1-year-old male rabbit, Oryctolagus cuniculus.Found dead without premonitory signs.
Poor body condition score (2/5). Bilateral diffuse lung congestion. Diffusely pale, enlarged liver containing multiple randomly distributed < 1mm diameter, ovoid pale yellow lesions.
Liver: Diffuse hepatocyte necrosis with the cells exhibiting hyper-eosinophilic, frequently shrunken and fragmented cytoplasmic outline and karyopyknosis/lysis. Entire hepatic acini are involved including cells of the limiting plates (massive necrosis), with associated light scatterings of heterophils (peracute event). Bile ducts are severely distended with florid papillomatous hyperplasia of the lining epithelium and in some cases by infiltrates of macrophages, multinucleate macrophage giant cells with surrounding mantles of admixed lymphocytes and plasma cells in the fibrotic portal stroma. Eosinophilic refractile debris along with myriad ovoid to oblong structures with well-defined cytoplasmic boundaries and contained crescent-shaped structures with eosinophilic cytoplasm and a single nucleus (unsporulated coccidial oocysts) and smaller, ovoid structures with basophilic granular content (macrogametes) noted in duct lumens.
Liver: 1. Hepatocyte necrosis, peracute, diffuse (massive), severe, consistent with a diagnosis of viral haemorrhagic disease of rabbits.
2. Bile duct hyperplasia and granulomatous cholangitis, chronic, severe with intra-luminal oocysts/macrogametes consistent with a diagnosis of hepatic coccidiosis.
Viral hemorrhagic disease; Eimeria stiedae
This case was unusual in that the rabbit presented with a severe, peracute necrotising hepatitis, consistent with a diagnosis of viral haemorrhagic disease of rabbits (VHD), superimposed on more longstanding lesions of hepatic coccidiosis. The most consistent pathological findings in VHD are severe necrotising hepatitis and disseminated intravascular coagulation (DIC).(2,5,13,14,17) Complete hepatic acini including the hepatocytes of the limiting plates are frequently affected in this case justifying the designation massive necrosis. In previous reports hepatocyte necrosis is described as confined to the peripheral zones of lobules.(5) Such extensive peracute hepatic necrosis in rabbits found dead without premonitory signs is considered pathognomonic for VHD.
VHD is an acute, highly fatal disease of European wild and domestic rabbits, first reported from the Peoples Republic of China in 1984.(5,10) The causative agent is a calicivirus.(10,13) The virus spreads via oral, nasal or parenteral transmission and rapidly replicates in the liver of adult rabbits resulting in death within 48 - 72 hours.(9,13) Virus-induced hepatocyte death is due to apoptosis and in experimentally induced disease, in-situ hybridization identified viral replication in both hepatocytes and macrophages, suggesting infected macrophages contribute to viral dissemination.(2,7,14) Naturally occurring VHD is rarely seen in rabbits less than two months of age, possibly due to differences in the leucocyte response to hepatocyte infection between adults and juveniles. The lymphocytic rather than heterophilic response observed in the more resistant younger rabbits possibly reflects a protective host response to viral antigens on the hepatocyte surface.(4)
Hepatic coccidiosis in rabbits occurs worldwide and is caused by infection with Eimeria stiedae.(6,16) The coccidial oocysts are highly resistant and remain viable in soil and on fomites for long periods. Rabbits become infected by ingesting sporulated oocysts which are broken down in the duodenum, releasing sporozoites that penetrate the intestinal mucosa and travel to the liver either via the blood stream or within macrophages in the lymphatic system. In the liver the sporozoites invade the bile duct epithelium. Following developmental and reproductive phases oocysts are produced and are passed via the bile ducts into the intestines. Oocysts become infective one to four days after they are shed in the faces.(6,16) The environment can become heavily contaminated in intensive conditions and wild rabbits can be a potential source of infection to domestic rabbits allowed access to grass grazed by wild rabbits.(6)
Infection of bile duct epithelial cells results in hyperplasia and inflammation with large numbers of ellipsoid oocysts in the walls and lumen of the bile ducts.(12) Destruction and regeneration of the bile duct epithelium results in significant cystic enlargement, papillomatous hyperplasia and duct reduplication. There is usually accompanying infiltration of plasma cells, lymphocytes, and occasional epithelioid cells.(12,15) Some enlarged bile ducts rupture initiating a granulomatous response and accompanying portal fibrosis is often prominent.(12) The oocysts may obstruct biliary outflow resulting in a distended bile duct and ischaemic necrosis can occur in the surrounding liver parenchyma due to compression of adjacent blood vessels by the bile duct enlargement.(12) In rabbits which survive such infection, fibrous tissue can replace much of the normal liver parenchyma.(16)
While there may be no clinical signs in mild infections, heavy infections can result in abdominal enlargement and ascites, with jaundice occurring in advanced stages of the disease. These signs reflect blockage of the bile ducts and interference with hepatic function.(12) Serum biochemistry may reveal elevated alkaline phosphatase, ALT and total bilirubin.(6)
1. Liver: Necrosis, massive, diffuse.
2. Liver: Cholangiohepatitis, proliferative and lymphoplasmacytic, diffuse, moderate, with coccidial oocysts and gametocytes.
Viral hemorrhagic disease virus (also referred to as rabbit hemorrhagic disease virus, RHDV) along with the closely-related and highly pathogenic European brown hair syndrome virus (EBHSV) make up the Lagovirus genus of the family Caliciviridae.(8) RHDV causes severe acute periportal to mid-zonal hepatic necrosis resulting in disseminated intravascular coagulopathy in rabbits in the Oryctolagus genus; whereas EBHSV causes similar disease in members of the Lepus genus. Recently there has been a report of an outbreak of a novel virus, designated Michigan rabbit calicivirus (MRCV), in a privately-owned New Zealand White rabbitry in Michigan. This outbreak was associated with a case fatality rate of 32.5%, with clinical signs including vulvar hemorrhage, epistaxis, ataxia, diarrhea, opisthotonos, ocular discharge, vocalization, hepatic necrosis and death. Experimental infection resulted in subclinical disease, and viral RNA sequencing and capsid amino acid sequencing indicated this calicivirus is distinct from other know lagoviruses.(3) It was suggested that MRCV represents a new variant of the nonpathogenic RCV-like group that includes RCV, Ashington and Lambay.(1) Further research is needed to fully elucidate the phylogenetic relationships of these viruses.(3)
Caliciviruses are non-enveloped, icosahedral, single stranded RNA viruses with characteristic 32 cup-shaped depressions on their surface. Other caliciviruses of veterinary importance include: vesicular exanthema of swine virus, San Miguel sea lion virus, feline calicivirus, and murine norovirus. Other caliciviruses in the genera Norovirus and Sapovirus have been reported to cause disease in cattle, pigs, wildlife, and non-human primates.(8)
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