Discrepancies between feline coronavirus antibody and nucleic acid detection in effusions of cats with suspected feline infectious peritonitis

Measurement of Feline Alpha-1 Acid Glycoprotein in Serum and Effusion Using an ELISA Method: Analytical Validation and Diagnostic Role for Feline Infectious Peritonitis

BACKGROUND

Alpha-1 acid glycoprotein (AGP) may support a clinical diagnosis of feline infectious peritonitis (FIP). In this study, we assessed the analytical and diagnostic performances of a novel ELISA method to measure feline AGP.

METHODS

AGP was measured in sera and effusions from cats with FIP (n = 20) or with other diseases (n = 15). Precision was calculated based on the coefficient of variation (CV) of repeated testing, and accuracy was calculated by linearity under dilution (LUD).

RESULTS

The test is precise (intra-assay CVs: <6.0% in individual samples, <15.0% in pooled samples; inter-assay CVs <11.0% and <15.0%) and accurate (serum LUD r2: 0.995; effusion LUD r2: 0.950) in serum and in effusions. AGP is higher in cats with FIP than in other cats in both serum (median: 1968, I-III interquartile range: 1216-3371 μg/mL and 296, 246-1963 μg/mL; p = 0.009) and effusion (1717, 1011-2379 μg/mL and 233, 165-566 μg/mL; p < 0.001). AGP discriminates FIP from other diseases (area under the receiver operating characteristic curve: serum, 0.760; effusion, 0.877), and its likelihood ratio is high (serum: 8.50 if AGP > 1590 μg/mL; effusion: 3.75 if AGP > 3780 μg/mL).

CONCLUSION

This ELISA method is precise and accurate. AGP in serum and in effusions is a useful diagnostic marker for FIP.

Feline Infectious Peritonitis: European Advisory Board on Cat Diseases Guidelines

Feline coronavirus (FCoV) is a ubiquitous RNA virus of cats, which is transmitted faeco-orally. In these guidelines, the European Advisory Board on Cat Diseases (ABCD) presents a comprehensive review of feline infectious peritonitis (FIP). FCoV is primarily an enteric virus and most infections do not cause clinical signs, or result in only enteritis, but a small proportion of FCoV-infected cats develop FIP. The pathology in FIP comprises a perivascular phlebitis that can affect any organ. Cats under two years old are most frequently affected by FIP. Most cats present with fever, anorexia, and weight loss; many have effusions, and some have ocular and/or neurological signs. Making a diagnosis is complex and ABCD FIP Diagnostic Approach Tools are available to aid veterinarians. Sampling an effusion, when present, for cytology, biochemistry, and FCoV RNA or FCoV antigen detection is very useful diagnostically. In the absence of an effusion, fine-needle aspirates from affected organs for cytology and FCoV RNA or FCoV antigen detection are helpful. Definitive diagnosis usually requires histopathology with FCoV antigen detection. Antiviral treatments now enable recovery in many cases from this previously fatal disease; nucleoside analogues (e.g., oral GS-441524) are very effective, although they are not available in all countries.

A Combined Method Based on the FIPV N Monoclonal Antibody Immunofluorescence Assay and RT-nPCR Method for the Rapid Diagnosis of FIP-Suspected Ascites

Feline infectious peritonitis (FIP), which is caused by feline infectious peritonitis virus (FIPV), is a fatal and immunologically mediated infectious disease among cats. At present, due to the atypical clinical symptoms and clinicopathological changes, the clinical diagnosis of FIP is still difficult. The gold standard method for the differential diagnosis of FIP is immunohistochemistry (IHC) which is time-consuming and requires specialized personnel and equipment. Therefore, a rapid and accurate clinical diagnostic method for FIPV infection is still urgently needed. In this study, based on the etiological investigation of FIPV in parts of southern China, we attempted to explore a new rapid and highly sensitive method for clinical diagnosis. The results of the etiological investigation showed that the N gene of the FIPV BS8 strain had the highest homology with other strains. Based on this, a specific FIPV BS8 N protein monoclonal antibody was successfully prepared by expression of the recombinant proteins, immunization of mice, fusion and selection of hybridoma cell lines, and screening and purification of monoclonal antibodies. Furthermore, we carried out a time-saving combination method including indirect immunofluorescence assay (IFA) and nested reverse transcription polymerase chain reaction (RT-nPCR) to examine FIP-suspected clinical samples. These results were 100% consistent with IHC. The results revealed that the combined method could be a rapid and accurate application in the diagnosis of suspected FIPV infection within 24 hours. In conclusion, the combination of IFA and RT-nPCR was shown to be a fast and reliable method for clinical FIPV diagnosis. This study will provide insight into the exploitation of FIPV N antibodies for the clinical diagnosis of FIP-suspected ascites samples.

