Elucidation of the pathology and tissue distribution of Lagovirus europaeus GI.2/RHDV2 (rabbit haemorrhagic disease virus 2) in young and adult rabbits (Oryctolagus cuniculus)

Elucidation of the pathology and tissue distribution of Lagovirus europaeus GI.2/RHDV2 (rabbit... Lagovirus europaeus GI.2, also known as RHDV2 or RHDVb, is an emerging virus that causes rabbit haemorrhagic dis‑ ease (RHD) in European rabbits (Oryctolagus cuniculus). In contrast to L. europaeus GI.1 (or RHDV/RHDVa) viruses that are only pathogenic for adults, GI.2 causes clinical disease in both adults and kittens. However, detailed descriptions of the pathology of this virus that may provide insight into its pathogenicity and emergence are lacking. Using an Australian GI.2 field strain isolated in 2015, we provide the first detailed description of pathology, viral antigen distribu ‑ tion and tissue load of GI.2 in adult and 5‑ week old New Zealand white rabbits using histology, immunohistochem‑ istry and RT‑ qPCR. Liver was the target organ, but in contrast to GI.1 viruses, lesions and inflammatory responses did not differ between adults and kittens. Lymphocytic inflammation, proposed to be protective in kittens infected with GI.1, was notably absent. We also present the first descriptions of bone marrow changes in RHD, including decreased myeloid‑ to‑ erythroid ratio. Consistent with other pathogenic lagoviruses, intracellular viral antigen was demonstrated in hepatocytes and cells of the mononuclear phagocytic system. In terminal stages of disease, viral loads were highest in liver, serum and spleen. Despite the small sample size, our data suggest that unlike early European GI.2 strains, the pathogenicity of the Australian GI.2 virus is similar to GI.1 viruses. Additionally, GI.2 was fatal for all (n = 5) inoculated kittens in this study. This may significantly alter RHD epidemiology in the field, and may impact biocontrol programs for invasive rabbits in Australia where GI.1 viruses are intentionally released. Introduction of RHD are all grouped within genogroup I (L. europaeus GI) (Table 1). Rabbit haemorrhagic disease (RHD) is a viral disease of First described in domestic rabbits in China in 1984, the European rabbit (Oryctolagus cuniculus) that primar- RHD was detected in Europe in 1986 and has been doc ily affects the liver [ 1]. The causative agents, currently - known as rabbit haemorrhagic disease virus (RHDV), the umented at some point on every continent except Ant- antigenic variant (RHDVa), and the recently described arctica [1]. In Australia and New Zealand, RHD is used RHDV2 or RHDVb, are lagoviruses within the Family intentionally for biocontrol of invasive, introduced Euro- Caliciviridae [1, 2]. Within a proposed new classification pean rabbits. Until 2010, the disease was almost exclu- system [3], all lagoviruses are reclassified into a single sively confined to wild and domestic European rabbits species, Lagovirus europaeus, and the causative viruses and was caused by viruses newly classified as genotype 1 of the L. europaeus GI group (i.e. L. europaeus GI.1 [3]), also known as RHDV and RHDVa (Table 1). L. europaeus GI.1 is further subdivided into variants denoted by lower case letters [3] (Table 1). *Correspondence: aleksija.neimanis@sva.se Department of Pathology and Wildlife Diseases, National Veterinary In 2010, a new genotype (L. europaeus GI.2 [3], also Institute (SVA), 751 89 Uppsala, Sweden known as RHDV2 or RHDVb, Table  1) was detected in Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/ publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Neimanis et al. Vet Res (2018) 49:46 Page 2 of 15 Table 1 A list of current names and proposed new, this study, rabbits were infected with a L. europaeus GI.2 phylogenetically derived names for lagoviruses according strain detected in Australia that is also closely related to to Le Pendu et al. [3] referred to in this manuscript circulating strains in southern Europe [22] and high qual- ity material from both adults and kittens was collected Current name Proposed new name [3] to describe the pathology and tissue distribution of this Rabbit haemorrhagic disease virus Lagovirus europaeus GI.1 virus in detail. We compare findings between the two age (RHDV ) groups and with previously published findings for GI.1 RHDVa Lagovirus europaeus GI.1a strains to provide further insight into pathogenesis. We RHDV G1 Lagovirus europaeus GI.1b also interpret our findings in conjunction with parallel RHDV2 or RHDVb Lagovirus europaeus GI.2 quantitative molecular analyses to evaluate virus localiza- Recombinant of RHDV G1 and RHDV2, Lagovirus europaeus GI.1bP–GI.2 tion and detection limits of our immunohistochemistry recombination break point between the polymerase and capsid coding analyses. regions Materials and methods The virus used in this study was the first L. euro - domestic and wild European rabbits in France [4]. It soon paeus GI.2 virus reported in Australia [9] (GenBank# was reported throughout Europe [2, 5–8] and reached KT280060). From here on this virus is referred to as Australia in 2015 [9], where it rapidly spread and appears BlMt-1, the name of this isolate. BlMt-1 is closely related to be replacing previously endemic GI.1 strains [10] to isolates described in the Iberian Peninsula [22] (>97% as has been described in some European countries [2, genetic identity), and is a recombinant virus with the 11–15]. While disease mimics the disease caused by L. capsid gene of GI.2 and the non-structural genes of GI.1 europaeus GI.1 viruses, disease course and levels of path- (GI.1bP–GI.2, according to the newly proposed nomen- ogenicity appeared to be more variable [2]. In addition, clature by Le Pendu et al. [3]) (Table 1). while animals younger than 5–8  weeks of age are highly Rabbits used in this study were sourced from the resistant to the development of clinical disease following domestic rabbit breeding colony at CSIRO Black Moun- infection with L. europaeus GI.1 viruses, kittens as young tain. Rabbits in this research colony are regularly tested as 11 days of age can succumb to disease and death from and confirmed to be free of antibodies to RHDV as well infection with L. europaeus GI.2 [13]. Finally, species bar- as the non-pathogenic lagovirus RCV-A1 [23, 24] that is riers are less rigid within the Leporidae family, as L. euro- endemic to Australia and can attenuate RHDV infections paeus GI.2 viruses can cause clinical disease and death in [25]. All procedures involving live animals were carried various hare species [16–20]. out in Australia and were in accordance with the Aus- While there are numerous descriptions of the pathol- tralian Code of Practice for the Care and Use of Animals ogy resulting from infection by L. europaeus GI.1 for Scientific Purposes (2013) and were approved by the (reviewed in [1]), to date, there are only a few, brief CSIRO Ecosystem Sciences Animal Ethics Committee descriptions for L. europaeus GI.2 [2, 5, 11]. Addition- (permits ESAEC 13-10, DOMRAB). ally, some disparities regarding pathology and patho- To generate material for further research, two adult genesis of disease caused by L. europaeus GI.1 have been New Zealand white rabbits were inoculated per os with reported. While many likely stem from differences in 1  mL of a clarified 2% w/v liver homogenate of BlMt-1 methodology as suggested by Abrantes et  al. [1], differ - containing 3 × 10 copies/mL. Although the infectivity ences between viral strains and hosts may also play a role. of this preparation was not experimentally determined in Finally, there appear to be strain differences in the type of rabbits, this likely constitutes a very high infectious dose. disease caused by L. europaeus GI.2. Le Gall-Reculé et al. Rabbits were monitored twice daily or more frequently [2] report that early strains often resulted in subacute to when required. Both animals were euthanized when they chronic disease, whereas experimental work by Capucci displayed signs of RHD, at 54 and 96  h post-inoculation et  al. [21] suggests that more recent L. europaeus GI.2 (hpi) (Table  2). Humane endpoints for terminal RHD strains result in predominantly acute disease. Detailed were defined as hypothermia (normal rectal temperature descriptions of disease caused by different L. europaeus is defined as 38.5–40 °C) after a fever episode (>40.4 °C), GI.2 viruses therefore are warranted and may provide depression with loss of muscle tone and/or anorexia and insight into mechanisms responsible for pathogenicity. >10% acute weight loss. For euthanasia, rabbits were Presently, lagoviruses cannot be cultured ex  vivo and first sedated with an intramuscular injection of Keta - experimental infections still are required to study cer- mine/Xylazine (30; 5  mg/kg) (Xylazil-20: Troy Labo- tain aspects of disease and generate material for further ratories, Smithfield, Australia; Ketamav 100: Mavlab, analyses, such as viral antigen for serological assays. For Logan, Australia), followed by intravenous injection of Neimanis et al. Vet Res (2018) 49:46 Page 3 of 15 Table 2 Domestic rabbits (Oryctolagus cuniculus) inoculated per os with Lagovirus europaeus GI.2 (BlMt-1) Animal Age Euthanized Time (hours Tissues for molecular Tissues for microscopic identification (E) or died post infection) analysis collected (Y = yes, analyses collected (Y = yes, number spontaneously (D) of euthanasia or death N = no) N = no) 282 Adult E 96 Y Y 283 Adult E 54 Y Y 302 Juvenile D <90 Y N 303 Juvenile D 66 Y Y 304 Juvenile D <66 Y N 305 Juvenile D <66 Y N 306 Juvenile E 75 Y Y Rabbit 130 (not shown), a healthy adult from the same research colony, was euthanized to serve as a negative control. Rabbit was found dead. pentobarbitone sodium (165  mg/kg) (Lethabarb: Virbac, at −20 °C. The three other kittens were found dead. Exact Milperra, Australia) in the marginal ear vein. Blood col- time of death was not known. The same subset of tissues lection and post-mortem examinations were performed was collected, but samples were saved at −20  °C only. immediately following euthanasia and the following suite Samples for microscopic examination were not collected. of tissues was collected: liver (hilar region and distal Viral load (capsid genome copies) was determined in lobe), gall bladder with bile, spleen, lung, heart, kidney, all tissue samples collected, including bile, serum and thymus, mesenteric lymph node, duodenum, jejunum, buffy coat. Samples were analysed as described previ - ileum, Peyer’s patches, appendix, sacculus rotundus, ously using a universal lagovirus quantitative reverse cecum, colon, trachea, femoral bone marrow and brain. transcriptase PCR (RT-qPCR) assay that detects all lago- The bone marrow was removed from the femur before viruses known to be present in Australia [26]. placement in formalin to eliminate the need for decalci- Following formalin fixation for 1  week, tissues were fication before processing for microscopic analysis. Each processed and embedded in paraffin at the Imaging and sample was split in two. One half was fixed in 10% neu - Cytometry facility, Australian National University for tral buffered formalin (NBF) while the other was frozen routine microscopic examination. at −20  °C for molecular analyses. A healthy adult rab- All microscopic analyses, including immunohisto- bit from the same rabbit colony was killed humanely as chemistry, were performed at the National Veterinary described above to serve as a negative control and the Institute in Sweden. To assess histopathology, sections same suite of samples were collected. (3–4  µm) were stained with Mayer’s haematoxylin and Blood samples were collected by cardiac puncture fol- eosin [27]. lowing euthanasia and centrifuged at 3000 g for 20 min to In order to perform a relative comparison of propor- separate the serum. White blood cells (buffy coat) were tions of different cell lines in the bone marrow of the two collected from the interphase and both serum and white infected adult rabbits and the control animal, estimates blood cells were stored at −20 °C until analysis. of the myeloid-to-erythroid ratio (M:E) and the propor- Young animals are resistant to the development of tion of early to later stage myeloid cells were determined. clinical disease following infection with L. europaeus Analyses were conducted on formalin-fixed bone marrow GI.1 viruses [1]. To investigate the susceptibility of young stained with haematoxylin and eosin as described above. animals to L. europaeus GI.2, five 5-week old New Zea - M:E ratios were calculated by classifying 300 nucleated land white rabbit kittens from the same rabbit colony cells at high power (100 cells per three different fields were inoculated per os with 0.5  mL of a clarified liver at 1000× magnification) into cell types and then divid - homogenate of BlMt-1. Again, all died or were eutha- ing the number of myeloid lineage cells by the number of nized because of terminal signs of RHD within 90  h of nucleated erythroid cells. Additionally, the proportion of inoculation (Table  2). One kitten that was euthanized early myeloid cells (myeloblasts, promyelocytes and mye- and one that died while euthanasia was about to be per- locytes as defined by Riedel et al. [28]) in relation to later formed were examined immediately after death and stage myeloid cells was estimated. This was done by clas - blood and the following subset of tissues were collected: sifying 200 myeloid cells (100 cells per two different fields liver, spleen, lung, heart, kidney, duodenum, jejunum, at 1000× magnification) as early [large cells with round ileum, Peyer’s patches, appendix and colon. Again, half of to oval nucleus, and at least one nucleolus (myeloblast) each sample was placed in 10% NBF and half was frozen or eosinophilic cytoplasmic granules (promyelocyte and Neimanis et al. Vet Res (2018) 49:46 Page 4 of 15 myelocyte)] or later stage myeloid