JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

October 2024

D-V04

 

SLIDE A: Signalment (JPC# 3167482):16-week-old female nude (NU-Foxn1nu) mouse (Mus musculus)

 

HISTORY: Submitted for routine health monitoring.

 

HISTOPATHOLOGIC DESCRIPTION: Liver: Multifocally and randomly affecting 40% of the liver are areas of hepatocellular degeneration, necrosis, and loss. Foci of necrosis are characterized by either loss of hepatic tissue and cellular architecture with replacement by eosinophilic cellular and basophilic karyorrhectic debris (lytic necrosis) or loss of differential staining and retention of hepatic architecture (coagulative necrosis). Foci of necrosis are admixed with neutrophils, macrophages (some of which are hemosiderin-laden), lymphocytes, and variable amounts of hemorrhage, fibrin, and edema. There are multinucleated hepatocyte viral syncytial cells with greater than 50 occasionally pyknotic often at the periphery of necrotic foci. In the adjacent parenchyma, hepatocytes are swollen and often vacuolated (degeneration) and compress the adjacent sinusoids. Rarely, there are endothelial viral syncytial cells within the tunica intima of vessels. Periportal areas are expanded by dilated lymphatics, increased clear space separating collagenous tissue (edema), and aggregates of previously described inflammatory cells. Multifocally, there is occasional extramedullary hematopoiesis. The capsule is multifocally expanded up to 100µm thick by fibrosis and moderate numbers of previously described inflammatory cells, and it is multifocally mildly undulant, indented in areas of parenchymal loss.

 

MORPHOLOGIC DIAGNOSIS: Liver: Hepatitis, necrotizing, acute, random, multifocal, moderate to marked, with numerous hepatocellular and few endothelial viral syncytia, nude (NU-Foxn1nu) mouse, rodent.

 

ETIOLOGIC DIAGNOSIS: Coronaviral hepatitis

 

SLIDE B: Signalment (JPC# 4205568): Juvenile female albino mouse (Mus musculus)

 

HISTORY: This mouse was submitted for necropsy with no history. Gross findings included minimal body fat, sunken eyes, milk in the stomach with a small amount of liquid ingesta in the intestines, no formed feces in the colon, mild splenomegaly, thymic hypoplasia, and scant diarrhea on the ventral tail.

 

HISTOPATHOLOGIC DESCRIPTION: Ileum: Diffusely, there is severe blunting and loss of 80% of the intestinal villar tips. Multifocally, the enterocytes lining the villar tips form viral syncytial cells that are up to 60µm diameter and contain pale, flocculent eosinophilic cytoplasm and greater than 30 nuclei. Syncytial cells occasionally contain 2-3µm eosinophilic intracytoplasmic viral inclusions. Crypt epithelium multifocally contains frequent mitotic figures and enlarged, hyperchromatic nuclei (regeneration) and crypt lumens contain neutrophils and cellular debris (crypt abscesses). The lamina propria contains a mild inflammatory infiltrate composed predominantly of lymphocytes and plasma cells. There is multifocal mixed inflammation within the serosa and adjacent mesentery.

 

Colon: Multifocally the colonic epithelium is ulcerated and replaced by eosinophilic proteinaceous fluid, cellular and karyorrhectic debris, and viable and degenerate neutrophils (lytic necrosis), and the subjacent lamina propria and submucosa contains a multifocally robust inflammatory infiltrate composed of neutrophils, lymphocytes, and plasma cells. In less affected areas, crypts are multifocally dilated up to 10 times normal diameter, lined by attenuated epithelium, and filled with viable and degenerate neutrophils and cellular debris. There is multifocal fusion of crypt epithelial cells (syncytia) occasionally containing 2-3µm eosinophilic, intracytoplasmic viral inclusions. Lymphatics are multifocally dilated (edema). There is multifocal mixed inflammation within the serosa and adjacent mesentery.

 

Lymph node: Within the subcapsular and medullary sinuses there are moderate numbers of neutrophils and histiocytes. 

 

Pancreas, ovary: No significant findings. 

 

MORPHOLOGIC DIAGNOSIS: Ileum, colon: Enterocolitis, necrotizing, subacute, multifocal, severe, with viral syncytia and intracytoplasmic viral inclusions, mouse, rodent.

