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In 2010, rabbit hemorrhagic disease virus (RHDV) GI.2 emerged, and unlike RHDV GI.1, it caused mortality in young rabbits, while existing vaccines were not fully protective. The GI.2-specific monoclonal antibody (mAb) 2D9 has been used as a tool to discriminate between these viruses in diagnostic tests. In this study, we mapped the binding epitope for 2D9 on the GI.2 The VP60 capsid protein demonstrated the neutralizing capacity of this mAb, which was able to prevent GI.2 infections in an experimental challenge. Our results suggest that external loops (1, 4 and 5) in the P2 subdomain of VP60 contribute to the discontinuous neutralizing epitope recognized by mAb 2D9. Moreover, analysis of naturally occurring RHDV GI.2 isolates revealed key residues involved in mAb 2D9 binding that are under selective pressure. The findings described in this work provide valuable information regarding our understanding of virus neutralization and immune escape, which may help in the development of novel antiviral compounds.

The Lagovirus rabbit hemorrhagic disease virus (RHDV), a member of the family Caliciviridae, severely affects European rabbit (Oryctolagus cuniculus) populations by causing rabbit hemorrhagic disease (RHD). RHDV is subdivided in six genogroups but, more recently, a new RHDV variant with a unique genetic and antigenic profile emerged. We performed a study in rabbits found dead in the field during 2013 and 2014 in Portugal to determine the prevalence of this new variant versus the classical RHDV. Fifty-seven liver samples were screened for the presence of RHDV and positive samples were genotyped. All cases of RHDV infection were caused by the new variant. The only former genogroup circulating in Portugal, G1, was not detected. We hence conclude that the new RHDV variant is replacing G1 in Portugal, probably due to a selective advantage. This sudden and rapid replacement emphasizes the necessity of continued monitoring of wild rabbit populations.

Over time, more and more is becoming known about micro-players of great significance. This is particularly the case for microRNAs (miRNAs; miR), which have been found to participate in the regulation of many physiological and pathological processes in both humans and animals. One such process is viral infection in humans and animals, in which the host miRNAs—alone or in conjunction with the virus—interact on two levels: viruses may regulate the host’s miRNAs to evade its immune system, while the host miRNAs can play anti- or pro-viral roles. The purpose of this comprehensive review is to present the key miRNAs involved in viral infections in humans and animals. We summarize the data in the available literature, indicating that the signature miRNAs in human viral infections mainly include 12 miRNAs (i.e., miR-155, miR-223, miR-146a, miR-122, miR-125b, miR-132, miR-34a, miR -21, miR-16, miR-181 family, let-7 family, and miR-10a), while 10 miRNAs are commonly found in animals (i.e., miR-155, miR-223, miR-146a, miR-145, miR-21, miR-15a/miR-16 cluster, miR-181 family, let-7 family, and miR-122) in this context. Knowledge of which miRNAs are involved in different viral infections and the biological functions that they play can help in understanding the pathogenesis of viral diseases, facilitating the future development of therapeutic agents for both humans and animals.

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.

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.

Rabbit hemorrhagic disease (RHD) is a lethal disease in rabbits caused by RHD virus (RHDV). Protection is only possible through vaccination. A new virus variant (RHDV2) which emerged in 2010 in France differed from the classical RHDV1 variant in certain aspects and vaccines against RHDV1 induced limited cross protection only. In a previous study, we designed a recombinant baculovirus based RHDV2-VP1 vaccine, which provided a protective immunity in rabbits against RHDV2. In the present study this newly created vaccine is characterized with regard to onset and duration of protection, and possible cross protection against classical RHDV1. Furthermore, humoral and cellular immune mechanisms in vaccinated and infected rabbits were analyzed. In all experiments, the recombinant vaccine was compared to a conventional liver-based RHDV2 vaccine. The RHDV2-VP1 vaccine induced a protective immune response already seven days after single vaccination and fully protected for at least 14 months. A booster vaccination 21 days after the first had a negative influence on long-term protection. The cross protection provided by the RHDV2-VP1 vaccine against classical RHDV1 was limited since only 50% of vaccinated rabbits survived the infection. Conclusively, the new, baculovirus-based RHDV2-VP1 vaccine has the potential to protect rabbits against the infection with RHDV2, blocks completely the disease progression and prevents the spread of RHDV2 at the population level.

