Genetic characterization of feline calicivirus strains associated with varying disease manifestations during an outbreak season in Missouri (1995–1996)

Molecular epidemiology and strain diversity of circulating feline Calicivirus in Thai cats

Feline calicivirus (FCV) is a significant viral pathogen causing upper respiratory tract and oral diseases in cats. The emergence of the virulent systemic FCV variant (VS-FCV) has raised global concern in the past decade. This study aims to explore the epidemiology, genetic characterization, and diversity of FCV strains circulating among Thai cats. Various sample types, including nasal, oral, and oropharyngeal swabs and fresh tissues, were collected from 184 cats across different regions of Thailand from 2016 to 2021. Using reverse transcription real-time polymerase chain reaction (RT-qPCR), FCV infection was investigated, with additional screening for feline herpesvirus-1 (FHV-1) by qPCR. The detection rates for FCV, FHV-1, and co-infection were 46.7, 65.8, and 31.5%, respectively. Significantly, the odds ratio (OR) revealed a strong association between the detection of a single FCV and the presence of gingivostomatitis lesions (OR: 7.15, 95% CI: 1.89-26.99, p = 0.004). In addition, FCV detection is notably less likely in vaccinated cats (OR: 0.22, 95% CI: 0.07-0.75, p = 0.015). Amino acid sequence analysis based on the VP1 major capsid protein gene of the 14 FCV-Thai (FCV-TH) strains revealed genetic diversity compared to the other 43 global strains (0 to 86.6%). Intriguingly, a vaccine-like FCV variant was detected in one cat. In summary, this study provides insights into the epidemiology and molecular characteristics of FCV diversity within the Thai cat population for the first time. The identification of unique physicochemical characteristics in the capsid hypervariable region of some FCV-TH strains challenges previous hypotheses. Therefore, further exploration of vaccine-like FCV variants is crucial for a comprehensive understanding and to improve viral prevention and control strategies.

Genetic Diversity and Evolution of Viruses Infecting Felis catus: A Global Perspective

Cats harbor many important viral pathogens, and the knowledge of their diversity has been greatly expanded thanks to increasingly popular molecular sequencing techniques. While the diversity is mostly described in numerous regionally defined studies, there lacks a global overview of the diversity for the majority of cat viruses, and therefore our understanding of the evolution and epidemiology of these viruses was generally inadequate. In this study, we analyzed 12,377 genetic sequences from 25 cat virus species and conducted comprehensive phylodynamic analyses. It revealed, for the first time, the global diversity for all cat viruses known to date, taking into account highly virulent strains and vaccine strains. From there, we further characterized and compared the geographic expansion patterns, temporal dynamics and recombination frequencies of these viruses. While respiratory pathogens such as feline calicivirus showed some degree of geographical panmixes, the other viral species are more geographically defined. Furthermore, recombination rates were much higher in feline parvovirus, feline coronavirus, feline calicivirus and feline foamy virus than the other feline virus species. Collectively, our findings deepen the understanding of the evolutionary and epidemiological features of cat viruses, which in turn provide important insight into the prevention and control of cat pathogens.

Molecular Characterization and Phylogenetic Analysis of Feline Calicivirus Isolated in Guangdong Province, China from 2018 to 2022

Feline calicivirus (FCV) is a common feline infectious pathogen that mainly causes upper respiratory tract disease. To investigate the prevalence of FCV in Guangdong Province in China, a total of 152 nasal and throat swabs from cats suspected of FCV infection were collected in veterinary clinics or shelters from 2018 to 2022. The positive detection rate of FCV was 28.9% (44/152) by RT-PCR. In addition, twenty FCV isolates were successfully isolated and purified. Eleven out of twenty isolates were selected for further phylogenetic analyses based on the capsid protein VP1; our results revealed that seven isolates were in genogroup I, and four were in genogroup II. Notably, according to the whole genome phylogenetic tree, FCV-SCAU-11 was in the same branch as Korean isolates, and recombination analysis revealed that the FCV-SCAU-11 isolate showed potential recombinant events between the FCV-SH isolate and FCV-GXNN03-20 isolate. Furthermore, the virus replication kinetics indicated that FCV-SCAU-10, with clinically severe symptoms in patient cats, performed a more efficient replication in vitro. In conclusion, this study revealed the genetic diversity of FCVs in Guangdong Province, providing a reference for novel vaccine candidate strains and the development of effective strategies for preventing FCV infection in cats.