cells (kidney bean- terminal RHD (depressed, little resistance to handling) shaped, band-shaped or segmented nucleus and eosino- and were euthanized or died just before euthanasia could philic cytoplasmic granules). be carried out. Both 303 and 306 also had subnormal To complement molecular analyses and provide fur- body temperature (< 38  °C). Rabbit 282 was active and ther insight into virus localization, distribution of GI.2/ behaving normally, but had lost > 10% of its body weight RHDV2 viral antigen in situ, and its relation to presence at 96  hpi and was euthanized. Rabbits 304, 305 and 302 of histological lesions, immunohistochemical staining were found dead. Time of death or euthanasia are sum- was carried out on all formalin-fixed and paraffin-embed - marized in Table 2. ded tissues available from the four infected animals (282, 283, 303 and 306) and the control animal (130). Briefly, Gross pathology 3–4  µm sections were deparaffinized and rehydrated. Rabbit 282 had a tan and mottled liver, enlarged spleen Slides were then placed in an automated Autostainer- and congested lungs. The tissues of rabbit 283 were unre - Plus (Agilent Technologies Sweden AB, Kista, Sweden). markable, except for an enlarged spleen. Endogenous peroxide activity was blocked with hydrogen All kittens had a tan and friable liver, and the liver was peroxide (Dako REAL Peroxidase-Blocking Solution, severely discoloured yellow-tan in rabbit 306. Rabbit 302 Agilent Technologies Sweden AB, Kista, Sweden) for showed subserosal petechial haemorrhages over Peyer’s 5 min. Sections were then treated with 2% bovine serum patches and multifocal haemorrhages around the mesen- albumin (BSA) for 20  min. Slides were then incubated teric lymph nodes. Rabbit 304 had copious foamy nasal for 30  min at room temperature with a 1:700 dilution discharge, and rabbit 305 had hay in the mouth, indicat- (diluted with Dako Antibody Diluent with Background ing peracute death while eating. Tissues were otherwise Reducing Components, Agilent Technologies Sweden unremarkable. AB, Kista, Sweden) of a primary monoclonal antibody cocktail (3H6 + 6G2 IgG1 mouse monoclonal antibodies, Histopathology concentration of 0.32  mg/mL) targeting the capsid pro- There was a clear difference in severity and distribution tein of lagoviruses [29] kindly provided by the OIE Ref- of lesions from the adult euthanized at 54  hpi (rabbit erence Laboratory for RHD, Brescia, Italy. Visualization 283) and the other three rabbits that were euthanized or of bound antibodies was performed by using the polymer died later (rabbits 282, 303 and 306, Table  2). Significant detection system Dako EnVision+ System-HRP Labelled lesions were seen in the liver, spleen and bone marrow of Polymer (anti-mouse) (Agilent Technologies Sweden all animals examined histologically, but these were less AB, Kista, Sweden) for 30  min followed by application severe in rabbit 283. Additional lesions were observed in of the chromogen diaminobenzidine (DAB) (Dako Liq- kidneys, lungs, heart and other lymphoid tissues of some, uid DAB+ Substrate Chromogen System, Agilent Tech- but not all, rabbits. nologies Sweden AB, Kista, Sweden) for 10 min. Sections Liver lesions of rabbit 283 were primarily in peripor- were then counterstained with Mayer’s haematoxylin for tal areas. Mild to moderate numbers of single or small 5 min. For the immunoglobulin negative control, a dupli- groups of periportal hepatocytes, including occasional cate section was incubated with a non-specific antibody cells of the limiting plate, were degenerate or dead and (Negative Control Mouse IgG1, Agilent Technologies usually were surrounded and infiltrated by moderate to Sweden AB, Kista, Sweden) with the same isotype as the large numbers of heterophils (Figure  1). Occasional het- primary antibody, diluted to the same protein concentra- erophils also infiltrated portal areas and were seen in tion as the primary antibody. A positive tissue control sinusoids. Affected hepatocytes had hypereosinophilic (liver from a wild rabbit in Sweden confirmed positive for cytoplasm, typically accompanied by a karyorrhectic or L. europaeus GI.2 by RT-qPCR, sequencing and immu- pyknotic nucleus. In advanced stages, cells or cell frag- nological characterization [8] and a negative tissue con- ments had rounded up, were diminished in size, often trol (liver from a healthy, adult rabbit from the same contained pyknotic nuclear fragments, and had detached Australian colony) were included on each slide to ensure from hepatic cords (consistent with apoptotic cells and immunolabelling was working for each slide and consist- apopotic bodies, Figure  1). Less frequently, affected ent among slides. hypereosinophilic hepatocytes retained their shape and nuclei were faded (karyolysis). In other areas, heterophils Results were seen amongst fragmented remnants of hepatocytes, Animals/clinical signs and where heterophil aggregates were more extensive, All inoculated rabbits developed a fever (tempera- hepatocytes were effaced, resulting in focal lytic necro - ture > 40.4  °C) between 30 and 52  hpi. Rabbits 283 sis and sometimes haemorrhage. Occasional hepatocytes (an adult), 303 and 306 (both kittens) showed signs of in midzonal regions were also affected but hepatocytes Neimanis et al. Vet Res (2018) 49:46 Page 5 of 15 Figure 1 Microscopic lesions in the liver of rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Asterisks denote portal areas. Haematoxylin and eosin stain. A Uninfected control rabbit (rabbit 130). 200× magnification. B Earlier stage of acute infection in an adult rabbit (rabbit 283). There is degeneration and death of hepatocytes at the limiting plate and periportally surrounded by intensive heterophilic inflammation. Arrows denote apoptotic cells. 200× magnification. Insert: high magnification of dead hepatocytes surrounded by heterophils. C Terminal stage of acute infection in a 5 week old rabbit (rabbit 303). There is massive hepatocellular degeneration and death. 200× magnification. D Terminal stage of acute infection in a 5 week old rabbit (rabbit 306). Arrowheads denote clusters of heterophils that are seen throughout the lobule and the arrow indicates an area of lytic necrosis. 400× magnification. Insert: high magnification of heterophils surrounding dead hepatocytes. around central veins were almost entirely spared. Fine In the liver of the other three rabbits, hepatocytes cytoplasmic vacuolation and mild to moderate small, showed the same changes indicative of degenera- distinct, round intracytoplasmic vacuoles were seen pre- tion and cell death as described for 283, but the extent dominantly in midzonal hepatocytes and occasionally, an and degree was much more severe (Figure  1). Although aggregate of hyaline, eosinophilic material was seen adja- there was a tendency for periportal hepatocytes to be cent to the nucleus. Kupffer cells were mildly enlarged targeted, including hepatocytes of the limiting plate, and contained a larger nucleus with an open chromatin affected hepatocytes could be seen throughout the lob - pattern, resulting in these cells being more prominent ule. The extent was so severe in the two kittens that cell than in the control animal. Bile ducts and vessels were death was classified as massive (Figure  1). Karyorrhexis unremarkable and no thrombi were seen. Similar to the was still common, but nuclei undergoing karyolysis were control rabbit, portal areas also displayed mild to mod- much more frequent than in rabbit 283. Round, hypere- erate bile duct hyperplasia, infiltration of scattered lym - osinophilic hepatocellular remnants with or without pyk- phocytes and plasma cells and mild, early fibrosis. notic nuclear fragments were occasionally to frequently Neimanis et al. Vet Res (2018) 49:46 Page 6 of 15 observed and individual dead cells that had lost cel- and lymphoid depletion. Again, whereas lesions were lular detail or were swollen and vacuolated instead of only mild to moderate in 283, they were similar or more shrunken were also seen (more consistent with single cell severe in 282 and 303. No other lymphoid tissues were necrosis as described in Elmore et  al. [30]). While occa- available for examination from animal 306. The thymus sional (303) to moderate numbers of heterophils (282 of rabbit 282 exhibited the most severe changes with and 306) were present, they were dispersed throughout marked, extensive lymphocytolysis and abundant tingi- sinusoids and infiltrated hepatic cords rather than being ble body macrophages containing apoptotic cellular and intensively aggregated periportally (Figure  1). Occasion- nuclear debris. ally, they clearly surrounded dead hepatocytes. Small, Pulmonary lesions were only seen in two of the four scattered foci of lytic necrosis and haemorrhage also were rabbits. In rabbit 282, moderate pulmonary congestion seen. Hepatocytes throughout the lobule often contained and oedema were accompanied by mildly increased num- distinct, round intracytoplasmic vacuoles consistent with bers of plump intra-alveolar macrophages. In the lungs lipid. Kupffer cells were plump, prominent and slightly to of the kitten that died spontaneously (303), infrequent moderately more numerous compared to the control ani- fibrin thrombi were seen in small vessels and alveolar mal. In one kitten, fibrin thrombi were very occasionally capillaries. Scattered to occasional pyknotic cells with seen in portal vessels, but in general, with the exception hypereosinophilic cytoplasm similar to those observed in of occasional plump, activated endothelium, vessels were the spleen were seen in alveolar septae of 282, 303 and unremarkable. Like the control animal and 283, there was 306. Trachea was only available from the two adult ani- mild (kittens) to moderate (282) bile duct hyperplasia, mals and moderate submucosal congestion of the trachea mild, early fibrosis and infiltration of scattered lympho - was seen in both. cytes and plasma cells in portal areas. Biliary epithelium Renal lesions were confined to the kittens. Glomerular and bile ducts showed no evidence of necrosis. tufts were severely congested, occasionally accompanied The spleen of all animals was moderately to severely by haemorrhage, and frequent fibrin thrombi were seen congested and abundant, activated macrophages (mac- both in glomerular capillaries and adjacent small vessels. rophage hyperplasia) displaying active phagocytosis were In scattered tubules, epithelium was either attenuated seen throughout the red pulp (Figure  2). In rabbits that or plump and regenerative, indicative of acute tubular died later (282, 303, 306), the stroma of the red pulp was nephrosis. hyaline, there was loss of cellular detail and numerous With the exception of rare thrombi seen in small ves- unidentifiable pyknotic cells were seen (Figure  2). Aggre- sels of the myocardium of rabbit 303, no other significant gates of amorphous eosinophilic material consistent with lesions were seen in the other tissues examined for all fibrin was seen scattered in the red pulp of rabbit 303. In animals. the white pulp, there was moderate to severe decrease in the proportion of lymphocytes to macrophages reflect - Immunohistochemistry ing general lymphoid depletion in comparison with the Viral antigen was visualized in the liver of all four rab- control animal. Active, mild to moderate lymphocytoly- bits and for animal 283, the liver was the only organ sis was observed, and within the kittens, frequent tingi- in which viral antigen was detected. In general, viral ble body macrophages were seen within the white pulp antigen was predominantly found within normal and (Figure 2). degenerate hepatocytes, where staining typically was Bone marrow was only available for examination from finely stippled and could be seen throughout the cyto - the adult rabbits and control animal. Similar to liver and plasm and/or concentrated around the periphery at the spleen, changes in the bone marrow were more marked cell membrane (Figure  4). Infrequently, intracytoplas- in the rabbit that was euthanized later following inocu- mic staining was confined to a focal area adjacent to the lation (282) (Figure  3). The M:E decreased and the pro - nucleus. Intranuclear staining, although it occurred, portion of immature myeloid cells increased (Table  3). was rare (Figure  4). Rare to occasional Kupffer cells In addition to the marked decrease in the proportion of also contained viral antigen. Within Kupffer cells, stain - myeloid to erythroid cells in rabbit 282, binucleated and ing was always intracytoplasmic and coarsely clumped. even multinucleated rubricytes that typically are indica- Additionally, coarsely clumped antigen often could tive of disrupted erythropoiesis were commonly seen be seen associated with hepatocellular remnants and (Figure 3). inflammatory cell infiltrates, but exact localization Within other lymphoid tissues (Peyer’s patches, sac- could not be readily discerned. In animals 282, 303 culus rotundus, cecal appendix, mesenteric lymph node, and 306, rare to infrequent intravascular leukocytes thymus), infected animals exhibited variable degrees of morphologically consistent with monocytes or mac- lymphocytolysis, presence of tingible body macrophages rophages showed coarsely clumped intracytoplasmic Neimanis et al. Vet Res (2018) 49:46 Page 7 of 15 Figure 2 Microscopic lesions and viral antigen localization in the spleen of a 5 week old rabbit (Oryctolagus cuniculus) (rabbit 306) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Asterisks denote the white pulp. A Uninfected control (rabbit 130). 200× magnification. B Terminal stage of acute infection. There is lymphoid depletion and tingible body macrophages (arrows) in the white pulp and macrophage hyperplasia in the adjacent red pulp. 200× magnification. C Red pulp of the uninfected control (rabbit 130). 