Objectives of the Presentation

 Review of the diagnosis of FIP

Overview of the Issue

 FIP presents many challenges but for the practitioner the major challenge is often in making a confident diagnosis pre-mortem.

 In most cases the diagnosis is suspected on the basis of the history of the case, the clinical features identified on clinical examination in the consulting room, supported by laboratory tests and imaging.

 No reliable tests to confirm a diagnosis are available based on either serology or detection of virus in blood

 Confirmation of diagnosis depends on histopathological investigation of biopsies or tissues obtained at post mortem examination supported by demonstration of virus in lesion using more specialized tests such as immunofluorescence.

Additional Detail

History

Historical features may help to increase the index of suspicion of FIP. It is most commonly seen in young pedigree cats derived from large, multicat households with clinical signs typically developing soon after moving to a new home for the breeder and sometimes linked to recent stresses such as vaccination and neutering.

Clinical Signs

A diagnosis cannot be made on clinical grounds alone since the clinical signs in FIP are so variable and many other feline diseases will result in similar clinical signs.

Body fluid accumulation is the most characteristic feature of FIP, particularly affecting the abdomen, leading to the classic presentation of ascites. Approximately 25% of effusive cases also show thoracic fluid accumulation which may be a pointer to differentiation of other causes of ascites and around 10% show fluid accumulation just within the thorax. Fluid accumulation occurs in around 75% of cases of FIP and may be present in some cats with predominantly non-effusive clinical features which can be of diagnostic value. The clinical features of non-effusive cases are very variable. Around 25% of cats with FIP have hyperbilirubinemia and some of these will have clinically detectable jaundice.

Imaging

Radiography and ultrasound scanning can help to identify effusions and organomegaly. Occasionally fibrinous tags may be detected in the abdomen and the liver may show a variable heterogenous appearance.

Routine Laboratory Changes

The most common laboratory changes are:

(Sparkes et al, 1994)

Serum protein electrophoresis frequently shows an increase in gamma globulins, sometimes appearing as a paraproteinemia, and increases in α2 globulins. Alpha acid glycoprotein levels may be raised (>3 mg/ml) (Paltrinieri et al, 2007).

Effusion Analysis

Effusions in FIP typically have very high protein contents consisting mainly of globulin, almost invariably with more globulin than albumin. Cell counts are variable but often low consisting of a mixture of inflammatory cells including neutrophilia and macrophages with RBCs.

CSF Analysis

Increased numbers of inflammatory cells and elevated protein levels (>50 mg/dl) may be found on CSF analysis of FIP cases with neurological signs, but frequently this is normal (Steinberg et al, 2008).

Demonstration of Coronavirus

Interpretation of coronavirus titres is complicated by the widespread occurrence of titres in cats with prior FCoV infection without FIP. High titres, however, do increase the suspicion of FIP. A negative (or low titre) does not eliminate the possibility of FIP.

Some studies have suggested that detection of virus in blood using PCR may help in diagnosis, on the basis that this indicates mutation of the virus which has permitted escape from the gut and dissemination (Herrewegh et al, 1995). However, low levels of virus may be detectable in cats with FCoV infection without FIP and the levels of viral RNA may be very low or undetectable in confirmed cases (Can-Sahna et al, 2007).

Attempts have been made to identify specific mutations associated with development of FIP which could be used as molecular tests but, although the sites of mutation have now been identified, the mutations are variable in different cases. If effusions are present, demonstration of virus using RT-PCR assay or immunofluorescence to detect virus in macrophages may help in diagnosis. However failure to demonstrate virus does not rule out the possibility of FIP.

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Summary

 FIP represents a major diagnostic challenge. In many cases a tentative diagnosis is made based on an index of suspicion. If fluid accumulation is present, laboratory analysis of the fluid is the most useful diagnostic investigation. Confirmation of diagnosis requires post mortem examination or examination of tissue samples obtained by pre-mortem biopsy.

References/Suggested Reading

1.  Can-Sahna K, Ataseven VS, Pinar D, Oğuzoqlu TC. (2007) The detection of feline coronaviruses in blood samples from cats by mRNA RT-PCR. J. Fel. Med. Surg. 9: 369-372.

2.  Herrewegh AA, de Groot RJ, Cepica A, Egberink HF, Horzinek MC, Rottier PJ.(1995) Detection of feline coronavirus RNA in feces, tissues, and body fluids of naturally infected cats by reverse transcriptase PCR. J. Clin. Microbiol 33: 684-9.

3.  Paltrinieri S, Giordano A, Tranquillo V, Guazetti S (2007) Critical assessment of the diagnostic value of feline alpha1-acid glycoprotein for feline infectious peritonitis using the likelihood ratios approach. J Vet Diagn Invest 19: 266-72.

4.  Sparkes AH, Gruffydd-Jones TJ, Harbour DA (1994) An appraisal of the value of laboratory tests in the diagnosis of feline infectious peritonitis. J Am Anim Hosp Assoc 30: 345.

5.  Steinberg TA, Boettcher IC, Matiasek K, Hirshvogel K, Hartmann K, Fischer A & Kuntz A (2008) Use of albumin quotient and IgG index to differentiate blood-vs. brain-derived proteins in the cerebrospinal fluid of cats with feline infectious peritonitis. Vet. Clin. Pathol 37: 207-216.