Dr Allison German BVSc MSc PhD MRCVS
Cats Protection Lecturer in Feline Medicine, University of Liverpool

Talk presented by Allison German to vets at the ESFM Feline Symposium 2010, pre-BSAVA Congress

Feline infectious peritonitis (FIP) remains one of the most frustrating and depressing diseases we see in feline medicine. It is a difficult disease to diagnose, a difficult (if not impossible) disease to treat and a source of distress and financial burden for our cat owners and breeders. This paper will give an update of the more recent literature to help you better diagnose the disease and choose treatment for appropriate cases.

What is feline coronavirus?
Feline coronavirus (FCoV) belongs to the group 1 species in the genus coronavirus, Family Coronaviridae, Order Nidovirales.  There are two serotypes, Type I is most prevalent in the field (70% of FIP cases in the UK are due to this strain). Type II is more closely related to canine coronavirus and transmissible gastroenteritis virus (shared immunodominant epitopes of spike proteins).

How does FCoV infect cats?
The virus is transmitted oro-faecally, so is a particular problem in multi-cat households and catteries.  Once infected, some cats remain as persistent shedders, others stop shedding and can become re-infected with the same or a different strain.  Some cats remain as shedders for life.  The virus is fragile and susceptible to common disinfectants, but some strains can persist in the environment for several weeks, which could be an alternative source of infection.

What is the pathogenesis?
There are two biotypes of FCoV. The common enteric form causes relatively few problems, but may be associated with a transient, self-limiting diarrhoea. Approximately 25-40% of household cats in the UK are infected. FIP is caused by the second biotype of FCoV. This is thought to arise de novo in each cat by mutation of an existing enteric FCoV strain, and usually dies with that cat.

What clinical signs are associated with FIP?
There are no clinical signs pathognomonic for FIP. ‘Wet’/effusive FIP develops in 60-70% of cases. A generalised vasculitis leads to effusions, particularly pleural and peritoneal.  Progression to dissemiated intravascular coagulopathy (DIC) and/or sepsis may occur.  The ‘dry’/proliferative form is characterised by granuloma and plaque formation on serosal surfaces; the abdominal organs, eyes and CNS are most commonly affected.

The disease may persist for several days to several months, although the wet form has a shorter duration. Initial signs are non-specific and include anorexia, lethargy and fluctuating pyrexia.  Gastrointestinal or respiratory signs may also be present.  Effusions can take weeks or months to occur.  Abdominal distension can be seen with abdominal effusions and dyspnoea with pleural effusions.  Other signs are also dependent on which organs become infected and can include uveitis, jaundice and neurological dysfunction.  With progression of disease, signs relating to immunosuppression (generalised infections) and coagulopathy (reduced clotting times, progressing to DIC) can be observed.

How do we diagnose disease?
The diagnosis of FIP is usually made through the history, clinical observations and diagnostic tests.  A clinical work-up should include a clinical examination, an ophthalmic examination, thoracic and abdominal radiographs, ultrasound and fluid collection and analysis. Additional investigations may include CSF analysis and MRI for hydrocephalus. Effusions have been reported in 60-97% of cats with FIP (Hartmann and others 2003; Lutz and others 1986; Ritz and others 2007). A definitive test is not available, though tissue biopsies can confirm the diagnosis (only if the cat is stable enough for the procedure).

A test with a high positive predictive value (PPV) is important to help the clinician determine whether a cat has FIP. The PPV varies depending on the prevalence of disease within a population. Therefore, a diagnostic test result is more reliable for a cat coming from a population with a high prevalence of FIP.

Haematology usually shows lymphopenia, neutrophilic leucocytosis (+/- left shift) and a mild to moderate non-regenerative anaemia.  Biochemistry shows hyperproteinaemia (hyperglobulinaemia) in 50-80% cats (Sparkes and others 1994), low-normal serum albumin (low albumin:globulin ratio), hyperbilirubinaemia in 25% cats and occasionally elevated liver enzymes.

The acute phase protein α1-acid glycoprotein (AGP) is often elevated >1500 mg/ml, although this may rise with other infectious diseases. An elevation in α1-AGP is significant if considered alongside a history and other test results supportive of FIP (Saverio and others 2007). If the supportive evidence for FIP is not so strong, α1-AGP levels >3000 mg/ml can be supportive (though not definitive) of a diagnosis. α1-AGP levels fluctuate in FCoV+ high prevalence households compared to SPF controls and low prevalence households (Paltrinieri and others 2007). This may just reflect the increase in viral load and virus turnover. However, as viral load is associated with increased risk of development of FIP, increasing α1-AGP levels may also be used as an indicator for the development of FIP.