400× magnification. D Red pulp of rabbit 306. Note the indistinct, hyaline stroma and presence of small, dark pyknotic cells (arrowheads). 400× magnification. E Rabbit 306. Active phagocytosis by macrophages in the red pulp (arrows) and numerous pyknotic cells (arrowheads). The macrophage indicated at the bottom of the image has phagocytized a pyknotic cell. 400× magnification. F Immunohistochemical visualization of viral capsid antigen (brown staining) in the cytoplasm of viable macrophages (arrows) in the terminal stage of acute infection. Note that no staining is associated with pyknotic cells. 400× magnification. Neimanis et al. Vet Res (2018) 49:46 Page 8 of 15 Figure 3 Microscopic lesions and viral antigen localization in the bone marrow of adult rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Arrows denote myeloid cells and arrowheads denote erythroid cells. A Uninfected control (rabbit 130). 600× magnification. B Bone marrow in an earlier stage of acute infection (rabbit 283). Note the decreasing proportion of myeloid to erythroid cells. 600× magnification. C Bone marrow at the terminal stage of acute infection (rabbit 282). Myeloid cells are scarce and erythroid cells at arrowheads are binucleated. 600× magnification. D Immunohistochemical visualization of viral capsid antigen (brown staining) in the cytoplasm of viable macrophages (arrows) in the terminal stage of acute infection (rabbit 282). 400× magnification. Table 3 Estimates of the myeloid-to-erythroid ratio and relative proportions of early stage myeloid cells versus later stage myeloid cells in the bone marrow of adult rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus GI.2 (BlMt-1) Rabbit Myeloid-to-erythroid ratio Proportion of early (myeloblasts, promyelocytes and myelocytes) versus later stage myeloid cells 130 (control) 2.08:1 16% versus 84% 283 (euthanized at 54 hpi) 0.99:1 24% versus 76% 282 (euthanized at 96 hpi) 0.47:1 55% versus 45% staining. Biliary epithelium and vascular endothelium was confined primarily to hepatocytes and hepatocellu - did not contain detectable viral antigen. lar remnants admixed with heterophil infiltration within Within the liver, the amount of viral antigen observed periportal areas (Figure  4). The other three rabbits con - and its distribution varied between rabbits. Rabbit 283 tained markedly more viral antigen, but whereas it was contained the least amount of viral antigen and staining primarily located within hepatocytes in the form of finely Neimanis et al. Vet Res (2018) 49:46 Page 9 of 15 Figure 4 Viral antigen localization in the liver of rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Asterisks denote portal areas. A Uninfected control (rabbit 130). No antigen (brown staining) is seen. 200× magnification. B Earlier stage of acute infection in an adult rabbit (rabbit 283). A small number of periportal hepatocytes contain viral antigen (brown staining, arrow). A cluster of heterophils surrounds necrotic, antigen‑ containing debris interpreted to be the remnants of an infected hepatocyte (arrowhead). 200× magnification. C Terminal stage of acute infection in an adult rabbit (rabbit 282). There is cytoplasmic staining in hepatocytes throughout the lobule (arrows). 200× magnification. D Terminal stage of acute infection in a 5 week old rabbit (rabbit 303). In addition to the finely stippled intracytoplasmic staining of infected hepatocytes, there is occasional intranuclear staining (arrow, arrowhead) and staining of the cell membrane (arrowhead). 400× magnification. stippled stain throughout the lobule in rabbits 282 and round, pyknotic bodies similar in appearance to the 303, in rabbit 306, antigen was more typically coarsely unidentified pyknotic cells in the red pulp. Otherwise, clumped and distributed throughout the lobule within the majority of pyknotic cells were not associated with hepatocytes, Kupffer cells and within inflammatory and viral antigen (Figure 2). No antigen was associated with necrotic debris. lymphocytolysis or observed within tingible body mac- Within the spleen of rabbits 282, 303 and 306, rophages of the white pulp. No antigen was observed in coarsely clumped viral antigen was seen within the the spleen of rabbit 283. cytoplasm of frequent (282) to abundant (303, 306) Similar to the spleen, occasional to more frequent macrophages (Figure  2). Antigen-containing mac- macrophages in the bone marrow of rabbit 282 con- rophages were numerous and conspicuous at the mar- tained coarsely clumped, intracytoplasmic viral antigen ginal zone of the white pulp and frequently within (Figure  3). Antigen-containing macrophages showed no sinus histiocytes of the red pulp (Figure  2). They also evidence of degeneration or cell death. Megakaryocytes occasionally contained intracytoplasmic, basophilic, and granulocytic and erythroid cell lines were devoid of Neimanis et al. Vet Res (2018) 49:46 Page 10 of 15 antigen. No viral antigen was seen in the bone marrow of were depressed, hypothermic and/or anorexic prior to rabbit 283 and bone marrow was unavailable for exami- euthanasia or death. This is compatible with the peracute nation in the kittens. and acute forms of RHD observed in GI.1 infected rab- A small amount of viral antigen was detected within bits [31, 32]. Although rabbit 282 was behaving normally the lungs of rabbits 282, 303 and 306. Staining primar- up to 96  hpi, there were widespread liver lesions, viral ily was confined to the alveolar septae, both clearly cell- loads in the serum and liver were very high and body associated as clumped intracytoplasmic material within weight had dropped more than 10%. It is not possible to cells morphologically consistent with macrophages, or predict whether this animal ultimately would have sur- within septal capillaries, often associated with material vived or succumbed to subacute RHD, but this variation consistent with fibrin. Scattered leukocytes with large, in disease progression and mortality rate among animals open nuclei and abundant cytoplasm (probable mono- is consistent with that seen in classic RHD caused by cytes or macrophages) within larger vessels also con- GI.1 and differs from the longer disease course and lower tained clumped intracytoplasmic antigen. No antigen case fatality rates described for early strains of GI.2 [2]. was detected within alveolar macrophages nor was anti- Although the number of animals used in this study was gen observed associated with pyknotic cells seen within small, our findings strongly suggest that BlMt-1 is much alveolar septae. more pathogenic than earlier reported strains of GI.2 that No viral antigen was detected in the kidneys of the two were detected in Europe [2]. This is in line with findings adults 283 and 282. In contrast, both kittens had a small by Capucci et al. [21] who carried out experimental infec- amount of detectable antigen which was seen associated tions of a small number of domestic rabbits with more with fibrin thrombi within glomeruli and small adjacent recent European GI.2 isolates, and similarly report very vessels. Rare leukocytes within larger vessels contained high case fatality rates. The European virus most closely intracytoplasmic antigen. Antigen was not detected in related to BlMt-1, a recombinant GI.1bP–GI.2 from tubular epithelium. 2014, has reportedly replaced previously circulating GI.1 Viral antigen was not detected in any other tissues, strains [12] and has led to substantial declines in wild including those that displayed microscopic lesions. No rabbit numbers in the Iberian Peninsula [33], indicative antigen was seen associated with lymphocytes, lympho- of a highly virulent virus. Capucci et al. [21] hypothesized cytolysis, tinigible body macrophages and/or lymphoid that GI.2 strains have evolved in their natural hosts since depletion in lymphoid tissues. Likewise, no staining was their emergence in 2010, and similar to what was pro- seen within the heart of 303 or the congested trachea of posed for GI.1 strains, selection pressure favours strains 282 and 283. Viral antigen was never detected in any tis- with higher pathogenicity [34]. Our data from this more sues of the control animal 130. recent isolate (BlMt-1) is consistent with this hypothesis. Consistent with all previous studies on RHD, the liver is the target organ of BlMt-1. Based on time of euthana- Molecular analyses sia after inoculation, lower viral tissues loads (Table  4) Viral RNA was detected by RT-qPCR in all tissues ana- and data from previous studies on GI.1 viruses (e.g. [35, lysed and viral loads (capsid gene copies per mg tissue) 36]), rabbit 283 represents an earlier stage of disease are presented in Table  4. For the majority of tissues ana- than rabbits 282, 303 and 306. By comparing results from lysed, Rabbit 283, the animal euthanized earliest after these animals, disease progression mimics that of other inoculation, had the lowest viral load, often by an order pathogenic lagoviruses. Infection in the liver began with of magnitude or more. Serum levels of virus in this rab- degeneration and death of periportal hepatocytes at and bit were 3–4 orders of magnitude lower than in other adjacent to the limiting plate and incited a marked het- animals. Highest viral loads were detected in the serum erophilic inflammatory response. As disease progressed, (except for animal 283) and liver, followed by the spleen. affected hepatocytes became more numerous and wide - spread throughout the lobule, although centrilobular Discussion hepatocytes still tended to be spared, reflecting disease In this study, clinical signs, disease and case fatality rate advancement from the periphery of lobules centrally. caused by the Australian strain of L. europaeus GI.2 Occasional to moderate numbers of heterophils were closely mimicked that of classic RHD in adult rabbits still present in the liver and often associated with dead caused by L. europaeus GI.1 strains [1]. Death or severe hepatocytes, but they were distributed throughout the disease necessitating euthanasia occurred within 96  hpi lobule rather than confined to periportal regions. These for all animals, with an average time to death or euthana- lesions were coupled with activation and hyperplasia of sia of 73 hpi. All animals developed a fever within 52 hpi Kupffer cells. The presence of bile duct hyperplasia, mild and, with the exception of 282, either died suddenly or mononuclear inflammation and mild fibrosis portally in Neimanis et al. Vet Res (2018) 49:46 Page 11 of 15 Table 4 Viral copies of Lagovirus europaeus GI.2 (BlMt-1) per milligram tissue calculated from RT-qPCR analyses Rabbit Adults 5 week old kittens 282 283 302 303 304 305 306 Liver (hilar) 2.42E+08 1.64E+08 n.c. n.c. n.c. n.c. n.c. Liver (distal) 3.03E+08 1.33E+08 n.c. n.c. n.c. n.c. n.c. Liver (not specified) n.c. n.c. 4.67E+08 6.16E+08 3.56E+08 9.01E+08 5.07E+08 Bile 1.46E+06 3.26E+06 n.c. n.c. n.c. n.c. n.c. Spleen 1.17E+08 2.36E+07 1.91E+08 1.02E+08 4.08E+07 8.55E+07 3.23E+08 Lung 7.42E+07 2.38E+06 1.30E+08 4.39E+07 9.94E+07 2.00E+08 2.77E+07 Heart 3.27E+06 1.02E+06 2.22E+07 2.16E+07 7.52E+06 n.c. 1.43E+07 Kidney 3.22E+06 6.76E+05 1.30E+07 3.91E+07 6.16E+06 1.20E+07 1.75E+07 Thymus 1.29E+06 1.15E+05 n.c. n.c. n.c. n.c. n.c. Mesenteric lymph node n.c. 4.24E+06 3.00E+06 n.c. 1.84E+06 n.c. 6.04E+06 Duodenum 5.80E+05 1.27E+06 3.23E+06 2.34E+06 2.61E+06 2.23E+06 1.47E+06 Jejunum 1.75E+06 3.64E+06 4.07E+06 3.70E+06 1.83E+06 5.15E+06 2.73E+06 Ileum 2.84E+06 3.96E+03 1.50E+06 1.26E+06 1.78E+06 1.87E+06 2.20E+06 Peyers patches 8.38E+05 1.25E+06 3.51E+06 1.40E+06 1.66E+06 7.12E+05 3.53E+06 Appendix 5.35E+05 1.12E+04 9.95E+05 6.78E+05 7.63E+05 8.16E+05 7.23E+05 Sacculus rotundus 4.74E+05 7.18E+04 n.c. n.c. n.c. n.c. n.c. Cecum 3.63E+05 4.98E+04 n.c. n.c. n.c. n.c. n.c. Colon (+contents) 1.50E+06 n.c. 1.34E+06 2.95E+05 5.24E+05 1.21E+06 3.12E+05 Trachea 2.27E+07 n.c. n.c. n.c. n.c. n.c. n.c. Bone marrow from femur 1.58E+07 3.85E+05 n.c. n.c. n.c. n.c. n.c. Brain 3.95E+06 2.55E+04 n.c. n.c. n.c. n.c. n.c. Buffy coat 5.35E+06 2.13E+04 n.c. n.c. n.c. n.c. n.c. Serum 1.29E+08 2.21E+05 2.42E+09 3.87E+08 2.43E+08 n.c. 7.42E+08 The analysis was done in technical duplicate per tissue and values represent the mean. n.c.: sample not collected. all rabbits, including the control, was interpreted to be in the liver in both BlMt-1 inoculated adults and kittens unrelated to RHD. Rather, this has been described as an was similar and heterophilic. This is not the case for GI.1, incidental finding in normal rabbits [37] or may reflect where adults display heterophilic inflammation through - previous, unrelated insult. To conclude, BlMt-1 induces out the disease course, but kittens develop lymphocytic the same lesions in both adults and kittens as reported inflammation by 48  hpi [41]. Mikami et  al. [42] also for GI.1 viruses in adults. showed that inflammation, predominantly lymphocytes In contrast to GI.1 strains, both Le Gall-Reculé et  al. admixed with macrophages and some heterophils contin- [2] and Dalton et  al. [13] report mortality rates of up ued to increase up to 96  hpi. Ferreira et  al. [41] further to 50% for kittens that were involved in RHD mortality hypothesize that these lymphocytes seen in infected kit- events caused by GI.2. Although sample size in this study tens respond to surface presentation of lagovirus antigen is small, all five inoculated kittens succumbed to disease. on hepatocytes and that this is important in modulating Four died and the fifth was euthanized. The fifth kitten resistance to development of clinical disease. Lympho- had severe, extensive liver lesions deemed incompatible cytic inflammation was not a feature in kittens succumb - with survival, supporting a case fatality rate of 5/5 kit- ing to BlMt-1. This suggests that GI.2 viruses may have tens for BlMt-1 in this study. Disease course was acute, the ability to replicate within hepatocytes without invok- with all kittens dying or euthanized by 90 hpi. This