Table 2: Common differential diagnoses for feline infectious peritonitis
Non-specific signsJaundiceEffusionOcularCNSMass lesionNotes
ToxoplasmosisHistory: Fed raw diet or hunter (also vertical transmission in kittens)
Differences: Hyperglobulinaemia uncommon
Diagnosis: Cytological identification of organisms on aspirates; PCR of aspirates or CSF; paired Toxoplasma serology (IgM and IgG)
Lymphocytic cholangitis
History: Persians may be over-represented
Differences: Usually (not always) associated with increased hepatic enzyme activities (primarily cholestatic). Cats often relatively well and normothermic
Diagnosis: Liver biopsy
Neoplasia (eg, lymphoma; carcinoma)Can affect cats of any age, particularly lymphoma. Jaundice may be present particularly with hepatic involvement
Diagnosis: Cytology of fluid or aspirates; biopsy
Mycobacterial disease
(often LNs)
History: Hunter or outdoor access (geographical variation), fed raw diet
Differences: Usually minimal to no effusions. Usually (not always) relatively well and normothermic. Pulmonary signs (tachypnoea; cough) not uncommon
Diagnosis: Ziehl-Neelsen stain of aspirates or biopsy; interferon-γ release assay; mycobacterial PCR or culture of aspirate or biopsy

Differences: Usually (not always) normothermic. Ascites, where present, usually small volume with high cellularity (non-degenerate neutrophils)
Diagnosis: Feline pancreatic lipase immunoreactivity; abdominal imaging
History: Outdoor or ‘stray’; entire adult with unknown mating activity (especially FIV)
Differences: Common differential for lymphadenopathy and/or uveitis. FeLV may be associated with neoplasia (especially lymphoma)
Diagnosis: FIV antibody/FeLV antigen serology (positive results should be confirmed)
SepsisInfection can involve different organ systems (eg, kidney, liver, uterus, heart) or body cavities (eg, pyothorax, septic peritonitis)Cats are often very sick (eg, pyrexia may have progressed to hypothermia with onset of shock)
Diagnosis: Haematology suggestive (leukocytosis or neutropenia; left shift and toxic change); hypoglycaemia may be present; imaging; cytology (degenerate neutrophils; intracellular bacteria) and culture of fluid or aspirates*
Septic peritonitis
Pyrexia common. Most frequently associated with gastrointestinal or urinary tract perforation
Differences: Ascites with high cellularity (degenerate neutrophils; intracellular bacteria)
Diagnosis: Cytology and culture of fluid or aspirates*
Usually pyrexic
Differences: Pleural effusion with high cellularity (degenerate neutrophils; intracellular bacteria)
Diagnosis: Cytology and culture of fluid or aspirates*
(pleural +/- ascites)
History: Some breeds are predisposed to cardiomyopathy (eg, ragdoll, Maine coon) with increased risk of CHF at a young age. Heart murmur (non-haemic), gallop sounds, arrhythmia, jugular vein distention and pulse may be present.
Differences: Low protein/low cellularity effusion. Hypothermia and/or hypotension are common. Pyrexia, hyperglobulinaemia and jaundice are not features
Diagnosis: Echocardiography
  • = feature shared with feline infectious peritonitis (NB: absence does not rule it out as a differential); * NB: risk of false-negative if collected after antibiotics are administered

  • CHF Congestive heart failure, CNS Central nervous system, CSF Cerebrospinal fluid, FelV Feline leukaemia virus, FIV Feline immunodeficiency virus, LNs Lymph nodes