Serum antibody and virus detection assays
Feline coronavirus-specific assays can generally be categorized as antibody measurement or virus detection assays. Because
of the inability to identify a consistent mutation correlating with FIP production, no FIP virus-specific test exists.
Serum antibody. Serologic analysis detects only antibody to the coronavirus and does not reflect the virus's biotype. While a high antibody
titer is consistent with a diagnosis of FIP, it is not confirmatory; in addition, some cats with FIP have low antibody titers
or are seronegative.33 This latter situation may occur in fulminant cases or may be due to high virus levels that bind antibody, making it undetectable
in the serologic assay.
Serologic assays for antibody to a single virus-specific protein (as opposed to antibody to multiple virus proteins) have
been developed. In particular, a serologic test for antibody to the 7b protein has been offered as a diagnostic aid to FIP.
This protein is a viral nonstructural protein whose function is unknown, but, as described above, it may play a role in disease
development. It has been theorized that this protein is not expressed in all feline coronavirus infections; when expression
does occur, perhaps because of a viral mutation allowing 7b expression, FIP may develop. Cats with high concentrations of
antibody to the 7b protein would, by definition, be infected with the FIP viral biotype. However, subsequent studies have
shown that 7b expression occurs in most infections; 7b-specific antibodies, while consistently present at high concentrations
in cats with FIP, are also present in healthy cats with feline coronavirus.34 Thus, while 7b seronegative status would lessen the likelihood of a diagnosis of FIP, this test (offered by Antech Diagnostics)
cannot be used to confirm FIP.
Virus detection. Virus detection assays also suffer from this nonspecificity for FIP virus. That is, finding the virus by antigen detection
(e.g. immunofluorescent staining of ascitic macrophages) or genetic detection (e.g. real-time polymerase chain reaction [PCR] testing of whole blood) is consistent with a diagnosis of FIP but is not necessarily
confirmatory. At least one laboratory (The Molecular Diagnostics Laboratory at Auburn University's College of Veterinary Medicine)
offers a real-time PCR assay that quantitates the level of viral mRNA in the monocytes of cats. While it is not known precisely
how the cutoff levels were determined, high levels of viral mRNA do reflect efficient viral replication in circulating monocytes.35 As stated above, high viral loads in the blood are consistent with FIP, especially in the end stage; however, high viral
loads in the blood are also found in healthy cats in endemically infected populations.10,36 Virus detection and quantitation is, thus, not confirmatory for FIP but does offer diagnostic information.
In addition, detecting virus in the feces by using PCR testing is the optimal method for identifying viral shedding. PCR testing
without quantitation is offered at many commercial laboratories. Testing multiple samples from an animal over time can identify
chronic shedding.3 Because these animals may shed the virus intermittently, test at least two, or preferably more, samples collected at a monthly
interval. An example regimen would be three samples collected daily, followed by three samples daily one month later. Some
laboratories may offer pooling of samples to reduce costs.
Histologic examination and immunohistochemistry
The gold standard for FIP diagnosis remains histologic examination and immunohistochemistry for feline coronavirus antigen.15,16 Granulomatous lesions are vascular and perivascular, involving small and medium veins primarily. Cellular composition is
mainly monocytes and macrophages with a minority of neutrophils. B lymphocytes and plasma cells may be found at the periphery
of lesions, while T lymphocytes are few. Detection of viral antigen (immunohistochemistry) or nucleic acid (in situ hybridization)
in infected cells within lesions is confirmatory; this testing is offered by some pathology laboratories.