differs ing this postulated protective lymphocytic inflammatory from GI.1 viruses where infected kittens of this age do response. Likewise, the other two proposed mechanisms not develop clinical disease, although the reasons are not of resistance to GI.1 strains in kittens apparently were fully understood. Viral receptor expression, innate immu- overcome and investigation into underlying mechanisms nity and changes in liver function around 5–6  weeks of would provide critical insight into pathogenicity determi- age have all been suggested to play a role [38–40]. One nants in lagoviruses. very clear finding of this study was that inflammation Neimanis et al. Vet Res (2018) 49:46 Page 12 of 15 Apoptotic programmed cell death is a common feature consumption of mature heterophils in the liver. While in viral hepatitides (e.g. [43, 44]) and has been shown to cytotoxic effects of the virus on circulating heterophils occur in RHD caused by GI.1 strains [45–47]. Hepatic or myeloid precursors cannot be excluded, the absence lesions in this study showed morphologic features of of viral antigen within circulating heterophils or granu- both apoptosis and necrosis. Earlier in infection (rab- locytic precursors in the bone marrow does not support bit 283), dead hepatocytes commonly display features direct viral cytotoxicity. M:E is used to detect variation in of apoptosis: shrinkage, hypereosinophilia, karyor- rates of differentiation of myeloid and erythroid precur - rhexis and pyknotic nuclear remnants, and rounding up sors. Although M:E for the control animal 130 was higher and dissociation of cells from the basement membrane than the reported mean of approximately 1.1:1 for labo- [30]. However, the intense, accompanying heterophilic ratory rabbits [54], the comparisons presented here are inflammation is not a feature typically associated with relative and are used as a tool to compare animals within apoptosis. Likewise, there also are foci of lytic necrosis this rabbit colony. The frequent bi- and multinucleate in areas of the inflammation, and in later stages of infec - rubricytes may represent erythroid dysplasia or dyseryth- tion, single cells with morphology more consistent with ropoiesis. While acquired dyserythropoiesis most often single cell necrosis (e.g. increased cell size, vacuolation, is associated with nutritional deficiencies [55], chemo - loss of cellular detail, karyolysis) are seen. Overlap of therapeutic drugs and myeloid neoplasms [56], dysplastic apoptosis and necrosis within the same disease process changes have also been reported in certain viral dis- is well-recognized and may occur, for example, when eases such as those caused by Feline immunodeficiency phagocytic capacity to clear apoptotic remnants is over- virus [57] and Feline Leukemia Virus [58]. Bone marrow whelmed [44]. Additionally, our current understanding of evaluation using formalin-fixed material has limitations programmed cell death is rapidly expanding and a host regarding more detailed discrimination of cells. There - of newly recognized mechanisms of programmed cell fore, further investigation, including concurrent evalua- death involving inflammation (e.g. necroptosis, NETo - tion of haematology and bone marrow cytology in rabbits sis, pyroptosis) have been described and can play critical with RHD, is required to better define the myeloid hypo - roles in antiviral immunity, particularly when viral mech- plasia and left shift, and to confirm and understand the anisms block apoptosis [48–50]. RHD has already been underlying mechanisms of the apparent erythroid dyspla- proposed as a model for acute fulminant liver disease in sia in RHD. In contrast to findings by Dalton et  al. [5], humans [51] and continued investigations into the cell no lesions were seen in the villi of the small intestine of death mechanisms in RHD may provide insight into the kittens, nor were they seen in the adults. pathogenesis of a wider range of liver diseases in different Using immunohistochemistry, presence of viral anti- species. gen was clearly associated with hepatic lesions and In extrahepatic tissues, lesions included thrombi and degenerate and dead hepatocytes and antigen spread haemorrhage typical of disseminated intravascular coag- from periportal areas towards the central vein as dis- ulation (DIC) [52], splenic necrosis, lymphocytolysis and ease progressed. Location of virus within hepatocytes lymphoid depletion. Within the bone marrow, there was was primarily intracytoplasmic and finely stippled, decreased M:E, increased proportion of immature mye- although rimming of the cell membrane, as described by loid cells and presence of atypical rubricytes. Both DIC Park and Itakura [59] for GI.1, was also seen (Figure  4). and lymphoid depletion have been described in GI.1 These authors suggest that the different staining patterns infections [1, 31, 36], but bone marrow changes have reflect different stages in infection as is observed in other not yet been described for RHD and provided additional viruses. Lagoviruses are thought to replicate within the insight into pathogenesis. In the adult rabbit that survived cytoplasm [1], but consistent with other reports [42, 59, longer (282), the proportion of heterophils and myeloid 60], infrequent antigen was also seen within the nucleus precursors decreased and of the myeloid cells remaining, (Figure 4). The presence of viral capsid protein within the the proportion of mature stages decreased. This left shift nucleus remains unexplained. However, as hypothesized in the bone marrow is consistent with the heteropenia by Ramiro-Ibanez et  al. [61], because so little is known described in RHD by Ferreira et  al. [53]. These authors about lagovirus replication, localization of the viral cap- proposed three mechanisms to account for the heterope- sid within the nucleus may be required for some stage in nia: sequestration of mature heterophils, cytotoxic effects the virus life cycle. of the virus on circulating heterophils and cytotoxic In addition to hepatocytes, viral antigen was also effects on granulocytic myeloid precursors in the bone detected within cytoplasm of cells of the mononuclear marrow. The moderate to marked heterophilic inflam - phagocytic system. These include Kupffer cells in the mation seen in the liver, particularly earlier in disease, liver, macrophages within the spleen, bone marrow favours the first hypothesis regarding sequestration and and lungs, as well as cells consistent with intravascular Neimanis et al. Vet Res (2018) 49:46 Page 13 of 15 monocytes or even tissue macrophages that have re- the bile, respectively. Additionally, the high viral load entered circulation in various tissues. While functions of in bone marrow may contribute to the epidemiology these cells are diverse, they are phagocytic and can play a of RHD in wild rabbit populations by prolonging envi- role in both innate and adaptive immunity [62]. Within ronmental persistence of virus when all other tissues these cells, staining was dark and coarsely clumped simi- except for the skeleton are gone, provided the virus lar to hepatocyte remnants in areas of lytic necrosis in material in bone marrow remains infectious. the liver, rather than finely stippled as seen within hepat - To conclude, unlike early GI.2 viruses [2], the path- ocytes. Additionally, whereas hepatocytes showed cyto- ogenicity of the 2015 BlMt-1 strain used in this study pathic effects from the virus, the phagocytic cells showed is comparable to that of GI.1 strains in adult rabbits. no signs of degeneration or death in this study. In these Additionally, and in contrast to GI.1, it is as pathogenic macrophages, antigen staining patterns may therefore for kittens as adults. The inflammatory response in the be more compatible with phagocytized material rather liver of kittens inoculated with BlMt-1 differs from than replicating virus. However, Kimura et  al. [63] show that reported from kittens infected with GI.1 strains, evidence for viral replication within macrophages using providing support for the proposed role of lympho- in  situ hybridization and further investigation into the cytes in modulating resistance to clinical disease from role of macrophages in RHD, including spread from the GI.1 infection. The strong susceptibility of kittens to site of entry to the liver, is needed. disease by the BlMt-1 virus adds a new dimension to Within lymphoid tissues, viral antigen was not pre- the epidemiology of RHD in wild rabbit populations sent in lymphocytes or associated with lymphocytolysis in Australia and will likely have implications for rab- or lymphoid depletion. This suggests that lymphocytoly - bit biocontrol in this country. A recent study reports sis is the result of a secondary mechanism, for example that the GI.1bP–GI.2 virus (to which BlMt-1 belongs) increased glucocorticoids associated with stress as is seen has replaced endemic GI.1 strains within 18  months in lymphoid tissues, especially the thymus [64]. of its arrival in Australia [10]. Mahar et al. [10] further Assessment of immunohistochemical staining of hypothesize that the ability of this virus to infect and inoculated animals in conjunction with both the nega- kill rabbits at a much younger age provides a key com- tive tissue and immunoglobulin controls allowed us to petitive advantage in the field, as new cohorts of rabbits conclude that the immunohistochemistry performed become susceptible to lethal infection at a much ear- on this material was very specific and that non-specific lier age compared to GI.1. Indeed, for 17 of the viruses staining and background staining were absent. How- sequenced in their study, the body mass of the deceased ever, the analysis required a large amount of virus to be rabbits was known, and eight of these isolates (47%) present before antigen could be detected in  situ. Anti- were from kittens weighing less than 600  g, corre- gen was readily detected in tissues with 10 viral copies sponding to 4–8 weeks of age. The differences in patho - per mg and in just over half of the tissues containing genicity between GI.2 strains, coupled with differences 7 6 10 viral copies per mg tissue. However, 10 viral cop- in susceptible age groups and host species when com- ies per mg tissue was below the detection limit for the pared to GI.1 viruses, provide an excellent opportunity immunohistochemical analysis of this material. We are to explore the genetic and immunological determinants confident that staining, when present, corresponds to of pathogenicity in lagoviruses. viral antigen, but we cannot exclude the possibility that tissues or cells containing lower amounts of virus were Abbreviations not detected with immunohistochemistry. While PCR RHDV: rabbit haemorrhagic disease virus; RHD: rabbit haemorrhagic disease; is more sensitive, it does not allow for specific in  situ hpi: hours post‑inoculation; M:E: myeloid‑to ‑ erythroid ratio; RT‑ qPCR: reverse transcriptase quantitative polymerase chain reaction. localization of virus within tissues nor can it be cor- related with microscopic lesions. Using immunohis- Competing interests tochemistry, intracellular virus presence was seen in The authors declare that they have no competing interests. liver and in cells of the mononuclear phagocytic system Authors’ contributions in the spleen, bone marrow, lungs and intravascular AN, DGW and TS conceived the study, all authors were involved in data monocytes. However, virus was also detected in high acquisition and analyses, AN wrote the paper, ULP, NH, DGW and TS revised the paper. All authors read and approved the final manuscript. levels in the serum and in reasonable amounts (10 copies per mg) in numerous other tissues, including Acknowledgements trachea and small intestine. The lack of lesions in these The authors thank Harold Tvedten and Helena Pettersson for assistance with interpretation of the haematopoietic tissues and Harold Tvedten for tissues suggests PCR is likely detecting viral RNA in Figures 3A–C. The authors also thank Peter Kerr and Jemma Bergfeld for their free viral particles in the blood in these tissues, as well helpful comments on the manuscript. as in the lumen of the intestine from virus excreted via Neimanis et al. Vet Res (2018) 49:46 Page 14 of 15 Ethics approval and consent to participate 11. Duarte M, Carvalho C, Bernardo S, Barros SV, Benevides S, Flor L, Monteiro All procedures involving animals were carried out in accordance with the Aus‑ M, Marques I, Henriques M, Banos SC, Fagulha T, Ramos F, Luis T, Fevereiro tralian Code of Practice for the Care and Use of Animals for Scientific Purposes M (2015) Rabbit haemorrhagic disease virus 2 (RHDV2) outbreak in (2013) and were approved by the CSIRO Ecosystem Sciences Animal Ethics Azores: disclosure of common genetic markers and phylogenetic segre‑ Committee (permits ESAEC 13‑10, DOMRAB). gation within the European strains. Infect Genet Evol 35:163–171 12. Lopes AM, Correia J, Abrantes J, Melo P, Ramada M, Magalhaes MJ, Alves Funding PC, Esteves PJ (2015) Is the new variant RHDV replacing genogroup 1 in Funding for the work carried out in Sweden was provided by the Swed‑ Portuguese wild rabbit populations? Viruses 7:27–36 ish Environmental Protection Agency, and the Research Council FORMAS 13. 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Elucidation of the pathology and tissue distribution of Lagovirus europaeus GI.2/RHDV2 (rabbit haemorrhagic disease virus 2) in young and adult rabbits (Oryctolagus cuniculus)

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Copyright © 2018 by The Author(s)
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Medicine & Public Health; Veterinary Medicine/Veterinary Science; Virology; Microbiology; Immunology
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Abstract