Serological evidence of exposure of Bornean wild carnivores to feline-related viruses at the domestic animal-wildlife interface

We conducted an exploratory serological survey to evaluate the exposure of Bornean wild carnivores to several viruses common to domestic felids, at interface areas between protected forest and industrial agriculture in the Kinabatangan floodplain (Sabah, Malaysia). Blood samples, collected from wild carnivores (n = 21) and domestic cats (n = 27), were tested for antibodies against feline coronavirus (FCoV), feline panleukopenia virus (FPLV), feline herpesvirus (FHV) and feline calicivirus (FCV), using commercial enzyme-linked immunosorbent assay (ELISA) test kits. Anti-FCoV antibodies were detected in most species, including one flat-headed cat (Prionailurus planiceps, [1/2]), leopard cats (Prionailurus bengalensis, [2/5]), Malay civets (Viverra tangalunga, [2/11]) and domestic cats (Felis catus, [2/27]). Anti-FCV antibodies were present in all domestic cats and one flat-headed cat, while anti-FPLV antibodies were identified in Sunda clouded leopards (Neofelis diardi, [2/2]), domestic cats [12/27] and Malay civets [2/11]. Anti-FHV antibodies were only detected in domestic cats [2/27]. Our findings indicate pathogen transmission risk between domestic and wild carnivore populations at the domestic animal-wildlife interface, emphasizing the concern for wildlife conservation for several endangered wild carnivores living in the area. Special consideration should be given to species that benefit from their association with humans and have the potential to carry pathogens between forest and plantations (e.g., Malay civets and leopard cats). Risk reduction strategies should be incorporated and supported as part of conservation actions in human-dominated landscapes.

ERDRP-0519 inhibits feline coronavirus in vitro

Background

Coronaviruses (CoVs) are major human and animal pathogens and antiviral drugs are pursued as a complementary strategy, chiefly if vaccines are not available. Feline infectious peritonitis (FIP) is a fatal systemic disease of felids caused by FIP virus (FIPV), a virulent pathotype of feline enteric coronavirus (FeCoV). Some antiviral drugs active on FIPV have been identified, but they are not available in veterinary medicine. ERDRP-0519 (ERDRP) is a non-nucleoside inhibitor, targeting viral RNA polymerase, effective against morbilliviruses in vitro and in vivo.

Results

The antiviral efficacy of ERDRP against a type II FIPV was evaluated in vitro in Crandell Reese Feline Kidney (CRFK) cells. ERDRP significantly inhibited replication of FIPV in a dose-dependent manner. Viral infectivity was decreased by up to 3.00 logarithms in cell cultures whilst viral load, estimated by quantification of nucleic acids, was reduced by nearly 3.11 logaritms.

Conclusions

These findings confirm that ERDRP is highly effective against a CoV. Experiments will be necessary to assess whether ERDRP is suitable for treatment of FIPV in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03153-3.

A retrospective study of clinical and laboratory features and treatment on cats highly suspected of feline infectious peritonitis in Wuhan, China

Feline infectious peritonitis (FIP) is a systemic, potentially fatal viral disease. The objectives of this study were to review clinical and laboratory features and treatment of cats highly suspected of FIP in Wuhan, China. The clinical records of 127 cats highly suspected of FIP were reviewed for history, clinical signs, physical findings, and diagnostic test results. Sex, neutering status, breed, age, and month of onset of disease were compared with the characteristics of the clinic population. Age and neutering status were significantly correlated with FIP-suspicion. Sex, breed and onset month were not associated with FIP. There were many more FIP-suspected cases in cats in young cats or male intact cats. Effusion was observed in 85.8% of the FIP-suspected cats. Increased serum amyloid A (SAA) and lymphopenia were common laboratory abnormalities in the FIP cases. Furthermore, 91.7% of the cats highly suspected of FIP had an albumin/globulin (A/G) ratio < 0.6, while 85.3% had an A/G ratio < 0.5. The mortality rate for FIP-suspected cats was 67%, and six submitted cases were confirmed by FIP-specific immunohistochemistry. Of the 30 cats treated with GS-441524 and/or GC376, 29 were clinically cured. The study highlights the diverse range of clinical manifestations by clinicians in diagnosing this potentially fatal disease. A/G ratio and SAA were of higher diagnostic value. GS-441524 and GC376 were efficient for the treatment of FIP-suspected cats.