 

ETIOLOGIC DIAGNOSIS: Intestinal coronavirosis

 

CAUSE: Mouse Hepatitis Virus (MHV, murine coronavirus)

 

SYNONYM: Lethal Intestinal Virus of Infant Mice (LIVIM)

 

GENERAL DISCUSSION: 

• Common, highly contagious, and highly mutagenic virus that affects all ages and strains of mice; most likely mouse disease to interfere with biological responses and contaminate transplantable tumors and cell lines
• Pleomorphic, enveloped, 80‑150 nm diameter RNA virus that replicates exclusively in the cytoplasm of infected cells
• High mortality in infant mice (LIVIM), immunodeficient mice, or genetically susceptible mice; neonates protected by passive immunity
• Coinfection of enterotropic MHV with bacteria (i.e., E.coli) or parasites (Spironucleus muris) common 
• MHV is divided into two biologically distinct groups:
  • Respiratory (Polytropic) (e.g., MHV-JHM, MHV-A59, MHV-S, MHV-3) initially replicates in nasal mucosa & disseminates to other organs
    • Immunocompromised animals: Systemic disease; acute necrosis with syncytia in liver, splenic red & white pulp, GALT, thymus, and bone marrow
    • Neonatal mice: Vascular-oriented necrotizing encephalitis with spongiosis and demyelination in brain stem
    • C3H mouse: Semi-susceptible
    • BALB/c & DBA/2 mice: Susceptible 
    • SJL and A/J mice: Resistant 
  • Enterotropic (MHV-S/CDC, MHV-Y, MHVR1, MHV-D) infects intestinal mucosa of terminal ileum, cecum, ascending colon & rarely disseminates
    • Neonatal mice: Severe necrotizing enterocolitis; high mortality
    • Adult mice: Minimal lesions - enterocyte syncytia in the cecum and ascending colon
    • Immunodeficient adult mice: Minimal hyperplasia in intestinal mucosa
    • Adult nude mice: Mild intestinal lesions with minimal mucosal hyperplasia; can develop a chronic hyperplastic typhlocolitis and mesenteric lymphadenopathy
    • All ages and strains of mice are susceptible, including the SJL mouse (which is resistant to polytropic strains)

 

PATHOGENESIS:

• Highly contagious by the oronasal route 
• Polytropic strains: Initial replication in the nasal mucosa → hematogenous or lymphatic dissemination (in susceptible strains, mice < 2 weeks old, immunocompromised mice) → 2º replication in endothelium and parenchyma of brain, liver, lymphoid organs, bone marrow and other sites → disease → immune-mediated viral clearance → by 3-4 weeks, resolution of infection or carrier state except in nude or SCID mice which develop progressively severe disease (unable to mount immune-mediated viral clearance)
• Infection of adult mouse brain can occur directly via the olfactory neural pathway
• Enterotropic strains: Selectively infect the intestinal mucosal epithelium and rarely disseminate (the severity of the disease is age-related in neonates, not immune-related; infected adult SCID and nude mice have minimal lesions)
• Host immunity is MHV strain specific, thus recovery from one strain will protect against re-exposure to that strain, but provides little to no protection against infection from heterotypic strain
• Innate and acquired cell mediated and humoral immunity are important for controlling infection
• T cells critical for viral clearance & recovery
• Maternal protection against polytropic MHV is via serum IgG passed in utero to the fetus or through intestinal IgG receptors in the post-natal period.
• Maternal protection against enterotropic MHV is luminal whey containing IgA or IgG
• Zona pellucida provides effective barrier against MHV infection

 

TYPICAL CLINICAL FINDINGS:

• The clinical signs in suckling, weanling and adult mice vary with the virus strain, tissue tropism, mouse strain, and age of mice
• Acute disease: Ruffled hair, depression, inanition, dehydration, rapid wasting, huddling, muscle tremors, dark yellow urine, and reluctance to move
• Chronic or latent infections: No signs or porencephaly, paralysis, hepatitis, encephalitis, lymph node adenopathy, vasculitis, and demyelination
• Neurologic signs, including vestibular signs and posterior paresis, are typically seen in immunodeficient mice; rare in immunocompetent mice
• Clinical pathology: Pancytopenia with polytropic MHV infections

 

TYPICAL GROSS FINDINGS:

• Liver: Multifocal, random necrosis (white foci)
• Intestines: May have necrotizing enterocolitis
  • Neonates: Thin-walled intestines filled with watery, yellow digesta
  • Weanlings and adults: Thickened bowel segments due to mucosal hyperplasia (ascending colon and ileocecal junction in older mice)
• Runted and jaundiced infant mice 
• Involution of lymph nodes, spleen, and thymus or splenomegaly (due to EMH), lymphadenopathy
• Ascites +/- peritoneal hemorrhagic exudate
• IFN-γ receptor -/- mice & IFN-γ -/- mice may develop granulomatous serositis with or without hepatic/intestinal lesions 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Polytropic:
  • Focal acute necrosis & syncytia of parenchymal cells & vascular endothelium of multiple organs
  • +/- focal peritonitis
  • Syncytia of pulmonary vascular endothelium common in immunodeficient mice
  • Liver: Necrotizing hepatitis, syncytial cells within necrotic foci (usually only apparent in immunodeficient mice), neutrophilic and histiocytic infiltrates, hepatocellular regeneration, and fibrosis
  • Bone marrow: Necrosis of hematopoietic cells, syncytial giant cells
  • CNS: Necrotizing encephalitis, meningitis, spongiosis, demyelination, syncytial giant cells (neonatal and immunodeficient mice)
  • Spleen, red pulp: Necrotizing splenitis, syncytial giant cells
  • Lymphoid tissue: Necrosis, syncytial giant cells
  • Olfactory mucosa, nerves, bulbs & tracts of the brain: Meningoencephalitis and demyelination
  • Hematopoietic hyperplasia (compensatory for pancytopenia) 
• Enterotropic: Dependent upon age of mouse
  • Terminal ileum, cecum, ascending colon
  • Villus attenuation, enterocyte syncytia (balloon cells), mucosal necrosis
  • Eosinophilic intracytoplasmic inclusion bodies 
  • Syncytial cells within mesenteric lymph nodes (lymphocytic syncytia) and endothelium of mesenteric vessels
  • Surviving mice develop compensatory intestinal mucosal hyperplasia

 

ULTRASTRUCTURAL FINDINGS:

 

ADDITIONAL DIAGNOSTIC TESTS:

• Active infection via immunohistochemistry or virus isolation
• PCR or serology
• Bioassay methods: MAP testing or infant/nude mouse inoculation

 

DIFFERENTIAL DIAGNOSIS:

Gastrointestinal disease:

• Adult mice:
  • Tyzzer’s disease (Clostridium piliforme, D-B06):Multifocal hepatic necrosis with intracytoplasmic bacilli; no giant cells
  • Salmonellosis (S. enteritidis, S, typhimurium, D-B01):multifocal necrosis progressing to granulomatous hepatic lesions (“paratyphoid nodules”); venous thrombi
  • Ectromelia (mouse pox; Orthopoxvirus, Poxviridae, I-V06):Splenic necrosis (“tiger striping”), skin lesions, multifocal hepatic necrosis; cytoplasmic eosinophilic inclusion bodies
• Infant mice:
  • Epizootic Diarrhea of Infant Mice (EDIM; Rotavirus A, D-V07):Less severe disease in neonatal mice; no syncytial cells or endothelial changes; epithelial vacuolar degeneration in ileum and jejunum; epithelial eosinophilic intracytoplasmic inclusion bodies
  • Reovirus-3 infection (orthoreovirus; Reoviridae):Foci of hepatic necrosis and CNS lesions; also, myocardial necrosis and pulmonary hemorrhages
  • Adenovirus: Intranuclear inclusion bodies
  • Salmonellosis (S. enteritidis, S, typhimurium, D-B01):multifocal necrosis; venous thrombi; granulomatous hepatic lesions
  • Cytomegalovirus (MCMV):Resistant strains include B6, B10, CBA, C3H; susceptible strains are BALB/c and A; macrophage is target cell

 

Neurologic disease:

• Mouse encephalomyelitis virus (MEV)
• Lactate dehydrogenase-elevating virus (LDV) in immunosuppressed AKR, C58 mice
• Polyoma virus (MPyV) in immunodeficient mice

            

COMPARATIVE PATHOLOGY:

Selected Coronaviruses of other species:

• Coronaviruses (CoV) affect many species and typically cause mild enteric or respiratory disease in affected species
  • With enteric infection, villous atrophy and fusion is a common lesion
• Coronaviruses that utilize divergent host receptors can jump to a different host species and/or affect a wide range of cell types within its host (Kenney, Vet Pathol 2021)

 

