Rabbit hemorrhagic disease virus 2, 2010-2023: a review of global detections and affected species

Rabbit hemorrhagic disease virus 2/genotype GI.2 (RHDV2/GI.2; Caliciviridae, Lagovirus) causes a highly contagious disease with hepatic necrosis and disseminated intravascular coagulation in several Leporidae species. RHDV2 was first detected in European rabbits (Oryctolagus cuniculus) in France in 2010 and has since spread widely. We gather here data on viral detections reported in various countries and affected species, and discuss pathology, genetic differences, and novel diagnostic aspects. RHDV2 has been detected almost globally, with cases reported in Europe, Africa, Oceania, Asia, and North America as of 2023. Since 2020, large scale outbreaks have occurred in the United States and Mexico and, at the same time, cases have been reported for the first time in previously unaffected countries, such as China, Japan, Singapore, and South Africa, among others. Detections have been notified in domestic and wild European rabbits, hares and jackrabbits (Lepus spp.), several species of cottontail and brush rabbits (Sylvilagus spp.), pygmy rabbits (Brachylagus idahoensis), and red rock rabbits (Pronolagus spp.). RHDV2 has also been detected in a few non-lagomorph species. Detection of RHDV2 causing RHD in Sylvilagus spp. and Leporidae species other than those in the genera Oryctolagus and Lepus is very novel. The global spread of this fast-evolving RNA virus into previously unexploited geographic areas increases the likelihood of host range expansion as new species are exposed; animals may also be infected by nonpathogenic caliciviruses that are disseminated by almost all species, and with which genetic recombination may occur.

Case series: Four fatal rabbit hemorrhagic disease virus infections in urban pet rabbits

Four pet rabbits (Oryctolagus cuniculus cuniculus) diagnosed with a fatal infection by rabbit hemorrhagic disease virus (RHDV GI.2) were identified in the same week and further investigated. All animals lived in an urban environment (Lisbon, Portugal), were between 8 months and 2 years old and none had been vaccinated against RHDV2 (GI.2). Three animals arrived at the clinic and died shortly afterward and it was only possible to collect material for RT-qPCR (RHDV) test. These rabbits tested positive for RHDV2, with high viral loads. In the fourth case, additional clinical and post-mortem gross and histological evaluations were performed. This 8 month old intact female indoor pet rabbit was presented with apathy, tachypnea and tachycardia. Radiographic projections revealed no clinical revealed no clinical abnormalities. Serum biochemistry revealed a significant increase in AST and ALT with a small hypoglycemia. Abdominal ultrasound revealed an acute hepatitis. Despite hospitalization support, after 30 h of admission, the rabbit lost consciousness and developed anorexia and pyrexia in the last minutes before death. Post-mortem analysis and molecular testing by RT-qPCR, confirmed the diagnosis of RHDV2 (GI.2) infection also with high viral load. In conclusion, this paper reports a case series that demonstrates the severe infectious ability and the high mortality associated with RHDV even in rabbits from urban environments. Furthermore, it highlights the importance of always considering rabbit hemorrhagic disease (RHD) as a differential diagnosis in pet rabbits with non-specific clinical signs, and should warn veterinarians that pet rabbits living indoors can also be infected with a fatal outcome.

Retrospective studies on rabbit haemorrhagic disease outbreaks caused by RHDV GI.2 virus on farms in France from 2013 to 2018

Rabbit haemorrhagic disease (RHD) is a critical health threat to the rabbit industry in Europe. In 2018, the French rabbit industry adopted a voluntary control plan against this disease. In this context, two epidemiological studies were conducted on RHD outbreaks that occurred between 2013 and 2018 in France. The objectives were to describe the spread of RHD due to the new genotype RHDV GI.2 (rabbit haemorrhagic disease virus GI.2) and to identify rearing factors influencing the occurrence of the disease in order to guide the prevention measures recommended in the control plan. An analysis of cases on 295 farms between 2013 and 2017 showed that 32% of farms were affected at least once; the incidence of the disease increased in 2016-2017 compared to 2013-2015. Farms already affected in 2013-2015 had a higher risk of being infected in 2016-2017 than those that remained unaffected until 2015 (Relative Risk and 95% Confident Interval 1.7 [1.1-2.7]). A case-control study carried out between 2016 and 2018 on 37 outbreaks and 32 control farms revealed variability in biosecurity and decontamination practices between farms. The risk of being infected tends to be linked to these practices, but certain structural factors (e.g. the manure disposal system, transfer of rabbits at weaning) could also influence the risk of virus introduction into farms. In the context of a limited vaccination coverage of the farms (only females are vaccinated), these hypotheses will be studied further, using information from the RHD outbreak monitoring system implemented at the same time as the control plan in 2018.

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CLINICAL SIGNIFICANCE

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

Infectivity of rabbit haemorrhagic disease virus excreted in rabbit faecal pellets

Rabbit haemorrhagic disease (RHD) is caused by a lagovirus affecting European rabbits (Oryctolagus cuniculus). Viral RNA is detected in tissues or faeces of convalescent rabbits, suggesting persistent infections; however, this RNA has not been shown to be related to infective viruses to date. In the present work, seven laboratory rabbits were challenged with the RHDV2/b virus variant. Viral RNA was individually detected by duplex qPCR in faeces collected for four weeks after infection, and the infective capacity of viral RNA excreted in the faeces of surviving rabbits was tested by challenging new rabbits with faecal inocula. As results, viral RNA was detected in faeces until the end of the assay. Viral RNA detected in the fourth week was infective only in the case of one rabbit that did not exhibit clear seroconversion, suggesting persistent infection as a result of an impaired immune response. Since the surviving rabbits were apparently healthy individuals, the importance of detecting carriers and the correct management of faeces to control RHD outbreaks in rabbitries are highlighted.

