General clinical signs of FIP include an antibiotic non-responsive fever, weight loss,
anorexia and lethargy. Transient upper respiratory signs have been reported in some
cats on initial infection with FCoV. Effusive FIP (wet form), dyspnea (due to pleural
effusion), progressive, non-painful abdominal distention (due to ascites) and pericardial
effusion are seen. No effusive FIP (dry form) any organ may be affected. Icterus,
splenomegaly, renal failure, renomegaly, abdominal masses, coughing and/or dyspnea
may be seen. Ocular involvement is commonly noted in the dry form and may cause
anterior uveitis, retinal detachments and hemorrhage. Neurologic signs may include
multifocal progressive signs such as ataxia, seizures, nystagmus, tremors, hyperesthesia,
decreased proprioception and behavioral changes. FIP is one of the most common
causes of neurologic disease in cats. Solitary intestinal masses can develop which may
cause intestinal obstruction.
Clinicopathologic findings include a mild to moderate regenerative anemia, lymphopenia,
and hyperproteinemia due to hyper-gammaglobulinemia (usually polyclonal
gammopathy). Serum albumin to globulin ratio (A:G ratio) has good diagnostic value, and
at values above 0.8, FIP is extremely unlikely. Azotemia, proteinuria, elevated liver
enzymes and coagulopathy may also be observed and are the result of secondary organ
damage. CSF analysis in cats with neuro signs may show hyperproteinemia, increased
neutrophils and increased lymphocytes (however this is not consistent). Immuno-
mediated glomerulonephritis has also been reported, and FIP always be considered in
cats with protein-losing nephropathy, which is otherwise rare in cats.
FIP effusions have a higher diagnostic value than blood tests. The effusions typically
have a very high protein content (>35 g/l) but a low cellularity (<5000 nucleated
cells/ml), consisting primarily of macrophages and neutrophils, and are clear to yellow
and may contain fibrin clots. When sufficient cells are present, the presence of viral
antigen in macrophages confirms the diagnosis with a very high positive predictive value
(PPV). The A:G ratio can also be measured in effusions; this test has a high PPV if the
ratio is <0.4 and a high negative predictive value (NPV) if the ratio is >0.8. The
demonstration of feline coronavirus-specific is only meaningful when the titer is high
(1:1600), whereas the absence of antibodies has a good NPV.
Serology (immunofluorescence, ELISA, rapid immunomigration), is commonly
employed in the diagnosis of FIP however positive results only indicate exposure to
feline coronavirus. Most tests can be run on blood as well as effusion. Negative antibody
titer helps to rule out FIP as less than 4% of cats with FIP will have a negative antibody
titer. There is still no test available to differentiate between strains and recent FIP
vaccination can also result in a positive titer. Very high titers (1:1600) in combination
with other tests which suggest FIP indicate an increased likelihood of FIP, unless
obtained from cats in an endemic environment. Positive CSF serology (>1:25) in cats
with neuro signs may help to support diagnosis of FIP. Serology results should not be
compared between laboratories due to different methodologies used. Real-time RT-PCR
is a sensitive method to detect virus RNA in a variety of samples (feces, blood, effusions,
and tissues) of feline coronavirus-infected cats and those with FIP; however, these still
cannot differentiate between the biotypes.