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Feline
infectious peritonitis (FIP) is a fatal viral disease of
cats. As a veterinarian, FIP is frustrating, as it can
be difficult to achieve a diagnosis. As a cat
specialist, I am frequently consulted for a second
opinion regarding a diagnosis of FIP. If I have a hunch
that a cat might have FIP, I try my best to disprove my
hunch, because a diagnosis of FIP is a death sentence.
The disease is caused by a type
of virus called a corona virus. Most cats are exposed to this virus as
kittens. On initial exposure, the virus causes mild enteritis
(intestinal inflammation), and maybe some mild diarrhea, or no clinical
signs at all. The immune system makes antibodies against the virus, but
does not eliminate it, and the virus continues to reside in the
intestinal tract, causing no problems.
Occasionally, the harmless
intestinal coronavirus mutates, and gains the ability to leave the
intestinal tract. Immune system cells, called macrophages, attack and
engulf the coronavirus, but they do not kill it. Instead, the virus
reproduces itself inside the macrophages. These cells travel
throughout lymphatic vessels, spreading the virus throughout the body.
This mutated intestinal coronavirus is now the evil FIP-inducing
coronavirus. The immune system tries to respond, however, the extent of
the response determines the clinical symptoms the cat will exhibit. A
strong cellular immune response doesn’t eliminate the virus completely,
but does contain it, preventing further dissemination. Cats can keep
the virus in check for months or years. Age, stress, and malnutrition
may reactivate the virus, resulting in full-blown FIP. In cats that do
not mount a cellular immune response, viral replication goes unchecked,
and the FIP-inducing coronavirus spreads. Damage to blood vessels
ultimately develops, and fluid leaks out through the damaged vessels.
Effusions (collections of fluid) can develop in the abdominal cavity,
chest cavity, and pericardium (the sac around the heart), resulting in
what is referred to as the “wet” form of FIP. In cats that mount a
partial cellular immune response, viral replication is slowed, and cats
develop nodular accumulations of inflammatory cells called granulomas
throughout the body. This form of the disease is known as the “dry”
form of FIP.
Diagnosis of the disease is
difficult because clinical signs are vague. Most cats are young
(usually less than one year), and show lethargy, weight loss, poor
appetite, and a fever that doesn’t respond to antibiotics. A serum
chemistry panel often only shows elevated protein (consisting mainly of
globulins), unless the virus has begun to affect the kidneys or liver,
in which case the liver and kidney parameters may be abnormal. The wet
form is easier to diagnose because the presence of fluid in the abdomen
or chest is relatively easy to detect, and fluid analysis can give
additional information supporting the diagnosis. The dry form remains
difficult to diagnose. Biopsy of the affected organs or tissues has
remained the only way to definitively diagnose FIP. A rapid, reliable
test is critical to allow veterinarians to make the diagnosis, to lessen
suffering in affected cats, and avoid euthanasia of unaffected cats.
Nearly every veterinary
diagnostic laboratory offers a “FIP test” to veterinarians. This test
is simply a test to measure the presence of antibodies against
coronaviruses. These coronavirus-specific antibodies are present in
80-90% of cats in catteries, and in 10-50% of cats in single-cat
households. The presence of antibodies in the blood stream DOES NOT
mean that the cat has FIP. Only 5-10% of coronavirus-infected cats
develop FIP in a cattery setting, and the incidence is much less in a
single-cat household.
A recent article in the Journal
of Veterinary Internal Medicine (November/December 2003) critically
analyzes the various methods used to diagnose FIP in clinical cases.
They studied 488 cats with biopsy-confirmed FIP, and compared the
results to that of 620 non-infected cats. Most of the affected cats
(80%) had fluid accumulation in a body cavity, although this alone was
not diagnostic for FIP, as there are many reasons for a cat to have
fluid accumulations. Many cats had elevated protein and globulin levels
in their serum, but this too proved to be a poor diagnostic test for
FIP. In cats with the wet form of FIP, measurement of the globulin
levels in the body cavity fluid had more diagnostic utility compared to
measuring globulins in the bloodstream, but this still was not
definitive. For cats with fluid accumulation, a simple test called
Rivalta’s test, can be used to differentiate fluid resulting from FIP
vs. fluid resulting from some other disease with very good accuracy, and
may be helpful for diagnosing the disorder in cats with the wet form of
the disease. Because it’s a test on body cavity fluid, it is not useful
in diagnosing the dry form of the disease.
The inadequacies and pitfalls
of measuring antibody levels as a diagnostic test for FIP have been
debated extensively amongst veterinarians for years. The researchers
examined the usefulness of measuring coronavirus-specific antibodies,
and they demonstrated that the presence of antibodies in general had a
poor predictive value in diagnosing FIP. More interestingly, it was
possible for cats to test negative (i.e. have no detectable antibodies
against the coronavirus) and still have FIP; 10% of cats who truly had
FIP had no detectable antibody on the test. Cats with a very high level
of antibodies, however, had a high probability of having FIP (94%). To
summarize: low or medium levels of coronavirus-specific antibody had no
diagnostic value, negative levels had limited value, and very high
levels, if present, raises the probability of FIP considerably.
Unfortunately, very high levels were present in only 36 cats in the
study.
One approach to the diagnosis
would be to try to detect the coronavirus, and then determine whether
the virus was the harmless intestinal virus or the mutated, evil FIP
virus. A relatively new test called polymerase chain reaction (PCR) is
very good at detecting the extremely small numbers of specific
infectious organisms in humans and animals, however, the investigators
found that while PCR is very sensitive at detecting coronavirus
infection, it cannot distinguish between the harmless intestinal
coronavirus and the mutated, FIP-inducing coronavirus, despite some
grandiose claims to the contrary.
Another method to
detect the virus is to search for it inside macrophages and other cells
present in body cavity fluid. A study published in 1995 showed the
presence of coronavirus in 34 out of 34 samples from cats with
FIP-induced fluid accumulations. In the present study, detection of
intracellular coronavirus in macrophages found in body cavity fluid was
evaluated, and indeed, there were no false-positive results. A positive
test, in other words, predicts with 100% certainty that the cat has
FIP. Unfortunately, this type of test tends to be performed in research
laboratories, and most commercial laboratories do not offer this test.
One must keep in mind that while a positive test is 100% diagnostic, a
negative test does not mean that the cat does not have FIP. Also, the
test is performed on body cavity fluid of affected cats. It is not
useful for cats with the dry form of the disease.
In conclusion, the only way to
definitively diagnose FIP is by biopsy, or by detection of coronavirus
in cells from body cavity fluid of affected cats. There is no simple
blood test that can make the diagnosis, and I shudder to think of how
many cats have been euthanized unnecessarily due to a mistaken belief
amongst many veterinarians that a positive antibody test is diagnostic
for FIP. FIP remains a death sentence for cats, and I urge all cat
owners whose cat is diagnosed with the disorder to question their
veterinarian extensively as to how the diagnosis was achieved, and seek
a second opinion if there are any doubts about the diagnosis.
Updated 2/9/06 |
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