Haemorrhagic Disease Viruses) in Terms of Their Pathogenicity and Immunogenicity

Rabbit haemorrhagic disease viruses (RHDV) belong to the family Caliciviridae, genus Lagovirus europaeus, genogroup GI, comprising four genotypes GI.1-GI.4, of which the genotypes GI.1 and GI.2 are pathogenic RHD viruses, while the genotypes GI.3 and GI.4 are non-pathogenic RCV (Rabbit calicivirus) viruses. Among the pathogenic genotypes GI.1 and GI.2 of RHD viruses, an antigenic variant of RHDV, named RHDVa-now GI.1a-RHDVa, was distinguished in 1996; and in 2010, a variant of RHDV-named RHDVb, later RHDV2 and now GI.2-RHDV2/b-was described; and recombinants of these viruses were registered. Pathogenic viruses of the genotype GI.1 were the cause of a disease described in 1984 in China in domestic (Oryctolagus (O.) cuniculus domesticus) and wild (O. cuniculus) rabbits, characterised by a very rapid course and a mortality rate of 90-100%, which spread in countries all over the world and which has been defined since 1989 as rabbit haemorrhagic disease. It is now accepted that GI.1-RHDV, including GI.1a-RHDVa, cause the predetermined primary haemorrhagic disease in domestic and wild rabbits, while GI.2-RHDV2/b cause it not only in rabbits, including domestic rabbits' young up to 4 weeks and rabbits immunised with rabbit haemorrhagic disease vaccine, but also in five various species of wild rabbits and seven different species of hares, as well as wild ruminants: mountain muskoxen and European badger. Among these viruses, haemagglutination-positive, doubtful and harmful viruses have been recorded and described and have been shown to form phylogenogroups, immunotypes, haematotypes and pathotypes, which, together with traits that alter and expand their infectious spectrum (rabbit, hare, wild ruminant, badger and various rabbit and hare species), are the determinants of their pathogenicity (infectivity) and immunogenicity and thus shape their virulence. These relationships are the aim of our consideration in this article.

First Detection and Circulation of RHDV2 in New Zealand

Rabbit haemorrhage disease virus 2 (RHDV2) is a highly pathogenic lagovirus that causes lethal disease in rabbits and hares (lagomorphs). Since its first detection in Europe in 2010, RHDV2 has spread worldwide and has been detected in over 35 countries so far. Here, we provide the first detailed report of the detection and subsequent circulation of RHDV2 in New Zealand. RHDV2 was first detected in New Zealand in 2018, with positive samples retrospectively identified in December 2017. Subsequent time-resolved phylogenetic analysis suggested a single introduction into the North Island between March and November 2016. Genetic analysis identified a GI.3P-GI.2 variant supporting a non-Australian origin for the incursion; however, more accurate identification of the source of the incursion remains challenging due to the wide global distribution of the GI.3P-GI.2 variant. Furthermore, our analysis suggests the spread of the virus between the North and South Islands of New Zealand at least twice, dated to mid-2017 and around 2018. Further phylogenetic analysis also revealed a strong phylogeographic pattern. So far, no recombination events with endemic benign New Zealand rabbit caliciviruses have been identified. This study highlights the need for further research and surveillance to monitor the distribution and diversity of lagoviruses in New Zealand and to detect incursions of novel variants.

First Detection of Benign Rabbit Caliciviruses in Chile

Pathogenic lagoviruses (Rabbit hemorrhagic disease virus, RHDV) are widely spread across the world and are used in Australia and New Zealand to control populations of feral European rabbits. The spread of the non-pathogenic lagoviruses, e.g., rabbit calicivirus (RCV), is less well studied as the infection results in no clinical signs. Nonetheless, RCV has important implications for the spread of RHDV and rabbit biocontrol as it can provide varying levels of cross-protection against fatal infection with pathogenic lagoviruses. In Chile, where European rabbits are also an introduced species, myxoma virus was used for localised biocontrol of rabbits in the 1950s. To date, there have been no studies investigating the presence of lagoviruses in the Chilean feral rabbit population. In this study, liver and duodenum rabbit samples from central Chile were tested for the presence of lagoviruses and positive samples were subject to whole RNA sequencing and subsequent data analysis. Phylogenetic analysis revealed a novel RCV variant in duodenal samples that likely originated from European RCVs. Sequencing analysis also detected the presence of a rabbit astrovirus in one of the lagovirus-positive samples.

A new HaCV-EBHSV recombinant lagovirus circulating in European brown hares (Lepus europaeus) from Catalonia, Spain

In 2020/2021, several European brown hare syndrome virus (EBHSV) outbreaks were recorded in European hares (Lepus europaeus) from Catalonia, Spain. Recombination analysis combined with phylogenetic reconstruction and estimation of genetic distances of the complete coding sequences revealed that 5 strains were recombinants. The recombination breakpoint is located within the non-structural protein 2C-like RNA helicase (nucleotide position ~ 1889). For the genomic fragment upstream of the breakpoint, a non-pathogenic EBHSV-related strain (hare calicivirus, HaCV; GII.2) was the most closely related sequence; for the rest of the genome, the most similar strains were the European brown hare syndrome virus (EBHSV) strains recovered from the same 2020/2021 outbreaks, suggesting a recent origin. While the functional impact of the atypical recombination breakpoint remains undetermined, the novel recombinant strain was detected in different European brown hare populations from Catalonia, located 20-100 km apart, and seems to have caused a fatal disease both in juvenile and adult animals, confirming its viability and ability to spread and establish infection. This is the first report of a recombination event involving HaCV and EBHSV and, despite the recombination with a non-pathogenic strain, it appears to be associated with mortality in European brown hares, which warrants close monitoring.

