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There is no single, predictable target organ for feline infectious peritonitis (FIP). The virus widely (systemically) disseminates in macrophages, and the clinical outcome is dependent on both the host immunity and the specific organ affected. Histopathology on biopsy or necropsy specimens remains the gold standard for diagnosis. Many of the pre‐mortem tests, and especially a cumulative amount of information, can be highly suggestive of the disease. If an animal dies or is euthanized with suspect FIP, a necropsy with histology would be diagnostic.

Gross findings: In the effusive form, there will be fluid within either or both the thoracic and abdominal cavities. The fluid is high in protein and may vary from slightly viscous to gelatinous. The surfaces of the viscera are covered with tiny (1–5 mm) friable, pale tan to white plaques (fibrin) that can give the surfaces a granular appearance (See Figure 5.9).

In the non‐effusive (chronic) form, there will be nodules (granulomas) of variable size present in one or multiple organs. These can vary in color from off white to light tan, in texture from slightly firm to soft. The nodules are typically associated with capsular or serosal vessels, although when abundant this can be difficult to distinguish. Within organs, the granulomas can be scattered throughout the parenchyma. Lymph nodes are often enlarged.

Figure 5.9 Feline infectious peritonitis (FIP) virus (feline coronavirus). In the effusive form of FIP, along with intracavitary fluid, the surfaces of abdominal and thoracic viscera are covered with small (1–2 mm) to coalescing pale tan plaques.

Formalin‐fixed tissues: Samples should be taken from affected organs (in the case of the non‐effusive form, this would be any viscera with detectable granulomas). In the case of the effusive form, multiple samples should be taken from affected viscera (liver, GI, lung). If the clinical presentation is limited to the nervous system and FIP is suspected, it is imperative to submit the brain. Brain lesions are, however, rarely uniquely present.