Feline Calicivirus Virulent Systemic Disease: Clinical Epidemiology, Analysis of Viral Isolates and In Vitro Efficacy of Novel Antivirals in Australian Outbreaks

Feline calicivirus (FCV) causes upper respiratory tract disease (URTD) and sporadic outbreaks of virulent systemic disease (FCV-VSD). The basis for the increased pathogenicity of FCV-VSD viruses is incompletely understood, and antivirals for FCV-VSD have yet to be developed. We investigated the clinicoepidemiology and viral features of three FCV-VSD outbreaks in Australia and evaluated the in vitro efficacy of nitazoxanide (NTZ), 2′-C-methylcytidine (2CMC) and NITD-008 against FCV-VSD viruses. Overall mortality among 23 cases of FCV-VSD was 39%. Metagenomic sequencing identified five genetically distinct FCV lineages within the three outbreaks, all seemingly evolving in situ in Australia. Notably, no mutations that clearly distinguished FCV-URTD from FCV-VSD phenotypes were identified. One FCV-URTD strain likely originated from a recombination event. Analysis of seven amino-acid residues from the hypervariable E region of the capsid in the cultured viruses did not support the contention that properties of these residues can reliably differentiate between the two pathotypes. On plaque reduction assays, dose–response inhibition of FCV-VSD was obtained with all antivirals at low micromolar concentrations; NTZ EC₅₀, 0.4–0.6 µM, TI = 21; 2CMC EC₅₀, 2.7–5.3 µM, TI > 18; NITD-008, 0.5 to 0.9 µM, TI > 111. Investigation of these antivirals for the treatment of FCV-VSD is warranted.

Prevalence of feline calicivirus and the distribution of serum neutralizing antibody against isolate strains in cats of Hangzhou, China

Background

Feline calicivirus (FCV) is a common pathogen of felids, and FCV vaccination is regularly practiced. The genetic variability and antigenic diversity of FCV hinder the effective control and prevention of infection by vaccination. Improved knowledge of the epidemiological characteristics of FCV should assist in the development of more effective vaccines.

Objectives

This study aims to determine the prevalence of FCV in a population of cats with FCV-suspected clinical signs in Hangzhou and to demonstrate the antigenic and genetic relationships between vaccine status and representative isolated FCV strains.

Methods

Cats (n = 516) from Hangzhou were investigated between 2018 and 2020. The association between risk factors and FCV infection was assessed. Phylogenetic analyses based on a capsid coding sequence were performed to identify the genetic relationships between strains. In vitro virus neutralization tests were used to assess antibody levels against isolated FCV strains in client-owned cats.

Results

The FCV-positive rate of the examined cats was 43.0%. Risk factors significantly associated with FCV infection were vaccination status and oral symptoms. Phylogenetic analysis revealed a radial phylogeny with no evidence of temporal or countrywide clusters. There was a significant difference in the distribution of serum antibody titers between vaccinated and unvaccinated cats.

Conclusions

This study revealed a high prevalence and genetic diversity of FCV in Hangzhou. The results indicate that the efficacy of FCV vaccination is unsatisfactory. More comprehensive and refined vaccination protocols are an urgent and unmet need.

Use of unbiased metagenomic and transcriptomic analyses to investigate the association between feline calicivirus and feline chronic gingivostomatitis in domestic cats

OBJECTIVE

To identify associations between microbes and host genes in cats with feline chronic gingivostomatitis (FCGS), a debilitating inflammatory oral mucosal disease with no known cause, compared with healthy cats and cats with periodontitis (control cats).

ANIMALS

19 control cats and 23 cats with FCGS.

PROCEDURES

At least 1 caudal oral mucosal swab specimen was obtained from each cat. Each specimen underwent unbiased metatranscriptomic next-generation RNA sequencing (mNGS). Filtered mNGS reads were aligned to all known genetic sequences from all organisms and to the cat transcriptome. The relative abundances of microbial and host gene read alignments were compared between FCGS-affected cats and control cats and between FCGS-affected cats that did and did not clinically respond to primary treatment. Assembled feline calicivirus (FCV) genomes were compared with reverse transcription PCR (RT-PCR) primers commonly used to identify FCV.