Lagovirus europaeus GI.2, also known as RHDV2 or RHDVb, is an emerging virus that causes rabbit haemorrhagic dis‑ ease (RHD) in European rabbits (Oryctolagus cuniculus). In contrast to L. europaeus GI.1 (or RHDV/RHDVa) viruses that are only pathogenic for adults, GI.2 causes clinical disease in both adults and kittens. However, detailed descriptions of the pathology of this virus that may provide insight into its pathogenicity and emergence are lacking. Using an Australian GI.2 field strain isolated in 2015, we provide the first detailed description of pathology, viral antigen distribu ‑ tion and tissue load of GI.2 in adult and 5‑ week old New Zealand white rabbits using histology, immunohistochem‑ istry and RT‑ qPCR. Liver was the target organ, but in contrast to GI.1 viruses, lesions and inflammatory responses did not differ between adults and kittens. Lymphocytic inflammation, proposed to be protective in kittens infected with GI.1, was notably absent. We also present the first descriptions of bone marrow changes in RHD, including decreased myeloid‑ to‑ erythroid ratio. Consistent with other pathogenic lagoviruses, intracellular viral antigen was demonstrated in hepatocytes and cells of the mononuclear phagocytic system. In terminal stages of disease, viral loads were highest in liver, serum and spleen. Despite the small sample size, our data suggest that unlike early European GI.2 strains, the pathogenicity of the Australian GI.2 virus is similar to GI.1 viruses. Additionally, GI.2 was fatal for all (n = 5) inoculated kittens in this study. This may significantly alter RHD epidemiology in the field, and may impact biocontrol programs for invasive rabbits in Australia where GI.1 viruses are intentionally released. Introduction of RHD are all grouped within genogroup I (L. europaeus GI) (Table 1). Rabbit haemorrhagic disease (RHD) is a viral disease of First described in domestic rabbits in China in 1984, the European rabbit (Oryctolagus cuniculus) that primar- RHD was detected in Europe in 1986 and has been doc ily affects the liver [ 1]. The causative agents, currently - known as rabbit haemorrhagic disease virus (RHDV), the umented at some point on every continent except Ant- antigenic variant (RHDVa), and the recently described arctica [1]. In Australia and New Zealand, RHD is used RHDV2 or RHDVb, are lagoviruses within the Family intentionally for biocontrol of invasive, introduced Euro- Caliciviridae [1, 2]. Within a proposed new classification pean rabbits. Until 2010, the disease was almost exclu- system [3], all lagoviruses are reclassified into a single sively confined to wild and domestic European rabbits species, Lagovirus europaeus, and the causative viruses and was caused by viruses newly classified as genotype 1 of the L. europaeus GI group (i.e. L. europaeus GI.1 [3]), also known as RHDV and RHDVa (Table 1). L. europaeus GI.1 is further subdivided into variants denoted by lower case letters [3] (Table 1). *Correspondence: aleksija.neimanis@sva.se Department of Pathology and Wildlife Diseases, National Veterinary In 2010, a new genotype (L. europaeus GI.2 [3], also Institute (SVA), 751 89 Uppsala, Sweden known as RHDV2 or RHDVb, Table  1) was detected in Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/ publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Neimanis et al. Vet Res (2018) 49:46 Page 2 of 15 Table 1 A list of current names and proposed new, this study, rabbits were infected with a L. europaeus GI.2 phylogenetically derived names for lagoviruses according strain detected in Australia that is also closely related to to Le Pendu et al. [3] referred to in this manuscript circulating strains in southern Europe [22] and high qual- ity material from both adults and kittens was collected Current name Proposed new name [3] to describe the pathology and tissue distribution of this Rabbit haemorrhagic disease virus Lagovirus europaeus GI.1 virus in detail. We compare findings between the two age (RHDV ) groups and with previously published findings for GI.1 RHDVa Lagovirus europaeus GI.1a strains to provide further insight into pathogenesis. We RHDV G1 Lagovirus europaeus GI.1b also interpret our findings in conjunction with parallel RHDV2 or RHDVb Lagovirus europaeus GI.2 quantitative molecular analyses to evaluate virus localiza- Recombinant of RHDV G1 and RHDV2, Lagovirus europaeus GI.1bP–GI.2 tion and detection limits of our immunohistochemistry recombination break point between the polymerase and capsid coding analyses. regions Materials and methods The virus used in this study was the first L. euro - domestic and wild European rabbits in France [4]. It soon paeus GI.2 virus reported in Australia [9] (GenBank# was reported throughout Europe [2, 5–8] and reached KT280060). From here on this virus is referred to as Australia in 2015 [9], where it rapidly spread and appears BlMt-1, the name of this isolate. BlMt-1 is closely related to be replacing previously endemic GI.1 strains [10] to isolates described in the Iberian Peninsula [22] (>97% as has been described in some European countries [2, genetic identity), and is a recombinant virus with the 11–15]. While disease mimics the disease caused by L. capsid gene of GI.2 and the non-structural genes of GI.1 europaeus GI.1 viruses, disease course and levels of path- (GI.1bP–GI.2, according to the newly proposed nomen- ogenicity appeared to be more variable [2]. In addition, clature by Le Pendu et al. [3]) (Table 1). while animals younger than 5–8  weeks of age are highly Rabbits used in this study were sourced from the resistant to the development of clinical disease following domestic rabbit breeding colony at CSIRO Black Moun- infection with L. europaeus GI.1 viruses, kittens as young tain. Rabbits in this research colony are regularly tested as 11 days of age can succumb to disease and death from and confirmed to be free of antibodies to RHDV as well infection with L. europaeus GI.2 [13]. Finally, species bar- as the non-pathogenic lagovirus RCV-A1 [23, 24] that is riers are less rigid within the Leporidae family, as L. euro- endemic to Australia and can attenuate RHDV infections paeus GI.2 viruses can cause clinical disease and death in [25]. All procedures involving live animals were carried various hare species [16–20]. out in Australia and were in accordance with the Aus- While there are numerous descriptions of the pathol- tralian Code of Practice for the Care and Use of Animals ogy resulting from infection by L. europaeus GI.1 for Scientific Purposes (2013) and were approved by the (reviewed in [1]), to date, there are only a few, brief CSIRO Ecosystem Sciences Animal Ethics Committee descriptions for L. europaeus GI.2 [2, 5, 11]. Addition- (permits ESAEC 13-10, DOMRAB). ally, some disparities regarding pathology and patho- To generate material for further research, two adult genesis of disease caused by L. europaeus GI.1 have been New Zealand white rabbits were inoculated per os with reported. While many likely stem from differences in 1  mL of a clarified 2% w/v liver homogenate of BlMt-1 methodology as suggested by Abrantes et  al. [1], differ - containing 3 × 10 copies/mL. Although the infectivity ences between viral strains and hosts may also play a role. of this preparation was not experimentally determined in Finally, there appear to be strain differences in the type of rabbits, this likely constitutes a very high infectious dose. disease caused by L. europaeus GI.2. Le Gall-Reculé et al. Rabbits were monitored twice daily or more frequently [2] report that early strains often resulted in subacute to when required. Both animals were euthanized when they chronic disease, whereas experimental work by Capucci displayed signs of RHD, at 54 and 96  h post-inoculation et  al. [21] suggests that more recent L. europaeus GI.2 (hpi) (Table  2). Humane endpoints for terminal RHD strains result in predominantly acute disease. Detailed were defined as hypothermia (normal rectal temperature descriptions of disease caused by different L. europaeus is defined as 38.5–40 °C) after a fever episode (>40.4 °C), GI.2 viruses therefore are warranted and may provide depression with loss of muscle tone and/or anorexia and insight into mechanisms responsible for pathogenicity. >10% acute weight loss. For euthanasia, rabbits were Presently, lagoviruses cannot be cultured ex  vivo and first sedated with an intramuscular injection of Keta - experimental infections still are required to study cer- mine/Xylazine (30; 5  mg/kg) (Xylazil-20: Troy Labo- tain aspects of disease and generate material for further ratories, Smithfield, Australia; Ketamav 100: Mavlab, analyses, such as viral antigen for serological assays. For Logan, Australia), followed by intravenous injection of Neimanis et al. Vet Res (2018) 49:46 Page 3 of 15 Table 2 Domestic rabbits (Oryctolagus cuniculus) inoculated per os with Lagovirus europaeus GI.2 (BlMt-1) Animal Age Euthanized Time (hours Tissues for molecular Tissues for microscopic identification (E) or died post infection) analysis collected (Y = yes, analyses collected (Y = yes, number spontaneously (D) of euthanasia or death N = no) N = no) 282 Adult E 96 Y Y 283 Adult E 54 Y Y 302 Juvenile D <90 Y N 303 Juvenile D 66 Y Y 304 Juvenile D <66 Y N 305 Juvenile D <66 Y N 306 Juvenile E 75 Y Y Rabbit 130 (not shown), a healthy adult from the same research colony, was euthanized to serve as a negative control. Rabbit was found dead. pentobarbitone sodium (165  mg/kg) (Lethabarb: Virbac, at −20 °C. The three other kittens were found dead. Exact Milperra, Australia) in the marginal ear vein. Blood col- time of death was not known. The same subset of tissues lection and post-mortem examinations were performed was collected, but samples were saved at −20  °C only. immediately following euthanasia and the following suite Samples for microscopic examination were not collected. of tissues was collected: liver (hilar region and distal Viral load (capsid genome copies) was determined in lobe), gall bladder with bile, spleen, lung, heart, kidney, all tissue samples collected, including bile, serum and thymus, mesenteric lymph node, duodenum, jejunum, buffy coat. Samples were analysed as described previ - ileum, Peyer’s patches, appendix, sacculus rotundus, ously using a universal lagovirus quantitative reverse cecum, colon, trachea, femoral bone marrow and brain. transcriptase PCR (RT-qPCR) assay that detects all lago- The bone marrow was removed from the femur before viruses known to be present in Australia [26]. placement in formalin to eliminate the need for decalci- Following formalin fixation for 1  week, tissues were fication before processing for microscopic analysis. Each processed and embedded in paraffin at the Imaging and sample was split in two. One half was fixed in 10% neu - Cytometry facility, Australian National University for tral buffered formalin (NBF) while the other was frozen routine microscopic examination. at −20  °C for molecular analyses. A healthy adult rab- All microscopic analyses, including immunohisto- bit from the same rabbit colony was killed humanely as chemistry, were performed at the National Veterinary described above to serve as a negative control and the Institute in Sweden. To assess histopathology, sections same suite of samples were collected. (3–4  µm) were stained with Mayer’s haematoxylin and Blood samples were collected by cardiac puncture fol- eosin [27]. lowing euthanasia and centrifuged at 3000 g for 20 min to In order to perform a relative comparison of propor- separate the serum. White blood cells (buffy coat) were tions of different cell lines in the bone marrow of the two collected from the interphase and both serum and white infected adult rabbits and the control animal, estimates blood cells were stored at −20 °C until analysis. of the myeloid-to-erythroid ratio (M:E) and the propor- Young animals are resistant to the development of tion of early to later stage myeloid cells were determined. clinical disease following infection with L. europaeus Analyses were conducted on formalin-fixed bone marrow GI.1 viruses [1]. To investigate the susceptibility of young stained with haematoxylin and eosin as described above. animals to L. europaeus GI.2, five 5-week old New Zea - M:E ratios were calculated by classifying 300 nucleated land white rabbit kittens from the same rabbit colony cells at high power (100 cells per three different fields were inoculated per os with 0.5  mL of a clarified liver at 1000× magnification) into cell types and then divid - homogenate of BlMt-1. Again, all died or were eutha- ing the number of myeloid lineage cells by the number of nized because of terminal signs of RHD within 90  h of nucleated erythroid cells. Additionally, the proportion of inoculation (Table  2). One kitten that was euthanized early myeloid cells (myeloblasts, promyelocytes and mye- and one that died while euthanasia was about to be per- locytes as defined by Riedel et al. [28]) in relation to later formed were examined immediately after death and stage myeloid cells was estimated. This was done by clas - blood and the following subset of tissues were collected: sifying 200 myeloid cells (100 cells per two different fields liver, spleen, lung, heart, kidney, duodenum, jejunum, at 1000× magnification) as early [large cells with round ileum, Peyer’s patches, appendix and colon. Again, half of to oval nucleus, and at least one nucleolus (myeloblast) each sample was placed in 10% NBF and half was frozen or eosinophilic cytoplasmic granules (promyelocyte and Neimanis et al. Vet Res (2018) 49:46 Page 4 of 15 myelocyte)] or later stage myeloid cells (kidney bean- terminal RHD (depressed, little resistance to handling) shaped, band-shaped or segmented nucleus and eosino- and were euthanized or died just before euthanasia could philic cytoplasmic granules). be carried out. Both 303 and 306 also had subnormal To complement molecular analyses and provide fur- body temperature (< 38  °C). Rabbit 282 was