Diagnostic Value of Detecting Feline Coronavirus RNA and Spike Gene Mutations in Cerebrospinal Fluid to Confirm Feline Infectious Peritonitis

BACKGROUND

Cats with neurologic feline infectious peritonitis (FIP) are difficult to diagnose. Aim of this study was to evaluate the diagnostic value of detecting feline coronavirus (FCoV) RNA and spike (S) gene mutations in cerebrospinal fluid (CSF).

METHODS

The study included 30 cats with confirmed FIP (six with neurological signs) and 29 control cats (eleven with neurological signs) with other diseases resulting in similar clinical signs. CSF was tested for FCoV RNA by 7b-RT-qPCR in all cats. In RT-qPCR-positive cases, S-RT-qPCR was additionally performed to identify spike gene mutations.

RESULTS

Nine cats with FIP (9/30, 30%), but none of the control cats were positive for FCoV RNA in CSF. Sensitivity of 7b-RT-qPCR in CSF was higher for cats with neurological FIP (83.3%; 95% confidence interval (95% CI) 41.8-98.9) than for cats with non-neurological FIP (16.7%; 95% CI 6.1-36.5). Spike gene mutations were rarely detected.

CONCLUSIONS

FCoV RNA was frequently present in CSF of cats with neurological FIP, but only rarely in cats with non-neurological FIP. Screening for spike gene mutations did not enhance specificity in this patient group. Larger populations of cats with neurological FIP should be explored in future studies.

Mutation analysis of the spike protein in Italian feline infectious peritonitis virus and feline enteric coronavirus sequences

Feline coronavirus (FCoV) exists as two different genotypes, FCoV type I and II, each including two biotypes, feline enteric coronavirus (FECV) and feline infectious peritonitis virus (FIPV), the latter being a virulent variant originating from the former virus. Recently, two amino acid substitutions, M1058L and S1060A, within the spike protein have been associated to the FECV/FIPV virulence change. In this study, we have analysed the frequency of detection of such mutations in FIPV and FECV strains circulating in Italian cats and obtained information about their evolutionary relationships with reference isolates. A total of 40 FCoV strains, including 19 strains from effusions or tissue samples of FIP cats and 21 strains from faecal samples of non-FIP cats, were analysed. Mutation M1058L was detected in 16/18 FCoV-I and 1/1 FCoV-II strains associated with FIP, while change S1060A was presented by two FIPV strains. By phylogenetic analysis, FCoV sequences clustered according to the genotype but not according to the biotype, with FECV/FIPV strains recovered from the same animal being closely related. Further studies are needed to better define the genetic signatures associated with the FECV/FIPV virulence shift.

Evidence of exposure to SARS-CoV-2 in cats and dogs from households in Italy

SARS-CoV-2 emerged from animals and is now easily transmitted between people. Sporadic detection of natural cases in animals alongside successful experimental infections of pets, such as cats, ferrets and dogs, raises questions about the susceptibility of animals under natural conditions of pet ownership. Here, we report a large-scale study to assess SARS-CoV-2 infection in 919 companion animals living in northern Italy, sampled at a time of frequent human infection. No animals tested PCR positive. However, 3.3% of dogs and 5.8% of cats had measurable SARS-CoV-2 neutralizing antibody titers, with dogs from COVID-19 positive households being significantly more likely to test positive than those from COVID-19 negative households. Understanding risk factors associated with this and their potential to infect other species requires urgent investigation.

Development of a recombinase polymerase amplification fluorescence assay to detect feline coronavirus

Feline coronavirus (FCoV) is classified into two pathotypes: the avirulent feline enteric coronavirus (FECV), and the virulent feline infectious peritonitis virus (FIPV). Rapid pathogen detection, which is efficient and convenient, is the best approach for early confirmatory diagnosis. In this study, we first developed and evaluated a rapid recombinase polymerase amplification (RPA) detection method for FCoV that can detect FCoV within 15 min at 39 °C. The detection limit of that assay was 233 copies/μL DNA molecules per reaction. The specificity was high: it did not cross-react with canine distemper virus (CDV), canine coronavirus (CCoV), canine adenovirus (CAV), feline calicivirus (FCV), feline herpesvirus (FHV), or feline parvovirus (FPV). This assay was evaluated using 42 clinical samples (30 diarrhea samples and 12 ascites samples). The coincidence rate between FCoV-RPA and RT-qPCR for detection in clinical samples was 95.2%. In summary, FCoV-RPA analysis provides an efficient, rapid, and sensitive detection method for FCoV.

