Dr. Elizabeth Colleran

How to confirm an FIP diagnosis - by Dr Elizabeth Colleran

January 23, 2023

The article below was written by Dr Elizabeth Colleran to help people and their vets understand how an FIP diagnosis can be made.   With the possibility of treating FIP now becoming available (See other posts about the unregulated drug market), it is more important than ever to arrive at a diagnosis quickly so a decision can be made whether to treat for FIP.   Email peter@ZenByCat.org with any questions and I will forward them to Dr Colleran.

The belief that FIP is difficult to diagnose is predicated on the fact that we, veterinarians, hate to diagnose it and so will go to great lengths to rule it out. Until recently, an FIP diagnosis was a “death sentence.” In addition, few veterinarians are aware of the great strides Dr. Pedersen and others have made toward a safe and effective treatment, even in the severely ill feline patient.

In 2018, a review of the diagnosis of FIP was published in the Journal of Feline Medicine and Surgery which mirrors my approach and views of the appropriate path to presumptive diagnosis. Given the recent findings that response to therapy is a significant indicator of FIP, a presumptive diagnosis should be adequate to seek treatment with a high degree of confidence.

The history is critical part of diagnosis as it can point toward an exploration of supporting (or discounting) evidence. FIP occurs most commonly in younger cats, less than 3 years old. A recent history of stress (sheltering, neutering, vaccination, another infection) may be present and play a role in triggering FIP a part in an FeCOV infected cat.

Clinical signs may vary but are often accompanied by a fever that fluctuates and is unresponsive to antibiotics or nonsteroidal anti-inflammatory drugs. Temporary normothermia may occur if corticosteroids are employed but the cat will remain quite ill. The effusive form will be accompanied by abdominal, pleural or pericardial effusion. The cat may present with apparent effusion and abdominal distension or dyspnea. The noneffusive form usually has some neurological component and develops more slowly than the effusive form. Posterior uveitis is common. Lymphadenopathy is common in both and peripheral nodes may be quite easily palpated.

Hematology can add to the likelihood of FIP as lymphopenia is common, as is neutrophilia with or without a left shift.

Serum biochemistry is also a helpful part of the picture. Hyperglobulinemia is seen in 89% of cases and hypoalbuminemia is common. The albumin:globulin ratio will likely be low. Cutoff values have been proposed, suggesting the an A:G ratio of < 0.4 is highly suggestive of FIP. However, a low A:G ratio in conjunction with other history and clinical findings is quite significant even if it does not fall that low. An A:G ratio, however of >0.6 lowers the likelihood of FIP considerably.

The presence of hyperbilirubinemia in the absence of elevated hepatic enzyme activities or anemia should raise the index of suspicion for FIP. Hyperbilirubinemia may not be seen until later stages of disease and can be caused by other conditions, such as pancreatitis.

Many clinically normal cats will have antibody titers to FeCOV which gives no indication of infection with FIP. It is even possible for cats with FIP to have no titers for FeCOV. Many cats, particularly in multicat households may have very high titers.

Both of these findings make FeCOV titers of very limited value in the diagnosis of FIP.

If an effusion is present, its analysis can provide very significant information toward a diagnosis. Like blood samples, effusions tend to have low A:G ratios. FIP effusions are poorly cellular (usually <5 x 109 cells/l), and are typically pyogranulomatous
in nature with macrophages, non-degenerate neutrophils and very few lymphocytes. The effusions are therefore often described as modified transudates based on cell counts (<5 x 109 cells/l) but as exudates based on protein concentrations(>35 g/l). Other tests have been used to rule out FIP for example, cytology of effusions that yield bacteria or neoplastic cells. RT-PCR has been suggested as useful, however, it does not distinguish FIP from FeCOV infection. When FCoV RT-PCR was performed on plasma or serum samples from FIP and non-FIP cats, none of the non-FIP cases and very few (9–15.4%) of the FIP cases gave positive results for FCoV RNA. A recent experimental study also failed to show any FCoV RNA in the plasma of three FCoV-infected cats over the first 12 weeks of infection. In another study, FCoV RNA was rarely detected in the blood of 20 FIP cases. Thus, use of FCoV RT-PCR on blood, plasma or serum samples is not helpful in the diagnosis of FIP due to low sensitivity. Other tests that may be definitive involve special stains and tests of either effusion or biopsy samples. These tend to be expensive and may take days or longer to be reported. Thus, my recommendation would be to look at the entire history, typical laboratory and physical exam findings and arrive at a presumptive diagnosis which would/could be confirmed by response to therapy.

A typical scenario that would make a presumptive diagnosis relatively straightforward is the following:
young male cat living in a multicat household, or newly adopted or otherwise some history of stress presents with a temperature of 104° (40 C). He is lethargic, has not been eating well. A full work up is declined so he is discharged with doxycycline to treat presumptive infection. He returns 4 days later unimproved and now has a mild uveitis in his left eye. A baseline work up is performed. He has a mild nonregenerative anemia, lymphopenia, neutrophilia without a left shift. Serum chemistries show normal liver enzymes, mild hyperbilirubinemia. His globulins are quite high; albumen low and an A:G ratio of 0.3. The likelihood of FIP could be confirmed by mRNA PCR if effusion is present but could take a week. On ultrasound,there does not appear to be ascites.

The same scenario can include effusion which would be helpful as the same characteristics would be present in fluid: yellow/clear, high protein, low A:G and low cellularity (monocytes and neutrophils predominate) with no organisms or evidence of neoplasia present.

Because of the costs and time required to do further testing and the likelihood, especially in the effusive form, for the cat to get worse pretty quickly. I’d be starting the conversation about response to treatment.

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