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.

On the virome's hidden diversity: lessons from RHDV

Emerging infectious diseases are a major challenge to human and animal health. While predicting the emergence of pathogens is complex, the advent of high-throughput sequencing technologies has allowed the rapid identification of unknown microbiology diversity within organisms. Here, we discuss an example of a metatranscriptomics output to decipher viral evolution.

Utilizing Molecular Epidemiology and Citizen Science for the Surveillance of Lagoviruses in Australia

Australia has multiple lagoviruses with differing pathogenicity. The circulation of these viruses was traditionally determined through opportunistic sampling events. In the lead up to the nationwide release of RHDVa-K5 (GI.1aP-GI.1a) in 2017, an existing citizen science program, RabbitScan, was augmented to allow members of the public to submit samples collected from dead leporids for lagovirus testing. This study describes the information obtained from the increased number of leporid samples received between 2015 and 2022 and focuses on the recent epidemiological interactions and evolutionary trajectory of circulating lagoviruses in Australia between October 2020 and December 2022. A total of 2771 samples were tested from January 2015 to December 2022, of which 1643 were lagovirus-positive. Notable changes in the distribution of lagovirus variants were observed, predominantly in Western Australia, where RHDV2-4c (GI.4cP-GI.2) was detected again in 2021 after initially being reported to be present in 2018. Interestingly, we found evidence that the deliberately released RHDVa-K5 was able to establish and circulate in wild rabbit populations in WA. Overall, the incorporation of citizen science approaches proved to be a cost-efficient method to increase the sampling area and enable an in-depth analysis of lagovirus distribution, genetic diversity, and interactions. The maintenance of such programs is essential to enable continued investigations of the critical parameters affecting the biocontrol of feral rabbit populations in Australia, as well as to enable the detection of any potential future incursions.

First detection and molecular characterization of rabbit hemorrhagic disease virus (RHDV) in Algeria

Since the first detection of rabbit hemorrhagic disease (RHD), the rabbit hemorrhagic disease virus (RHDV) has been responsible for high morbidity and mortality worldwide, both in domestic and in wild rabbits. Despite the apparent control of RHD in rabbitries through vaccination, several studies highlighted the rapid evolution of RHDV by recombination, which may facilitate the emergence of new pathogenic strains. The aim of this study was to confirm the presence and characterize RHDV in Algeria. For this, rabbit samples were collected in the north of Algeria, between 2018 and 2021, from small farms where the virus was suspected after the sudden death of a high number of rabbits, and from healthy hunted wild rabbits. The domestic rabbits revealed clinical signs and lesions that were suggestive of RHD. RT-PCR showed that 79.31% of the domestic rabbit samples were positive for RHDV, while in 20.69%, including the hunted rabbits, the virus was not detected. Phylogenetic analysis of the Algerian strains allowed the confirmation and identification as GI.2 (RHDV2), and showed a close relation to GI.3P-GI.2 recombinant strains, suggesting a potential introduction from other countries, with an older strain potentially originated from neighboring Tunisia, while more recent isolates grouped with strains from North America. Our study reports for the first time the presence of GI.2 (RHDV2) in Algeria with multiple routes of introduction. Consequently, we propose that RHDV control in Algeria should be based on epidemiological surveys in association with an adequate prophylactic program.

First report of GI.1aP-GI.2 recombinants of rabbit hemorrhagic disease virus in domestic rabbits in China

The rabbit hemorrhagic disease virus 2 (RHDV2 or GI.2) is a highly contagious agent leading to lethal disease in rabbits. It frequently recombines with other Lagovirus genus, generating epidemical variants with high pathogenicity. In this study, twenty-two liver samples tested positive for GI.2 VP60 gene, were collected in rabbit farms from several geographical regions in China. All GI.2 positive specimens were submitted for RT-PCR detection, nucleotide sequencing and phylogenetic analysis. In addition, suspected GI.2 recombinants were evaluated for virus virulence. The results showed that nine presumptive recombinants were identified by testing for RdRp-VP60 recombination. In these recombinants, four were selected to fully characterize the genome of novel GI.2 recombinant variants, which were described as GI.1aP-GI.2. The nucleotide sequence of these novel variants showed unique recombination pattern and phylogenetic features compared to currently prevalent GI.2 variants. Furthermore, this distinctive recombination of new variant SCNJ-2021 moderately enhanced the virulence of GI.2, even for rabbits vaccinated against parental GI.2. In conclusion, the novel GI.1aP-GI.2 recombinants were identified in rabbit industry in China for the first time, which expanded the knowledge on the phylodynamics and genomic diversity of GI.2 genotype. The rapid molecular evolution and varied pathogenicity of these virus recombinants highlight the urgent need for epidemiological surveillance and for future prevention of these neglected GI.2 variants.

