Feline infectious peritonitis (FIP) is feared by people who breed cats. It is also frustrating to the veterinarian because
of the difficulties in making a definitive diagnosis. The gaps in the information needed for advising the owners about prevention
are also frustrating. Our understanding of this disease is still incomplete but considerable information has emerged in the
last few years.
The definitive diagnosis of the "dry form" of FIP can be difficult because even a complete necropsy may produce a vague report
of "compatible with FIP." A clinical history of a cat under a year of age with lethargy and a persistent fever unresponsive
to antibiotic treatment suggest FIP. Sparkes studied a number of cats with FIP as well as control cats with other diseases
to identify laboratory findings that are supportive of FIP. He found that cats with compatible signs and a combination of
lymphopenia, hyperglobulinemia and FIP titers above 1:160 had a positive predictive value (PPV) of 89% while cats that had
none of these changes had a negative predictive value (NPV) of 98.8%. The only definitive way to establish a diagnosis is
from histopathology of affected organs such as liver, kidney or omentum. In equivocal cases, immunostaining for coronavirus
of tissues with granulomas will confirm the diagnosis but there may be only small amounts of virus even in cats with large
The "wet form" is somewhat easier to diagnose from the typical pleural or peritoneal effusion. Effusions with total protein
values of greater than 3.5 g/dl in which globulins make up greater than 50% of the protein have a PPV of 94% and a NPV of
100%. The diagnosis can be confirmed by immunostaining of macrophages in the effusion for coronavirus.
Polymerase chain reaction (PCR) techniques have emerged as a tool to identify the presence of coronavirus in the plasma or
tissues of cats suspected of having FIP. The PCR technique was an especially worthy advance to study feline coronaviruses
because standard virus isolation techniques in tissue culture do not routinely recover coronaviruses. At this time, I don't
know of any studies that have identified the genetic difference between nonpathogenic feline coronaviruses that cause mild
diarrhea and highly lethal coronaviruses that cause FIP. The primers currently used for PCR identify the feline coronavirus
group. When the genetic differences between the virulent and avirulent viruses are known, then reliable primers can be made
to separate the biotypes of feline coronavirus. The expression of the 7b protein in pathogenic coronaviruses has been proposed
as a method of separating pathogenic and nonpathogenic viruses. A commercially available test is available to identify antibodies
to the 7b protein. No convincing data has been presented by the company that gives the sensitivity and specificity of the
assay. Dr. Kennedy at UT has identified 7b antibodies in 5 cats with confirmed FIP but has also found antibodies in 2 healthy
The detection by PCR of coronavirus in the cat is important information because cats not infected with coronavirus don't get
FIP. Coronavirus is endemic in multiple cat households and catteries. Surveys done in California and in Switzerland have not
identified active catteries free of coronavirus. Li and Scott found coronavirus by PCR in spleen, liver and kidneys of 8 of
9 cats with FIP but they also found coronavirus in 61% of cats that died of diseases other than FIP. Until specific primers
are developed, PCR remains a research tool to study the transmission of coronavirus in catteries.
Previously, the presence of coronavirus systemically was considered an indication of FIP because the avirulent virus was believed
to be confined to the gut. With PCR techniques coronavirus was found in plasma of 37% of cats in a breeding colony. The cats
remained healthy for 8 months after testing.
When a diagnosis of FIP is made, what should we recommend doing with the other cats in the household? To make valid recommendations
we need to better understand coronavirus transmission between cats and the events that lead to FIP. Coronavirus is enzootic
in almost all households where there are 10 or more cats. Yet, epidemics of FIP are uncommon. Addie showed that kittens from
households infected with coronavirus were no more likely to develop FIP if there had been a death from FIP in the household.
This is not consistent with an infectious disease that progresses rapidly through the cat population. This is more consistent
with a widely distributed virus that in certain cats mutates to a virulent form and causes disease if the cat is not able
to contain the infection.