The most consistent finding in cats with FIP are elevated globulins and a decreased albumin:globulin ratio. The lower the A:G ratio, the more likely the diagnosis of FIP; the albumin:globulin ratio has a greater diagnostic utility than just considering the separate protein amounts. In FIP cases, the α2-globulins rise, followed by the γ-globulins just prior to clinical signs. The combination of elevated globulins and FCoV titre is consistent with an active (yet ineffective/inappropriate) immune response to FCoV infection. When the ratio is less than 0.6, FIP is possible; when 0.3 or lower, FIP is highly likely. Care must be taken when interpreting high total protein or globulin values as gingivostomatitis complex, upper respiratory disease, multiple myeloma and other inflammatory conditions may similarly elevate the globulin fraction.

Fluid analysis: This is one of the most useful diagnostic tests in effusive FIP.  Fluid is generally viscous, straw-coloured, with a high protein level (>35g/l, usually 60g/l, with globulins comprising at least 50%). The fluid froths on shaking and may clot after standing for a few hours at room temperature; an albumin:globulin ratio <0.4 is highly suggestive of FIP and >0.8 is unlikely to indicate FIP. The fluid has a low cell count (<5x106/ml), mainly consisting of neutrophils and macrophages, and is sterile.

If the cat presents with neurological disease, analysis of the CSF should reveal elevated cell counts (particularly neutrophils) and elevated total protein.  The CSF:serum antibody ratio should also be higher than the CSF:serum total protein ratio.

Rivalta’s test can be used to distinguish transudates from exudates. 5ml distilled water is placed in a sample tube with 1 drop of acetic acid (98%). 1 drop of the effusion is applied to the surface of the solution. In a negative test (transudate), the drop will disappear; in a positive test, the drop will stay formed at the surface or slowly sink to the bottom of the tube. A recent evaluation of the Rivalta’s test reported specificity as 80% and sensitivity at 98% (Hartmann and others 2003). The test depends on the effusion being high in protein, fibrin and inflammatory mediators. False positives may arise in cats with bacterial peritonitis or lymphoma.

Coronavirus serology may be performed by indirect immunofluorescent antibody testing or ELISA.  A positive result just indicates exposure to FCoV. A POSITIVE TITRE DOES NOT MEAN THAT THE CAT HAS FIP.  In a study of cats with clinical signs suggestive of FIP, 44% of cats with a measurable antibody titre had FIP (Hartmann and others 2003). However, the presence of a very high antibody titre did have a greater diagnostic utility in determining whether a cat had FIP.  In effusive FIP, titres are very variable and cats may even be negative on serology. This is because large amounts of virus bind to antibody in the cat, so no free antibody is left when the cat’s serum is tested. In Hartmann’s study (2003), 10% of FIP cats had a negative FCoV antibody titre. Cats with “dry” FIP generally have a high antibody titre. Measurement of anti-coronavirus IgG in the CSF is of equivocal clinical use as their presence is dependent on a high serum titre rather than neurological disease (Boettcher and others 2007). Another important element to consider in interpreting the serology results is what antigen is used to detect the FCoV antibodies. Antigen derived from coronaviruses from different species to the cat may have a reduced sensitivity. The detection of FCoV antibodies in effusion as a diagnostic test has been reported with specificity 85% and sensitivity 86% (Hartmann and others 2003). However, the magnitude of the titre does not correlate with the diagnosis of FIP and, in one study, mid range titres were obtained from cats without FIP (Kennedy and others 1998).

Histopathology is still considered as the “gold standard” for diagnosis. Biopsies (surgical or ultrasound-guided percutaneous) should preferably be taken from affected organs, though more accessible tissues may be examined for evidence of disease, as the disease is systemic.  At post mortem examination, fibrinous to granulomatous peritonitis/pleuritis, effusions and granulomatous lesions in various organs and lymph nodes may be observed. Confirmation, by immunohistology will detect the presence of FCoV antigen within macrophages in lesions. Alternatively, effusion fluid can be centrifuged and a smear made of the cellular pellet. Immunofluorescent staining of FCoV antigen in macrophages is consistent with a positive FIP diagnosis (specificity 100% and sensitivity 57% (Hartmann and others 2003), meaning that when this test is positive, the cat has FIP).