The potential role of scavenging flies as mechanical vectors of Lagovirus europaeus/GI.2

The European rabbit (Oryctolagus cuniculus) populations of the Iberian Peninsula have been severely affected by the emergence of the rabbit haemorrhagic disease virus (RHDV) Lagovirus europaeus/GI.2 (RHDV2/b). Bushflies and blowflies (Muscidae and Calliphoridae families, respectively) are important RHDV vectors in Oceania, but their epidemiological role is unknown in the native range of the European rabbit. In this study, scavenging flies were collected between June 2018 and February 2019 in baited traps at one site in southern Portugal, alongside a longitudinal capture-mark-recapture study of a wild European rabbit population, aiming to provide evidence of mechanical transmission of GI.2 by flies. Fly abundance, particularly from Calliphoridae and Muscidae families, peaked in October 2018 and in February 2019. By employing molecular tools, we were able to detect the presence of GI.2 in flies belonging to the families Calliphoridae, Muscidae, Fanniidae and Drosophilidae. The positive samples were detected during an RHD outbreak and absent in samples collected when no evidence of viral circulation in the local rabbit population was found. We were able to sequence a short viral genomic fragment, confirming its identity as RHDV GI.2. The results suggest that scavenging flies may act as mechanical vectors of GI.2 in the native range of the southwestern Iberian subspecies O. cuniculus algirus. Future studies should better assess their potential in the epidemiology of RHD and as a tool for monitoring viral circulation in the field.

Genetics of the rabbit haemorrhagic disease virus strains responsible for rabbit haemorrhagic disease outbreaks in Japan between 2000 and 2020

Rabbit haemorrhagic disease (RHD) is a highly contagious and fatal disease in rabbits caused by the rabbit haemorrhagic disease virus (RHDV), which includes two genotypes, RHDV-GI.1 and RHDV2-GI.2. RHDVs tend to recombine among different strains, resulting in significant genetic evolution. This study evaluated the genetics of Japanese RHDV strains associated with six outbreaks between 2000 and 2020 using whole-genome sequencing, genomic recombination and phylogenetic analyses. Genomic recombination analysis using near-complete genomic sequences revealed that two Japanese strains detected in 2000 and 2002 were non-recombinant GI.1 (variant RHDVa-GI.1a) strains of different origins, most closely related to strains identified in PR China in 1997 and the USA in 2001, respectively. In contrast, four recent Japanese GI.2 strains detected between 2019 and 2020 were recombinant viruses harbouring structural protein (SP) genes from GI.2 strains and non-SP (NSP) genes from a benign rabbit calicivirus (RCV) strain of genotype RCV-E1-GI.3 (GI.3P-GI.2) or an RHDV G1-GI.1b variant (GI.1bP-GI.2). Phylogenetic analysis based on SP and NSP regions revealed that the GI.1bP-GI.2 recombinant virus detected in Ehime prefecture and the GI.3P-GI.2 recombinant viruses detected in Ibaraki, Tochigi and Chiba prefectures were most closely related to recombinant viruses identified in Australia in 2017 and Germany in 2017, respectively. These results suggested that past RHD outbreaks in Japan did not result from the evolution of domestic RHDVs but rather represented incursions of foreign RHDV strains, implying that Japan is constantly at risk of RHDV incursion from other countries.

Genetic Characteristics and Phylogeographic Dynamics of Lagoviruses, 1988-2021

Rabbit haemorrhagic disease virus (RHDV), European brown hare syndrome virus (EBHSV), rabbit calicivirus (RCV), and hare calicivirus (HaCV) belong to the genus Lagovirus of the Caliciviridae family that causes severe diseases in rabbits and several hare (Lepus) species. Previously, Lagoviruses were classified into two genogroups, e.g., GI (RHDVs and RCVs) and GII (EBHSV and HaCV) based on partial genomes, e.g., VP60 coding sequences. Herein, we provide a robust phylogenetic classification of all the Lagovirus strains based on full-length genomes, grouping all the available 240 strains identified between 1988 and 2021 into four distinct clades, e.g., GI.1 (classical RHDV), GI.2 (RHDV2), HaCV/EBHSV, and RCV, where the GI.1 clade is further classified into four (GI.1a-d) and GI.2 into six sub-clades (GI.2a-f). Moreover, the phylogeographic analysis revealed that the EBHSV and HaCV strains share their ancestor with the GI.1, while the RCV shares with the GI.2. In addition, all 2020-2021 RHDV2 outbreak strains in the USA are connected to the strains from Canada and Germany, while RHDV strains isolated in Australia are connected with the USA-Germany haplotype RHDV strain. Furthermore, we identified six recombination events in the VP60, VP10, and RNA-dependent RNA polymerase (RdRp) coding regions using the full-length genomes. The amino acid variability analysis showed that the variability index exceeded the threshold of 1.00 in the ORF1-encoded polyprotein and ORF2-encoded VP10 protein, respectively, indicating significant amino acid drift with the emergence of new strains. The current study is an update of the phylogenetic and phylogeographic information of Lagoviruses that may be used to map the evolutionary history and provide hints for the genetic basis of their emergence and re-emergence.

