• This article is an extract from “FIP – causes, signs, diagnosis, treatment and prevention update”, published in Vet Times 53.27.

 

Feline coronaviruses (FCoV) include feline enteric coronavirus (FECV), which has affinity for enterocytes; and FIP virus (FIPV), which replicates in monocytes and macrophages (Pedersen, 2009).

This change in cell tropism, where FECV mutates to become FIPV and moves from replicating in enterocytes to monocytes, is known as the internal mutation theory (Pedersen, 2009).

FECV infection is usually subclinical, but can result in mild gastrointestinal signs. In contrast, FIPV causes systemic infection commonly referred to as FIP.

Feline coronavirus infection is very common in cats, with up to 50% of cats in single‑cat households and up to 90% of cats in multi‑cat households being seropositive for FCoV antibodies (Pedersen, 1976; Addie et al, 1995).

Although the majority of cats infected with FCoV remain healthy, up to 12% of cats may develop FIP (Addie et al, 1995).

FIP usually affects young cats – most cases are younger than a year of age, typically from shelters or breeding catteries, with certain breeds being more predisposed; however, breed predispositions may vary geographically (Pedersen, 2009).

It is easy to underestimate how common FIP is, and it varies around the world. Therefore, it is worth thinking about how many cats a year in the UK may be developing this devastating disease.

The prevalence of cats being highly suspected of having FIP varies in the general cat population from 0.02% to 1.42% (Addie and Jarrett, 1992; Riemer et al, 2016), increasing to 5% to 10% in catteries, particularly pedigree breeding households (Pedersen, 1976; Addie and Jarrett, 1992).

The UK is home to between 9 million and 11 million pet cats, of which approximately 12% are pedigree (PDSA, 2020), although this increased to 26% by 2022 (Cats Protection, 2022). Approximately a third of these are likely to be young cats, of which about 10% may develop FIP (Addie et al, 1995).

This suggests up to 30,000 cats develop FIP in the UK each year.

We owe it to those cats to know how best to treat them.

Until recently, FIP was a fatal disease, with median survival time of eight days with effusive FIP (Fischer et al, 2011). This changed in 2019, when Niels Pedersen and his team studied the effect of nucleoside analogues in the treatment of FIP. They showed marked improvement and prolonged survival times (Pedersen et al, 2019; Dickinson et al, 2020; see the section about how to treat cases of FIP).

Clinical features of FIP

The effusive (wet) form of FIP is generally believed to develop in cats with poor cell‑mediated immune responses, while the non‑effusive (dry) form develops in cats with partial cell-mediated immunity.

The effusive form of FIP is an immune complex vasculitis characterised by leakage of protein-rich fluid into the pleural space, the peritoneal cavity, the pericardial space, and the subcapsular space of the kidneys.

Vascular permeability factors – including vascular endothelial growth factor produced by infected monocytes and macrophages – may increase the vascular permeability in cats with effusive FIP (Takano et al, 2011).

In the non-effusive form, pyogranulomatous or granulomatous lesions develop in multiple tissues – particularly the eyes, brain, kidneys, omentum, spleen and liver.

Some affected cats have characteristics of both forms of FIP, with non-effusive forms often becoming terminally effusive. With treatment this is something we hope to see a lot less frequently now.

The main differential diagnoses for cats with FIP can be seen in Table 1.