Reverse transcription polymerase chain reaction (RT-PCR) is of limited use, as a positive result indicates the presence of coronavirus, not an FIP strain.  This is because there is no particular mutation linked with pathogenicity.  RT-PCR on exudates may be more helpful, but may still give false positives.  RT-PCR of biopsies can be diagnostic.  Viral loads are significantly higher in haemolymphatic tissues in cats that die from FIP, indicating these cats have either a higher rate of viral replication or a reduced ability to clear the virus due to immunosuppression (Kipar and others 2006). RT-PCR on faeces may help detect cats shedding virus, though this should be performed daily for 4-5 days as cats fluctuate in the amount of virus that they shed.  Proper handling and storage of the sample is essential to prevent RNA degradation. Additionally, PCR inhibitors are often present in faeces, so test results should be interpreted with caution (Dye and others 2008).

What differential diagnoses should I consider?
An important differential diagnosis for cats with ascites is lymphocytic cholangitis.  This disease produces a similar abdominal effusion and biochemical profile to FIP. However, pleural fluid does not develop, so thoracic radiography can help differentiate cases, as 25% of ascitic cats will have pleural effusion.  Other differential diagnoses include cardiomyopathy, lymphoma and hepatopathies. A retrovirus screen should be performed as retroviruses can mimic most differentials, or can cause immunosuppression, encouraging replication of FCoV and the generation of a pathogenic viral strain. As the clinical signs associated with FIP are variable depending on which organ is affected, a wide differential list should be considered with respect to the predominant clinical signs.

What is the prognosis?
FIP is invariably progressive and fatal. In cats who have a poor quality of life alongside refractory disease (high effusions, moderate to severe anaemia, low lymphocyte count, progressive liver damage), the decision to euthanase sooner rather than later should be made. Treatment can be continued for those cats responding favourably within the first 48 hours (a reduction in the volume of effusion/the lack of re-development of effusion and an improvement in haematological/ biochemical parameters). For monitoring those we do decide to treat, I would base treatment decisions on the cat’s progression to remission. Remission can be considered as a lack of development of a pleural/abdominal effusion; a lack or reduction in clinical signs of disease; a reduction in the globulin level; an elevation of the A:G ratio to 0.6 or above; a reduction in α1-AGP; an improvement in haematocrit and an increase in bodyweight. FCoV titres can be monitored every 2-3 months to see if they are reducing. Regardless of our ability to predict survival, the outcome and general prognosis is grave and there are no confirmed cases of cats surviving definitively diagnosed FIP. I would never talk an owner out of euthanasing the cat if that is their wish.  However, consideration of these parameters may influence our decision of which cats to persevere with palliative treatment.

How do we treat FIP?
Treatment is supportive and symptomatic and may include fluid therapy, drainage of effusions, nutritional support, warmth, and appetite stimulants.  Euthanasia is advisable once welfare is compromised.  Prednisolone (2-4mg/kg sid) or cyclophosphamide (2.2 mg/kg po 4 times weekly on consecutive days, or 200-300 mg/m2 every 2-3 weeks) have been reported to slow disease progression.  Broad spectrum antibiotics should be administered, for example cefovecin 8mg/kg subcutaneously every 14 days.  Immunomodulators (Propionobacterium acnes, acemannan) and anti-virals have not yet proved beneficial.  Anecdotal reports of the use of interferon (human interferon α 2x106 IU/Kg/d for effusive FIP, 30 IU/d for 7 days on alternate weeks for non-effusive FIP; feline interferon omega – see below) may indicate some benefits, but this has not been scientifically confirmed.  Other supportive drugs may include aspirin (10mg/kg q 48-72h), anabolic steroids, ascorbic acid (125mg bid), vitamin A (not beta-carotene formulation, 200IU/Kg/d) and vitamin B1 (thiamine, 100µg/d).

Studies investigating the benefit of feline interferon omega have now been performed. Ishida and colleagues (2004) recommended a treatment regime of 1 MU recombinant feline IFN-ω /kg subcutaneously every other day until remission, followed by weekly subcutaneous injections with the same dosage. Glucocorticoid was also administered as an intrathoracic injection of 1 mg/kg dexamethasone (in cats with compromising pleural effusion), followed by oral doses of 2 mg/kg prednisolone daily, tapering to 0.5 mg/kg every other day. Complete remission was reported in a third of the cats. However, FIP was not definitively diagnosed in these cats and there were no matched case controls. More recently, a study based on this protocol was performed as a double-blinded placebo-controlled trial in cats with a diagnosis of FIP confirmed by immunohistology of effusion or tissue biopsies (Ritz and others 2007). This study showed there was no statistically significant difference between the treatment and placebo group for any study variable apart from lymphocyte count (lymphocyte count was lower in the treatment group). Survival ranged from 3-200 days, with the median survival time 9 days for the treatment group and 8 days for the placebo group.