Immune response in the recombinant strain of Lagovirus europaeus GI.1a

Lagovirus europaeus/GI.1 is the virus that causes severe and dangerous rabbit haemorrhagic disease (RHD) in rabbits. Recombination formation in RHD viruses is common. Recombination is thought to play a key role in the evolution of lagoviruses and therefore most likely influences the pathogenicity of L. europaeus/GI strains. Immunological events also play a key role in the control of RHD, and an in-depth knowledge of these phenomena provides insights into the characteristics of the infection, which can help implement appropriate infection control measures. To obtain a more complete picture of RHD caused by different GI.1 strains, it is necessary to correlate the genetic diversity within L. europaeus/GI.1 strains and the immune picture in response to infection. We performed a phylogenetic analysis of the L. europaeus/GI strains and compared the recombinant L. europaeus/GI.1 strain with the GI.1a strain on the basis of a thorough statistical analysis of immunological traits performed previously. Our phylogenetic analysis based on the sequence of the gene encoding the VP60 capsid protein of 34 strains of Lagovirus europaeus showed that the Hartmannsdorf strain forms a separate clade from the other GI.1a strains and is separate from the GI.1b-d strains. Next, we showed significant differences in the levels of individual parameters for non-specific cellular and humoral immunity in infection with the GI.1a strain and the Hartmannsdorf recombinant strain. Against the background of this study, our results indicate that the characteristics of each recombinant should be considered individually.

Refining the emergence scenario of the invasive recombinant Tomato yellow leaf curl virus -IS76

TYLCV-IS76, a unique recombinant between tomato yellow leaf curl virus (TYLCV) and tomato yellow leaf curl Sardinia virus (TYLCSV), has replaced its parental viruses in southern Morocco. To refine its emergence scenario, its fitness was monitored experimentally in conditions aiming at reproducing natural situations, i.e. superinfection of plants already infected with parental viruses and competition with other TYLCV/TYLCSV recombinants (LSRec) automatically generated in plants coinfected with TYLCV and TYLCSV. TYLCV-IS76 accumulated significantly more than parental viruses regardless of plant age and superinfection delay. Although TYLCV-IS76 and LSRec both accumulated more than parental viruses in laboratory conditions, LSRec were displaced by TYLCV-IS76 in nature like parental viruses were. TYLCV-IS76 did not exhibit any vector transmission advantage over LSRec and TYLCV the most competitive parental virus. Thus, it is apparently only in the plant compartment that the recombination event that generated TYLCV-IS76, induced the competitiveness advantage by which the last became first.

Confirmation of the Rabbit Hemorrhagic Disease Virus Type 2 (GI.2) Circulation in North Africa

Rabbit hemorrhagic disease (RHD) is a highly contagious viral disease that causes fatal acute hepatitis in domestic and wild lagomorphs. It has taken on major economic importance in countries like Morocco. In addition to the classical virus (RHDV), a novel emerged genotype (RHDV2) is circulating, especially in the north shore of the Mediterranean basin since 2010. Many small animal farmers reported clinical cases from several rabbitries in Agadir (Morocco) despite systematic vaccination against the RHDV. The main objective was to characterize the current RHDV strains circulating in the studied area to help to choose an adequate vaccine. For that, we extracted viral RNA from rabbit livers, carried out the PCR analyses, and we sequenced the viral structural capsid protein (VP60) of the RHDV. The phylogenetic analysis results allowed us to state that the novel genotype (RHDV2) is circulating in the studied geographical area, and to characterize the isolated sequences. As a conclusion, we recommend updating RHD epidemiological relating data and reviewing the vaccine protocols by both targeting RHDV (GI.1) and RHDV2 (GI.2) in any future preventive program.

The pathogenicity comparison of Lagovirus europaeus GI.1 and GI.2 strains in China by using relative quantitative assay

Lagovirus europaeus GI.1 belongs to Lagovirus in the Caliciviridae family. GI.1 causes an acute, septic, and highly lethal disease in rabbits. Lagovirus europaeus GI.2, a new variant of GI.1, has caused explosive mortality in rabbits of all ages in Sichuan Province, China. To explore the differences in pathogenicity of rabbits infected with GI.1/GI.2, we investigated the virulence and disease progression of a naturally occurring GI.1/GI.2 in 4-week-old, 13-week-old, and 25-week-old New Zealand White laboratory rabbits after GI.1/GI.2 infection. Objective measures of disease progression were recorded using continuous body-temperature monitoring. We observed the kittens were infected with GI.2 during the most urgent course of the disease, and GI.1 was not lethal to kittens. We found that the target organ of both GI.1 and GI.2 was the liver, but the disease course of the two viruses was differed. Our study enriches the research on the pathogenicity of GI.1 and GI.2 under the same conditions.

