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FIP Testing? There is No Such Thing!

Joanne Lewis BSc BVMS (Hons) MRCVS

Feline Infectious Peritonitis (FIP) still keeps vets and clinical pathologists on their toes largely because we are still not able to accurately differentiate between the most pathogenic (FIP-causing) strains of coronavirus and those that cause little or no disease. As many as 40% of cats in the general population test positive for coronavirus (FCoV) and this increases to 90% in catteries/multi-cat households.

FIP Testing

A positive coronavirus titre does not mean a diagnose of FIP!

A relatively small percentage of the cats that have been infected with coronavirus may be unfortunate enough to go on and develop fatal FIP. It is suspected that FIP is a consequence of exposure to specific FCoV strains that undergo mutations during viral replication, becoming a virulent FIP-causing strain within the individual cat. The chances of this happening may also be dependent on other factors.

  • Whether or not the immune system in the infected cat is competent plays an important role, hence affected cats are often less than 2 years of age.
  • Genetic susceptibility is thought to be relevant too, with breeds such as Persians, Birmans, Burmese, Siamese and Bengals being more likely to develop FIP.
  • Stressors such as concurrent illness, vaccination, neutering, rehoming, boarding, originating from a breeding cattery/rescue centre or being resident in a multicat household are all thought to be significant factors that increase the risk of a positive coronavirus titre becoming FIP.

Many cats will present with the more acute and classic "wet"/"effusive" clinical signs. Not all cats however, will present with the textbook symptoms, particularly those that have the more chronic "dry" granulomatous form of the disease. Either way, the underlying process is that of an immune-mediated vasculitis and as blood vessels are everywhere, it is easy to see why there is such a wide array of clinical presentations!

Enhancing you index of suspicion for FIP

A coronavirus titre is a necessary but rather small component of a workup for cat with suspected FIP. Other diagnostic tests have an important role to play in building up a portfolio of information for a cat that you think may have disease resembling FIP.

  • Haematology - lymphopaenia, neutrophilia with left shift and mild to moderate non-regenerative anaemia. These changes are more likely in cats with non-effusive disease.
  • Biochemistry - hyperglobulinaemia, hence hyperproteinaemia +/- hypoalbuminaemia, hyperbilirubinaemia and hepatopathy.
  • AGP (Alpha 1 Glycoprotein) is an acute phase protein that is produced by the liver. It can be elevated with inflammation in general so is not diagnostic for FIP; however levels > 1500 ug/ml in effusive disease and > 1000 ug/ml in non-effusive disease may increase the index of suspicion for FIP. AGP can be measured on serum or fluid samples.
  • SPE (Serum Protein Electrophoresis) - Antibodies produced in chronic inflammatory disease (including FIP) can produce broad-based gammaglobulin peaks, evident as a polyclonal gammopathy. You may also expect to see elevations in the acute phase proteins AGP, SSA (serum amyloid A) and haptoglobin.
  • Cytology

Cavitary Fluids - Typically modified transudates (> 35 g/L, low to moderate WBC count 2-6 x 10^9/L, mainly non-degenerate to mildly degenerate neutrophils, fewer numbers of macrophages, small lymphocytes and occasional plasma cells)

CSF (cerebrospinal fluid) - Highly variable cell count and protein results with a pleocytosis that can vary from suppurative to mononuclear in nature.

  • Coronavirus Antibody Titre (IFA) - Antibody titres of between 80 and 10540 are suggestive only of exposure to FCoV and can be run on serum, cavitary effusions, CSF or aqueous humour.
  • Coronavirus RT-qPCR - not currently widely available in the UK and its use at present seems limited diagnostically as it still doesn't differentiate pathogenic or non-pathogenic strains of FCoV. It may prove useful for monitoring faecal viral shedding in catteries/multicat households.
  • Direct Immunofluorescence - detects large amounts of virus present in the macrophages of effusions. This may be a helpful test in those cats with such high viral loads that serum antibody detection testing has failed to show a positive titre. It does require intact cells for accurate results so there can be false positives with sample /cell deterioration.
  • SPE (Serum Protein Electrophoresis) - Antibodies produced in chronic inflammatory disease (including FIP) can produce broad-based gammaglobulin peaks, evident as a polyclonal gammopathy. You may also expect to see elevations in the acute phase proteins AGP, SSA (serum amyloid A) and haptoglobin.
  • Histopathology and Immunohistochemistry - still the gold standard test. Immunohistochemistry is used to demonstrate the distribution of coronaviral antigens within the tissues once FIP is suspected on histopathology.

Summary

It is diagnostically challenging without histopathology to actually get close to a diagnosis of FIP. A confirmed post-mortem histopathological diagnosis of FIP can be made in cats that have presented with no haematological/biochemical changes and conversely, there are many feline conditions that look for the entire world like they have FIP clinically, but do not. Emphasis relies on our approach to a potential case of FIP. We need to improve our index of suspicion by collecting a thorough history, conducting a full physical examination and requesting the appropriate diagnostic tests... not just a coronavirus titre!