Feline Infectious Peritonitis

VIRUSES CORONAVIRUSES

Antigenicity

• FIP occurs in 5-10% of cats infected with Feline Enteric Coronavirus (FECoV), which is quite common
• It is therefore antigenically indistinguishable from FECoV

Hosts

• Domestic and wild cats

Pathogenesis

• FECoV may cause mild respiratory symptoms and diarrhoea but is often asymptomatic
• Weeks, months or years may intervene between localized primary FECoV infection and FIP development
• FECoV replicates in the gut, but FIP spreads systemically in the circulation
• FIP gains ability to replicate in monocytes and macrophages
• Almost invariably fatal
• Failure of the immune system to clear antibody-antigen complexes leads to immune-mediated disease
  • Deposited complexes cause inflammation and exudation
  • This leads to characteristic oedema as fibrin-rich serum escapes to intercellular spaces
  • Pyogranulomas can develop in major organs as a result of the immune response and the body's failure to clear away excess neutrophils
• Cats previously exposed to coronavirus (and therefore with circulating antibody) may be at greater risk as they are more susceptible to taking up virus into mononuclear cells
• Cats making a biased Th-1 response are more likely to evade infection, whereas cats making a balanced response are at moderate risk and cats making a biased Th-2 response are at greater risk, as the virus is best tackled by cell mediation and not antibody
• Cats compromised by immunosuppression (either iatrogenic or disease-related) are at a greater risk of developing FIP

Clinical signs:

• Chronic weight loss
• Anorexia
• Pyrexia
• Depression
• Fluid in the abdomen, thorax or pericardium symptomatic of wet, or exudative FIP
• Granulomatous change in the organs symptomatic of dry, or nonexudative FIP

Can be shown to cause:

• Uveitis
• Hydrocephalus
• Neurological symptoms, such as ataxia or seizures
• Chronic diarrhoea

Epidemiology

• FECoV is endemic worldwide, with the majority of cats showing a subclinical seroconversion
• Orofecal, aerosol, and contact transmission
• Particular concern for catteries and homes with multiple cats
• FIP arises from a mutation of FECoV (in 5-10% of chronically infected cats) and not directly from cat to cat

Diagnosis

• Clinical signs
  • FIP should be suspect in all cases of chronic weight loss or recurrent fever unresponsive to antibiotics, particularly in multiple cat situations
• Simple serology is impossible as most cats will have antibody to FECoV
• However, 4 indicators can be used to cross reference:
  • High FECoV Ab titres
  • Low albumin:globulin ratio in plasma/ascites (globulin levels rise in FIP)
  • High levels of glycoprotein alpha 1-acid glycoprotein (AGP)
  • Low white cell counts
• FIP antigen detection by immunofluorescence in macrophages gives a definite positive diagnosis
• PM: look for characteristic lesions in vascular immune complex disease and lymphoid infiltration

Control

• Conventional vaccination is counterproductive as antibody worsens infection
• A non-systemic vaccine (Primucell) is available outside the UK
  • Temperature-sensitive mutant
  • Replication confined to nasal mucosa, providing local immunity and cell-mediated immunity
  • Cannot protect cats already infected with FECoV
  • Kittens must be isolated until old enough to vaccinate at 16 weeks
• Antibody tests are available to certify "FECoV-free" cat houses