Potent Protease Inhibitors of Highly Pathogenic Lagoviruses: Rabbit Hemorrhagic Disease Virus and European Brown Hare Syndrome Virus

Rabbit hemorrhagic disease (RHD) and European brown hare syndrome (EBHS) are highly contagious diseases caused by lagoviruses in the Caliciviridae family. These infectious diseases are associated with high mortality and a serious threat to domesticated and wild rabbits and hares, including endangered species such as riparian brush rabbits (Sylvilagus bachmani riparius). In the United States (U.S.), only isolated cases of RHD had been reported until Spring 2020. However, RHD caused by GI.2/rabbit hemorrhagic disease virus (RHDV)2/b was unexpectedly reported in April 2020 in New Mexico and has subsequently spread to several U.S. states, infecting wild rabbits and hares and making it highly likely that RHD will become endemic in the U.S. Vaccines are available for RHD; however, there is no specific treatment for this disease. Lagoviruses encode a 3C-like protease (3CLpro), which is essential for virus replication and a promising target for antiviral drug development. We have previously generated focused small-molecule libraries of 3CLpro inhibitors and demonstrated the in vitro potency and in vivo efficacy of some protease inhibitors against viruses encoding 3CLpro, including caliciviruses and coronaviruses. Here, we report the development of the enzyme and cell-based assays for the 3CLpro of GI.1c/RHDV, recombinant GI.3P-GI.2 (RHDV2/b), and GII.1/European brown hare syndrome virus (EBHSV) as well as the identification of potent lagovirus 3CLpro inhibitors, including GC376, a protease inhibitor being developed for feline infectious peritonitis. In addition, structure-activity relationship study and homology modeling of the 3CLpro and inhibitors revealed that lagovirus 3CLpro share similar structural requirements for inhibition with other calicivirus 3CLpro. IMPORTANCE Rabbit hemorrhagic disease (RHD) and European brown hare syndrome (EBHS) are viral diseases that affect lagomorphs with significant economic and ecological impacts. RHD vaccines are available, but specific antiviral treatment for these viral infections would be a valuable addition to the current control measures. Lagoviruses encode 3C-like protease (3CLpro), which is essential for virus replication and an attractive target for antiviral drug discovery. We have screened and identified potent small-molecule inhibitors that block lagovirus 3CLpro in the enzyme- and cell-based assays. Our results suggest that these compounds have the potential for further development as antiviral drugs for lagoviruses.

Pathogenicity of the newly emerged Lagovirus europaeus GI.2 strain in China in experimentally infected rabbits

In April 2020, rabbit hemorrhagic virus type 2 (Lagovirus europaeus GI.2), which causes highly infectious fatal rabbit hemorrhagic disease, was emerged in China. The phylogenetic analyses of the complete genome sequence of GI.2 showed that it belonged to the non-recombinant GI.3/GI.2 genotype. However, the pathogenicity of this GI.2 strain differed from that of early typical GI.2 strains in Europe. To prevent the spread of the new strain in China, its pathogenicity urgently needs to be studied. Thus, viral shedding and distribution as well as clinical symptoms, histopathological changes, and serum cytokines were studied in experimentally GI.2/SC2020-infected rabbit adults and kits. The kit group showed a shorter survival time after the challenge than the adult group did. The mortality rate was higher in the kits (80 %) than in the adults (30 %). Viral RNA could be detected in both nasal and fecal swabs, and the main dissemination route appeared to be the fecal route. Viral RNA rapidly increased in the blood of the adults and kits at 6 h post-infection, indicating that blood viral load testing can be used for early diagnosis. The most affected organs were the liver and spleen, and the lesions were more severe in the kits than in the adults. The liver contained the highest viral RNA levels. Moreover, serum interleukin (IL)-6, IL-8, IL-10, and tumor necrosis factor-alpha levels were increased in the infected rabbits. In conclusion, our findings will help to understand the evolutionary trends and pathogenic characteristics of GI.2 strains in China.

Horizontal gene transfer as a mechanism for the promiscuous acquisition of distinct classes of IRES by avian caliciviruses

In contrast to members of Picornaviridae which have long 5'-untranslated regions (5'UTRs) containing internal ribosomal entry sites (IRESs) that form five distinct classes, members of Caliciviridae typically have short 5'UTRs and initiation of translation on them is mediated by interaction of the viral 5'-terminal genome-linked protein (VPg) with subunits of eIF4F rather than by an IRES. The recent description of calicivirus genomes with 500-900nt long 5'UTRs was therefore unexpected and prompted us to examine them in detail. Sequence analysis and structural modelling of the atypically long 5'UTRs of Caliciviridae sp. isolate yc-13 and six other caliciviruses suggested that they contain picornavirus-like type 2 IRESs, whereas ruddy turnstone calicivirus (RTCV) and Caliciviridae sp. isolate hwf182cal1 calicivirus contain type 4 and type 5 IRESs, respectively. The suggestion that initiation on RTCV mRNA occurs by the type 4 IRES mechanism was confirmed experimentally using in vitro reconstitution. The high sequence identity between identified calicivirus IRESs and specific picornavirus IRESs suggests a common evolutionary origin. These calicivirus IRESs occur in a single phylogenetic branch of Caliciviridae and were likely acquired by horizontal gene transfer.