•

FCoV RPA has the shortest reaction time within 15 min among all the PCR-based methods.

•

FCoV RPA detection method has no cross-reactivity with other pathogens.

•

The sensitivity of FCoV RPA was consistent with real-time PCR, as low as 204 copies/μL of DNA molecules per reaction.

Getting Insights from a Large Corpus of Scientific Papers on Specialisted Comprehensive Topics - the Case of COVID-19

COVID-19 is one of the most important topics these days, specifically on search engines and news. While fake news is easily shared, scientific papers are reliable sources where information can be extracted. With about 24,000 scientific publications on COVID-19 and related research on PubMed, automatic computer-assisted analysis is required. In this paper, we develop two methodologies to get insights on specific sub-topics of interest and latest research sub-topics. These rely on natural language processing and graph-based visualizations. We run these methodologies on two cases: the virus origin and the uses of existing drugs.

Evidence of exposure to SARS-CoV-2 in cats and dogs from households in Italy

SARS-CoV-2 originated in animals and is now easily transmitted between people. Sporadic detection of natural cases in animals alongside successful experimental infections of pets, such as cats, ferrets and dogs, raises questions about the susceptibility of animals under natural conditions of pet ownership. Here we report a large-scale study to assess SARS-CoV-2 infection in 817 companion animals living in northern Italy, sampled at a time of frequent human infection. No animals tested PCR positive. However, 3.4% of dogs and 3.9% of cats had measurable SARS-CoV-2 neutralizing antibody titers, with dogs from COVID-19 positive households being significantly more likely to test positive than those from COVID-19 negative households. Understanding risk factors associated with this and their potential to infect other species requires urgent investigation.

One Sentence Summary

SARS-CoV-2 antibodies in pets from Italy.

Origin and transmission of Feline coronavirus type I in domestic cats from Northern Italy: a phylogeographic approach

•

FCoV type I only was observed in cats from Northern Italy.

•

A high genetic variability of the Italian FCoV circulating strains was observed.

•

Different strains of FCoV cocirculate in Northern Italy.

•

FCoV likely originated in USA in the 1950s-1970s and thereafter spread to Europe.

•

In the last decade FCoV segregates mainly on the basis of geographic origin.

Feline coronavirus (FCoV) is responsible, along with an inadequate immune response of the host, for Feline infectious peritonitis (FIP), one of the most frequent and deadly infectious feline disease worldwide. This study analyzed the genetic characteristics of the spike (S) gene of 33 FCoVs circulating in Northern Italy between 2011 and 2015 in cats with or without FIP. In order to reconstruct the most probable places of origin and dispersion of FCoV among Italian cats, a phylogeographic approach was performed based on 106 FCoV S gene partial sequences from cats, including the 33 novel Italian sequences and 73 retrieved from public databases. Only FCoV type I was found in the Italian cats. The estimated mean evolutionary rate of FCoV was 2.4 × 10^-2 subs/site/year (95% HPD: 1.3-3.7 × 10^-2), confirming the high genetic variability in the circulating strains. All the isolates clustered in a unique highly significant clade that likely originated from USA between the 1950s and the 1970s, confirming the first descriptions of the disease in American cats. Our results suggest that from USA the virus likely entered Germany and thereafter spread to other European countries. Phylogeography showed that sequences segregated mainly by geographical origin. In the 2010s Italian sequences clustered in different subclades, confirming that different strains cocirculate in Italy. Further studies on archival samples and other genetic regions of FCoV are suggested in order to confirm the present results and to reconstruct a more in-depth detailed virus dispersion pattern for the definition of possible control measures.

Effect of cat litters on feline coronavirus infection of cell culture and cats

OBJECTIVES

Feline infectious peritonitis (FIP) is caused by infection with feline coronavirus (FCoV). FCoV is incredibly contagious and transmission is via the faecal-oral route. FCoV infection, and therefore FIP, is most common in breeder and rescue catteries, where many cats are kept indoors, using litter trays. Whether it is possible to break the cycle of FCoV infection and reinfection using cat litters has never been investigated. The aim of the study was to examine the effect of cat litters on FCoV infectivity and virus load in multi-cat households, and transmission frequency.

