Feline Infectious Peritonitis Virus (FIPV) & DNA Testing

Feline Infectious Peritonitis (FIP) is an immune-mediated disease which may arise in any cat infected with any strain of Feline Enteric Coronavirus (FECV). The outcome following exposure to FECV depends on the interaction of a complex set of factors, including characteristics of the virus and the immunocompetence of the host. Protective immunity is cell-mediated only, and the presence of high titres of antibody may, in fact, be detrimental.

FECVs show a continuum of virulence from the so-called non-pathogenic strains to those which are highly virulent. The degree of virulence is associated with mutations in a specific segment of the viral genome termed the hyper-variable region. This region is present in all FECVs, allowing highly virulent strains to arise occasionally in individuals previously infected with a strain of the virus associated with low virulence.

The highly virulent strains have previously been designated as FIP viruses (FIPV). However, in an immunocompromised host, such as a cat recently infected with FIV, even the so-called non-pathogenic strains may result in lesions characteristic of FIP, while a fully immunocompetent host may resist challenge with even a highly virulent strain of the virus.

There are a number of serologic tests which detect exposure to FECV but there are no such tests available which will determine whether or not a particular individual has the disease FIP. Currently a definitive diagnosis of FIP can only be established by demonstration of characteristic lesions in tissue biopsy specimens. The ELISA test is a highly sensitive screening method which will detect even very low levels of antibody confirming exposure to a coronavirus. Titres are determined in an attempt to differentiate exposure from likely clinical disease. The assumption is that a cat with a high titre is more likely to be clinically affected. Unfortunately, even cats which are sero-negative may have FIP and those with high titres may be clinically healthy.

Recently, we have introduced a PCR-based test which detects the presence of FECV nucleic acid. By means of this test, virus can be detected in blood, feces, effusion fluids or tissue biopsy samples. This offers the advantage of differentiating between individuals previously exposed to the agent versus those actively infected. The PCR test, however, does not differentiate between the various strains of FECV or between individuals who simply harbor the virus, and those who are or ever will develop FIP. Viremia is frequently detected in uninfected cats, however, shedding of the virus in feces is episodic and therefore a positive fecal result confirms the presence of the virus, but a negative fecal result does not confirm a virus-free state. Once again, the presence of the virus does not mean that the cat has or will develop the disease FIP. This technology has simply given us another diagnostic tool to use in the battle against FIP but has not yet allowed us to win.

So, what do we do? The approach to the problem of FIP continues to be multi-faceted. Serologic testing should be used to identify individuals at risk by virtue of previous exposure to the virus. Titres will help to identify individuals at high risk since those individuals with large amounts of antibody present are most likely to have had exposure to high levels of potentially virulent strains of the virus. The PCR-based test can be used to identify chronic shedders of the virus in catteries or multi-cat households, differentiate between cats which have antibodies due to previous exposure or vaccination versus those cats which are, and can confirm a diagnosis of FIP when effusion fluids or tissue samples are tested. Detection of virus in blood or feces of kittens will identify individuals who are actively infected rather than those with passively acquired maternal antibody. In the majority of cases, however, a definitive diagnosis of the disease FIP, will still require histologic demonstration of lesions in tissue biopsy specimens.

What about the cat with confirmed FIP? Recent work suggests that, since the disease is immune-mediated, use of immunosuppressive agents such as corticosteroids may be beneficial. There is little information available, however, regarding the success of treatment.

In conclusion, control of the disease FIP will require control or elimination of FECV, all FECVs. This may prove to be too daunting a task. Approximately 85% of all cats are infected with FECV. Fortunately, of these, only 2% will develop the disease FIP. Vaccination with currently available products is effective in preventing disease development, however, since most kittens are infected between 4 and 6 weeks of age, vaccination generally comes too late. The efficacy of the vaccine is highly dependent on the dose of challenge virus, with failure of protection occurring when cats are exposed to large amounts of virus. Antibody dependent enhancement of disease has been shown to occur experimentally, but does not appear to occur in naturally exposed cats.

We at VITA-TECH have embarked on a research program to develop improved detection methods for the presence of FECV as well as approaches to help differentiate between infection and lesion development. Together it should be possible to control this most frustrating and devastating of feline diseases.