JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
December 2021
D-V31

 

Signalment:  Rabbit (Oryctolagus cuniculus)

 

HISTORY:  None.

 

HISTOPATHOLOGIC DESCRIPTION:  Liver:  Diffusely affecting periportal areas and often variably extending into the midzonal and centrilobular areas (panlobular/massive), there is lytic necrosis characterized by loss of hepatic cord architecture and replacement with eosinophilic cellular and karyorrhectic debris, hemorrhage, fibrin, edema, and rare heterophils, as well as coagulative necrosis characterized by retention of hepatic cord and hepatocellular architecture and loss of differential staining.  Diffusely, remaining hepatocytes within midzonal regions and extending into centrilobular areas display one or more of the following changes: cell swelling with pale, discretely vacuolated cytoplasm (lipid-type degeneration) or eosinophilic, lacy cytoplasm (glycogen-type degeneration); or are shrunken and hypereosinophilic with a pyknotic nucleus (single cell death).  Within affected areas, sinusoids are congested or compressed, and occasionally Kupffer cells are in aggregates (micronodules) and contain yellow to brown cytoplasmic pigment (hemosiderin or bile).  Multifocally, portal areas are expanded by few lymphocytes, plasma cells, and occasional heterophils, and portal lymphatics are ectatic.

 

MORPHOLOGIC DIAGNOSES:  Liver:  Hepatocellular degeneration and necrosis, diffuse, massive, acute, severe, European rabbit, Oryctolagus cuniculus.

 

ETIOLOGIC DIAGNOSIS:  Caliciviral hepatitis

 

CAUSE:  Calicivirus: Rabbit hemorrhagic disease virus (RHDV)

 

CONDITION: Rabbit hemorrhagic disease virus (RHDV)

 

GENERAL DISCUSSION:

• There are three pathogenic lagoviruses (genus), all are within Caliciviridae family, non-enveloped, ssRNA, icosahedral viruses:
• Rabbit hemorrhagic disease (RHD) virus: only affects Oryctolagus species of rabbits; Sylvilagus spp. rabbits found in the Americas are not affected
• European brown hare syndrome virus (EBHS) affects only hare species (wild & farmed): Lepus europaeus (European brown hare), Lepus timidus (mountain or northern hare) and Lepus timidus (varying hare)
• RHDV2: recently emerged and kills rabbits under 30 days of age, including those vaccinated for RHDV, has 20% (5-80%) mortality, affects rabbits and hares, and RHD ELISA does not detect it
• EBHS and RHD represent distinct, though closely related agents, causing similar clinical signs and pathological and histological changes, mortality rates, virion morphology and antigenicity; however, there is no cross-species infection and no cross-species protection
• RHD:
• Affects older animals with >80% mortality: young rabbits and hares < 4-8 weeks of age do not develop clinical disease
• Rabbits between 4-8 weeks of age are protected by maternal antibodies and if infected during this time, can develop immunity
• OIE reportable disease, depopulation is required for control

 

PATHOGENESIS: 

• Spread by direct and indirect routes: oral fecal, aerosol, fomites, +/- vectors
• Target organs: Liver, lungs, spleen
• Viral replication occurs in the hepatocytes and causes apoptosis via VP10;
• Young animals have virus in their hepatocytes, but it does not cause apoptosis
• RHDV binds specific host-cell histo-blood group antigens (HBGA) that are expressed on the surface of epithelial cells of the upper respiratory and digestive tracts; possible site of entry into host;
• Younger animals have fewer HBGA possibly contributing to resistance
• Death is due to multiple organ failure secondary to consumptive coagulopathy (DIC); DIC may result from systemic endothelial damage from viremia and/or as a consequence of massive hepatic necrosis, with activation of extrinsic coagulation factors and failure of clearance of activated procoagulant factors

 

TYPICAL CLINICAL FINDINGS:

• 3 different clinical courses:
  • Peracute: No clinical signs; sudden death within 1 day
  • Acute: anorexia, apathy, congestion of the palpebral conjunctiva, and neurological symptoms (opisthotonus, excitement, paralysis, and ataxia), with variable respiratory signs (tracheitis, dyspnea, and cyanosis) and epistaxis; death within 3 days of clinical signs
  • Subacute: Similar, though milder clinical signs than acute form; usually death within 1-3 weeks with few survivors
• Mortality highest in fall when population most dense

 

TYPICAL GROSS FINDINGS: 

• Pale, fatty liver with attenuated lobular pattern & hemorrhage
• Multisystemic hemorrhage especially the lungs, heart, kidneys, and from nares

 

CLINICAL PATHOLOGY:

• Aspartate aminotransferase (AST), alanine aminotransferase (ALT), bilirubin all elevated shortly before death
• Hepatic calcium content in affected hares is increased up to 20 times normal

 

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

• Liver:
  • Periportal to massive hepatocellular necrosis and degeneration with lipid-type vacuolar change
  • Acidophilic bodies: spherical, eosinophilic globules (condensed cytoplasm of apoptotic hepatocytes) within sinusoids to be phagocytized by Kupffer cells
  • +/- inflammatory cells (macrophages, heterophils) at portal areas
• Lymphopenia (T and B-cell) in both liver and spleen
• Splenic congestion & necrosis
• Pulmonary edema & hemorrhage
• Segmental necrotizing enteritis in small intestine
• Multisystemic hemorrhage and fibrin thrombi in small blood vessels

