How to differentiate between dry and wet feline infectious peritonitis (FIP)

Feline Infectious Peritonitis (FIP) remains one of the most challenging feline diseases to diagnose accurately. The disease manifests predominantly in two forms: dry (non-effusive) and wet (effusive). Differentiating between these two phenotypes is crucial for prognosis, treatment decisions, and understanding the disease progression.

Clinical Presentation and Symptomatology

Dry FIP:

Exhibits a more insidious onset with subtle signs.

Commonly presents with weight loss, lethargy, anorexia, and variable fever.

Neurological signs may be present if the CNS is involved, such as ataxia or behavioral changes.

Ocular abnormalities, including uveitis or chorioretinitis, are frequently observed.

Wet FIP:

Characterized by an acute presentation with rapid development.

Features include abdominal distension due to fluid accumulation, often accompanied by dyspnea if thoracic effusion is present.

Fever tends to be high and persistent.

General malaise and depression are prominent.

Physical Examination Indicators

Dry Form:

Usually no palpable fluid in body cavities.

Lymphadenopathy may be evident.

Neurological deficits or ocular changes might be detected upon examination.

Wet Form:

Notable abdominal enlargement due to peritoneal effusion.

In cases of thoracic involvement, the cat may show labored breathing.

Mild dehydration or signs of systemic illness may be apparent alongside fluid accumulation.

Laboratory and Diagnostic Tests

Complete blood count (CBC):

Both forms often display lymphopenia and anemia.

Elevated globulin levels with a decreased albumin-to-globulin ratio are common.

Serology and PCR:

Serologic tests indicating coronavirus exposure are supportive but not definitive.

Polymerase chain reaction (PCR) assays may detect FIP virus RNA but can produce false positives, emphasizing the need for combined diagnostic criteria.

Biochemical Analysis:

Hyperglobulinemia is prevalent in both forms.

Elevated total protein and decreased albumin levels can suggest FIP.

Cerebrospinal Fluid (CSF) and Ocular Fluids:

In dry FIP with neurological involvement, CSF may show elevated protein content and pleocytosis.

Ocular fluids may demonstrate similar protein abnormalities and contain viral particles in confirmed cases.

Imaging:

Ultrasound or radiography can reveal characteristic findings.

Wet FIP is confirmed by identifying free or loculated fluid.

Dry FIP may show organomegaly or granulomatous lesions.

Fluid Analysis and Cytology

In wet FIP, abdominal or thoracic fluids are typically straw-colored, viscous, and contain high protein content.

Cytology often reveals a mononuclear cell predominance, especially macrophages, with some neutrophils.

Immunohistochemistry (IHC) staining can reveal FIP virus within effusions, providing a more definitive diagnosis.

Dry FIP:

Usually lacks or has minimal effusions.

Lesions are granulomatous, with tissue biopsies revealing pyogranulomatous inflammation.

Fine-needle aspirates from affected organs may help identify granulomas.

Histopathology and Definitive Diagnosis

Tissue biopsies remaining the gold standard, demonstrating granulomatous vasculitis in dry FIP.

Wet FIP shows widespread serositis with fibrinous exudates and vasculitis.

IHC detection of FIP virus antigen in tissue samples confirms diagnosis in both forms.

Prognosis and Response to Treatment

While both forms have historically carried poor prognoses, recent advances suggest potential therapeutic benefits.

Wet FIP often allows early intervention with supportive care, although survival remains limited.

Dry FIP’s less aggressive progression may extend survival if diagnosed early, but persistent granulomatous inflammation poses challenges.

Independent Insights and Future Outlook

Emerging research indicates that biomarkers such as serum or effusion apolipoproteins could enhance differentiation accuracy. Additionally, advanced imaging techniques like contrast-enhanced ultrasound may uncover distinct vascular patterns associated with each form. Understanding host immune responses and genetic factors could unlock targeted therapies, transforming the management landscape for FIP in the near future.

Identifying subtle clinical and laboratory differences, combined with evolving diagnostic technologies, enables veterinarians to distinguish these FIP variants more precisely. As our grasp of feline immunopathology deepens, so does the potential for more effective and tailored treatment plans, ultimately improving outcomes for affected cats.