Recombinant myxoma virus infection associated with high mortality in rabbit farming (Oryctolagus cuniculus)

Myxomatosis is an emergent disease in the Iberian hare, having been considered a rabbit disease for decades. Genome sequencing of the strains obtained from Iberian hares with myxomatosis showed these to be distinct from the classical ones that circulated in rabbits since the virus introduction in Europe, in 1952. The main genomic difference in this natural recombinant hare myxoma virus (ha-MYXV) is the presence of an additional 2.8 kb region disrupting the M009L gene and adding a set of genes homologous to the myxoma virus (MYXV) genes M060R, M061R, M064R, M065R and M066R originated in Poxviruses. After the emergence of this recombinant virus (ha-MYXV) in hares, in the summer of 2019, the ha-MYXV was not detected in rabbit surveys, suggesting an apparent species segregation with the MYXV classic strains persistently circulating in rabbits. Recently, a group of six unvaccinated European rabbits (Oryctolagus cuniculus cuniculus) from a backyard rabbitry in South Portugal developed signs of myxomatosis (anorexia, dyspnoea, oedema of eyelids, head, ears, external genitals and anus, and skin myxomas in the base of the ears). Five of them died within 24-48 hr of symptom onset. Molecular analysis revealed that only the recombinant MYXV was present. This is the first documented report of a recombinant hare myxoma virus in farm rabbits associated with high mortality, which increases the concern for the future of both the Iberian hare and wild rabbits and questions the safety of the rabbit industry. This highlights the urgent need to evaluate the efficacy of available vaccines against this new MYXV.

Rabbit haemorrhagic disease: Cross-protection and comparative pathogenicity of GI.2/RHDV2/b and GI.1b/RHDV lagoviruses in a challenge trial

European rabbits (Oryctolagus cuniculus) are severely affected by rabbit haemorrhagic disease (RHD). Caused by a lagovirus, the disease leads to losses in the rabbit industry and has implications for wildlife conservation. Past RHD outbreaks have been caused by GI.1/RHDV genotype viruses. A new virus belonging to the GI.2/RHDV2/b genotype emerged in 2010, quickly spreading and replacing the former in several countries; however, limited data are available on its pathogenicity and epidemiological factors. The present work extends these issues and evaluates cross-protection between both genotypes. Ninety-four and 88 domestic rabbits were challenged with GI.2/RHDV2/b and GI.1b/RHDV variant isolates, respectively. Cross-protection was determined by a second challenge on survivors with the corresponding strain. Mortality by GI.2/RHDV2/b was highly variable due to unknown individual factors, whereas mortality by GI.1b/RHDV was associated with age. Mortality in rabbits < 4 weeks old was 84%, higher than previously reported. Cross-protection was not identical between the two viruses because the ratio of mortality rate ratios for the first and second challenges was 3.80 ± 2.68 times higher for GI.2/RHDV2/b than it was for GI.1b/RHDV. Rabbit susceptibility to GI.2/RHDV2/b varied greatly and appeared to be modulated by the innate functionality of the immune response and/or its prompt activation by other pathogens. GI.1b/RHDV pathogenicity appeared to be associated with undetermined age-related factors. These results suggest that GI.2/RHDV2/b may interact with other pathogens at the population level but does not satisfactorily explain the GI.1b/RHDV virus's quick replacement.

Clinical chemistry, haematology, immune response and histological evaluation of rabbits after immunisation and challenge with rabbit haemorrhagic disease (RHD) virus

<p>Following their immunisation and infection with a VSHI-CN-6 viral strain, a group of 15 rabbits were examined in a study of Rabbit Haemorrhagic Disease (RHD). Serum samples were collected from the external ear vein at 0, 15, 30 and 60 days post-immunisation. The recorded platelet numbers were closer to the lower physiological limit, indicating a mild thrombocytopenia, with values ranging from 26.6 to 30.43×10⁴/mm³. The phagocytic index revealed significant differences (P&lt;0.001) between the mean values obtained before vaccination (day 0) and the 3 post-vaccination measurements, confirming the increase in phagocytic capacity after immunisation. Additionally, the serum lysozyme average value equalled 9.14 mg/mL post-vaccination. The analysis of variance revealed significant statistical differences (P&lt;0.05) between the average values obtained before vaccination (0) and the post-vaccination values, measured on day 14 and 30, respectively. The morphology of the samples collected from the main organs involved in immune protection, spleen and gastric and portal lymph nodes highlighted changes corresponding to the post-vaccination immunological response. The white pulp of the spleen appeared as a diffuse lymphoid tissue, presenting with primary and secondary lymphoid follicles. The ratio of white/red pulp was in favour of the white pulp and multiple lymphoid follicles were present, indicating their reactivation. In the medullary area of gastric and portal lymph nodes, narrow lymphoid cords, circumscribed by relatively large lymphatic sinuses, and well defined lymphocytolysis were observed. Moreover, the exudate and lymphoid follicles during activation were noted in the cortical area. Furthermore, the inflammatory processes were identified, morphologically manifested by the thickening of connective tissue in the lymph node capsule, dilacerations of the connective fibres and the presence of light acidophilic serous exudate with rare inflammatory cells (serous lymphoreticulitis).</p>