JPC SYSTEMIC PATHOLOGY
SPECIAL SENSES 
April 2021
S-V03

Signalment (JPC #1725431):  Six-month-old female cat

HISTORY:  Tissue from a six-month-old female cat that developed an eye infection that was treated with topical medication for several weeks.  During this time, the cat became lethargic and ataxic.  After a period of anorexia lasting three weeks, the cat was euthanized.

HISTOPATHOLOGIC DESCRIPTION:  Eye:  All parts of the uvea (the choroid, ciliary body, and iris) are moderately expanded by multifocal, often perivascular infiltrates of neutrophils, macrophages, and fewer lymphocytes and plasma cells, admixed with abundant fibrin and edema.  Multifocally throughout the uvea and adjacent sclera, blood vessels are lined by reactive endothelium with occasional expansion of the tunica media and adventitia by edema and infiltration by a similar population of inflammatory cells (vasculitis).  The retina is detached from the underlying hypertrophic retinal pigment epithelium (RPE), and is folded, and there is eosinophilic proteinaceous material and few neutrophils (exudate) and sloughed RPE cells in the post-retinal space.  There is multifocal loss of photoreceptor cells, loss of cells in the inner nuclear layer, loss or degeneration of remaining ganglion cells, and vacuolation of the nerve fiber layer (axonal degeneration or spongiosis) with exposure of prominent Muller’s fibers.  Multifocally, the optic nerve is vacuolated, characterized by dilated myelin sheaths that occasionally contain cellular debris and foamy, vacuolated macrophages (digestion chambers).  Within the anterior chamber there is abundant eosinophilic proteinaceous fluid admixed with fibrin and moderate numbers of neutrophils and macrophages (hypopyon).  This proteinaceous material and inflammatory population bilaterally fills and obscures the iridocorneal drainage angles and also fills lymphatics. Both the bulbar and palpebral conjunctivae are moderately expanded by fibrin, edema, and lymphoplasmacytic nodular and perivascular infiltrates.

MORPHOLOGIC DIAGNOSIS:  Eye: Uveitis and scleritis, pyogranulomatous and lymphoplasmacytic, marked, with pyogranulomatous vasculitis, retinal detachment, hypopyon, and optic nerve degeneration, breed unspecified, feline.

ETIOLOGIC DIAGNOSIS:  Feline coronaviral uveitis

CAUSE:  Mutated feline enteric coronavirus (feline infectious peritonitis virus)

GENERAL DISCUSSION:

• Family Coronaviridae, genus Alphacoronavirus - enveloped, single stranded, positive sense RNA viruses
• Feline infectious peritonitis (FIP) is a worldwide, invariably fatal, sporadic, low prevalence viral disease of domestic and wild felids caused by feline coronavirus (FCoV)
• Purebred domestic cats and certain species of large cats (e.g. cheetahs, lions), as well as young intact males, may be predisposed
• FECV infects and replicates only in enterocytes, causing diarrhea or asymptomatic infection; FIPV infects and replicates primarily in macrophages, resulting in macrophage activation and systemic infection
• Most cats that die of FIP have ocular involvement; this is detected by coagulation of the aqueous with acidic fixatives (indicating increased aqueous protein)

PATHOGENESIS:

• Fecal-oral transmission and possibly inhalation of FCoV > replication in enterocytes / lymphoid system > mutation (FIPV), virus able to replicate in macrophages > secondary macrophage associated viremia > dissemination to multiple organs and vessels > host immune response
• Mutation is presumed to occur at 3c gene of feline enteric coronavirus
• The progression of disease depends on the cat’s immune response:
  • Strong cell-mediated immune response: Results in activation of macrophages, FIP virus replication is terminated, and cleared 
  • Weak or ineffective cell-mediated response: Delayed (Type IV) hypersensitivity response;  non-effusive (dry form) syndrome ensues, with a less florid macrophage response in tissue and reduced virus production; this form has a more prolonged clinical course (1-6 months)
  • No/ineffective cell-mediated immunity: Antibody is produced, but there is a failure to generate a cell-mediated response and cats develop effusive disease (wet form); vasculitis results from both Type III hypersensitivity response [primary immune complex deposition] and activation of macrophages; this syndrome has a rapid clinical course, progressing to death in 1-12 weeks
• Granulomatous inflammation and granulomatous phlebitis (due to enhanced adhesion of virally infected monocytes/macrophages to endothelial cells)
• Antibody-mediated lysis of infected macrophages in or around vessels (predominantly veins) may enhance the inflammatory reaction
• Release of substance from infected cells causes apoptosis of bystander lymphocytes
• Immunity may be associated with low TNF-alpha and high IFN-gamma responses, whereas the converse favors disease

TYPICAL CLINICAL FINDINGS:

• Noneffusive (dry form): Typically granulomatous inflammation, localized in the lymph nodes, kidneys, uvea, meninges, ependyma, and choroid plexus of the brain and spinal cord with vague signs of dullness, weight loss, and anorexia, and CNS signs (ataxia, nystagmus, seizures)
  • Ophthalmic signs are far more common in the noneffusive form and include anterior uveitis, chorioretinitis, miosis, nystagmus, aqueous flare, hypopyon, hyphema, blepharospasm, and epiphora
  • Clinicopathological features:

• Neutrophilia with left shift
• Lymphopenia
• Nonregenerative anemia ([HCT] <30%; anemia of chronic disease)
• Cerebral spinal fluid elevated protein levels (56-348 mg/dL with normal <25 mg/dL) and pleocytosis (100-10,000 nucleated cells/mL)

• Effusive (wet form): Weight loss, dyspnea, tachypnea, mild pyrexia, icterus, scrotal enlargement, palpable abdominal distention and masses (from adhesions)
  • Clinicopathological Features:

• Plasma proteins typically elevated because of hypergammaglobulinemia (may be polyclonal or monoclonal) due to chronic inflammation and antibody production
• Effusion is a high protein exudate or modified transudate with a high protein concentration (>3.5g/dL)
• Albumin:globulin ratio less than 0.8 and typically 0.45 or less (albumin level remains normal or falls slightly and globulin levels increase, possibly through stimulation of B cells by IL-6)
• Elevated Alpha1-acid glycoprotein (AGP) >1500 ug/mL
• Neutrophillia with left shift

TYPICAL GROSS FINDINGS: 

• Noneffusive form – Granulomatous lesions in various organs (on surface and throughout)
  • Eye: Keratic precipitates in anterior chamber, uveitis, panophtalmitis, and fibrin within anterior chamber, hypopyon, hyphema, corneal edema
  • Colon: Thickened with a gross appearance similar to alimentary lymphosarcoma
  • Abdominal and thoracic lymph nodes: Lymphadenopathy
  • Kidneys: Enlarged with vasculocentric pyogranulomas
  • Brain: Hydrocephalus with gelatinous foci resembling cryptococcosis possible in cats with neurologic involvement
• Effusive form – Pleural effusion (40%) and effusive peritonitis (60‑70%) with up to 1L of viscous clear to yellow fluid
  • The surfaces of abdominal and/or thoracic contents covered with small (1-2 mm) white plaques of fibrin with a granular appearance; large amounts of fibrin can result in adhesions on visceral and peritoneal surfaces