Condition	Animal/Species	Disease/Signs/Lesion
Avian infectious bronchitis virus (IBV) Gammacoronavirus (gCoV)	Chicken	Tracheobronchitis, nephritis, wrinkled eggs, decreased egg production, uremia
Bluecomb disease virus Gammacoronavirus (gCoV)	Turkeys	Enteritis; infectious diarrhea, cyanosis of the comb
Bovine coronavirus (BCoV) Betacoronavirus (bCoV)	Bovine	Gastroenteritis, coronavirus implicated;diarrhea in neonatal calves; pneumonia in calves; winter dysentery in adults
Canine enteric coronavirus (D-V03) Alphacoronavirus (aCoV)	Canine	Gastroenteritis and diarrhea; villous atrophy; severe enteritis and leukopenia less frequently
Canine respiratory coronavirus Betacoronavirus (bCoV)	Canine	Respiratory disease
Equine coronavirus  Betacoronavirus (bCoV)	Horses	Gastroenteritis
Feline infectious peritonitis virus (FIP; P-V15; N-V17; S-V03) Alphacoronavirus (aCoV)	Feline	Caused by mutated form of FECV; peritonitis, pneumonia, meningoencephalitis, panophthalmitis; granulomatous vasculitis
Feline enteric coronavirus (FECV) Alphacoronavirus (aCoV)	Feline	Diarrhea in kittens; mild villous atrophy
Ferret coronavirus (Epizootic catarrhal enteritis; ECE)	Ferret	Profuse, green mucoid diarrhea in adults; diffuse lymphocytic enteritis; villous atrophy, fusion, and blunting
Ferret systemic coronavirus Alphacoronavirus (aCoV)	Ferret	Granulomatous systemic inflammation similar to the dry form of FIP in cats (Doria-Torra, Vet Pathol. 2016)
Mouse hepatitis virus  Betacoronavirus (bCoV)	Human	Progressive multifocal leukoencephalopathy
SARS-CoV-1 Betacoronavirus (bCoV)	Humans	Severe Acute Respiratory Syndrome;  zoonotic with bats and civet cats as natural reservoir
SARS-CoV-2 Betacoronavirus (bCoV)	Humans/Mink	Coronavirus Disease 2019; respiratory disease zoonotic with bats; unknown natural reservoir
MERS-CoV Betacoronavirus (bCoV)	Humans	Middle Eastern Respiratory Syndrome; zoonotic with bats and dromedary camels as natural host
Porcine transmissible gastroenteritis virus (TGEV; D-V06) Alphacoronavirus (aCoV)	Porcine	Gastroenteritis; diarrhea/vomiting in all age groups with high piglet mortality
Porcine hemagglutinating encephalomyelitis virus Betacoronavirus (bCoV)	Porcine	Vomiting, wasting, and encephalomyelitis (usually no diarrhea)
Porcine epidemic diarrhea virus (PEDV) Alphacoronavirus (aCoV)	Porcine	Gastroenteritis (western Europe); identical clinical, gross, microscopic lesions to TGE; requires PCR or IHC to distinguish from TGE
Porcine respiratory coronavirus (PRCoV) Alphacoronavirus (aCoV)	Porcine	Mild respiratory disease
Swine acute diarrhea syndrome (SADS) - CoV Alphacoronavirus (aCoV)	Porcine	Currently emerging; gastroenteritis; watery diarrhea, vomiting, dehydration
Porcine deltacoronavirus (PDCoV) Deltacoronavirus (dCoV)	Porcine	All age groups exhibit diarrhea/vomiting; gastroenteritis in sows and nursing pigs; mortality rates in nursing piglets lower than with PEDV; clinically indistinguishable from TGEV and PEDV
Mouse hepatitis virus(MHV; D-V04) Betacoronavirus (bCoV)	Mouse	Polytropic strains: Hepatic necrosis, enteritis, encephalomyelitis; syncytia formation
Parkers rat coronavirus (PRC)	Rat	Primarily lung lesions with only minimal salivary or lacrimal involvement
Rat coronavirus Betacoronavirus (bCoV)	Rat	Rhinitis, tracheitis, pneumonitis in young
Rat coronavirus - Sialodacryoadenitis virus  (SDAV; D-V05; S-V02) Betacoronavirus (bCoV)	Rat	Sialodacryoadenitis, porphyrin released from damaged harderian gland, squamous metaplasia of ducts
Rabbit coronavirus	Rabbits	Enteritis, myocarditis

 

Information in the above table was adapted from U-V06 with additional details from Kenney, Vet Pathol 2021.

 

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents & Rabbits. 4th ed. Ames, IA: John Wiley & Sons, Inc.; 2016:27-31.
2. Delaney MA, Treuting PM, Rothenburger JL. Rodentia. In: Terio KA, McAloose D, Judy St. Leger J, ed. Pathology of Wildlife and Zoo Animals. Cambridge, MA Academic Press; 2018:507.
3. Doria-Torra G, Vidaña B, Ramis A, Amarilla SP, Martínez J. Coronavirus Infection in Ferrets: Antigen Distribution and Inflammatory Response. Vet Pathol. 2016;53(6):1180-1186.
4. Kenney SP, Wang Q, Vlasova A, Jung K, Saif L. Naturally Occurring Animal Coronaviruses as Models for Studying Highly Pathogenic Human Coronaviral Disease. Vet Pathol. 2021;58(3):438-452.
5. Miller AD, Porter, BF. Nervous System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:908.
6. Uzal FA, Plattner BL, Hostetter JM.Alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:146-151, 529, 541-542. 

 

 

 

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