FIP remains the one of the most challenging diseases to vets to diagnose.  In the past a cat presenting with a swollen abdomen was always thought to have wet FIP.  We now know that this is a misconception as there is more than a handful of other disease including parasitic infection that can result in a swollen or distended abdomen. A swollen abdomen could be as a result of bacterial peritonitis, cholangiohepatitis, lymphoma, trauma to just name a few. So what do you do if faced with a suspected diagnosis of wet FIP due to a cat presenting with a swollen abdomen?

Wet or effusive FIP occurs when fluid accumulates in either the abdomen or the chest cavity.  Most wet FIP cases are abdominal while pleural cases represent 35% of all wet FIP cases.  Wet FIP results due to vasculitis, which in simple terms is when the blood vessels leak plasma into the surrounding cavity.  Wet FIP is the more acute / virulent form of FIP.  It is this form that is predominantly found in younger cats and this is due to their immune systems being very badly compromised.  FIP is an immune mediated disease; if the immune system is very weak or non-existent then the effusive form results but if the immune system is mounting a partial response then the more chronic form, which is non-effusive or dry FIP, results.

FIP tummies usually feel like a water-filled balloon. But feeling is not enough to go on because after all FIP is fatal and you do want to ensure the best possible accuracy in diagnosis. Firstly, insist on having fluid drawn. This serves two purposes in that it will provide a sample to send for testing and secondly it gives the cat a measure of comfort.  Up to 2 litres of fluid have been drawn off a cat with effusive FIP! Imagine how uncomfortable that must be. Once the fluid is drawn examine it visually followed by a Rivalta’s test and then submit the fluid for FCoV  RT-PCR

Typically FIP effusion is yellow to straw coloured.  It is what is called an exudate or modified transudate. But yellow fluid can present in other ailments, which is again why complete testing needs to be followed through. In the past s definitive diagnosis of wet FIP could only be made either post mortem through an autopsy.  With the advent of better testing laboratories and more knowledge on the disease we now know that a FCoV RT-PCR test is viable as this indicates the presence of coronavirus in the fluid and the only way for it to found in fluid is if it has passed through the macrophages and into the body.  Coronavirus is a disease that is confined to the enteric system unless mutation has occurred.

What about the many other tests that are offered such as immunoperoxidase staining of the fluid?  As per Dr Diane Addie this test is prone to give false positives and while suggestive of FIP is not diagnostic of FIP.  A Rivalta’s test is also by no means a sure fire indicator. All it does is draw attention to the possibility of FIP. The Rivaltas test highlights that the fluid is a modified transudate or exudate – it has protein in it. The cause or origin of that has to be investigated further.

As with dry FIP, bloodwork is only indicative.  It is not a 100% guarantor of FIP. Some levels will be out but these may be exceeding the norms due to any inflammation or infection, not only FIP. Wet FIP is more subtle and difficult to diagnose when it comes to bloodwork and often what will be off the chart for dry FIP is hardly evident in the bloodwork for wet FIP.

Rule of thumb when faced with a possible diagnosis of wet FIP

  • Step 1 – Download and go through Dr Diane Addie ‘s FIP Diagnostic Flowchart. This is the gold standard.
  • Step 2 – Evaluate the cat in terms of history and clinical signs.
  • Step 3 – Do the Rivalta’ s test. If negative then it definitely isn’t FIP as it is over 95% accurate on negative results.  If positive it MAY be FIP but then the fluid has to be sent for.
  • Step 4 – Request a FCoV RT-PCR on the fluid.

Cats with wet FIP do not always exhibit the same symptoms as cats with dry FIP. Many still continue to eat with healthy appetites or are still active.

Remember euthanasia is final. No going back. Always ensure you have eliminated every possible other differential when it comes to a diagnosis. These are your babies.

Written by Aurora Lambrecht