Session #1001

Abstract

Rabbit hemorrhagic disease (RHD) is a highly infectious disease that causes a fatal hepatitis in European rabbits (Oryctolagus cuniculus). The RHD virus (RHDV, currently designated GI.1) belongs to the genus Lagovirus (family Caliciviridae). In 2010, a new genotype of RHDV (RHDV2, currently designated GI.2) was identified in France. The virus was affecting both domestic rabbits, even those vaccinated for the classical RHDV1 genotypes. The new variant has spread around the world, causing disease in domestic and wild rabbits and hares. This review provides a broad coverage and description of the disease caused by both viruses, with a special focus on RHDV2.

Introduction

The first outbreak of rabbit hemorrhagic disease (RHD) occurred in the 1980s in Chinese rabbitries.¹ The outbreak rapidly spread over European countries and northern Africa causing significant economic losses to rabbit farmers.^2,3 Affected rabbits suffered from hemorrhagic lesions and hepatic necrosis.⁴ The etiologic agent was identified by a European team as the rabbit hemorrhagic disease virus (RHDV). In 2010, a new pathogenic Lagovirus emerged in France and was designated as RHDV2. This new RHDV variant was responsible for atypical RHD in RHDV-vaccinated does from French rabbitries and in wild rabbits.⁵ Since its first description, RHDV2 has been described in many other countries in the world (Spain,⁶ Portugal,^7,8 Italy,^9,10 Australia,¹¹ Great Britain,¹¹ Ireland,¹² United States of America,¹³ and China¹⁴). Recent publications demonstrate that the former circulating RHDV or RHDV1 is progressively overcome by RHDV2.^15-17 This new variant is phylogenetically distinct from the former RHDV1 and differs also by its antigenic profile and pathogenicity.^15,18

Etiological Agent

The etiological agent of the disease, the rabbit hemorrhagic disease virus (RHDV), is a non-enveloped single-stranded positive-sense RNA virus. There are two recognized subtypes RHDV and RHDVa that are both pathogenic.⁴ Nowadays, RHDV is commonly named as RHDV1, especially after the identification of the new Lagovirus RHDV2.¹⁵ RHDV belongs to the genus Lagovirus of the family Caliciviridae.^19,20 The genus Lagovirus includes both RHDV and the European brown hare syndrome virus (EBHSV) described in Sweden in the early 1980s. Despite their host specificity, RHDV and EBHSV share several similarities such as clinical signs, pathological and histopathological alterations, mortality rates, virion morphology, and antigenicity.²¹ RHDV virions are non-enveloped and small sized (between 35–40 nm in diameter). The RNA material is enclosed in a capsid arranged in a T=3 icosahedral symmetry. The unique arrangement of the capsid, composed of 90 arch-like dimers of the capsid protein which form 32 cup-shaped depressions (calix in Latin for cup or chalice), is at the origin of the family name Caliciviridae.²² The most exposed region of the capsid, called P2, displays the greatest genetic variation to escape from immune recognition of the host.²³ Another type of virus particle, the RHDV core-like particles (CLP), can be found in large amounts in the liver and spleen. They are thought to be the result of the degradation of the RHDV-IgM immune-complexes formed during the humoral response.²¹ The genome of RHDV is composed with the genomic RNA (gRNA) and an additional RNA species with 2.2 kb called subgenomic RNA (sgRNA), which terminates the 3’ end of the genomic RNA. Subgenomic RNA is involved in the production of high levels of products required during the intermediate and late stages of infection (e.g., structural proteins).¹⁹

In 2010, a new pathogenic Lagovirus was identified in France, formerly designated as RHDV2.^5,15 This emerging Lagovirus was demonstrated to be phylogenetically different from the other Lagovirus with a unique antigenic profile.¹⁵ Phylogenetic analyses based on the capsid protein VP60 allowed the determination of the phylogenetic relationship between known pathogenic and non-pathogenic rabbit lagoviruses.¹⁵ The new variant RHDV2 was shown to be genetically related to but distant from RHDV and RHDVa isolates. The average nucleotide identity between RHDV2 and RHDV-RHDVa was 82.4% when it was 70.4% with EBHSV, showing that RHDV2 is closer to rabbit lagoviruses.¹⁵ The RHDV2 spread worldwide and is nowadays replacing the circulating RHDV/RHDVa strains in Europe and other remote countries, in both wild and domestic lagomorph populations.^16,17,24,25

Epidemiology

Host

The classical RHDV1 causes rabbit hemorrhagic disease in wild and domesticated adult European rabbits (Oryctolagus cuniculus).⁴ Other rabbit species including cottontails (Sylvilagus floridanus), black-tailed jackrabbits (Lepus californicus), and volcano rabbits (Romerolagus diazzi) are not susceptible to RHDV.²⁶

The new genotype RHDV2 also affects the species Oryctolagus cuniculus. But, it exhibits a larger host range than the former genotype RHDV. RHDV2 causes an RHDV-like disease in several hare species, such as the Sardinian cape hare (L. capensis var mediterraneus), the Italian hare (L. corsicanus), the European brown hare (L. europaeus), and the mountain hare (L. timidus).^10,28

Age

Rabbits of all ages can be infected by RHDV1. Young age seems to be protective. Indeed, rabbits younger than 10 weeks of age have a natural resistance to RHDV.¹⁴ This age resistance is not evident in RHDV2. The virus can fatally infect rabbits younger than two months.^15,29

Transmission

Transmission of RHDV is by direct contact, through the oral, nasal, or conjunctival routes, but also after exposure to carcasses or fur from affected rabbits, or by means of fomites, contaminated food, bedding, water, cage, clothing, and utensils. Blood-feeding insects have also been shown to be very efficient mechanical vectors. Predators (dogs, foxes) can excrete RHDV in faces after eating infected lagomorphs. Importation of infected rabbit meat could be the main way of transmission to a new area as contaminated meat can contain a high level of virus which survive freezing. The principal mode of transmission is the fecal-oral route in natural infections. The duration of the infected status of lagomorphs after a recovery from RHDV1 and RHDV2 remains unknown.^7,26

The disease caused by RHDV1 is highly infectious and is associated with high rates of both morbidity (40–100%) and mortality (approaching 100%). Higher rates of morbidity and mortality are seen in unvaccinated populations. The incubation period is 1 to 3 days. Outbreak peaks occur 2 to 3 days after the infection, and the disease course last 7 to 13 days.

