Feline infectious peritonitis (FIP) is a condition caused by feline coronavirus (FCoV). FCoV is a large, enveloped RNA virus, which is very common in cats (around 40% of the domestic cat population are seropositive, increasing to 90% in multi-cat households), and generally causes only mild intestinal signs.
In a small percentage of cats (around 10%), FCoV infection results in FIP. The factors that contribute to the development of FIP are viral, host and environmental. Regarding viral factors, mutations in the Spike protein gene (although these can also be present in non-FIP cats), the tropism of the FCoV for macrophages and its sustained replication in monocytes leading to a disseminated infection, are thought to be correlated with FIP after FCoV infection. The host immune response and the level of stress/overcrowding are also thought to contribute to the development of FIP.
What is the signalment and clinical signs?
FIP is most common in young cats (particularly those under 2 years). There is a small subset that present older than 10 years old. Male cats are over-represented, and a recent history of stress may contribute. Living in a multi-cat household increases the risk of FCoV seropositivity and pedigrees may be predisposed.
There are two main forms of FIP:
- Effusive (or ‘wet’)
- Non-effusive (or ‘dry’)
Wet FIP is present in up to 80% of FIP cases due to a vasculopathy (the effusion can be abdominal, pleural and/or pericardial).
Dry FIP is caused by the presence of granuloma formation. It is typically associated with neurological and/or ocular signs (e.g uveitis). However, there is overlap between the two forms, with cats that present with the non-effusive disease going on to develop effusions, and vice versa. Frequent non-specific clinical signs include lethargy, anorexia, weight loss as well as fluctuating pyrexia and mild jaundice. Lymphadenomegaly can also be present.
How is the diagnosis made?
A presumptive diagnosis is made by a combination of the clinical suspicion based on the signalment and clinical signs, alongside clinical pathology, imaging, and molecular diagnostic tests.
- Clinical pathology: lymphopenia (common), neutrophilia with a left shift, mild to moderate non regenerative anaemia, microcytosis (IMHA is uncommon), hyperglobulinaemia (common), with low-normal albumin and a low albumin: globulin (A: G) ratio < 0.4 (> 0.8 makes FIP unlikely) and hyperbilirubinaemia.
- Acute phase proteins (α-1 acid glycoprotein: AGP): often markedly elevated (>1.5mg/ml).
- Diagnostic imaging: effusion in body cavities, granulomas in organs, large intra-abdominal and mesenteric lymph nodes or thickening of the ileocecocolic region. MRI changes (e.g obstructive hydrocephalus, meningeal enhancement).
- Analysis of effusions: usually clear, viscous/sticky, straw yellow. It is typically pyogranulomatous (macrophages, non-degenerate neutrophils) with low cell counts of < 5 x 109 cells/L and high proteins of > 35 g/L and A:G ratio and AGP like in serum. CSF may show elevated protein concentration and increased cell counts.
- FCoV serology: A positive results indicates the exposure to FCoV. Although FIP cats tend to have higher FCoV antibody titres than non-FIP cats, there is much overlap. Also, around 10% of cats with FIP are seronegative.
- Molecular tests:
- FCoV RNA RT-PCR (not specific for FIP associated FCoV) performed in blood (not helpful due to low sensitivity), CSF, aqueous humour, effusions, or cytology of organs (the latter two are particularly helpful if high viral load and cytological features of FIP e.g pyogranulomatous)
The definitive diagnosis is only confirmed with positive immunostaining for coronavirus antigen in macrophages in formalin-fixed tissue samples. Given that obtaining tissue for histopathology may not be possible in a cat suffering FIP, demonstration of FCoV antigen by immunostaining in effusions, CSF, or cytology of aspirates from abnormal organs, in association with typical cytological features of FIP, is very supportive of a diagnosis.
Given that there are multiple other differential diagnoses for the clinical signs and clinical pathology, it is important to consider other conditions such as immune-mediated conditions (lymphocytic cholangitis), infectious diseases (toxoplasmosis, mycobacterial infection, retroviral infection, septic peritonitis, pyothorax), neoplasia (lymphoma) or others (e.g congestive heart failure or pancreatitis).
Tasker S. Diagnosis of feline infectious peritonitis. ‘Update on evidence supportive available tests’ Journal of Feline Medicine and Surgery (2018) 20, 228–243
Tasker S. and members of the European Advisory Board for Cat Diseases. Feline Infectious Peritonitis Guidelines. 2021 http://www.abcdcatsvets.org/feline-infectious-peritonitis
Kim, Y.; Liu, H.; Galasiti Kankanamalage, A.C.; Weerasekara, S.; Hua, D.H.; Groutas, W.C.; Chang, K.O.; Pedersen, N.C. Reversal of the progression of fatal coronavirus infection in cats by a broad-spectrum coronavirus protease inhibitor. PLoS Pathog. 2016, 12, e1005531.
Pedersen, N.C.; Kim, Y.; Liu, H.; Galasiti Kankanamalage, A.C.; Eckstrand, C.; Groutas, W.C.; Bannasch, M.; Meadows, J.M.; Chang, K.O. Efficacy of a 3C-like protease inhibitor in treating various forms of acquired feline infectious peritonitis. J. Feline Med. Surg. 2018, 20, 378–392.
Murphy, B.G.; Perron, M.; Murakami, E.; Bauer, K.; Park, Y.; Eckstrand, C.; Liepnieks, M.; Pedersen, N.C. The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies. Vet. Microbiol. 2018, 219, 226–233.
Pedersen, N.C.; Perron, M.; Bannasch, M.; Montgomery, E.; Murakami, E.; Liepnieks, M.; Liu, H. Efficacy, and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. J. Feline Med. Surg. 2019, 21, 271–281.
Dickinson PJ, Bannasch M, Thomasy SM, et al. Antiviral treatment using the adenosine nucleoside analogue GS-441524 in cats with clinically diagnosed neurological feline infectious peritonitis. Journal of Veterinary Internal Medicine. 2020, 34, 1587-1593.
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