Detection of rabbit haemorrhagic disease virus particles in the rabbit liver tissues

PseudoRHDV constructed with feline calicivirus genome as vector has the characteristics of well proliferation in vitro

Rabbit hemorrhagic disease virus (RHDV) is a major member of the Caliciviridae. which is fatal to wild and domestic European rabbit. Because RHDV does not reproduce stably in vitro, molecular studies on this pathogen have been limited. Feline calicivirus (FCV), also a member of the Caliciviridae, reproduces well in vitro and is a good viral vector. As these viruses share similar genomic structures, we hypothesized that a chimeric infectious clone could be constructed by replacing the corresponding regions of the FCV genome with the structural proteins VP60 and VP10 and the 3' non-translated region of the RHDV genome. Transfection of the infectious clone into RK13 cells made it possible to rescue the chimeric virus, named pseudoRHDV, which reproduced in an RK13 cell line with high titer. An infectious pseudoRHDV was produced, which proliferated in RK13 cells to at least 15 generations. PseudoRHDV caused significant cytopathic changes in the RK13 cells, with a viral titer was 9.74 log₁₀ TCID₅₀ / mL. The pseudoRHDV constructed in this study will be helpful for investigating the molecular biology of RHDV, especially its interaction with the host. The model can also be used to explore some common laws between FCV and RHDV.

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.

An outbreak of rabbit hemorrhagic disease (RHD) caused by Lagovirus europaeus GI.2/rabbit hemorrhagic disease virus 2 (RHDV2) in Ehime, Japan

A total of ten 1–2-year-old rabbits died within 2 weeks at a facility in Ehime prefecture in May 2019. Necropsy revealed liver discoloration and fragility, hemorrhage of some organs and blood coagulation failure. On histopathologic examination, necrotizing hepatitis was a common finding, together with fibrin thrombi in the small vessels and hemorrhage in some organs. Rabbit hemorrhagic disease (RHD) virus gene was detected in liver samples, and viral particles of approximately 32 nm in diameter were found in the cytoplasm of degenerated hepatocytes by electron microscopy. Phylogenetic analysis based on the partial VP60 gene sequence classified it as Lagovirus europaeus GI.2/RHDV2. This is the first confirmed outbreak of RHD caused by globally emerging GI.2/RHDV2 in Japan.

Glycosphingolipids as receptors for non-enveloped viruses

Glycosphingolipids are ubiquitous molecules composed of a lipid and a carbohydrate moiety. Their main functions are as antigen/toxin receptors, in cell adhesion/recognition processes, or initiation/modulation of signal transduction pathways. Microbes take advantage of the different carbohydrate structures displayed on a specific cell surface for attachment during infection. For some viruses, such as the polyomaviruses, binding to gangliosides determines the internalization pathway into cells. For others, the interaction between microbe and carbohydrate can be a critical determinant for host susceptibility. In this review, we summarize the role of glycosphingolipids as receptors for members of the non-enveloped calici-, rota-, polyoma- and parvovirus families.

Hyperlipidemia in rabbit hemorrhagic disease

Clinically healthy rabbits were inoculated with rabbit hemorrhagic disease virus (RHDV) and the kinetics of their serum lipid parameters and liver enzymes were monitored. After RHDV inoculation, hyperlipidemia was observed (P(triglyceride)<0.0001, P(cholesterol)=0.0003) along with significant increases in serum aspartate aminotransferase, alanine aminotransferase and gamma-glutamyltransferase (P<0.0001). An exponential increase in serum triglyceride was also seen. Thus, the presence of hyperlipidemia (from 30 h post-inoculation) in the infected rabbits points to impairment in lipid metabolism. This is the first demonstration that RHDV infection leads to hyperlipidemia, probably due to the disorder of liver enzymes associated with lipid metabolism.

Binding of rabbit hemorrhagic disease virus to antigens of the ABH histo-blood group family

The ability of rabbit hemorrhagic disease virus to agglutinate human erythrocytes and to attach to rabbit epithelial cells of the upper respiratory and digestive tracts was shown to depend on the presence of ABH blood group antigens. Indeed, agglutination was inhibited by saliva from secretor individuals but not from nonsecretors, the latter being devoid of H antigen. In addition, erythrocytes of the rare Bombay phenotype, which completely lack ABH antigens, were not agglutinated. Native viral particles from extracts of infected rabbit liver as well as virus-like particles from the recombinant virus capsid protein specifically bound to synthetic A and H type 2 blood group oligosaccharides. Both types of particles could attach to adult rabbit epithelial cells of the upper respiratory and digestive tracts. This binding paralleled that of anti-H type 2 blood group reagents and was inhibited by the H type 2-specific lectin UEA-I and polyacrylamide-conjugated H type 2 trisaccharide. Young rabbit tissues were almost devoid of A and H type 2 antigens, and only very weak binding of virus particles could be obtained on these tissues.