The new variant RHDV2 is less virulent, with lower observed mortality rates than RHDV1. Under experimental conditions, mortalities seem to occur later after the infection (3–9 days after inoculation) and over a longer period (5 days), compared with RHDV1.¹⁵

Host Specificity

Several caliciviruses use the carbohydrate portion present in the host-cell histo-blood group antigens (HBGA) for attachment to initiate their replication. Histo-blood group antigens are complex glycans attached to proteins or lipids present on the surface of epithelial cells and erythrocytes and can also be found in biological fluids (blood, intestinal content, saliva, milk). The specificity of the HBGA on the host cells determines the infectivity of the caliciviruses.⁴

Resistance to the RHDV in young individuals might be explained in part by the genetically determined absence or weak expression of the histo-blood group antigens on the capsid surface.

New Nomenclature

The historical nomenclature originally classified the lagoviruses based on the pathogenicity and the host. Due to the discovery of new strains and their frequent recombination, a new nomenclature based on phylogenetic relationships recently emerged. According to the new Lagovirus nomenclature, RHDV belongs to genogroup Gl, which further divides into genotype Gl.1 (former G1−G6), genotype Gl.2 (RHDV2), and other genotypes.³⁰

Clinical Signs

Rabbit hemorrhagic disease should be suspected when there are several cases of sudden death, especially if multiple rabbits are affected and unvaccinated. The incubation period for RHDV1 is 1–2 days and it is slightly longer, 3–5 days, for RHDV2.^4,5,26,31 Rabbits younger than 6–8 weeks may develop a subclinical form of RHDV1 disease. The peracute and the acute disease with an onset of fever (>40°C) are the most common in adult individuals. Main symptoms are observed during the acute infection: apathy, fever, neurological symptoms (excitement, opisthotonos, ataxia, paralysis), and respiratory signs (dyspnea, epistaxis). Death usually occurs 12–36 hours after the onset of fever. Few individuals rarely survive RHDV1 disease and may develop a chronic form of the disease characterized by jaundice. RHDV1 disease is a highly lethal disease associated with a mortality of 80–90%.^4,26,31

The clinical signs caused by RHDV2 are similar to the illness caused by RHDV. Under experimental conditions, RHDV2 disease differs from the classical RHDV1 in terms of disease duration, higher occurrence of subacute/chronic forms, and mortality.^15,31 A lower mortality rate is observed in rabbits infected by RHDV2 (5 to 70%), suggesting less virulence of the new variant.³¹

Diagnosis

Tissue samples should be made on fresh liver, spleen, and blood for detection using real-time PCR. Histopathological examination is recommended on formalin-fixed samples of liver, spleen, lung, kidney, and other organs. Although emphasis is given to PCR testing, post-mortem examination and histopathology can provide valuable information and should always be used in the presumptive diagnosis of RHD disease.¹⁷ Serum should be collected for serology.^26,31

Postmortem Findings

The disease is characterized by an acute necrotizing hepatitis and splenomegaly. Hemorrhages in the lungs, heart, and kidneys are also described and are due to disseminated intravascular coagulation. In subacute and chronic forms, icterus can be seen on the conjunctiva, ears, and subcutis.^4,26,31

Prevention and Control

Vaccination

Different types of vaccines against RHDV2 (Eravac, Filavac VHD K C+V), RHDV, or both serotypes (bivalent vaccines; Nobivac® Myxo-RHD PLUS) are commercially available and commonly used.^32,33 In France and Spain, vaccines specific for the new RHDV2 have been approved for the vaccination of domestic meat rabbits.^26,31

In the USA, an RHDV2 vaccine developed by Medgene Labs has been developed after several recent outbreaks. The vaccine is available in 45 states and has demonstrated efficacy against RHDV2.³⁴ Only killed RHD vaccines, FILAVAC and ERAVAC, can be imported under special authorization.

There is limited or no cross-protection between RHDV1 and RHDV2 as they belong to different genogroups.³¹ Vaccines developed against classic RHDV1 do not confer protection against RHDV2. Under experimental conditions, the recovery of some convalescent RHDV2 rabbits when challenged with classical RHDV, show that cross-protection between RHDV1 and RHDV2 is partial.¹⁵

Adverse side effects of the vaccine may include lethargy, slight fever, swelling at the vaccination site, and an anaphylactic reaction. A recent report describes fatalities in RHDV2-vaccinated companion rabbits due to rabbit hemorrhagic disease virus 2 infection.³⁵

Disinfection

RHDV can be inactivated with 4–10% sodium hydroxide or 1–2% formalin. Other disinfectants such as 0.5% sodium hypochlorite (10% household bleach) and 2% One-stroke Environ® (Vestal Lab, Inc., St. Louis, MO) seem to be efficient.²⁶

Zoonotic Potential

There is no indication that any lagoviruses infect humans.²⁶

References

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