Frequent intergenotypic recombination between the non-structural and structural genes is a major driver of epidemiological fitness in caliciviruses

The diversity of lagoviruses (Caliciviridae) in Australia has increased considerably in recent years. By the end of 2017, five variants from three viral genotypes were present in populations of Australian rabbits, while prior to 2014 only two variants were known. To understand the evolutionary interactions among these lagovirus variants, we monitored their geographical distribution and relative incidence over time in a continental-scale competition study. Within 3 years of the incursion of rabbit haemorrhagic disease virus 2 (RHDV2, denoted genotype GI.1bP-GI.2 [polymerase genotype]P-[capsid genotype]) into Australia, two novel recombinant lagovirus variants emerged: RHDV2-4e (genotype GI.4eP-GI.2) in New South Wales and RHDV2-4c (genotype GI.4cP-GI.2) in Victoria. Although both novel recombinants contain non-structural genes related to those from benign, rabbit-specific, enterotropic viruses, these variants were recovered from the livers of both rabbits and hares that had died acutely. This suggests that the determinants of host and tissue tropism for lagoviruses are associated with the structural genes, and that tropism is intricately connected with pathogenicity. Phylogenetic analyses demonstrated that the RHDV2-4c recombinant emerged independently on multiple occasions, with five distinct lineages observed. Both the new RHDV2-4e and -4c recombinant variants replaced the previous dominant parental RHDV2 (genotype GI.1bP-GI.2) in their respective geographical areas, despite sharing an identical or near-identical (i.e. single amino acid change) VP60 major capsid protein with the parental virus. This suggests that the observed replacement by these recombinants was not driven by antigenic variation in VP60, implicating the non-structural genes as key drivers of epidemiological fitness. Molecular clock estimates place the RHDV2-4e recombination event in early to mid-2015, while the five RHDV2-4c recombination events occurred from late 2015 through to early 2017. The emergence of at least six viable recombinant variants within a 2-year period highlights the high frequency of these events, detectable only through intensive surveillance, and demonstrates the importance of recombination in lagovirus evolution.

Comparative susceptibility of eastern cottontails and New Zealand white rabbits to classical rabbit haemorrhagic disease virus (RHDV) and RHDV2

Rabbit haemorrhagic disease virus (RHDV) is associated with high morbidity and mortality in the European rabbit (Oryctolagus cuniculus). In 2010, a genetically distinct RHDV named RHDV2 emerged in Europe and spread to many other regions, including North America in 2016. Prior to this study it was unknown if eastern cottontails (ECT(s); Sylvilagus floridanus), one of the most common wild lagomorphs in the United States, were susceptible to RHDV2. In this study, 10 wild-caught ECTs and 10 New Zealand white rabbits (NZWR(s); O. cuniculus) were each inoculated orally with either RHDV (RHDVa/GI.1a; n = 5 per species) or RHDV2 (a recombinant GI.1bP-GI.2; n = 5 per species) and monitored for the development of disease. Three of the five ECTs that were infected with RHDV2 developed disease consistent with RHD and died at 4 and 6 days post-inoculation (DPI). The RHDV major capsid protein/antigen (VP60) was detected in the livers of three ECTs infected with RHDV2, but none was detected in the ECTs infected with RHDV. Additionally, RHD viral RNA was detected in the liver, spleen, intestine and blood of ECTs infected with RHDV2, but not in the ECTs infected with RHDV. RHD viral RNA was detected in urine, oral swabs and rectal swabs in at least two of five ECTs infected with RHDV2. One ECT inoculated with RHDV2 seroconverted and developed a high antibody titre by the end of the experimental period (21 DPI). ECTs inoculated with the classic RHDV did not seroconvert. In comparison, NZWRs inoculated with RHDV2 exhibited high mortality (five of five) at 2 DPI and four of five NZWRs inoculated with RHDV either died or were euthanized at 2 DPI indicating both of these viruses were highly pathogenic to this species. This experiment indicates that ECTs are susceptible to RHDV2 and can shed viral RNA, thereby suggesting this species could be involved in the epidemiology of this virus.

Multiple Introductions of Rabbit Hemorrhagic Disease Virus Lagovirus europaeus/GI.2 in Africa

Rabbit hemorrhagic disease (RHD) causes high mortality and morbidity in European rabbits (Oryctolagus cuniculus). In Africa, the presence of the causative agent, the rabbit hemorrhagic disease virus (RHDV), was first confirmed in 1992 (genotype Lagovirus europaeus/GI.1). In 2015, the new genotype Lagovirus europaeus/GI.2 (RHDV2/b) was detected in Tunisia. Currently, GI.2 strains are present in several North and Sub-Saharan African countries. Considerable economic losses have been observed in industrial and traditional African rabbitries due to RHDV. Like other RNA viruses, this virus presents high recombination rates, with the emergence of GI.2 being associated with a recombinant strain. Recombination events have been detected with both pathogenic (GI.1b and GII.1) and benign (GI.3 and GI.4) strains. We obtained complete genome sequences of Tunisian GI.2 strains collected between 2018 and 2020 and carried out phylogenetic analyses. The results revealed that Tunisian strains are GI.3P-GI.2 strains that were most likely introduced from Europe. In addition, the results support the occurrence of multiple introductions of GI.2 into Africa, stressing the need for characterizing complete genome sequences of the circulating lagoviruses to uncover their origin. Continued monitoring and control of rabbit trade will grant a better containment of the disease and reduce the disease-associated economic losses.