METHODS

Fifteen cat litters were mixed and incubated with FCoV, centrifuged and the supernatants tested in vitro for the ability to prevent virus infection of cell culture. To test applicability of in vitro results to real life, virus load was measured in two households in a double crossover study of four Fuller's earth-based cat litters by testing rectal swabs using FCoV reverse transcriptase quantitative PCR.

RESULTS

Four litters abrogated FCoV infection of cell culture, nine reduced it to a greater or lesser extent and two had no effect. One brand had different virus inhibitory properties depending on where it was manufactured. Fuller's earth-based litters performed best, presumably by adsorbing virus. In the field study, there appeared to be less virus shedding on one Fuller's earth-based cat litter.

CONCLUSIONS AND RELEVANCE

The in vitro study successfully identified cat litters that inactivate FCoV; such litters exist so do not need to be developed. Fuller's earth-based litters best prevented infection of cell culture, but did not completely abrogate FCoV transmission in two multi-cat households. A dust-free clumping Fuller's earth litter appeared to fare best, but virus shedding also varied on the control litters, complicating interpretation. Sawdust-based cat litters are not useful in FCoV-endemic households because they track badly and have a poor effect on virus infection.

Oral Mutian®X stopped faecal feline coronavirus shedding by naturally infected cats

Feline coronavirus (FCoV) is common among cats living indoors in groups. In about 10% of infected cats, a potentially lethal disease, feline infectious peritonitis (FIP) occurs. Virus transmission is faecal-oral. Mutian® Xraphconn (Mutian X) is a product marketed to treat cats with FIP but is also being used to stop virus shedding, although no clear guidelines exist for its use for this purpose. The aim of this study was to establish the minimum dose and treatment duration required to ensure viral clearance from the faeces of asymptomatic virus-shedding cats. In five multicat households, 29 cats naturally infected with FCoV and actively shedding virus in the faeces were given Mutian X pills. Virus shedding was monitored using reverse-transcription quantitative polymerase chain reaction (RT-qPCR) controlled for faecal inhibitors to ensure sensitivity. Mutian X given orally cleared the virus in 29 cats; although four cats required a repeated course to finally stop virus shedding. A dose of 4 mg/kg q24 h for four days was found to be the optimal treatment protocol: 2 mg/kg cleared only 80% of cats. Post-treatment using a sensitive RT-qPCR test was essential to ensure that virus clearance had been achieved, since failure to clear even one cat can result in re-infection of the others. Records of virus shedding by cats before treatment provided a retrospective control: significantly more cats stopped shedding virus after Mutian X than recovered from infection during the control period (p < .00001). This is the first report of the successful elimination of faecal FCoV shedding in chronically infected cats.

•

Feline infectious peritonitis (FIP) is caused by feline coronavirus infection

•

The source of feline coronavirus (FCoV) infection is faeces of virus shedding cats

•

Treatment of 29 cats for 4 days with Mutian X pills stopped FCoV shedding

•

Treated cats tested for up to 157 days following treatment remained virus-free

Diagnosis of Feline Infectious Peritonitis: A Review of the Current Literature

Feline infectious peritonitis (FIP) is a fatal disease that poses several challenges for veterinarians: clinical signs and laboratory changes are non-specific, and there are two pathotypes of the etiologic agent feline coronavirus (FCoV), sometimes referred to as feline enteric coronavirus (FECV) and feline infectious peritonitis virus (FIPV) that vary fundamentally in their virulence, but are indistinguishable by a number of diagnostic methods. This review focuses on all important steps every veterinary practitioner has to deal with and new diagnostic tests that can be considered when encountering a cat with suspected FIP with the aim to establish a definitive diagnosis. It gives an overview on all available direct and indirect diagnostic tests and their sensitivity and specificity reported in the literature in different sample material. By providing summarized data for sensitivity and specificity of each diagnostic test and each sample material, which can easily be accessed in tables, this review can help to facilitate the interpretation of different diagnostic tests and raise awareness of their advantages and limitations. Additionally, diagnostic trees depict recommended diagnostic steps that should be performed in cats suspected of having FIP based on their clinical signs or clinicopathologic abnormalities. These steps can easily be followed in clinical practice.

Differential susceptibility of macrophages to serotype II feline coronaviruses correlates with differences in the viral spike protein

•

Differences in the S protein modulate serotype II FCoV infection of macrophages.

•

Critical residues in the spike S2 domain of type II FCoV affecting cell tropism.

•

Cooperativity at 5 positions in the S protein modulates FCoV macrophage entry.

