Diagnosing Non-Effusive (Dry) FIP

Presenter: Dr Diane Addie, Honorary Senior Research Fellow (School of Medicine), University of Glasgow

Part two of Dr Diane Addie’s set of webinars covering the diagnosis of dry and wet FIP was every bit as good as her first webinar delivered last year. Last week’s discussion covered the topic of dry FIP which, according to Dr Addie, is one of the most over and under diagnosed conditions in the cat. This is obviously a contradiction but Dr Addie explained that with a differential list as long as your arm, there are a number of diseases that dry FIP can mimic. However, it could also be possible to assume a positive diagnosis of FIP when in fact a cat has a different condition altogether.

With this in mind Dr Addie took us back to the FIP diagnostic flow chart available on her website, catvirus.com as following this in a step by step approach is key to ensuring the best possible chance of a correct diagnosis. Before taking us through this flow chart Dr Addie explained that wet and dry FIP are on an FIP spectrum, and are not two separate diseases. Wet FIP is an acute form of the disease and dry FIP is the chronic form where the virus load is lower and there are often less pyogranulomatous lesions which may be bigger.

Anecdotally Dr Addie believes cats with dry FIP stand a much better chance of being significantly helped if diagnosed early. History and physical examination are always the best place to start and Dr Addie’s flow charts help to ensure no clinical signs are missed. Ocular signs are, according to Dr Addie, present in all cats with dry FIP and include uveitis, aqueous flare, retinal vessel cuffing and keratitic precipitates. However ocular signs associated with dry FIP can be subtle and easy to miss, so Dr Addie always advises checking both eyes thoroughly and to remember to check for ocular signs under third eyelids.

Standard laboratory tests are, as expected, always necessary and certain values can help point in the direction of FIP. For example studies have shown that 83% of cats with FIP have a non-regenerative anaemia, often with a haematocrit of <30%. FIP cases also tend to have raised globulins and an albumin:globulin ratio of <0.7. As for wet FIP, testing for FCoV antibody titre is also useful, however the test must be sensitive enough as a negative result will rule out FIP altogether. Dr Addie cited a paper written by herself discussing the accuracy of number of commercially available FCoV titre tests, with some faring better than others.

The use of specialist laboratory testing is also invaluable for diagnosing FIP and Dr Addie finds measuring alpha-1acid glycoproteins (AGP) incredibly useful. AGP increases with any acute infection/inflammation or trauma within the body and tends to be very high in cases of wet FIP and 2-3 times the norm in cases of dry FIP. Measuring AGP levels is also really useful in differentiating FIP from neoplasia and hyperthyroidism which tends to be normal in cats with these diseases.

The gold standard for diagnosing dry FIP is to use gross pathology and histopathology to confirm the presence of pyogranulomatous lesions, either at post-mortem or biopsy. Dr Addie warned us however, that it can be easy to miss these lesions on biopsy, especially as some may be quite small. Discussions also centred around the use of RT-PCR for FCoV as this could be used on effusions in wet FIP to provide a definitive diagnosis. However Dr Addie explained in dry FIP this test is pointless if being used on blood, as the majority of samples will be negative. There may be merit in performing RT-PCR on fine needle aspirates of the mesenteric lymph nodes which could spare the use of more invasive techniques to obtain biopsies.

This webinar delivered a huge amount of information and everything mentioned within this blog is discussed in much greater depth by Dr Addie. With FIP being one of the most difficult conditions to diagnose correctly, investing time to learn more about this condition from a well renowned leader in the field of feline medicine has to be worthwhile for every small animal vet.

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