Feline Infectious Peritonitis

By Chick Newman, Ph.D, DVM 1 black cat

Causes, Clinical Disease, Diagnosis, Treatment, Control

I. Causes

Feline Infectious Peritonitis (FIP) is caused by a coronavirus that can infect any cat, though young cats and very old cats (14yr and up) appear most susceptible. The FIP virus (FIPV) is very similar to the coronavirus that causes a transient, usually mild, self-limiting diarrhea (Feline Enteric Corona Virus, FECV). In fact, there is considerable evidence that FECV actually mutates to FIPV in individual cats. This means that even an indoor-only cat in a single-cat household may, in theory, develop this disease if the cat carries the enteric corona virus. Indeed, there is now the general belief that mutation from FECV to FIP is the most probable means of acquiring FIP! However, there are some doubters. Recently, the mutation theory of FIP origin has been questioned (J. Feline Med Surg 9:202, 2007). In that paper only a small portion of the entire genome of an enteric corona virus and a non-enteric corona virus isolate (presumed to be the FIP variant) were compared. The portions compared were identical...i.e. no mutations were discovered. Nevertheless, it behooves the authors to compare the entire genome sequences, as well as prove that the viruses being compared were native and FIP variants of corona virus before making conclusions refuting or supporting the mutation concept explaining the derivation of FIP.

Though still debatable, it appears unlikely that the virus can be passed to unborn kittens or via the milk to newborns. Some apparently healthy cats may carry the virus which can be shed intermittently in bodily fluids or feces, with feces being the most common source of oral infection (which is considered an uncommon means of infection). Interestingly, and most likely due to the primarily mutational mechanism of acquired FIP actual mortality from environmental exposure to the virus (i.e from animals shedding virus) is sporadic, even in a population of cats where FIPV- carriers are known to be present.


II. Clinical Disease

The type and development of disease is quite complex and, in large part, dependent on the status of the animal’s immune system. In some instances, the immune system’s response to infection may actually worsen the clinical signs. In the Effusive Form of the disease there is accumulation of substantial quantities of fluid in body cavities (abdomen and chest). Some of these animals appear profoundly "pot-bellied", while the Dry Form of the disease does not present this way. In both forms, clinical signs can be quite variable; virtually any organ or soft tissue system can become affected, thus mimicking many diseases. The most common clinical signs are non-specific and include fluctuating fever, inappetance, lethargy and weight loss. Sometimes, if the central nervous system is affected, neurological abnormalities are apparent.

III Diagnosis

Routine Labwork:

May show a profile consistent with (but not specific for) FIP. The presence of a "typical" FIP profile coupled with "typical" clinical signs provides a strong index of suspicion for the presence of the disease.

Fluid Cytology:

In the Effusive Form of the disease, body cavity fluid analysis can provide useful information with high predictive value for the presumptive diagnosis of FIP infection. Similarly, when neurological abnormalities are present, analysis of cerebral spinal fluid can produce findings strongly supporting the presence of this disease.

Histopathological Examination:

Histological examination (microscopic evaluation of tissue samples) of affected organs is the most specific of all the available tests for the definitive diagnosis of FIP.

Serum Antibody Tests (Elisa, IFA, AGI, PH):

This is an area of considerable controversy among veterinarians. In the past it was assumed that a high level of antibody to the FIPV along with signs of the disease (which may or may not be specific) was diagnostic for active FIPV infection. We now know that is NOT true:

The antibody tests for FIPV also test for antibodies to FECV, i.e. any
a) cat exposed to any feline corona virus may test "positive" or even "strongly positive".
b) cats given FIPV vaccine may also test positive.

The antibody tests for FIPV may be negative in FIP-positive cats, because
a) The immune system components may actually be involved in the progression of the disease and be "consumed" in the disease process.
b) Early in disease—there is not enough time to develop antibodies.
Some animals are immune-suppressed from concurrent diseases (e.g. feline AIDS).
c) Antibody levels fluctuate up and down, seemingly in random fashion, in either FIPV or FECV infected cats. There is
d) More recent research concerning the disease, including the use/misuse of serum serology to diagnose FIP is presented here.

PCR Test ("Polymerase-Chain Reaction")

This tests for the presence of virus…not antibody. Suffice it to say that at the time of this publication, this test also does not distinguish FECV from FIPV.

IV. Treatment

At present, there is no specific treatment for FIP! As there appears to be an immune-mediated component to the progression and severity of the disease, some have tried immune-suppressive and/or immune-modulating drugs. Some antiviral drugs alone or in combination with immune modulators are being investigated. Massive doses of interferon may help some severely debilitated cats...though effusive FIP has been much more difficult to treat.

More recently, there have been some anecdotal but promising reports from a preliminary study by Dr. Al Legendre, professor of medicine at the University of Tennessee College of Veterinary Medicine. He is experimenting with the drug polyprenyl immunostimulant (PI) on cats with confirmed dry-form FIP. The study so far is on a limited population of patients and the results, while preliminary, are "promising". You can learn a little more about this by reading the article here . You can contact Dr. Legendre if you are interested in having your cat included in his study, as well.

V. Control

Keep in mind...as environmental exposure to FIP from shedding animals appears to represent a minor or inconsequential source of acquired FIP, the measures to eradicate or to minimize prevalence of the virus and FIP infection within the population is not yet established.


There is an FDA approved
intranasal (drops) vaccine
available for cats who are at least 16 weeks old. Initially, two doses are given separated by three weeks; thereafter an annual booster is required.

The extent of protection provided by the vaccine, according to the manufacturer, is claimed to be 50% to 70%, but there are independent studies which question whether the vaccine is beneficial. One older (1992) study, from Cornell University, showed that vaccine-induced antibody resulted in enhancement of the the disease! Several anecdotal reports of cats dieing from FIP within weeks of vaccination are published (albeit not considered scientific proof of cause & effect).

The vaccine will not help a cat that is currently infected or recently exposed to FIPV (or FECV). Therefore, there is NO POINT in vaccinating a cat that already has a corona virus antibody titer or has been exposed to other infected cats.

There is a more recent publication (Kirk's Current Veterinary Medicine, XIV pp1295, 2008 ) which reviews some of the older work also supporting the claim that the older vaccine is protective, providing the cat is not already infected with any corona virus. That publication argues that all cats, especially breeding cats and cats that will be boarded, should be kept free of any corona virus exposure until such time as it can be vaccinated. That admonition is not commonly supported by other veterinarians with special interests in infectious diseases.


The risk of acquired FIP is small even in cat populations with proven FIP infection. Risk of exposure is, nevertheless, greatest in enclosed multicat facilities and much less via casual contact with neighboring cats. The least risk is in the single, indoor-only cat.


The virus may persist in the environment for several weeks, but is easily killed with common household disinfectants. However, consider that cats with clear symptoms of FIP infection are rarely shedding virus and that true (mutated corona) FIP virus is poorly transmitted to cats.

More information and other resources can be found at the sockit.org website.

1 The information contained herein represents the opinion and judgement of the author, based upon his interpretation of revered veterinary journals, comments and discussions with respected colleagues, anecdotal reports and comments from and to knowledgeable veterinarians world-wide via the Veterinary Information Network(VIN). This information is subject to extensive revision based upon the availability of new and pertinent clinical data. The author wishes to thank in particular Dr Alice Wolfe, for her helpful comments, published insights and advice as a VIN consultant on this and other important subjects.

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