JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1492980): 3-month-old cat
HISTORY: Section of lung from a cat killed two days after aerosol infection with an experimental agent.
HISTOPATHOLOGIC DESCRIPTION: Lung: There is consolidation and atelectasis affecting approximately 70% of the lung. Within areas of consolidation, alveolar septa are fragmented, discontinuous and replaced by fibrin and eosinophilic cellular and karyorrhectic debris (septal necrosis), lined by hyperplastic cuboidal epithelium (type II pneumocyte hyperplasia) or expanded up to 8 times normal by fibrin, edema, neutrophils and macrophages. Alveolar, and to a lesser extent bronchiolar and bronchial lumina are filled with variable amounts of fibrin, edema, hemorrhage, eosinophilic cellular and karyorrhectic debris (necrosis), neutrophils and alveolar macrophages. There is bronchiolar epithelial hyperplasia. Multifocally, blood vessels are lined by plump endothelial cells (reactive endothelium). There is perivascular edema and hemorrhage and the pleura is mildly thickened by fibrin and hemorrhage.
MORPHOLOGIC DIAGNOSIS: Lung: Pneumonia, interstitial, necrotizing, focally extensive, marked, with type II pneumocyte hyperplasia, breed unspecified, feline.
ETIOLOGIC DIAGNOSIS: Caliciviral pneumonia
CAUSE: Feline calicivirus (FCV)
- Common cause of upper respiratory tract disease that can also cause oral ulcers, chronic stomatitis, pneumonia, systemic disease, or lameness
- 15-25% of cats are subclinical carriers
- Small (35 to 40 nm), non-enveloped, single-stranded RNA virus in the genus Vesivirus
- Feline junctional adhesion molecule A (fJAM-A), localized at cell-cell junctions of epithelial and endothelial cells, is a functional receptor for feline calicivirus
- Epitheliotropic (replicates in the cytoplasm)
- Transmitted by direct contact or via infected fomites
- Recovery often results in persistent infection
- Pneumotropic form affects upper respiratory tract, occasionally producing pneumonia; the less common rheumatic form, “limping kitten syndrome,” causes joint pain and lameness; clinical manifestation is not related to identifiable differences in the virus subtype
- In addition to upper respiratory infections, ulcerative stomatitis may occur
- Virulent Systemic Feline Calicivirus (VS-FCV) is a rare manifestation, primarily affects adult vaccinated cats and can have a mortality of 33 to 60%
- Primary target cells are the squamous epithelial cells of the tongue, pharynx, tonsil and alveoli; virus causes lytic infection of type I pneumocytes followed by septal thickening and type II pneumocyte hyperplasia
- Virus ingested or inhaled > replication in oropharyngeal tissues > degeneration and necrosis of cells > local inflammation with neutrophils, hemorrhage and edema causing vesicle formation > rupture of vesicles releasing large amounts of infective virus into the environment > occasional spread to lungs either directly or due to viremia
- Excretion of FCV in oropharyngeal fluids of cats usually persists for 10 to 14 days
- The site of chronic infection appears to be the tonsils and associated pharyngeal mucosa
- Cats may often be co-infected with feline herpes virus type 1, Chlamydophila felis, Bordetella bronchiseptica and/or Mycoplasma sp.
- Antigenic variability of the capsid protein, which is the target of the immune response, allows viral persistence in spite of immune response
- Viral infection may persist after resolution of clinical signs; cats may shed months to years
TYPICAL CLINICAL FINDINGS:
- Ulceration of the oral cavity (tongue, gingiva), tonsil and oropharyngeal mucosa
- Fever, anorexia, rhinitis, conjunctivitis and occasionally patchy pneumonia
- Death due to FCV infection is uncommon and associated with pneumonia primarily in kittens in catteries where large infective doses of virus are generated
- A less common acute febrile lameness syndrome has been recognized, which may be accompanied by respiratory signs and is characterized by fever, malaise, joint swelling and pain, muscle soreness, hyperesthesia and limping in post-weaning kittens; the pathogenesis of this syndrome is unknown
- VS-FCV: Ulcerations most commonly seen in the oral cavity, but also seen in the pinnae, nares, footpads and skin
- Finding of oral ulcers is suggestive of calicivirus infection
TYPICAL GROSS FINDINGS:
- Mucosal vesicles and ulceration that affect the tongue, hard palate, nasal philtrum, lip or periodontal gingival
- Pneumonia involving the cranioventral margins of the lungs
- Hepatic, pancreatic necrosis, subcutaneous edema, foci of fat necrosis, DIC, and bronchointerstitial to interstitial pneumonia all reported with VS-FCV
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Initial stage: Interstitial pneumonia with viral-induced necrosis of type I pneumocytes, serofibrinous exudation, formation of hyaline membranes and high numbers of neutrophils
- As the disease progresses: Type II pneumocyte proliferation and the inflammatory cells are predominately mononuclear
- After 30 days: Few residual changes other than scarring of the alveolar walls
- Lesions in the upper respiratory mucosa and conjunctiva are confined to moderate neutrophilic infiltration, edema, necrosis and sloughing of individual cells
- VS-FCV: Epithelial necrosis and ulceration, bronchointerstitial pneumonia, hepatic changes ranging from disruption of the parenchyma to extensive disruption of hepatocellular plates, intestinal crypt necrosis and pancreatic necrosis
- 35 to 40 nm virions
- Characteristic cuplike depressions in their surface
- In the cytoplasm there are either scattered single particles in characteristic linear arrays along microfibrils, as small clusters slightly larger than free ribosomes, or as paracrystalline arrays within membranous structures
- Degenerative changes in infected cells include dilatation and invagination of the rough endoplasmic reticulum and dense cytoplasmic masses of nucleoprotein-like material
ADDITIONAL DIAGNOSTIC TESTS:
- PCR and virus isolation from oropharyngeal fluids, oral swabbing of the tonsils, urine, feces, blood and visceral tissues
- Definitive diagnosis of FCV requires lab support
Feline respiratory diseases:
- Feline rhinotracheitis virus (feline herpesvirus type I): Usually a more severe necrotizing disease; may have intranuclear inclusion bodies; definitive diagnosis by viral isolation on conjunctival swabs
- Chlamydophila felis (Chlamydia psittaci var felis): Conjunctivitis and upper respiratory tract disease; definitive diagnosis by cell culture or ELISA
- Bordetella bronchiseptica: Believed to be a secondary infection in respiratory infections; definitive diagnosis by culture
- Felids: Feline calicivirus is thought to infect all felids
- Dogs: Feline calicivirus has been reported to cause glossitis in dogs; has been isolated from dogs with vesicular genital disease
- Swine: Vesicular Exanthema of Swine (VES) causes fever, vesicles on the snout, oral cavity and feet and abortion
- Lagomorphs: Rabbit Viral Hemorrhagic Disease (RVHD) described in European rabbits (Oryctolagus cuniculus) and is characterized by hemorrhagic lesions especially in the lung and liver; European Brown Hare Syndrome (EBHS) affecting Lepus europaeus is characterized by necrotizing hepatitis, nephrosis, congestion and hemorrhages of the tracheal mucosa and lungs and splenic vascular congestion
- Pinnipeds: Calicivirus is ubiquitous in marine environment; cause severe vesicular stomatitis to dermatitis; indistinguishable from vesicular stomatitis of swine
- Bovine: Bovine enteric calicivirus-associated with diarrhea in young calves
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