Feline infectious peritonitis (FIP) is caused by a pathogenic strain of coronavirus belonging to a large family of pleomorphic RNA viruses. These viruses generally infect cats by inhalation or ingestion. Other non-pathogenic coronaviruses also infect cats, the most notable being feline enteric coronavirus (FECV), which do not produce the characteristic lesions of FIP.

FIP exhibits a variety of clinical and pathologic signs. Younger cats are most at risk (6 months to 5 years old), although it may infect cats of any age. The outcome of FIP virus (FIPV) infection can be extremely variable and is influenced by many factors. A large number of cats may remain virus carriers. The infection may be self-limiting and cleared by the immune surveillance response of some cats, while others suppress the virus to a state of chronic subclinical infection, and still others progress to develop an effusive ("wet") or noneffusive ("dry") form of the disease.

Molecular biologic techniques and research have identified certain regions of the FIPV genome that need to be present and functional for cats to develop FIP. A specific region on the FEPV genome, referred to as the 7B gene, is absent or truncated in many FECV strains and is absent from the commercial FIP vaccine. However, all pathogenic strains of FIPV have intact and functional 7B regions. The 7B gene produces a specific protein (the 7B protein) which apparently acts as a viral growth factor and allows the FIPV to infect macrophages. Therefore, a test to determine if cats have been exposed to a coronavirus producing 7B protein, should be beneficial in determining if the cat is at risk of developing FIP.

Diagnosis of FIP has remained difficult since the first coronavirus serologic titer test appeared two decades ago. The presence of classic histopathologic lesions, and/or the identification of viral proteins in classic histologic lesions, are currently the most definitive methods of diagnosis. However, the FIP-PCR (see Antech News, May 1997), is helpful in confirming the diagnosis of FIP in symptomatic cats. This test is best performed on effusive fluids rather than peripheral blood, and has limited use as a screening method for multicat households or catteries. The compilation of data from: physical exam, analysis of body cavity effusions, serum protein/ globulin levels, serum protein electrophoresis patterns showing marked polyclonal gammopathy, and elevated serologic titers to the feline coronavirus family, have been used to make a presumptive diagnosis.

The FIP antibody titers routinely measured today are nonspecific, as they recognize an animal's exposure to any member of the coronavirus family. These use indirect fluorescent antibody (IFA) techniques and fail to differentiate the host cat's immunologic response to FECV, FIPV, or even canine or human coronaviruses. Many normal cats, including those never exposed to the pathogenic strains of FIPV, may have elevated feline coronavirus titers by the routine IFA method. This creates a serious dilemma for the clinician attempting to determine a specific diagnosis, upon which to advise multicat owners or catteries.

For example, a high, rising IFA titer could be indicative of active infection, whereas a waning titer could represent recovery. However, if the active infection is caused by nonpathogenic FECV, the cat should never develop FIP and need not be culled from a multicat household or cattery. Conversely, the fulminant terminal stage of FIP often shows depressed IFA titers. Thus, overinterpretation of the feline coronavirus IFA titer has resulted in the inadvertent euthanasia of cats erroneously thought to be infected with FIPV, and an overly optimistic prognosis for cats dying of FIP. There exists a clear need, therefore, for a serologic screening test capable of detecting antibodies to FIPV to the exclusion of all other coronaviruses.

One of the most important aspects of the new forms is the section that allows you to list which tests have priority. When sample volumes are limited and we have to decide which tests to run, it saves a lot of time if we know which ones are most important to you. Another improvement is the listing of the sample amounts required for each test and profile. If you send in less volume than stated on the form, we will attempt to get the most information out of what we have. If there is any question about sample size requirements, please call the laboratory beforehand. These new sample submission procedures ensure relevant, quality results!

Another change concerns the blood tubes that are required for specific samples. Almost every chemistry test we offer analyzes plasma, not serum. We get a larger volume of sample by using plasma. Please use the green top tube (heparin) microtainers we supply rather than the serum microtainers. Some of you have had problems with abnormally elevated results for phosphorus and/or potassium in your samples. These problems almost always occur because of leakage of potassium or phosphorus from the red blood cells, even in the absence of gross hemolysis. One easy way around this problem is to separate the plasma from the blood cells before you submit the sample to the lab. Spin your sample in the green microtainer and decant the plasma into a serum microtainer tube and send it to us. As a CBC cannot be run on the plasma, you still need to send us blood cells in either a green top (heparin) or lavender top (EDTA) tube. We also recommend sending one or two freshly made blood smears.

Antech Diagnostics, in collaboration with Engene and Synbiotics Corp. has developed an ELISA-based serologic test that measures the presence of antibodies to the 7B protein of FIPV.

To create the FIP-specific test, the 7B gene identified by the FIP-PCR was cut out from the FIPV genome by recombinant DNA technology, inserted into an E. coli bacterium, and the 7B protein produced by the gene was purified. The purified 7B protein is coated onto ELISA plates and sera from cats to be tested is allowed to bind to the plates. Antibodies specific for the 7B protein, if present in the test samples, are then detected by use of antifeline antibody reagents. Appropriate control samples are run to correct for any nonspecific background binding. Titers range from negative (<1:40) to 1:640, lower than typical IFA titers, but reflecting the proportion of antibodies truly specific for FIPV.

This test should allow better differentiation of cats exposed to nonpathogenic FECV strains rather than to pathogenic strains of FIPV, since expression of 7B correlates with nonpathogenic to pathogenic conversion. Cats having positive antibody titers to the 7B protein of FIPV are at risk of developing FIP, while those cats without antibodies to this protein, apparently would not have been exposed to pathogenic FIPV. Furthermore, early research with this test shows that cats vaccinated with FIP vaccine do not develop antibody titers to the 7B protein, because the attentuated FIP strains created for vaccine purposes are selected for nonvirulence and in doing so lose 7B expression. However, vaccinated cats do develop positive titers with the routine IFA coronavirus test, making it of little diagnostic use in this situation. Clearly, as with any diagnostic serologic test, the presence of antibodies only indicates exposure, and does not (at the current point in time) confirm that the animal has the disease in question, or will develop the disease at some future point. Also, there likely will be numerous cats that have a positive feline coronavirus IFA titer of 1:400 or even 1:1600 that have negative FIP 7B-ELISA titers. The interpretation here would be that the cat has been exposed to nonpathogenic coronavirus and has not been exposed to a pathogenic strain of FIPV. Similarly, vaccinated cats can be coronavirus IFA positive but remain FIP 7B-ELISA negative. Antech Diagnostics refers to the new FIP 7B assay as the "FIP-Specific ELISA".