active and ther insight into virus localization, distribution of GI.2/ behaving normally, but had lost > 10% of its body weight RHDV2 viral antigen in situ, and its relation to presence at 96  hpi and was euthanized. Rabbits 304, 305 and 302 of histological lesions, immunohistochemical staining were found dead. Time of death or euthanasia are sum- was carried out on all formalin-fixed and paraffin-embed - marized in Table 2. ded tissues available from the four infected animals (282, 283, 303 and 306) and the control animal (130). Briefly, Gross pathology 3–4  µm sections were deparaffinized and rehydrated. Rabbit 282 had a tan and mottled liver, enlarged spleen Slides were then placed in an automated Autostainer- and congested lungs. The tissues of rabbit 283 were unre - Plus (Agilent Technologies Sweden AB, Kista, Sweden). markable, except for an enlarged spleen. Endogenous peroxide activity was blocked with hydrogen All kittens had a tan and friable liver, and the liver was peroxide (Dako REAL Peroxidase-Blocking Solution, severely discoloured yellow-tan in rabbit 306. Rabbit 302 Agilent Technologies Sweden AB, Kista, Sweden) for showed subserosal petechial haemorrhages over Peyer’s 5 min. Sections were then treated with 2% bovine serum patches and multifocal haemorrhages around the mesen- albumin (BSA) for 20  min. Slides were then incubated teric lymph nodes. Rabbit 304 had copious foamy nasal for 30  min at room temperature with a 1:700 dilution discharge, and rabbit 305 had hay in the mouth, indicat- (diluted with Dako Antibody Diluent with Background ing peracute death while eating. Tissues were otherwise Reducing Components, Agilent Technologies Sweden unremarkable. AB, Kista, Sweden) of a primary monoclonal antibody cocktail (3H6 + 6G2 IgG1 mouse monoclonal antibodies, Histopathology concentration of 0.32  mg/mL) targeting the capsid pro- There was a clear difference in severity and distribution tein of lagoviruses [29] kindly provided by the OIE Ref- of lesions from the adult euthanized at 54  hpi (rabbit erence Laboratory for RHD, Brescia, Italy. Visualization 283) and the other three rabbits that were euthanized or of bound antibodies was performed by using the polymer died later (rabbits 282, 303 and 306, Table  2). Significant detection system Dako EnVision+ System-HRP Labelled lesions were seen in the liver, spleen and bone marrow of Polymer (anti-mouse) (Agilent Technologies Sweden all animals examined histologically, but these were less AB, Kista, Sweden) for 30  min followed by application severe in rabbit 283. Additional lesions were observed in of the chromogen diaminobenzidine (DAB) (Dako Liq- kidneys, lungs, heart and other lymphoid tissues of some, uid DAB+ Substrate Chromogen System, Agilent Tech- but not all, rabbits. nologies Sweden AB, Kista, Sweden) for 10 min. Sections Liver lesions of rabbit 283 were primarily in peripor- were then counterstained with Mayer’s haematoxylin for tal areas. Mild to moderate numbers of single or small 5 min. For the immunoglobulin negative control, a dupli- groups of periportal hepatocytes, including occasional cate section was incubated with a non-specific antibody cells of the limiting plate, were degenerate or dead and (Negative Control Mouse IgG1, Agilent Technologies usually were surrounded and infiltrated by moderate to Sweden AB, Kista, Sweden) with the same isotype as the large numbers of heterophils (Figure  1). Occasional het- primary antibody, diluted to the same protein concentra- erophils also infiltrated portal areas and were seen in tion as the primary antibody. A positive tissue control sinusoids. Affected hepatocytes had hypereosinophilic (liver from a wild rabbit in Sweden confirmed positive for cytoplasm, typically accompanied by a karyorrhectic or L. europaeus GI.2 by RT-qPCR, sequencing and immu- pyknotic nucleus. In advanced stages, cells or cell frag- nological characterization [8] and a negative tissue con- ments had rounded up, were diminished in size, often trol (liver from a healthy, adult rabbit from the same contained pyknotic nuclear fragments, and had detached Australian colony) were included on each slide to ensure from hepatic cords (consistent with apoptotic cells and immunolabelling was working for each slide and consist- apopotic bodies, Figure  1). Less frequently, affected ent among slides. hypereosinophilic hepatocytes retained their shape and nuclei were faded (karyolysis). In other areas, heterophils Results were seen amongst fragmented remnants of hepatocytes, Animals/clinical signs and where heterophil aggregates were more extensive, All inoculated rabbits developed a fever (tempera- hepatocytes were effaced, resulting in focal lytic necro - ture > 40.4  °C) between 30 and 52  hpi. Rabbits 283 sis and sometimes haemorrhage. Occasional hepatocytes (an adult), 303 and 306 (both kittens) showed signs of in midzonal regions were also affected but hepatocytes Neimanis et al. Vet Res (2018) 49:46 Page 5 of 15 Figure 1 Microscopic lesions in the liver of rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Asterisks denote portal areas. Haematoxylin and eosin stain. A Uninfected control rabbit (rabbit 130). 200× magnification. B Earlier stage of acute infection in an adult rabbit (rabbit 283). There is degeneration and death of hepatocytes at the limiting plate and periportally surrounded by intensive heterophilic inflammation. Arrows denote apoptotic cells. 200× magnification. Insert: high magnification of dead hepatocytes surrounded by heterophils. C Terminal stage of acute infection in a 5 week old rabbit (rabbit 303). There is massive hepatocellular degeneration and death. 200× magnification. D Terminal stage of acute infection in a 5 week old rabbit (rabbit 306). Arrowheads denote clusters of heterophils that are seen throughout the lobule and the arrow indicates an area of lytic necrosis. 400× magnification. Insert: high magnification of heterophils surrounding dead hepatocytes. around central veins were almost entirely spared. Fine In the liver of the other three rabbits, hepatocytes cytoplasmic vacuolation and mild to moderate small, showed the same changes indicative of degenera- distinct, round intracytoplasmic vacuoles were seen pre- tion and cell death as described for 283, but the extent dominantly in midzonal hepatocytes and occasionally, an and degree was much more severe (Figure  1). Although aggregate of hyaline, eosinophilic material was seen adja- there was a tendency for periportal hepatocytes to be cent to the nucleus. Kupffer cells were mildly enlarged targeted, including hepatocytes of the limiting plate, and contained a larger nucleus with an open chromatin affected hepatocytes could be seen throughout the lob - pattern, resulting in these cells being more prominent ule. The extent was so severe in the two kittens that cell than in the control animal. Bile ducts and vessels were death was classified as massive (Figure  1). Karyorrhexis unremarkable and no thrombi were seen. Similar to the was still common, but nuclei undergoing karyolysis were control rabbit, portal areas also displayed mild to mod- much more frequent than in rabbit 283. Round, hypere- erate bile duct hyperplasia, infiltration of scattered lym - osinophilic hepatocellular remnants with or without pyk- phocytes and plasma cells and mild, early fibrosis. notic nuclear fragments were occasionally to frequently Neimanis et al. Vet Res (2018) 49:46 Page 6 of 15 observed and individual dead cells that had lost cel- and lymphoid depletion. Again, whereas lesions were lular detail or were swollen and vacuolated instead of only mild to moderate in 283, they were similar or more shrunken were also seen (more consistent with single cell severe in 282 and 303. No other lymphoid tissues were necrosis as described in Elmore et  al. [30]). While occa- available for examination from animal 306. The thymus sional (303) to moderate numbers of heterophils (282 of rabbit 282 exhibited the most severe changes with and 306) were present, they were dispersed throughout marked, extensive lymphocytolysis and abundant tingi- sinusoids and infiltrated hepatic cords rather than being ble body macrophages containing apoptotic cellular and intensively aggregated periportally (Figure  1). Occasion- nuclear debris. ally, they clearly surrounded dead hepatocytes. Small, Pulmonary lesions were only seen in two of the four scattered foci of lytic necrosis and haemorrhage also were rabbits. In rabbit 282, moderate pulmonary congestion seen. Hepatocytes throughout the lobule often contained and oedema were accompanied by mildly increased num- distinct, round intracytoplasmic vacuoles consistent with bers of plump intra-alveolar macrophages. In the lungs lipid. Kupffer cells were plump, prominent and slightly to of the kitten that died spontaneously (303), infrequent moderately more numerous compared to the control ani- fibrin thrombi were seen in small vessels and alveolar mal. In one kitten, fibrin thrombi were very occasionally capillaries. Scattered to occasional pyknotic cells with seen in portal vessels, but in general, with the exception hypereosinophilic cytoplasm similar to those observed in of occasional plump, activated endothelium, vessels were the spleen were seen in alveolar septae of 282, 303 and unremarkable. Like the control animal and 283, there was 306. Trachea was only available from the two adult ani- mild (kittens) to moderate (282) bile duct hyperplasia, mals and moderate submucosal congestion of the trachea mild, early fibrosis and infiltration of scattered lympho - was seen in both. cytes and plasma cells in portal areas. Biliary epithelium Renal lesions were confined to the kittens. Glomerular and bile ducts showed no evidence of necrosis. tufts were severely congested, occasionally accompanied The spleen of all animals was moderately to severely by haemorrhage, and frequent fibrin thrombi were seen congested and abundant, activated macrophages (mac- both in glomerular capillaries and adjacent small vessels. rophage hyperplasia) displaying active phagocytosis were In scattered tubules, epithelium was either attenuated seen throughout the red pulp (Figure  2). In rabbits that or plump and regenerative, indicative of acute tubular died later (282, 303, 306), the stroma of the red pulp was nephrosis. hyaline, there was loss of cellular detail and numerous With the exception of rare thrombi seen in small ves- unidentifiable pyknotic cells were seen (Figure  2). Aggre- sels of the myocardium of rabbit 303, no other significant gates of amorphous eosinophilic material consistent with lesions were seen in the other tissues examined for all fibrin was seen scattered in the red pulp of rabbit 303. In animals. the white pulp, there was moderate to severe decrease in the proportion of lymphocytes to macrophages reflect - Immunohistochemistry ing general lymphoid depletion in comparison with the Viral antigen was visualized in the liver of all four rab- control animal. Active, mild to moderate lymphocytoly- bits and for animal 283, the liver was the only organ sis was observed, and within the kittens, frequent tingi- in which viral antigen was detected. In general, viral ble body macrophages were seen within the white pulp antigen was predominantly found within normal and (Figure 2). degenerate hepatocytes, where staining typically was Bone marrow was only available for examination from finely stippled and could be seen throughout the cyto - the adult rabbits and control animal. Similar to liver and plasm and/or concentrated around the periphery at the spleen, changes in the bone marrow were more marked cell membrane (Figure  4). Infrequently, intracytoplas- in the rabbit that was euthanized later following inocu- mic staining was confined to a focal area adjacent to the lation (282) (Figure  3). The M:E decreased and the pro - nucleus. Intranuclear staining, although it occurred, portion of immature myeloid cells increased (Table  3). was rare (Figure  4). Rare to occasional Kupffer cells In addition to the marked decrease in the proportion of also contained viral antigen. Within Kupffer cells, stain - myeloid to erythroid cells in rabbit 282, binucleated and ing was always intracytoplasmic and coarsely clumped. even multinucleated rubricytes that typically are indica- Additionally, coarsely clumped antigen often could tive of disrupted erythropoiesis were commonly seen be seen associated with hepatocellular remnants and (Figure 3). inflammatory cell infiltrates, but exact localization Within other lymphoid tissues (Peyer’s patches, sac- could not be readily discerned. In animals 282, 303 culus rotundus, cecal appendix, mesenteric lymph node, and 306, rare to infrequent intravascular leukocytes thymus), infected animals exhibited variable degrees of morphologically consistent with monocytes or mac- lymphocytolysis, presence of tingible body macrophages rophages showed coarsely clumped intracytoplasmic Neimanis et al. Vet Res (2018) 49:46 Page 7 of 15 Figure 2 Microscopic lesions and viral antigen localization in the spleen of a 5 week old rabbit (Oryctolagus cuniculus) (rabbit 306) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Asterisks denote the white pulp. A Uninfected control (rabbit 130). 200× magnification. B Terminal stage of acute infection. There is lymphoid depletion and tingible body macrophages (arrows) in the white pulp and macrophage hyperplasia in the adjacent red pulp. 200× magnification. C Red pulp of the uninfected control (rabbit 130). 400× magnification. D Red pulp of rabbit 306. Note the indistinct, hyaline stroma and presence of small, dark pyknotic cells (arrowheads). 400× magnification. E Rabbit 306. Active phagocytosis by macrophages in the red pulp (arrows) and numerous pyknotic cells (arrowheads). The macrophage indicated at the bottom of the image has phagocytized a pyknotic cell. 400× magnification. F Immunohistochemical visualization of viral capsid antigen (brown staining) in the cytoplasm of viable macrophages (arrows) in the terminal stage of acute infection. Note that no staining is associated with pyknotic cells. 