Gene transfer using recombinant rabbit hemorrhagic disease virus capsids with genetically modified DNA encapsidation capacity by addition of packaging sequences from the L1 or L2 protein of human papillomavirus type 16

The aim of this study was to produce gene transfer vectors consisting of plasmid DNA packaged into virus-like particles (VLPs) with different cell tropisms. For this purpose, we have fused the N-terminally truncated VP60 capsid protein of the rabbit hemorrhagic disease virus (RHDV) with sequences which are expected to be sufficient to confer DNA packaging and gene transfer properties to the chimeric VLPs. Each of the two putative DNA-binding sequences of major L1 and minor L2 capsid proteins of human papillomavirus type 16 (HPV-16) were fused at the N terminus of the truncated VP60 protein. The two recombinant chimeric proteins expressed in insect cells self-assembled into VLPs similar in size and appearance to authentic RHDV virions. The chimeric proteins had acquired the ability to bind DNA. The two chimeric VLPs were therefore able to package plasmid DNA. However, only the chimeric VLPs containing the DNA packaging signal of the L1 protein were able efficiently to transfer genes into Cos-7 cells at a rate similar to that observed with papillomavirus L1 VLPs. It was possible to transfect only a very limited number of RK13 rabbit cells with the chimeric RHDV capsids containing the L2-binding sequence. The chimeric RHDV capsids containing the L1-binding sequence transfer genes into rabbit and hare cells at a higher rate than do HPV-16 L1 VLPs. However, no gene transfer was observed in human cell lines. The findings of this study demonstrate that the insertion of a DNA packaging sequence into a VLP which is not able to encapsidate DNA transforms this capsid into an artificial virus that could be used as a gene transfer vector. This possibility opens the way to designing new vectors with different cell tropisms by inserting such DNA packaging sequences into the major capsid proteins of other viruses.

Apoptosis in rabbit haemorrhagic disease

Rabbit haemorrhagic disease virus (RHDV) causes an acute hepatitis and disseminated intravascular coagulation. Six rabbits were inoculated experimentally with RHDV to investigate any potential relationship between infection and apoptosis in the liver. Two rabbits were killed at 12 h post-inoculation (PI) and two at 24 h PI. The remaining two rabbits died at 30 h and 31 h PI. Immunohistochemical labelling for RHDV antigen-positive cells, TUNEL assay for apoptotic cells, and DNA analysis were performed on samples of liver. The four rabbits that died or were killed 24-31 h PI had acute hepatitis with infiltration of heterophils and necrotic hepatocytes. RHDV antigen-positive cells and apoptotic cells appeared in the centriacinar areas at 12 h PI; subsequently they spread to periacinar areas and increased in number, but the viral antigen-positive cells outnumbered apoptotic cells. At 24-31 h PI, few apoptotic cells were recognized in the areas infiltrated with lymphocytes and heterophils. The results suggested an association between RHDV infection and apoptosis of hepatocytes.

Immunohistochemical localisation of rabbit haemorrhagic disease virus VP-60 antigen in early infection of young and adult rabbits

This study evaluated the time course distribution of rabbit haemorrhagic disease virus (RHDV) structural protein VP60 in tissues from experimentally infected rabbits from three different age groups. Viral VP60 antigen could not be detected in tissue samples from animals under four weeks, and only a few hepatocytes (0.01 to 0.2 per cent) were stained in the 6-week-old animals. A 6-week-old rabbit euthanised at 72 hpi showed VP60-labelling in hepatocytes and macrophages close to areas of inflammation. Viral VP60 antigen was detected as early as 12 hpi in a few hepatocytes (0.03 per cent) from adult animals. Within this age group, the extent of hepatocyte labelling considerably increased at 18 (3.0 per cent), 24 (25.5 per cent), 36 (50 per cent) and 48 (60 per cent) hpi. Extrahepatic viral VP60 antigen was also detected at 36 and 48 hpi in spleen macrophages and lymphocytes from adult rabbits. These findings support the hypothesis that the hepatocyte is the only cell type in the liver able to support RHDV replication almost immediately after viral infection.

Hepatic lesions in young rabbits experimentally infected with rabbit haemorrhagic disease virus

Twenty young rabbits (eleven 2-week-old and nine 4-week-old) were experimentally infected with rabbit haemorrhagic disease virus (RHDV) to clarify susceptibility. They were killed chronologically up to 96 hours post-inoculation (PI) and examined for lesions. All inoculated rabbits were clinically normal, but grossly minute white or grey spots were detected throughout the liver. Histologically, the lesions consisted of aggregates of lymphocytes, macrophages and heterophils, with or without acidophilic bodies and necrotic hepatocytes. Immunohistochemically, RHDV antigens were found in the degenerated hepatocytes and in macrophages. The cellular aggregates were considered to be a reaction to necrotic hepatocytes infected with RHDV. It was concluded that some hepatocytes are susceptible to RHDV in young rabbits.