Table 1. Potential differential diagnoses for FIP
Disease	Comparison and diagnostics to differentiate
Pancreatitis	Often cats involved in fights. Abdominal and even pleural effusions have high total proteins. With bacterial infection, cell counts are high, with degenerative neutrophils and intracellular bacteria. Culture can reveal bacterial infection. Abdominal ultrasound and/or measurement of feline pancreatic lipase immunoreactivity can be helpful.
Septic abdomen	Usually very sick, may present in shock. Effusions have high cell counts, with degenerative neutrophils and intracellular bacteria (although these may be few). Culture can reveal bacterial infection.
Pyothorax	Anorexia, vomiting and weight loss is common.
Lymphocytic cholangitis	Usually more chronic. Liver enzyme activities (such as alkaline phosphatase and alanine aminotransferase) are increased; fine-needle aspiration of bile or liver, and/or liver biopsy is diagnostic.
Toxoplasmosis	Can involve multiple organs. Diagnosis by high IgM antibodies or rising IgG titres.
Mycobacterial infections	Fever, lymphadenopathy, respiratory signs, abdominal masses and uveitis are common. Severe hyperglobulinaemia is less common. Cytology may show acid-fast bacteria in tissue samples. PCR and/or culture can be useful, but diagnosis can be challenging.
Congestive heart failure	Heart murmur and/or gallop may be present. Mostly modified transudate. Echocardiography is useful to make diagnosis.
Lymphoma	Multiple organ involvement. Cytology can reveal neoplastic cells.
Carcinomatosis	Can present as single or bicavitary effusion. Cytology can reveal neoplastic cells, but often challenging diagnosis antemortem.

Anorexia, weight loss, lethargy and pyrexia are common presenting complaints. Owners may notice icterus, ocular inflammation, abdominal distention, dyspnoea, or CNS abnormalities such as ataxia or seizures.

In cats with effusive disease, abdominal distention is common, and pleural effusion can result in dyspnoea and a restrictive breathing pattern, as well as muffled heart and lung sounds. Male cats sometimes have scrotal swelling.

Masses (pyogranulomas or lymphadenopathy) can occasionally be palpated in the abdomen.

Diagnosis

Enteric coronavirus

No assay is currently available that can determine whether enteric coronaviruses are associated with diarrhoea in cats. Cats with coronavirus RNA in faeces should be evaluated for other causes of diarrhoea.

The only purpose of measuring coronavirus in the faeces, via PCR, is to screen for cats that are chronically shedding FCoV. This is, however, an expensive process as repeat monthly samples are needed over a longer time period (ideally a minimum of nine months).

FIP

The diagnosis of FIP (see flow diagram) can be challenging as routine laboratory testing gives no specific findings. Table 2 summarises available laboratory tests.