TYPICAL LIGHT MICROSCOPIC FINDINGS

• Phlebitis (vessels typically surrounded by a zone of necrosis and a mixed inflammatory cell infiltrate); most severe in the anterior uvea with extension into the anterior and posterior chambers
• Presence of large globular accumulations of macrophages and neutrophils adherent to corneal endothelium (keratic precipitates) is an important clinical hallmark
• Pyogranulomas, large or small, consolidated or numerous, focal tissue necrosis
• Leukocytes infiltration is heaviest in ciliary body and limbic sclera and usually is an even mix of leukocytes, but can be purely histiocytic
• The most common ocular manifestation is granulomatous anterior uveitis with variable chorioretinitis, retinal separation, peripheral anterior synechia, preiridal fibrovascular membrane, lens luxation, and iridocorneal angle closure
• Increased GFAP expression in the retina in FIP cases and proliferation of Müller cells in cases of retinal detachment
• In cases of severe inflammation, B-cells and plasma cells predominate over T-cells and macrophages; macrophages expressing FCoV antigens are believed to be derived from blood (i.e. calprotectin immunolabeling)

ULTRASTRUCTURAL FINDINGS: 

• Coronaviruses are 80-160 nm in diameter and have a distinctive fringe of petal-shaped peplomers or spikes that resemble a crown or corona; peplomers are approximately 20 nm long and 7 nm wide at the tip
• Virions may form paracrystalline arrays

ADDITIONAL DIAGNOSTIC TESTS: 

• Gold standard for diagnosis is IHC demonstrating viral antigen within uveal WBCs
• Clinical signs and typical gross and histologic findings are critical diagnostically
• There is a lack of specificity of serologic tests for the FIP virus due to cross reaction with other antigenically related coronaviruses
• Rivalta’s test of abdominal effusion
• Many cats have coronaviral antibodies from prior FECV infection, which are indistinguishable from FIPV antibodies; however, a negative or very high (1:1600) FIP (coronaviral) titer can exclude or support an FIP diagnosis
• Detection of intracellular FCoV antigen by immunoflourescence or immunohistochemistry (CCV2-2 more sensitive than FIPV3-70)
• Virus detection tests include:
  • RT-PCR (reverse transcriptase polymerase chain reaction) – can perform on abdominal or thoracic effusions
  • Direct FA – monoclonal antibodies against FIP N protein
  • Immunohistochemistry
  • Electron microscopy

DIFFERENTIAL DIAGNOSIS:

• Anterior uveitis in the cat: Neoplasia (malignant lymphoma), trauma, infectious (feline leukemia virus, feline immunodeficiency virus, toxoplasmosis, protothecosis, cryptococcosis, bacterial septicemia)

COMPARATIVE PATHOLOGY:

Selected other coronaviruses:

Bovine coronavirus (winter dysentery)	Bovine	Gastroenteritis, coronavirus implicated
Canine enteric coronavirus (D-V03)	Canine	Enteritis
Canine respiratory coronavirus	Canine	Respiratory disease
Feline infectious peritonitis virus (P-V15; N-V17; S-V03)	Feline	Peritonitis, pneumonia, meningoencephalitis, panophthalmitis; granulomatous vasculitis
Feline enteric coronavirus	Feline	Diarrhea in kittens; mild villous atrophy
Mouse hepatitis virus     (D-V04)	Mouse	Polytropic strains: Hepatic necrosis, enteritis, encephalomyelitis; syncytia formation
Porcine transmissible gastroenteritis virus (TGE) (D-V06)	Porcine	Gastroenteritis
Porcine hemagglutinating encephalomyelitis virus	Porcine	Vomiting, wasting, and encephalomyelitis (usually no diarrhea)
Porcine epidemic diarrhea virus	Porcine	Gastroenteritis (western Europe, similar to TGE)
Porcine respiratory coronavirus	Porcine	TGEV mutation, often subclinical
Rat coronavirus	Rat	Rhinitis, tracheitis, pneumonitis in young
Sialodacryoadenitis virus (D-V05; S-V02)	Rat	Sialodacryoadenitis, porphyrin released from damaged harderian gland, squamous metaplasia of ducts
Avian infectious bronchitis	Chickens	Tracheobronchitis, nephritis, wrinkled eggs
Bluecomb (turkeys)	Turkeys	Enteritis, cyanosis of the comb
Rabbit coronavirus	Rabbits	Enteritis, myocarditis
SARS-CoV-1	Humans	Severe Acute Respiratory Syndrome (civet – source/amplification host)
SARS-CoV-2	Humans/Mink	Coronavirus Disease 2019
MERS-CoV	Humans	Middle Eastern Respiratory Syndrome (camels – reservoir host)
Epizootic catarrhal enteritis (ECE)	Ferrets	Profuse, green mucoid diarrhea in adults
Systemic Coronavirus-Associated Disease	Ferrets	Pyogranulomatous inflammation similar to FIP in cats within numerous organs but no effusion, icterus, or increased bilirubin
Coronavirus-like infection	Guinea pigs	Enteritis, wasting syndrome