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

Rabbit haemorrhagic disease virus type 2 (RHDV2) causes a severe systemic disease with hepatic necrosis. Differently from classic RHDV, which affects only European rabbits (Oryctolagus cuniculus), RHDV2 can affect many leporid species, including hares (Lepus spp.) and cottontail rabbits (Sylvilagus spp.). RHDV2 emerged in Europe in 2010 and spread worldwide. During the last 5 years, there have been multiple outbreaks in North America since the first known event in 2016 in Quebec, Canada, including several detections in British Columbia, Canada, between 2018 and 2019, Washington State and Ohio, USA, in 2018 and 2019, and New York, USA, in 2020. However, the most widespread outbreak commenced in March 2020 in the southwestern USA and Mexico. In California, RHDV2 spread widely across several southern counties between 2020 and 2021, and the aim of this study was to report and characterize these early events of viral incursion and circulation within the state. Domestic and wild lagomorphs (n = 81) collected between August 2020 and February 2021 in California with a suspicion of RHDV2 infection were tested by reverse transcription quantitative real-time PCR on the liver, and histology and immunohistochemistry for pan-lagovirus were performed on liver sections. In addition, whole genome sequencing from 12 cases was performed. During this period, 33/81 lagomorphs including 24/59 domestic rabbits (O. cuniculus), 3/16 desert cottontail rabbits (Sylvilagus audubonii), and 6/6 black-tailed jackrabbits (Lepus californicus) tested positive. All RHDV2-positive animals had hepatic necrosis typical of pathogenic lagovirus infection, and the antigen was detected in sections from individuals of the three species. The 12 California sequences were closely related (98.9%-99.95%) to each other, and also very similar (99.0%-99.4%) to sequences obtained in other southwestern states during the 2020-2021 outbreak; however, they were less similar to strains obtained in New York in 2020 (96.7%-96.9%) and Quebec in 2016 (92.4%-92.6%), suggesting that those events could be related to different viral incursions. The California sequences were more similar (98.6%-98.7%) to a strain collected in British Columbia in 2018, which suggests that that event could have been related to the 2020 outbreak in the southwestern USA.

Spillover event of recombinant Lagovirus europaeus/GI.2 into the Iberian hare (Lepus granatensis) in Spain

Viruses that affect lagomorphs have decades of reported history of spillover events. One of these viruses is the causative agent of the so-called rabbit or 'lagomorph' haemorrhagic disease (e.g. Lagovirus europaeus/GI.1 and L. europaeus/GI.2). In particular, L. europaeus/GI.2 has shown a great capacity to recombine with existing lagoviruses. In fact, it has replaced the former GI.1 genotype in the wild, and recently, an increase on spillover events has been detected among several lagomorph species including European and North American species of hares. In this study, we report for the first time the infection of a wild Iberian hare with GI.2 (RHDV2/b), potential shedding and associated histopathological alterations. We identify the recombinant GI.4P-GI.2 as causative of the infection and discuss plausible causes regarding the origin of the spillover event and its potential consequences for the Iberian hare wild populations, which is an endemic species of the Iberian Peninsula as well as an important game and prey species for many predators, including endangered species.

Rabbit haemorrhagic disease virus Lagovirus europaeus/GI.1d strain: genome sequencing, in vivo virus replication kinetics, and viral dose effect

BACKGROUND

Rabbit haemorrhagic disease virus Lagovirus europaeus/GI.1d variant (GI.1d/RHDV) was identified in 1990 in France, and until the emergence of the new genotype GI.2, it was the main variant circulating in the country. The early stages of RHDV infection have been described in a few studies of rabbits experimentally infected with earlier strains, but no information was given on the minimum infective dose. We report the genomic and phenotypic characterisation of a GI.1d/RHDV strain collected in 2000 in France (GI.1d/00-21).

RESULTS

We performed in vivo assays in rabbits to study virus replication kinetics in several tissues at the early stage of infection, and to estimate the minimum infective dose. Four tested doses, negligible (10- 1 viral genome copies), low (104), high (107) and very high (1011) were quantified using a method combining density gradient centrifugation of the viral particles and an RT-qPCR technique developed to quantify genomic RNA (gRNA). The GI.1d/00-21 genome showed the same genomic organisation as other lagoviruses; however, a substitution in the 5' untranslated region and a change in the potential p23/2C-like helicase cleavage site were observed. We showed that the liver of one of the two rabbits inoculated via the oral route was infected at 16 h post-infection and all tissues at 39 h post-infection. GI.1d/00-21 induced classical RHD signs (depression) and lesions (haemorrhage and splenomegaly). Although infective dose estimation should be interpreted with caution, the minimum infective dose that infected an inoculated rabbit was lower or equal to 104 gRNA copies, whereas between 104 and 107 gRNA copies were required to also induce mortality.

CONCLUSIONS

These results provide a better understanding of GI.1d/RHDV infection in rabbits. The genome analysis showed a newly observed mutation in the 5' untranslated region of a lagovirus, whose role remains unknown. The phenotypic analysis showed that the pathogenicity of GI.1d/00-21 and the replication kinetics in infected organs were close to those reported for the original GI.1 strains, and could not alone explain the observed selective advantage of the GI.1d strains. Determining the minimum dose of viral particles required to cause mortality in rabbits is an important input for in vivo studies.

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.

A Review on the Methods Used for the Detection and Diagnosis of Rabbit Hemorrhagic Disease Virus (RHDV)

Since the early 1980s, the European rabbit (Oryctolagus cuniculus) has been threatened by the rabbit hemorrhagic disease (RHD). The disease is caused by a lagovirus of the family Caliciviridae, the rabbit hemorrhagic disease virus (RHDV). The need for detection, identification and further characterization of RHDV led to the development of several diagnostic tests. Owing to the lack of an appropriate cell culture system for in vitro propagation of the virus, much of the methods involved in these tests contributed to our current knowledge on RHD and RHDV and to the development of vaccines to contain the disease. Here, we provide a comprehensive review of the RHDV diagnostic tests used since the first RHD outbreak and that include molecular, histological and serological techniques, ranging from simpler tests initially used, such as the hemagglutination test, to the more recent and sophisticated high-throughput sequencing, along with an overview of their potential and their limitations.

