In some cases, the diagnosis can be elusive and may depend on the logical exclusion of other possibilities. Clinicians often
have to piece together supportive evidence and weigh the likelihood of FIP in a particular patient. Since feline coronavirus
is endemic in many cat populations, FIP cannot be definitively diagnosed based solely on evidence of exposure to the virus.
Instead, evidence is needed that the virus has moved beyond the gastrointestinal tract and is replicating in the internal
organs, as this behavior differentiates the virulent virus from the nonvirulent forms. Unfortunately, proving this key piece
of evidence can be difficult.
FIP is commonly reported in cats < 2 years of age, although it may arise in older cats from shelter environments. Generally,
affected cats have an insidious onset of clinical signs with episodes of fever, malaise, and hyporexia. Kittens may fail to
thrive and appear stunted.
Cats with effusive disease may present with abdominal distention, dyspnea, or both. Abdominal palpation may indicate mesenteric
lymphadenopathy, renomegaly, or areas of thickened intestine.
Uveitis or chorioretinitis may be noted in cats with the dry form, and a careful ophthalmologic examination should be performed
in any cat with an unexplained fever. Neurologic problems, including seizures, changes in mentation, and spinal compromise,
are routinely reported in cats with dry FIP.2
Routine laboratory findings
A mature neutrophilia is expected in cats with FIP, along with lymphopenia. Many cats have concurrent nonregenerative anemia,
attributed to chronic inflammatory disease and secondary suppression of erythropoiesis.5 Hyperglobulinemia is routinely reported in cats with FIP, particularly the dry form. The albumin:globulin ratio is often
< 0.8 (a ratio < 0.6 is highly suggestive of FIP).2 Hyperbilirubinemia is also commonly noted, although overt icterus is unusual.
FIP is the most common cause of abdominal effusion in cats < 2 years of age.2 An examination of abdominal or pleural effusion often provides strong supportive evidence of FIP. The fluid is usually light
to dark yellow and may be cloudy or mucinous.5 The total protein content is predictably > 3.5 g/dl, with an albumin:globulin ratio < 0.6. The nucleated cell count is generally
< 5,000/μl and consists primarily of macrophages and nontoxic neutrophils.
The Rivalta test is a simple in-house test you can perform to exclude FIP as a cause of effusion; however, it lacks specificity,
as any protein-rich fluid is likely to produce a positive result.6 Mix one drop of 98% acetic acid with 5 ml of distilled water in a test tube. Place a drop of the effusion on top of this
solution and observe its motion. If the effusion dissipates, the fluid is transudate and is not consistent with FIP. If the
effusion is still clearly visible, it is consistent with an exudate but not diagnostic for FIP.
Radiography can confirm the presence of free fluid in effusive cases but has limited value in cats with dry FIP. Abdominal
ultrasonography permits the identification and collection of ascitic fluid along with an evaluation of the internal organs.
Renomegaly, lymphadenopathy, and regional thickening of the small bowel or colon are commonly noted in affected cats.
Gross surgical or postmortem findings
In cats with effusive FIP, serosal surfaces are often covered in pyogranulomatous lesions of varying size.2 The omentum is often markedly thickened and contracted. Firm white or mottled granulomas may be seen on the surface of affected
organs. Lymphadenopathy is expected, and affected nodes are firm and nodular. In cats with dry FIP, fewer but larger lesions
are noted. These start on the serosal surface of the organs and extend into the parenchyma.
Cytologic and histologic examination
The results of a cytologic examination of affected tissues in cats with effusive FIP show a pyogranulomatous reaction with
clusters of macrophages and neutrophils.2 A histologic evaluation of affected tissues reveals widespread perivascular inflammation, with a predominance of macrophages.
Edema and necrosis are also evident. In cats with dry FIP, dense lymphocytic aggregates are often noted around affected vessels.
Necrosis and fibrin deposition are less dramatic than in the effusive form.