400× magnification. Neimanis et al. Vet Res (2018) 49:46 Page 8 of 15 Figure 3 Microscopic lesions and viral antigen localization in the bone marrow of adult rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Arrows denote myeloid cells and arrowheads denote erythroid cells. A Uninfected control (rabbit 130). 600× magnification. B Bone marrow in an earlier stage of acute infection (rabbit 283). Note the decreasing proportion of myeloid to erythroid cells. 600× magnification. C Bone marrow at the terminal stage of acute infection (rabbit 282). Myeloid cells are scarce and erythroid cells at arrowheads are binucleated. 600× magnification. D Immunohistochemical visualization of viral capsid antigen (brown staining) in the cytoplasm of viable macrophages (arrows) in the terminal stage of acute infection (rabbit 282). 400× magnification. Table 3 Estimates of the myeloid-to-erythroid ratio and relative proportions of early stage myeloid cells versus later stage myeloid cells in the bone marrow of adult rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus GI.2 (BlMt-1) Rabbit Myeloid-to-erythroid ratio Proportion of early (myeloblasts, promyelocytes and myelocytes) versus later stage myeloid cells 130 (control) 2.08:1 16% versus 84% 283 (euthanized at 54 hpi) 0.99:1 24% versus 76% 282 (euthanized at 96 hpi) 0.47:1 55% versus 45% staining. Biliary epithelium and vascular endothelium was confined primarily to hepatocytes and hepatocellu - did not contain detectable viral antigen. lar remnants admixed with heterophil infiltration within Within the liver, the amount of viral antigen observed periportal areas (Figure  4). The other three rabbits con - and its distribution varied between rabbits. Rabbit 283 tained markedly more viral antigen, but whereas it was contained the least amount of viral antigen and staining primarily located within hepatocytes in the form of finely Neimanis et al. Vet Res (2018) 49:46 Page 9 of 15 Figure 4 Viral antigen localization in the liver of rabbits (Oryctolagus cuniculus) inoculated with Lagovirus europaeus G1.2 (BlMt-1). Asterisks denote portal areas. A Uninfected control (rabbit 130). No antigen (brown staining) is seen. 200× magnification. B Earlier stage of acute infection in an adult rabbit (rabbit 283). A small number of periportal hepatocytes contain viral antigen (brown staining, arrow). A cluster of heterophils surrounds necrotic, antigen‑ containing debris interpreted to be the remnants of an infected hepatocyte (arrowhead). 200× magnification. C Terminal stage of acute infection in an adult rabbit (rabbit 282). There is cytoplasmic staining in hepatocytes throughout the lobule (arrows). 200× magnification. D Terminal stage of acute infection in a 5 week old rabbit (rabbit 303). In addition to the finely stippled intracytoplasmic staining of infected hepatocytes, there is occasional intranuclear staining (arrow, arrowhead) and staining of the cell membrane (arrowhead). 400× magnification. stippled stain throughout the lobule in rabbits 282 and round, pyknotic bodies similar in appearance to the 303, in rabbit 306, antigen was more typically coarsely unidentified pyknotic cells in the red pulp. Otherwise, clumped and distributed throughout the lobule within the majority of pyknotic cells were not associated with hepatocytes, Kupffer cells and within inflammatory and viral antigen (Figure 2). No antigen was associated with necrotic debris. lymphocytolysis or observed within tingible body mac- Within the spleen of rabbits 282, 303 and 306, rophages of the white pulp. No antigen was observed in coarsely clumped viral antigen was seen within the the spleen of rabbit 283. cytoplasm of frequent (282) to abundant (303, 306) Similar to the spleen, occasional to more frequent macrophages (Figure  2). Antigen-containing mac- macrophages in the bone marrow of rabbit 282 con- rophages were numerous and conspicuous at the mar- tained coarsely clumped, intracytoplasmic viral antigen ginal zone of the white pulp and frequently within (Figure  3). Antigen-containing macrophages showed no sinus histiocytes of the red pulp (Figure  2). They also evidence of degeneration or cell death. Megakaryocytes occasionally contained intracytoplasmic, basophilic, and granulocytic and erythroid cell lines were devoid of Neimanis et al. Vet Res (2018) 49:46 Page 10 of 15 antigen. No viral antigen was seen in the bone marrow of were depressed, hypothermic and/or anorexic prior to rabbit 283 and bone marrow was unavailable for exami- euthanasia or death. This is compatible with the peracute nation in the kittens. and acute forms of RHD observed in GI.1 infected rab- A small amount of viral antigen was detected within bits [31, 32]. Although rabbit 282 was behaving normally the lungs of rabbits 282, 303 and 306. Staining primar- up to 96  hpi, there were widespread liver lesions, viral ily was confined to the alveolar septae, both clearly cell- loads in the serum and liver were very high and body associated as clumped intracytoplasmic material within weight had dropped more than 10%. It is not possible to cells morphologically consistent with macrophages, or predict whether this animal ultimately would have sur- within septal capillaries, often associated with material vived or succumbed to subacute RHD, but this variation consistent with fibrin. Scattered leukocytes with large, in disease progression and mortality rate among animals open nuclei and abundant cytoplasm (probable mono- is consistent with that seen in classic RHD caused by cytes or macrophages) within larger vessels also con- GI.1 and differs from the longer disease course and lower tained clumped intracytoplasmic antigen. No antigen case fatality rates described for early strains of GI.2 [2]. was detected within alveolar macrophages nor was anti- Although the number of animals used in this study was gen observed associated with pyknotic cells seen within small, our findings strongly suggest that BlMt-1 is much alveolar septae. more pathogenic than earlier reported strains of GI.2 that No viral antigen was detected in the kidneys of the two were detected in Europe [2]. This is in line with findings adults 283 and 282. In contrast, both kittens had a small by Capucci et al. [21] who carried out experimental infec- amount of detectable antigen which was seen associated tions of a small number of domestic rabbits with more with fibrin thrombi within glomeruli and small adjacent recent European GI.2 isolates, and similarly report very vessels. Rare leukocytes within larger vessels contained high case fatality rates. The European virus most closely intracytoplasmic antigen. Antigen was not detected in related to BlMt-1, a recombinant GI.1bP–GI.2 from tubular epithelium. 2014, has reportedly replaced previously circulating GI.1 Viral antigen was not detected in any other tissues, strains [12] and has led to substantial declines in wild including those that displayed microscopic lesions. No rabbit numbers in the Iberian Peninsula [33], indicative antigen was seen associated with lymphocytes, lympho- of a highly virulent virus. Capucci et al. [21] hypothesized cytolysis, tinigible body macrophages and/or lymphoid that GI.2 strains have evolved in their natural hosts since depletion in lymphoid tissues. Likewise, no staining was their emergence in 2010, and similar to what was pro- seen within the heart of 303 or the congested trachea of posed for GI.1 strains, selection pressure favours strains 282 and 283. Viral antigen was never detected in any tis- with higher pathogenicity [34]. Our data from this more sues of the control animal 130. recent isolate (BlMt-1) is consistent with this hypothesis. Consistent with all previous studies on RHD, the liver is the target organ of BlMt-1. Based on time of euthana- Molecular analyses sia after inoculation, lower viral tissues loads (Table  4) Viral RNA was detected by RT-qPCR in all tissues ana- and data from previous studies on GI.1 viruses (e.g. [35, lysed and viral loads (capsid gene copies per mg tissue) 36]), rabbit 283 represents an earlier stage of disease are presented in Table  4. For the majority of tissues ana- than rabbits 282, 303 and 306. By comparing results from lysed, Rabbit 283, the animal euthanized earliest after these animals, disease progression mimics that of other inoculation, had the lowest viral load, often by an order pathogenic lagoviruses. Infection in the liver began with of magnitude or more. Serum levels of virus in this rab- degeneration and death of periportal hepatocytes at and bit were 3–4 orders of magnitude lower than in other adjacent to the limiting plate and incited a marked het- animals. Highest viral loads were detected in the serum erophilic inflammatory response. As disease progressed, (except for animal 283) and liver, followed by the spleen. affected hepatocytes became more numerous and wide - spread throughout the lobule, although centrilobular Discussion hepatocytes still tended to be spared, reflecting disease In this study, clinical signs, disease and case fatality rate advancement from the periphery of lobules centrally. caused by the Australian strain of L. europaeus GI.2 Occasional to moderate numbers of heterophils were closely mimicked that of classic RHD in adult rabbits still present in the liver and often associated with dead caused by L. europaeus GI.1 strains [1]. Death or severe hepatocytes, but they were distributed throughout the disease necessitating euthanasia occurred within 96  hpi lobule rather than confined to periportal regions. These for all animals, with an average time to death or euthana- lesions were coupled with activation and hyperplasia of sia of 73 hpi. All animals developed a fever within 52 hpi Kupffer cells. The presence of bile duct hyperplasia, mild and, with the exception of 282, either died suddenly or mononuclear inflammation and mild fibrosis portally in Neimanis et al. Vet Res (2018) 49:46 Page 11 of 15 Table 4 Viral copies of Lagovirus europaeus GI.2 (BlMt-1) per milligram tissue calculated from RT-qPCR analyses Rabbit Adults 5 week old kittens 282 283 302 303 304 305 306 Liver (hilar) 2.42E+08 1.64E+08 n.c. n.c. n.c. n.c. n.c. Liver (distal) 3.03E+08 1.33E+08 n.c. n.c. n.c. n.c. n.c. Liver (not specified) n.c. n.c. 4.67E+08 6.16E+08 3.56E+08 9.01E+08 5.07E+08 Bile 1.46E+06 3.26E+06 n.c. n.c. n.c. n.c. n.c. Spleen 1.17E+08 2.36E+07 1.91E+08 1.02E+08 4.08E+07 8.55E+07 3.23E+08 Lung 7.42E+07 2.38E+06 1.30E+08 4.39E+07 9.94E+07 2.00E+08 2.77E+07 Heart 3.27E+06 1.02E+06 2.22E+07 2.16E+07 7.52E+06 n.c. 1.43E+07 Kidney 3.22E+06 6.76E+05 1.30E+07 3.91E+07 6.16E+06 1.20E+07 1.75E+07 Thymus 1.29E+06 1.15E+05 n.c. n.c. n.c. n.c. n.c. Mesenteric lymph node n.c. 4.24E+06 3.00E+06 n.c. 1.84E+06 n.c. 6.04E+06 Duodenum 5.80E+05 1.27E+06 3.23E+06 2.34E+06 2.61E+06 2.23E+06 1.47E+06 Jejunum 1.75E+06 3.64E+06 4.07E+06 3.70E+06 1.83E+06 5.15E+06 2.73E+06 Ileum 2.84E+06 3.96E+03 1.50E+06 1.26E+06 1.78E+06 1.87E+06 2.20E+06 Peyers patches 8.38E+05 1.25E+06 3.51E+06 1.40E+06 1.66E+06 7.12E+05 3.53E+06 Appendix 5.35E+05 1.12E+04 9.95E+05 6.78E+05 7.63E+05 8.16E+05 7.23E+05 Sacculus rotundus 4.74E+05 7.18E+04 n.c. n.c. n.c. n.c. n.c. Cecum 3.63E+05 4.98E+04 n.c. n.c. n.c. n.c. n.c. Colon (+contents) 1.50E+06 n.c. 1.34E+06 2.95E+05 5.24E+05 1.21E+06 3.12E+05 Trachea 2.27E+07 n.c. n.c. n.c. n.c. n.c. n.c. Bone marrow from femur 1.58E+07 3.85E+05 n.c. n.c. n.c. n.c. n.c. Brain 3.95E+06 2.55E+04 n.c. n.c. n.c. n.c. n.c. Buffy coat 5.35E+06 2.13E+04 n.c. n.c. n.c. n.c. n.c. Serum 1.29E+08 2.21E+05 2.42E+09 3.87E+08 2.43E+08 n.c. 7.42E+08 The analysis was done in technical duplicate per tissue and values represent the mean. n.c.: sample not collected. all rabbits, including the control, was interpreted to be in the liver in both BlMt-1 inoculated adults and kittens unrelated to RHD. Rather, this has been described as an was similar and heterophilic. This is not the case for GI.1, incidental finding in normal rabbits [37] or may reflect where adults display heterophilic inflammation through - previous, unrelated insult. To conclude, BlMt-1 induces out the disease course, but kittens develop lymphocytic the same lesions in both adults and kittens as reported inflammation by 48  hpi [41]. Mikami et  al. [42] also for GI.1 viruses in adults. showed that inflammation, predominantly lymphocytes In contrast to GI.1 strains, both Le Gall-Reculé et  al. admixed with macrophages and some heterophils contin- [2] and Dalton et  al. [13] report mortality rates of up ued to increase up to 96  hpi. Ferreira et  al. [41] further to 50% for kittens that were involved in RHD mortality hypothesize that these lymphocytes seen in infected kit- events caused by GI.2. Although sample size in this study tens respond to surface presentation of lagovirus antigen is small, all five inoculated kittens succumbed to disease. on hepatocytes and that this is important in modulating Four died and the fifth was euthanized. The fifth kitten resistance to development of clinical disease. Lympho- had severe, extensive liver lesions deemed incompatible cytic inflammation was not a feature in kittens succumb - with survival, supporting a case fatality rate of 5/5 kit- ing to BlMt-1. This suggests that GI.2 viruses may have tens for BlMt-1 in this study. Disease course was acute, the ability to replicate within hepatocytes without invok- with all kittens dying or euthanized by 90 hpi. This differs ing this postulated protective lymphocytic inflammatory from GI.1 viruses where infected kittens of this age do response. Likewise, the other two proposed mechanisms not develop clinical disease, although the reasons are not of resistance to GI.1 strains in kittens apparently were fully understood. Viral receptor expression, innate