References:

1. Addie DD. Feline coronavirus infections. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. Philadelphia, PA: Saunders Elsevier; 2012:92-108.
2. Agnew D. Camelidae. In: Terio KA, McAloose D, St. Leger J., eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018: 197.
3. Church ME, Terio KA, Keel MK. Procyonidae, Viverridae, Hyenidae, Herpestidae, Eupleridae, and Prionodontidae. In: Terio KA, McAloose D, St. Leger J., eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018: 312.
4. Felten S, Hartmann K, Doerfelt S, et al. Immunocytochemistry of mesenteric lymph node fine-needle aspirates in the diagnosis of feline infectious peritonitis. J Vet Diagn Invest. 2019; 31(2): 210-216.
5. Labelle P. The eye. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2016: 1317-1318.
6. MacLachlan NJ, Dubovi EJ. Fenner’s Veterinary Virology. 5th ed. London, UK: Academic Press; 2017:444-459.
7. Malbon AJ, Meli ML, Barker EN et al. Inflammatory mediators in the mesenteric lymph nodes, site of a possible intermediate phase in the immune response to feline coronavirus and the pathogenesis of feline infectious peritonitis? J Comp Pathol. 2019; 166: 69-86.
8. Miller AD, Zachary JF. Nervous system. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2016: 896-897.
9. Rakich PM, Latimer KS. Cytology. In: Latimer KS, ed. Duncan & Prasse’s Veterinary Laboratory Medicine Clinical Pathology. 5th ed. Ames, Ia: John Wiley & Sons; 2011:350.
10. Rissi DR. A retrospective study of the neuropathology and diagnosis of naturally occurring feline infectious peritonitis. J Vet Diagn Invest. 2018; 30(3): 392-399.
11. Sangl L, Felten S, Matiasek K, et al. Detection of feline coronavirus RNA, spike gene mutations, and feline coronavirus antigen in macrophages in aqueous humor of cats in the diagnosis of feline infectious peritonitis. J Vet Diagn Invest. 2020; 32(4): 527-534.
12. Stranieri A, Giordano A, Paltrinieri S, et. al. Comparison of the performance of laboratory tests in the diagnosis of feline infectious peritonitis. J Vet Diagn Invest. 2018; 30(3): 459-463.
13. Stranieri A, Paltrinieri S, Giordano A. Diagnosing feline infectious peritonitis using sysmex XT-2000iV based on frozen supernatants from cavitary effusions. J Vet Diagn Invest. 2017; 29(3): 321-324.
14. Terio KA, McAloose D, Mitchell E. Felidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018: 273.
15. 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; 2016:253-255.
16. Wilcox BP, Njaa BL: Special senses. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol. 1, 6th ed. St. Louis, MO: Elsevier; 2016:453.
17. Williams BH, et al. Mustelidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018: 296.
18. Zachary JF. Mechanisms of microbial infections. In: Zachary JF. ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2012:217-218.
19. Ziółkowska N, Paździor-Czapula K, Lewczuk B, Mikulska-Skupień E, Przybylska-Gornowicz B, Kwiecińska K, Ziółkowski H. Feline infectious peritonitis: Immunohistochemical features of ocular inflammation and the distribution of viral antigens in structures of the eye. Vet Pathol. 2017;54(6):933-944.

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