Table 2. Useful diagnostics for FIP
Test	Description
Haematology	Changes common, but not specific. Lymphopenia, anaemia with or without neutrophilia.
Serum biochemistry	Dependent on organ(s) affected. Increase in bilirubin and globulin levels. Albumin:globulin (A:G) ratio often lower than 0.4 (not FIP if higher than 0.8). Increase in α1-acid glycoprotein (AGP; higher than 1,500ug/ml, often higher than 3,000ug/ml).
Fluid analysis – biochemistry and cytology	Can be performed on abdominal or pleural fluid (also pericardial if accessible), abdominal lavage. In cats with FIP, the fluid might be yellow, viscous and clots. Protein higher than 35g/L; A:G ratio lower than 0.8, often lower than 0.4 (not FIP if higher than 0.8). Rivalta test: To 7ml to 8ml distilled water in a 10ml tube, add one drop (20ul to 30ul) of 98 per cent to 100 per cent acetic acid (or household vinegar), mix well, then place one drop of body fluid on the top of this solution. If it keeps its shape, this detects increased protein with inflammation. Relatively low cell counts (often fewer than 5 × 109/L) – non-toxic neutrophils and macrophages = non-septic exudate. Culture and sensitivity (should be negative unless repeated tapping for fluid previously). Increase in AGP (higher than 1,500ug/mL).
Quantitative reverse transcriptase PCR (qRT-PCR) – detects feline coronavirus (FCoV) RNA	Performed at Axiom/Finn, Veterinary Pathology Group/Synlab, Glasgow and Langford. Detection of high levels of FCoV RNA in samples correspond with increasing likelihood of FIP, but negative results do not rule out a diagnosis of FIP. Should be interpreted with rest of clinical picture, including signalment. Can be performed on fluid samples (pleural, abdominal, pericardial or retroperitoneal effusions, CSF, or aqueous humour): minimum 100μl blood and serum samples generally not recommended Fine-needle aspirates (FNAs) can be submitted: Mix with a small amount of saline, ethylenediaminetetraacetic acid (EDTA)‑saline or, ideally, RNAlater if available in sample tube. Add several FNA aspirates to sample tube until the solution to submit is cloudy, indicating cell presence. If the sample is already on a slide, rub it off with a cotton swab, place the swab in saline, shake, spin down, remove supernatant, and re-suspend in saline to approximately 100μl. Tissue samples require extra processing: Ideally, fresh-frozen samples. Fixed tissue blocks can also be used (but formalin cross-linkage and wax inhibitors can reduce PCR sensitivity); 5μm sections.	Idexx FIP Virus RealPCR test When high levels of FCoV present, S mutation analysis is possible to determine if certain mutations are present in the S gene. Performed on same samples as qRT-PCR. Positive S mutation testing corresponds to a likelihood of FIP similar to the finding of high levels of FCoV RNA on qRT-PCR
Immunocytochemistry – detects FCoV antigen on cytology samples	Contact lab beforehand to discuss cellular harvest, preparation, costs and postage requirements to ensure successful sampling and submission. Performed at universities of Edinburgh and Liverpool, plus Glasgow. Detects and visualises FCoV antigen within macrophages. Can be performed on effusions, CSF or aqueous humour, and sometimes FNAs (lab dependent). Smears prepared on glass slides are not good as cells fall off; it is better to prepare two cytospins on to charged glass slides if possible. If neither cytospins nor preparation of cell pellets is possible (see below for method), submit fluid samples for qRT-PCR directly (for FNAs, place several FNAs into small volume of sterile saline or into EDTA – ensure the fluid is cloudy) and submit ASAP, although yield may be poor. Most labs prefer fluid submitted as a cell pellet (see below for method). Preparing a cell pellet – use body fluid (for example, ascitic, pleural, CSF) or tissue FNAs: Prepare 1ml or 2 ml of saline, or autologous serum in plain tube. Aspirate from the tissue/organ and place the cellular content from the needle directly into the saline/serum tube (as if rinsing the needle). Repeat until becomes turbid. Make sure saline/serum solution becomes turbid; the more turbid the solution is, the more cellular the pellet will be. Centrifuge at 5,000rpm for five minutes. Verify presence of a visible cell button at the bottom of the tube. Carefully remove the supernatant with pipette, leaving cell button (pellet/sediment) intact at bottom of the tube. Add 2ml of buffered formalin, mix thoroughly using vortex or re-suspend using a pipette. Send ASAP to lab – next‑day delivery.
Immunohistochemistry – detects FCoV antigen on histology samples	Contact lab beforehand to discuss sample preparation and costs. Performed at universities of Edinburgh and Liverpool, plus Glasgow. Detects and visualises FCoV antigens within cells of a tissue sample. FCoV in macrophages within a pyogranulomatous lesion is consistent with FIP. Recommended tissues to sample depend on pathology present, but include liver, kidney, spleen and mesenteric lymph nodes. Can also be done on cell pellets prepared from fluid or FNA samples (see above).

Complete blood cell count (CBC) usually shows mild, non‑regenerative anaemia, microcytosis, neutrophilia (with or without toxic changes), lymphopenia and thrombocytopenia or thrombocytosis.

Serum biochemical findings often include hyperproteinaemia, hyperglobulinaemia, hypoalbuminaemia, increased liver enzyme activities, hyperbilirubinaemia and electrolyte imbalances.

Low albumin:globulin ratio (A:G) can aid in diagnosis of FIP, with A:G 0.4 or lower making FIP likely. In one study, an A:G of 0.5 had a positive predictive value of 89%, while an A:G of 1.0 had a negative predictive value of 91% (Hartmann et al, 2003).