immu- overcome and investigation into underlying mechanisms nity and changes in liver function around 5–6  weeks of would provide critical insight into pathogenicity determi- age have all been suggested to play a role [38–40]. One nants in lagoviruses. very clear finding of this study was that inflammation Neimanis et al. Vet Res (2018) 49:46 Page 12 of 15 Apoptotic programmed cell death is a common feature consumption of mature heterophils in the liver. While in viral hepatitides (e.g. [43, 44]) and has been shown to cytotoxic effects of the virus on circulating heterophils occur in RHD caused by GI.1 strains [45–47]. Hepatic or myeloid precursors cannot be excluded, the absence lesions in this study showed morphologic features of of viral antigen within circulating heterophils or granu- both apoptosis and necrosis. Earlier in infection (rab- locytic precursors in the bone marrow does not support bit 283), dead hepatocytes commonly display features direct viral cytotoxicity. M:E is used to detect variation in of apoptosis: shrinkage, hypereosinophilia, karyor- rates of differentiation of myeloid and erythroid precur - rhexis and pyknotic nuclear remnants, and rounding up sors. Although M:E for the control animal 130 was higher and dissociation of cells from the basement membrane than the reported mean of approximately 1.1:1 for labo- [30]. However, the intense, accompanying heterophilic ratory rabbits [54], the comparisons presented here are inflammation is not a feature typically associated with relative and are used as a tool to compare animals within apoptosis. Likewise, there also are foci of lytic necrosis this rabbit colony. The frequent bi- and multinucleate in areas of the inflammation, and in later stages of infec - rubricytes may represent erythroid dysplasia or dyseryth- tion, single cells with morphology more consistent with ropoiesis. While acquired dyserythropoiesis most often single cell necrosis (e.g. increased cell size, vacuolation, is associated with nutritional deficiencies [55], chemo - loss of cellular detail, karyolysis) are seen. Overlap of therapeutic drugs and myeloid neoplasms [56], dysplastic apoptosis and necrosis within the same disease process changes have also been reported in certain viral dis- is well-recognized and may occur, for example, when eases such as those caused by Feline immunodeficiency phagocytic capacity to clear apoptotic remnants is over- virus [57] and Feline Leukemia Virus [58]. Bone marrow whelmed [44]. Additionally, our current understanding of evaluation using formalin-fixed material has limitations programmed cell death is rapidly expanding and a host regarding more detailed discrimination of cells. There - of newly recognized mechanisms of programmed cell fore, further investigation, including concurrent evalua- death involving inflammation (e.g. necroptosis, NETo - tion of haematology and bone marrow cytology in rabbits sis, pyroptosis) have been described and can play critical with RHD, is required to better define the myeloid hypo - roles in antiviral immunity, particularly when viral mech- plasia and left shift, and to confirm and understand the anisms block apoptosis [48–50]. RHD has already been underlying mechanisms of the apparent erythroid dyspla- proposed as a model for acute fulminant liver disease in sia in RHD. In contrast to findings by Dalton et  al. [5], humans [51] and continued investigations into the cell no lesions were seen in the villi of the small intestine of death mechanisms in RHD may provide insight into the kittens, nor were they seen in the adults. pathogenesis of a wider range of liver diseases in different Using immunohistochemistry, presence of viral anti- species. gen was clearly associated with hepatic lesions and In extrahepatic tissues, lesions included thrombi and degenerate and dead hepatocytes and antigen spread haemorrhage typical of disseminated intravascular coag- from periportal areas towards the central vein as dis- ulation (DIC) [52], splenic necrosis, lymphocytolysis and ease progressed. Location of virus within hepatocytes lymphoid depletion. Within the bone marrow, there was was primarily intracytoplasmic and finely stippled, decreased M:E, increased proportion of immature mye- although rimming of the cell membrane, as described by loid cells and presence of atypical rubricytes. Both DIC Park and Itakura [59] for GI.1, was also seen (Figure  4). and lymphoid depletion have been described in GI.1 These authors suggest that the different staining patterns infections [1, 31, 36], but bone marrow changes have reflect different stages in infection as is observed in other not yet been described for RHD and provided additional viruses. Lagoviruses are thought to replicate within the insight into pathogenesis. In the adult rabbit that survived cytoplasm [1], but consistent with other reports [42, 59, longer (282), the proportion of heterophils and myeloid 60], infrequent antigen was also seen within the nucleus precursors decreased and of the myeloid cells remaining, (Figure 4). The presence of viral capsid protein within the the proportion of mature stages decreased. This left shift nucleus remains unexplained. However, as hypothesized in the bone marrow is consistent with the heteropenia by Ramiro-Ibanez et  al. [61], because so little is known described in RHD by Ferreira et  al. [53]. These authors about lagovirus replication, localization of the viral cap- proposed three mechanisms to account for the heterope- sid within the nucleus may be required for some stage in nia: sequestration of mature heterophils, cytotoxic effects the virus life cycle. of the virus on circulating heterophils and cytotoxic In addition to hepatocytes, viral antigen was also effects on granulocytic myeloid precursors in the bone detected within cytoplasm of cells of the mononuclear marrow. The moderate to marked heterophilic inflam - phagocytic system. These include Kupffer cells in the mation seen in the liver, particularly earlier in disease, liver, macrophages within the spleen, bone marrow favours the first hypothesis regarding sequestration and and lungs, as well as cells consistent with intravascular Neimanis et al. Vet Res (2018) 49:46 Page 13 of 15 monocytes or even tissue macrophages that have re- the bile, respectively. Additionally, the high viral load entered circulation in various tissues. While functions of in bone marrow may contribute to the epidemiology these cells are diverse, they are phagocytic and can play a of RHD in wild rabbit populations by prolonging envi- role in both innate and adaptive immunity [62]. Within ronmental persistence of virus when all other tissues these cells, staining was dark and coarsely clumped simi- except for the skeleton are gone, provided the virus lar to hepatocyte remnants in areas of lytic necrosis in material in bone marrow remains infectious. the liver, rather than finely stippled as seen within hepat - To conclude, unlike early GI.2 viruses [2], the path- ocytes. Additionally, whereas hepatocytes showed cyto- ogenicity of the 2015 BlMt-1 strain used in this study pathic effects from the virus, the phagocytic cells showed is comparable to that of GI.1 strains in adult rabbits. no signs of degeneration or death in this study. In these Additionally, and in contrast to GI.1, it is as pathogenic macrophages, antigen staining patterns may therefore for kittens as adults. The inflammatory response in the be more compatible with phagocytized material rather liver of kittens inoculated with BlMt-1 differs from than replicating virus. However, Kimura et  al. [63] show that reported from kittens infected with GI.1 strains, evidence for viral replication within macrophages using providing support for the proposed role of lympho- in  situ hybridization and further investigation into the cytes in modulating resistance to clinical disease from role of macrophages in RHD, including spread from the GI.1 infection. The strong susceptibility of kittens to site of entry to the liver, is needed. disease by the BlMt-1 virus adds a new dimension to Within lymphoid tissues, viral antigen was not pre- the epidemiology of RHD in wild rabbit populations sent in lymphocytes or associated with lymphocytolysis in Australia and will likely have implications for rab- or lymphoid depletion. This suggests that lymphocytoly - bit biocontrol in this country. A recent study reports sis is the result of a secondary mechanism, for example that the GI.1bP–GI.2 virus (to which BlMt-1 belongs) increased glucocorticoids associated with stress as is seen has replaced endemic GI.1 strains within 18  months in lymphoid tissues, especially the thymus [64]. of its arrival in Australia [10]. Mahar et al. [10] further Assessment of immunohistochemical staining of hypothesize that the ability of this virus to infect and inoculated animals in conjunction with both the nega- kill rabbits at a much younger age provides a key com- tive tissue and immunoglobulin controls allowed us to petitive advantage in the field, as new cohorts of rabbits conclude that the immunohistochemistry performed become susceptible to lethal infection at a much ear- on this material was very specific and that non-specific lier age compared to GI.1. Indeed, for 17 of the viruses staining and background staining were absent. How- sequenced in their study, the body mass of the deceased ever, the analysis required a large amount of virus to be rabbits was known, and eight of these isolates (47%) present before antigen could be detected in  situ. Anti- were from kittens weighing less than 600  g, corre- gen was readily detected in tissues with 10 viral copies sponding to 4–8 weeks of age. The differences in patho - per mg and in just over half of the tissues containing genicity between GI.2 strains, coupled with differences 7 6 10 viral copies per mg tissue. However, 10 viral cop- in susceptible age groups and host species when com- ies per mg tissue was below the detection limit for the pared to GI.1 viruses, provide an excellent opportunity immunohistochemical analysis of this material. We are to explore the genetic and immunological determinants confident that staining, when present, corresponds to of pathogenicity in lagoviruses. viral antigen, but we cannot exclude the possibility that tissues or cells containing lower amounts of virus were Abbreviations not detected with immunohistochemistry. While PCR RHDV: rabbit haemorrhagic disease virus; RHD: rabbit haemorrhagic disease; is more sensitive, it does not allow for specific in  situ hpi: hours post‑inoculation; M:E: myeloid‑to ‑ erythroid ratio; RT‑ qPCR: reverse transcriptase quantitative polymerase chain reaction. localization of virus within tissues nor can it be cor- related with microscopic lesions. Using immunohis- Competing interests tochemistry, intracellular virus presence was seen in The authors declare that they have no competing interests. liver and in cells of the mononuclear phagocytic system Authors’ contributions in the spleen, bone marrow, lungs and intravascular AN, DGW and TS conceived the study, all authors were involved in data monocytes. However, virus was also detected in high acquisition and analyses, AN wrote the paper, ULP, NH, DGW and TS revised the paper. All authors read and approved the final manuscript. levels in the serum and in reasonable amounts (10 copies per mg) in numerous other tissues, including Acknowledgements trachea and small intestine. The lack of lesions in these The authors thank Harold Tvedten and Helena Pettersson for assistance with interpretation of the haematopoietic tissues and Harold Tvedten for tissues suggests PCR is likely detecting viral RNA in Figures 3A–C. The authors also thank Peter Kerr and Jemma Bergfeld for their free viral particles in the blood in these tissues, as well helpful comments on the manuscript. as in the lumen of the intestine from virus excreted via Neimanis et al. Vet Res (2018) 49:46 Page 14 of 15 Ethics approval and consent to participate 11. Duarte M, Carvalho C, Bernardo S, Barros SV, Benevides S, Flor L, Monteiro All procedures involving animals were carried out in accordance with the Aus‑ M, Marques I, Henriques M, Banos SC, Fagulha T, Ramos F, Luis T, Fevereiro tralian Code of Practice for the Care and Use of Animals for Scientific Purposes M (2015) Rabbit haemorrhagic disease virus 2 (RHDV2) outbreak in (2013) and were approved by the CSIRO Ecosystem Sciences Animal Ethics Azores: disclosure of common genetic markers and phylogenetic segre‑ Committee (permits ESAEC 13‑10, DOMRAB). gation within the European strains. Infect Genet Evol 35:163–171 12. Lopes AM, Correia J, Abrantes J, Melo P, Ramada M, Magalhaes MJ, Alves Funding PC, Esteves PJ (2015) Is the new variant RHDV replacing genogroup 1 in Funding for the work carried out in Sweden was provided by the Swed‑ Portuguese wild rabbit populations? Viruses 7:27–36 ish Environmental Protection Agency, and the Research Council FORMAS 13. Dalton KP, Nicieza I, Abrantes J, Esteves PJ, Parra F (2014) Spread of new (Contract 221‑2014‑1841) as part of the ECALEP project funded by the EU variant RHDV in domestic rabbits on the Iberian Peninsula. Vet Microbiol Animal Health and Welfare ERA‑Net (ANIHWA). CSIRO and the Invasive Animals 169:67–73 Cooperative Research Centre supported work carried out in Australia. 14. Calvete C, Santo P, Calvo AJ, Monroy F, Calvo JH (2014) Could the new rabbit haemorrhagic disease virus variant (RHDVb) be fully replacing clas‑ Author details sical RHD strains in the Iberian Peninsula? World Rabbit Sci 22:91 Department of Pathology and Wildlife Diseases, National Veterinary Insti‑ 15. Martin‑Alonso A, Martin‑ Carrillo N, Garcia‑livia K, Valladares B, Foronda tute (SVA), 751 89 Uppsala, Sweden. 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Veterinary ResearchSpringer Journals

Published: Jun 5, 2018

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