Although there have been a number of different treatment protocols advocated for FIP, the majority of studies have failed to definitively diagnose FIP or provide a control group. Interferon only appears to be effective when administered before experimental infection of cats with pathogenic FCoV-FIP strains. Administration of interferon to the cat after clinical signs are present appears to be too late to make a difference. However, the long surviving cat in Ritz’s study (2007) was treated with interferon; whether this is an effect seen due to administration of interferon or due to the individual cat is not known.

Polyprenyl Immunostimulant (Sass & Sass Inc.), a mixture of phosphorylated, linear polyisoprenols, has been shown to upregulate Th1-cytokine mRNA biosynthesis. Oral administration of Polyprenyl Immunostimulant to three cats with dry form FIP has shown favourable results, with two cats living over two years and one 14 months (the latter cat stopped treatment after five months) (Legendre and Bartges 2009). This compound is currently not commercially available and requires further testing.

References

BOETTCHER, I. C., STEINBERG, T., MATIASEK, K., GREENE, C. E., HARTMANN, K. &  FISCHER, A. (2007) Use of anti-coronavirus antibody testing of cerebrospinal fluid for diagnosis of feline infectious peritonitis involving the central nervous system in cats. J Am Vet Med Assoc 230, 199-205
DYE, C., HELPS, C. R. &  SIDDELL, S. G. (2008) Evaluation of real-time RT-PCR for the quantification of FCoV shedding in the faeces of domestic cats. J Feline Med Surg 10, 167-174
HARTMANN, K., BINDER, C., HIRSCHBERGER, J., COLE, D., REINACHER, M., SCHROO, S., FROST, J., EGBERINK, H., LUTZ, H. &  HERMANNS, W. (2003) Comparison of different tests to diagnose feline infectious peritonitis. J Vet Intern Med 17, 781-790
ISHIDA, T., SHIBANAI, A., TANAKA, S., UCHIDA, K. &  MOCHIZUKI, M. (2004) Use of recombinant feline interferon and glucocorticoid in the treatment of feline infectious peritonitis. J Feline Med Surg 6, 107-109
KENNEDY, M. A., BRENNEMAN, K., MILLSAPS, R. K., BLACK, J. &  POTGIETER, L. N. D. (1998) Correlation of genomic detection of feline coronavirus with various diagnostic assays for feline infectious peritonitis. Journal of Veterinary Diagnostic Investigation 10, 93-97
KIPAR, A., BAPTISTE, K., BARTH, A. &  REINACHER, M. (2006) Natural FCoV infection: cats with FIP exhibit significantly higher viral loads than healthy infected cats. J Feline Med Surg 8, 69-72
LEGENDRE, A. M. &  BARTGES, J. W. (2009) Effect of Polyprenyl Immunostimulant on the survival times of three cats with the dry form of feline infectious peritonitis. Journal of Feline Medicine and Surgery 11, 624-626
LUTZ, H., HAUSER, B. &  HORZINCK, M. C. (1986) Feline Infectious Peritonitis (Fip) - the Present State of Knowledge. Journal of Small Animal Practice 27, 108-116
PALTRINIERI, S., METZGER, C., BATTILANI, M., POCACQUA, V., GELAIN, M. E. &  GIORDANO, A. (2007) Serum alpha1-acid glycoprotein (AGP) concentration in non-symptomatic cats with feline coronavirus (FCoV) infection. J Feline Med Surg 9, 271-277
RITZ, S., EGBERINK, H. &  HARTMANN, K. (2007) Effect of feline interferon-omega on the survival time and quality of life of cats with feline infectious peritonitis. J Vet Intern Med 21, 1193-1197
SAVERIO, P., ALESSIA, G., VITO, T. &  STEFANO, G. (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, 266-272
SPARKES, A. H., GRUFFYDD-JONES, T. J. &  HARBOUR, D. A. (1994) An Appraisal of the Value of Laboratory Tests in the Diagnosis of Feline Infectious Peritonitis. Journal of the American Animal Hospital Association 30, 345-350

Recommended general references
ADDIE, D., BELAK, S. et al. (2009) Feline infectious peritonitis ABCD guidelines on prevention and management. Journal of Feline Medicine and Surgery 11(7), 594-604
PEDERSEN, N. C. (2009) A review of feline infectious peritonitis virus infection: 1963-2008. Journal of Feline Medicine and Surgery 11(4), 225-258