The ability to infect and replicate in monocytes/macrophages is a critically distinguishing feature between the two feline coronavirus (FCoV) pathotypes: feline enteric coronavirus (FECV; low-virulent) and feline infectious peritonitis virus (FIPV; lethal). Previously, by comparing serotype II strains FIPV 79-1146 and FECV 79-1683 and recombinant chimeric forms thereof in cultured feline bone marrow macrophages, we mapped this difference to the C-terminal part of the viral spike (S) protein (S2). In view of the later identified diagnostic difference in this very part of the S protein of serotype I FCoV pathotypes, the present study aimed to further define the contribution of the earlier observed ten amino acids difference to the serotype II virus phenotype in macrophages. Using targeted RNA recombination as a reverse genetics system we introduced the mutations singly and in combinations into the S gene and evaluated their effects on the infection characteristics of the mutant viruses in macrophages. While some of the single mutations had a significant effect, none of them fully reverted the infection phenotype. Only by combining five specific mutations the infections mediated by the FIPV and FECV spike proteins could be fully blocked or potentiated, respectively. Hence, the differential macrophage infection phenotype is caused by the cooperative effect of five mutations, which occur in five functionally different domains of the spike fusion subunit S2. The significance of these observations will be discussed, taking into account also some questions related to the identity of the virus strains used.

Detection of feline Coronavirus in effusions of cats with and without feline infectious peritonitis using loop-mediated isothermal amplification

•

Diagnosis of feline infectious peritonitis is critical in terms of time.

•

Rapid detection methods for coronavirus are needed in cats.

•

Isothermal amplification assays are suitable as point-of-care tests.

•

The sensitivity of isothermal amplification assays for coronavirus detection is low.

Feline infectious peritonitis (FIP) is a fatal disease in cats worldwide. The aim of this study was to test two commercially available reaction mixtures in a reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay to detect feline Coronavirus (FCoV) in body cavity effusions of cats with and without FIP, in order to minimize the time from sampling to obtaining results.

RNA was extracted from body cavity effusion samples of 71 cats, including 34 samples from cats with a definitive diagnosis of FIP, and 37 samples of control cats with similar clinical signs but other confirmed diseases. Two reaction mixtures (Isothermal Mastermix, OptiGene Ltd.and PCRun™ Molecular Detection Mix, Biogal) were tested using the same primers, which were designed to bind to a conserved region of the FCoV membrane protein gene. Both assays were conducted under isothermal conditions (61 °C–62 °C). Using the Isothermal Mastermix of OptiGene Ltd., amplification times ranged from 4 and 39 min with a sensitivity of 35.3% and a specificity of 94.6% for the reported sample group. Using the PCRun™ Molecular Detection Mix of Biogal, amplification times ranged from 18 to 77 min with a sensitivity of 58.8% and a specificity of 97.3%.

Although the RT-LAMP assay is less sensitive than real time reverse transcription PCR (RT-PCR), it can be performed without the need of expensive equipment and with less hands-on time. Further modifications of primers might lead to a suitable in-house test and accelerate the diagnosis of FIP.

Diagnosis of feline infectious peritonitis: Update on evidence supporting available tests

Practical relevance: Feline coronavirus (FCoV) infection is very common in cats, usually causing only mild intestinal signs such as diarrhoea. Up to 10% of FCoV infections, however, result in the fatal disease feline infectious peritonitis (FIP). Clinical challenges: Obtaining a definitive diagnosis of FIP based on non-invasive approaches is difficult. Confirmation of the disease relies on finding appropriate cytological or histopathological changes in association with positive immunostaining for FCoV antigen. In FIP cases with effusions, cytology and immunostaining on effusion samples can be relatively easy to perform; otherwise obtaining diagnostic samples is more challenging and collection of biopsies from tissues with gross lesions is necessary. In the absence of a definitive diagnosis, a high index of suspicion of FIP may be obtained from the cat's signalment and history, combined with findings on clinical examination and laboratory test results. If largely consistent with FIP, these can be used as a basis for discussion with the owner about whether additional, more invasive, diagnostic tests are warranted. In some cases it may be that euthanasia is discussed as an alternative to pursuing a definitive diagnosis ante-mortem, especially if financial limitations exist or where there are concerns over a cat's ability to tolerate invasive diagnostic procedures. Ideally, the diagnosis should be confirmed in such patients from samples taken at post-mortem examination. Global importance: FIP occurs wherever FCoV infection is present in cats, which equates to most parts of the world. Evidence base: This review provides a comprehensive overview of how to approach the diagnosis of FIP, focusing on the tests available to the veterinary practitioner and recently published evidence supporting their use.