The Rabbit Hemorrhagic Disease Virus (RHDV) represents a significant threat to rabbit populations globally, affecting both wild and domesticated rabbits, with mortality rates ranging from 50% to 90%. Despite its severity, there is currently no specific treatment available for RHDV. This study investigates the potential of natural compounds derived from Nigella sativa as antiviral agents against RHDV. Molecular docking analysis was conducted to explore the interaction between eleven compounds from Nigella sativa and the two key proteins of RHDV, viral protein genome-linked (VPg) and RNA-dependent RNA polymerase (RdRP), as key proteins involved in viral replication. Explicit-solvent MD (100 ns, 310 K) was performed for four top complexes (VPg/RdRP with nigellidine and dithymoquinone), tracking backbone/ligand RMSD, radius of gyration, H-bond counts, and per-residue RMSF, with equilibrated frames analyzed by PCA and MM-GBSA. The results revealed successful docking of all compounds from Nigella sativa to both VPg and RdRP proteins. From Nigella sativa compounds, Nigellidine and Dithymoquinone displayed strong interactions with VPg and RdRP and formed various hydrogen bonds and hydrophobic interactions, indicating their potential as inhibitors of viral replication. Interestingly, all ligands demonstrated favorable drug-likeness properties, adhering to Lipinski’s Rule of Five and exhibiting desirable pharmacokinetic profiles. Thymohydroquinone and nigellidine displayed the highest lipophilicity, suggesting their potential for efficient tissue penetration and distribution. Complexes remained stable and retained poses, with reduced pocket flexibility, favorable MM-GBSA ΔG_bind, and tighter PCA clustering—supporting sustained binding and pocket stabilization. These findings suggest that compounds from Nigella sativa show promise as natural antiviral agents against RHDV. Nevertheless, additional experimental validation through in vitro and in vivo studies is essential to confirm the effectiveness and safety of these compounds for treating RHDV infection.

Quantifying the infectious titre of preparations containing rabbit haemorrhagic disease virus (RHDV) is an essential virological technique during RHDV research. The infectious titre of an RHDV preparation is determined using a bioassay to identify the endpoint dilution at which 50% of rabbits become infected (RID50). Previous publications have briefly described the method for estimating the infectious titre of RHDV preparations by challenging rabbits with 10-fold serial dilutions. However, these descriptions lack the critical considerations for a standardised method to estimate RID50. These details are presented here, along with a comparison between the Reed–Muench, Dragstedt–Behrens, Spearman–Kärber, and probit regression methods for calculating the RID50. All the statistical approaches demonstrated a high level of agreement in calculating the RID50. To help assess the precision of the estimated infectious titre, the improved Spearman–Kärber and probit regression methods provide the 95% confidence intervals. The method outlined improves the accuracy of results when undertaking studies of pathogenicity, host resistance, and the production of vaccines against RHDV.

MicroRNAs (miR) are a group of small, non-coding RNAs of 17–25 nucleotides that regulate gene expression at the post-transcriptional level. Dysregulation of miRNA expression or function may contribute to abnormal gene expression and signaling pathways, leading to disease pathology. Lagovirus europaeus (L. europaeus) causes severe disease in rabbits called rabbit hemorrhagic disease (RHD). The symptoms of liver, lung, kidney, and spleen degeneration observed during RHD are similar to those of acute liver failure (ALF) and multi-organ failure (MOF) in humans. In this study, we assessed the expression of miRs and their target genes involved in the innate immune and inflammatory response. Also, we assessed their potential impact on pathways in L. europaeus infection—two genotypes (GI.1 and GI.2)—in the liver, lungs, kidneys, and spleen. The expression of miRs and target genes was determined using quantitative real-time PCR (qPCR). We assessed the expression of miR-155 (MyD88, TAB2, p65, NLRP3), miR-146a (IRAK1, TRAF6), miR-223 (TLR4, IKKα, NLRP3), and miR-125b (MyD88). We also examined biomarkers of inflammation: IL-1β, IL-6, TNF-α, and IL-18 in four tissues at the mRNA level. Our study shows that the main regulators of the innate immune and inflammatory response in L. europaeus/GI.1 and GI.2 infection, as well as RHD, are miR-155, miR-223, and miR-146a. During infection with L. europaeus/RHD, miR-155 has both pro- and anti-inflammatory effects in the liver and anti-inflammatory effects in the kidneys and spleen; miR-146a has anti-inflammatory effects in the liver, lungs and kidneys; miR-223 has anti-inflammatory effects in all tissues; however, miR-125b has anti-inflammatory effects only in the liver. In each case, such an effect may be a determinant of the pathogenesis of RHD. Our research shows that miRs may regulate three innate immune and inflammatory response pathways in L. europaeus infection. However, the result of this regulation may be influenced by the tissue microenvironment. Our research shows that infection of rabbits with L. europaeus/GI.1 and GI.2 genotypes causes an overexpression of two critical acute phase cytokines: IL-6 in all examined tissues and TNF-α (in the liver, lungs, and spleen). IL-1β was highly expressed only in the lungs after L. europaeus infection. These facts indicate a strong and rapid involvement of the local innate immune and inflammatory response in L. europaeus infection—two genotypes (GI.1 and GI.2)—and in the pathogenesis of RHD. Profile of biomarkers of inflammation in rabbits infected with L. europaeus/GI.1 and GI.2 genotypes are similar regarding the nature of changes but are different for individual tissues. Therefore, we propose three inflammation profiles for L. europaeus infection for both GI.1 and GI.2 genotypes (pulmonary, renal, liver, and spleen).

Lagovirus europaeus GI.1/GI.2 is an etiological agent causing the highly dangerous rabbit hemorrhagic disease (RHD). Molecular research is the basic tool today that can help solve epidemic problems related to the expansion of pathogens in the world. By using the real-time polymerase chain reaction technique (PCR), we detected three different strains of Lagovirus europaeus/GI.1, which is an RNA virus infecting mainly rabbits. The results showed that the method used was fast, very specific, and effective.