When hyperglobulinaemia is detected, protein electrophoresis usually shows polyclonal gammopathy with greatest increases in the gamma‑globulin fractions (Taylor et al, 2010).

Other diagnostics can be variably helpful in making a diagnosis of FIP. Acute phase proteins – particularly α1-acid glycoprotein (AGP) – is often increased (1,500μg/ml and higher), but it can also be increased with other inflammatory and neoplastic diseases.

Measuring FCoV antibodies in the blood – or effusive fluid of cats with FIP – is not helpful as most cats with FCoV infection will be seropositive regardless of whether they go on to develop FIP, and about 10% of cats with FIP are seronegative.

Ultrasound examination often confirms pleural or peritoneal effusion in case of effusive FIP (Figure 1), and abdominal lymphadenopathy and renal medullary rim sign can be seen in both effusive and non-effusive FIP.

Fluid analysis can be helpful in cases of effusive FIP. The effusions are mostly consistent with modified transudate or exudate, being high in protein (3.5g/dL and higher) and having low cellularity (nucleated cells fewer than 5,000cells/μL). Mixed inflammatory cell populations occur most commonly; non‑toxic neutrophils predominate in most cases, while in some cats, macrophages are the primary cell type.

Measuring protein and AGP concentrations in effusions – and calculation of the A:G ratio – can be very helpful in making the diagnosis of effusive FIP.

The gold standard for diagnosing FIP is immunohistochemistry (IHC), which detects FCoV antigen in macrophages by immunofluorescence or immunoperoxidase methods. Where effusions are present, they can be spun down and the cell pellet processed as for IHC (see Table 2 for lab test availability).

FCoV RNA can also be detected in fluid, tissue aspirates or biopsies by reverse transcriptase PCR (RT-PCR). The specificity and sensitivity of RT-PCR varies based on the test and sample used for diagnostics, with highest specificity being on pleural and peritoneal effusion, as FCoV is unlikely to be in effusions from other causes.

Quantitative RT-PCR (qRT‑PCR) is very useful, as finding a large amount of FCoV in an effusion or cell pellet would not be expected in non‑FIP cases. The IDEXX FIP Virus RealPCR Test detects certain spike mutations, which may be present in approximately 70% of cases, and offers another possibility.

Diagnosis of non-effusive FIP can be even more challenging due to the absence of effusion. Fine-needle aspirates of enlarged kidneys or abdominal lymph nodes can be used to generate a cell suspension that can be used for qRT-PCR, or for IHC after making a cell pellet.

Abdominal lavage with sterile saline may also be useful at generating cell suspension to process to IHC.

Surgical biopsies of lymph nodes and/or other organs can be taken for histopathology and IHC to confirm FIP; however, this is not always possible in an already very sick kitten.

In cases of neurological FIP, CSF can be taken for analysis and qRT-PCR, and specific MRI changes are almost pathognomonic for FIP.

Cats with ocular involvement can undergo aqueocentesis and qRT-PCR performed on aqueous humour.

A summary of useful diagnostics for FIP can be seen in Table 2. The European Advisory Board on Cat Diseases also has guidelines for diagnosis.

In most cases of the non‑effusive (and sometimes the effusive) form, it is possible to get to a presumptive diagnosis of FIP. It is usually based on the combination of clinical and clinicopathological findings, and the exclusion of other causes for the presenting clinical signs.

It is important to consider the high costs of treatment, so it is best not to use all available funds trying to completely confirm an FIP diagnosis. Take the aspirates and submit them, including sending effusions for culture, then start treating pending getting the results.

• Some drugs mentioned in this article series are not licensed for veterinary use.

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Also read:

• FIP: treatment with antivirals
• FIP: treatment with financial limitations, and black market antivirals warning
• FIP: monitoring, drug combinations, and treatment of cats with faecal shedding