Macrophage tropism of rabbit hemorrhagic disease virus is associated with vascular pathology

To delineate the interactions between rabbit hemorrhagic disease virus (RHDV) and host cells, organ and cellular targets of infection were identified in vivo. Viral specific antigens were detected by immunohistochemistry in liver, lung, spleen and lymph nodes cells. Also, intravascular infected cells were detected in most organs including kidneys, myocardium, thymus and central nervous system. To further characterize infected target cells, viral proteins and cell-specific surface antigens were identified simultaneously in double labeling experiments. Numerous lymphoid organ macrophages, from the splenic red pulp, circulating monocytes, alveolar macrophages and Kupffer cells were double labeled, demonstrating that cells of the mononuclear phagocyte lineage are major hosts for RHDV. Double labeling for other specific cell markers were negative. The distribution of viral antigens in these tissues coincided with those areas where cells presented morphology of apoptosis. Association of intravascular monocyte infection and apoptosis, could represent a possible mechanism to develop disseminated intravascular coagulation.

Early stages of rabbit haemorrhagic disease virus infection monitored by polymerase chain reaction

In order to define more accurately the initial events that take place during rabbit haemorrhagic disease virus (RHDV) infection, different organs of experimentally infected rabbits were analysed for the presence of the virus and correlated with histopathological observations. A total of 24 rabbits were intranasally inoculated with a viral suspension, and tissue samples were taken from the liver, spleen, kidney, lung, thymus, lymph node and tonsil at different intervals post-inoculation (2, 4, 6, 12, 18, 24, 30, 36, 48, 50, 51, 70 and 72 h). Histopathological observations revealed the presence of the first significant lesions at 30 h post-inoculation (p.i.) in the liver. Using an ELISA and a haemagglutination test (HAT), the virus was detected in the liver at 36 h p.i. The reverse transcriptase-polymerase chain reaction (RT-PCR) showed that the RHDV RNA was present as early as 18 h p.i. in the liver and spleen, whereas thymus, kidney, tonsil and lymph node were found to be positive after more than 36 h p.i. The lungs presented a variable positivity between 0 and 36 h p.i., but remained positive after this time.

A sequential study of the light and electron microscopic liver lesions of infectious anemia in Atlantic salmon (Salmo salar L.)

The present study describes light and electron microscopic changes in the liver of Atlantic salmon during the development of infectious salmon anemia (ISA). Atlantic salmon postsmolts weighing 80-100 g were infected by intraperitoneal injections, and liver samples were collected sequentially between day 0 and day 25 post infection (p.i.), with time intervals of 3-4 days. At each collection time, livers from five infected fish and two control fish were examined. Changes involving the perisinusoidal macrophages were observed by transmission electron microscopy, from day 4 p.i. Large vacuoles, containing a fine-granular material with low electron density, accumulated in the cytoplasm. These changes persisted and became more severe throughout the investigation, leading to a considerable increase in the size of the cells. At day 14 p.i., degenerative features of the sinusoidal endothelium were observed. By day 18 p.i., areas of the liver were devoid of a sinusoidal endothelial lining, bringing hepatocytes in direct contact with blood cells. At this stage, the sinusoids were moderately congested. From day 21 p.i., heavy sinusoidal congestion, peliosis hepatis, and degeneration of the hepatocytes were observed. No virus was observed in any of the inhabitant cell types of the liver. Gross and light microscopic changes were first recorded at day 18 p.i., as was a significant decrease in the hematocrit values. By day 25 p.i., characteristic multifocal, confluent, hemorrhagic necroses were present. Results of the present investigation suggest that the liver lesions observed with ISA are not the result of the development of an anemia alone or caused by direct viral damage to hepatocytes. Hepatocellular degeneration succeeded changes in the perisinusoidal macrophages and degeneration of the sinusoidal endothelium. These changes may have impeded the sinusoidal blood flow and hence caused an ischemic hepatocellular necrosis.

Partial characterization of the human erythrocyte receptor for rabbit haemorrhagic disease virus

An important, well known property of the rabbit haemorrhagic disease virus is its ability to agglutinate human red blood cells. Accordingly, red cells from human adult donors were agglutinated despite their blood group ABO status, and treatments with proteases or glycosidases did not prevent agglutination. However, we discovered that the cells from human umbilical cords or foetuses were not agglutinated. In order to identify the viral receptor on human erythrocytes, glycolipids and glycoproteins from adult red cells were separated and tested for their potency in inhibiting agglutination. The bulk of the biological activity was associated with the highly glycosylated glycolipids (polyglycosylceramides), whereas a lower but significant activity was also associated with neutral glycolipids. No activity was found in the lipid-free sialoglycoprotein fractions. All these data strongly suggest that the RHDV receptor on human red cells corresponds to a development antigen which is not expressed on foetal cells and is mainly carried by glycolipids. Faint activity was also found in membranes from sheep red cells, suggesting that a similar glycolipid component is carried by these animal cells.