 

ULTRASTRUCTURAL FINDINGS:

• 27-39 nm in diameter; has scalloped border with 32 cup-shaped surface indentations arranged in icosahedral pattern

 

ADDITIONAL DIAGNOSTIC TESTS: 

• ELISA, RT-PCR
• Immunohistochemistry: Demonstration of viral antigen in hepatocytes, mononuclear cells

 

DIFFERENTIAL DIAGNOSIS: 

For hepatic necrosis in hares:

• Toxoplasma gondii (P-P01, N-P02, R-P01)
• Tyzzer’s disease (Clostridium piliforme; D-B06): Characteristic bacilli within the cytoplasm of hepatocytes along the periphery of the lesion
• Tularemia/rabbit fever (Francisella tularensis; D-B14): Necrosis in liver, spleen, and bone marrow; acute suppurative lesions; chronic granulomatous lesions; many gram-negative bacteria
• Bubonic plague (Yersinia pestis): Rare in domestic species; necrosis of liver, spleen, cecum, occasionally reproductive tract and lymph nodes; large bacterial colonies
• Salmonella (D-B01): Hepatic necrosis with paratyphoid nodules
• Listeriosis (Listeria monocytogenes): Hepatic necrosis; abortions in pregnant does
• Liver lobe torsion: caudate lobe most commonly; ischemic coagulative necrosis with sinusoidal dilation

 

COMPARATIVE PATHOLOGY: 

Caliciviridae of veterinary importance, four genera:

• Lagovirus
• RHDV: RHDV viral RNA has been detected in small mammals (mice and shrew species) sympatric to wild rabbits
• EBHSV
  • RHDV2: A recent study describing the pathologic findings of an RHDV2 outbreak that occurred in Washington state, in July 2019, reports multifocal, random hepatocellular necrosis as the most consistent pathologic finding (Williams, J Vet Diagn Invest. 2021)

 

• Vesivirus
• Feline calicivirus (P-V08): Important cause of respiratory disease in cats; ulcerative stomatitis, mucopurulent conjunctivitis, and oculonasal discharge; virulent systemic strains associated with profound fever; anorexia; marked subcutaneous edema (esp. limbs and face); icterus; alopecia; crusting; ulceration of the nose, lips, pinnae, and feet; bronchointerstitial pneumonia; and pancreatic, hepatic, and splenic necrosis in addition to oculonasal discharge; adult cats at higher risk for severe disease and death
• San Miguel sea lion virus (I-V14): Vesicular lesions on flippers; infects a variety of sea mammals; causes vesicular exanthema of swine when infected sea mammal carcasses are fed to pigs
• Vesicular exanthema of swine: Grossly indistinguishable from foot-and-mouth disease (Picornaviridae, Aphthovirus), swine vesicular disease (Picornaviridae, Enterovirus), and vesicular stomatitis (Rhabdoviridae, Vesiculovirus); eradicated from the U.S.; virus still present in marine mammals

 

• Norovirus
• Norwalk virus: humans

 

• Sapovirus
• Causes gastroenteritis in pigs (genogroup III) and humans (genogroup I, II, IV, V)

 

• Hepevirus
• Avian hepatitis E virus
• Swine hepatitis E virus
• Hepatitis E virus (human, monkey, pig)

 

REFERENCES:

1. Asin J, Nyaoke AC, Moore JD, et al. Outbreak of rabbit hemorrhagic disease virus 2 in the southwestern United States: first detections in southern California. J Vet Diagn Invest. 2021;33(4):728-731.
2. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4^th Ames, IA: John Wiley & Sons, Inc.; 2016:264-266.
3. Cheville NF, Lehmkuhl H. Cytopathology of viral diseases. In: Cheville NF, ed. Ultrastructural Pathology: the comparative cellular basis of disease. 2nd ed. Ames, IA: Wiley-Blackwell; 2009:399-401.
4. Delaney MA, Treuting PM, Rothenburger JL. Lagomorpha. In: Terio K, McAloose D, Leger J, ed. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018: 490-491.
5. O'Toole AD, Zhang J, Williams LBA, Brown CC. Detection of rabbit hemorrhagic disease virus 2 in formalin-fixed, paraffin-embedded tissues via in situ hybridization. J Vet Diagn Invest. 2021; Sep 23:10406387211047561. (Epub ahead of print)
6. Parker J, Pesavento P. Family In: Machlachlan HN, Dubovi EJ, eds. Fenner’s Veterinary Virology. 5th ed. San Diego, CA: Elsevier; 2017:497-506.
7. Williams LBA, Edmonds SE, Kerr SR, et al. Clinical and pathologic findings in an outbreak in rabbits of natural infection by rabbit hemorrhagic disease virus 2 in the northwestern United States. J Vet Diagn Invest. 2021;33(4):732-735.

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