Immunity against Lagovirus europaeus and the Impact of the Immunological Studies on Vaccination

In the early 1980s, a highly contagious viral hemorrhagic fever in rabbits (Oryctolagus cuniculus) emerged, causing a very high rate of mortality in these animals. Since the initial occurrence of the rabbit hemorrhagic disease virus (RHDV), several hundred million rabbits have died after infection. The emergence of genetically-different virus variants (RHDV GI.1 and GI.2) indicated the very high variability of RHDV. Moreover, with these variants, the host range broadened to hare species (Lepus). The circulation of RHDV genotypes displays different virulences and a limited induction of cross-protective immunity. Interestingly, juvenile rabbits (<9 weeks of age) with an immature immune system display a general resistance to RHDV GI.1, and a limited resistance to RHDV GI.2 strains, whereas less than 3% of adult rabbits survive an infection by either RHDV GI.1. or GI.2. Several not-yet fully understood phenomena characterize the RHD. A very low infection dose followed by an extremely rapid viral replication could be simplified to the induction of a disseminated intravascular coagulopathy (DIC), a severe loss of lymphocytes—especially T-cells—and death within 36 to 72 h post infection. On the other hand, in animals surviving the infection or after vaccination, very high titers of RHDV-neutralizing antibodies were induced. Several studies have been conducted in order to deepen the knowledge about the virus’ genetics, epidemiology, RHDV-induced pathology, and the anti-RHDV immune responses of rabbits in order to understand the phenomenon of the juvenile resistance to this virus. Moreover, several approaches have been used to produce efficient vaccines in order to prevent an infection with RHDV. In this review, we discuss the current knowledge about anti-RHDV resistance and immunity, RHDV vaccination, and the further need to establish rationally-based RHDV vaccines.

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.

Full-genome sequencing of German rabbit haemorrhagic disease virus uncovers recombination between RHDV (GI.2) and EBHSV (GII.1)

Rabbit haemorrhagic disease virus (RHDV; genotypes GI.1 and GI.2) and European brown hare syndrome virus (EBHSV; genotype GII.1) are caliciviruses belonging to the genus Lagovirus. These viruses pose a serious threat to wild and domestic rabbit and hare populations around the world. In recent years, an expanding genetic diversity has been described within the genus, with recombination events occurring between the different genotypes. Here, we generated and analysed 56 full-genome sequences of RHDV and EBHSV from rabbit and hare livers, collected in Germany between the years 2013 and 2020. We could show that genotype Gl.2 (RHDV-2) almost entirely replaced Gl.1 (classical RHDV) in the German rabbit population. However, GI.1 is still present in Germany and has to be included into disease control and vaccination strategies. Three recombinant strains were identified from rabbit samples that contain the structural genes of genotype Gl.2 and the non-structural genes of genotype Gl.1b. Of special interest is the finding that sequences from two hare samples showed recombination events between structural genes of RHDV Gl.2 and non-structural genes of EBHSV GII.1, a recombination between different genogroups that has not been described before. These findings lead to the assumption that also a recombination of the non-structural genes of RHDV Gl.2 with the structural genes of EBHSV Gll.1 might be possible and therefore increase the potential genetic variability of lagoviruses immensely. Our findings underline the importance of whole genome analysis with next-generation sequencing technology as one of new tools now available for in-depth studies that allow in depth molecular epidemiology with continuous monitoring of the genetic variability of viruses that would otherwise likely stay undetected if only routine diagnostic assays are used.

Retrospective Analysis Shows That Most RHDV GI.1 Strains Circulating Since the Late 1990s in France and Sweden Were Recombinant GI.3P-GI.1d Strains

Recombination is one of the major sources of genetic variation in viruses. RNA viruses, such as rabbit hemorrhagic disease virus (RHDV), are among the viruses with the highest recombination rates. Several recombination events have been described for RHDV, mostly as a consequence of their genomic architecture. Here, we undertook phylogenetic and recombination analyses of French and Swedish RHDV strains from 1994 to 2016 and uncovered a new intergenotypic recombination event. This event occurred in the late 1990s/early 2000s and involved nonpathogenic GI.3 strains as donors for the nonstructural part of the genome of these recombinants, while pathogenic GI.1d strains contributed to the structural part. These GI.3P-GI.1d recombinant strains did not entirely replace GI.1d (nonrecombinant) strains, but became the dominant strains in France and Sweden, likely due to a fitness advantage associated with this genomic architecture. GI.3P-GI.1d (P stands for polymerase) strains persisted until 2013 and 2016 in Sweden and France, respectively, and cocirculated with the new genotype GI.2 in France. Since strains from the first GI.2 outbreaks were GI.3P-GI.2, we hypothesize that GI.3P-GI.1d could be the parental strain. Our results confirm the outstanding recombination ability of RHDV and its importance in the evolution of lagoviruses, which was only revealed by studying complete genomic sequences.