RESULTS

The only microbe strongly associated with FCGS was FCV, which was detected in 21 of 23 FCGS-affected cats but no control cats. Problematic base pair mismatches were identified between the assembled FCV genomes and RT-PCR primers. Puma feline foamy virus was detected in 9 of 13 FCGS-affected cats that were refractory to treatment and 5 healthy cats but was not detected in FCGS-affected cats that responded to tooth extractions. The most differentially expressed genes in FCGS-affected cats were those associated with antiviral activity.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that FCGS pathogenesis has a viral component. Many FCV strains may yield false-negative results on RT-PCR-based assays. Coinfection of FCGS-affected cats with FCV and puma feline foamy virus may adversely affect response to treatment.

Multiple Correspondence Analysis on Amino Acid Properties within the Variable Region of the Capsid Protein Shows Differences between Classical and Virulent Systemic Feline Calicivirus Strains

Feline calicivirus (FCV) is a widespread and highly prevalent pathogen of domestic cats, responsible for mild upper respiratory tract disease. Outbreaks of severe virulent systemic disease (VSD) associated with FCV infection have been reported worldwide. VSD FCV strains have a broader tropism and cause a systemic vascular compromise. Despite clear differences in the pathogenesis of VSD and oral respiratory infections, attempts to identify specific molecular markers of VSD strains on the major capsid protein VP1 have failed. Region E of VP1 is responsible for the interaction with the cell receptor Junctional Adhesion Molecule JAM-1 (FeJAM-1) and with VP2 minor capsid protein during the entry of the virus. We carried out an original analysis on the sequences from region E of VSD and classical strains. A Multiple Correspondence Analysis was performed on a Boolean matrix built by coding sequences on the basis of their amino acid properties. For the first time, this approach was able to differentiate VSD and classical FCV. Seven remarkable residue positions were shown to be statistically significant for pathotype differentiation, mainly located in the N-terminal hypervariable part of region E. As structural analysis suggested an interaction of these residues with FeJAM-1 or VP2, post-binding events, and specific conformational changes may explain the difference of pathogenesis between pathotypes.

Temporally separated feline calicivirus isolates do not cluster phylogenetically and are similarly neutralised by high-titre vaccine strain FCV-F9 antisera in vitro

OBJECTIVES

Feline calicivirus (FCV) is a highly variable and globally important feline pathogen for which vaccination has been the mainstay of control. Here, we test whether the continued use of FCV-F9, one of the most frequently used vaccine strains globally, is driving the emergence of vaccine-resistant viruses in the field.

METHODS

This study made use of two representative panels of field isolates previously collected from cats visiting randomly selected veterinary practices across the UK as part of separate cross-sectional studies from 2001 and 2013/2014. Phylogenetic analysis and in vitro virus neutralisation tests were used to compare the genetic and antigenic relationships between these populations and FCV-F9.

RESULTS

Phylogenetic analysis showed a typically radial distribution dominated by 52 distinct strains, with strains from both 2001 and 2013/2014 intermingled. The sequence for FCV-F9 appeared to be integral to this phylogeny and there were no significant differences in the genetic distances within each studied population (intra-population distances), or between them (inter-population distances), or between each population and FCV-F9. A 1 in 8 dilution neutralised 97% and 100% of the 2001 and 2013/14 isolates, respectively, and a 1 in 16 dilution neutralised 87% and 75% of isolates, respectively. There was no significant difference either in variance between the FCV-F9 neutralising titres for the two populations, or in the distribution of neutralisation titres across the two populations.

CONCLUSIONS AND RELEVANCE

Although FCV is a highly variable virus, we found no evidence for a progressive divergence of field virus from vaccine strain FCV-F9, either phylogenetically or antigenically, with FCV-F9 antisera remaining broadly and equally cross-reactive to two geographically representative and temporally separated FCV populations. We suggest this may be because the immunodominant region of the FCV capsid responsible for neutralisation may have structural constraints preventing its longer term progressive antigenic evolution.

Isolation and molecular characterization of a virulent systemic feline calicivirus isolated in China

In this study, a virulent systemic (VS) feline calicivirus (FCV) strain, SH, was isolated from a household cat with severe systemic clinical signs, and its full-length genome was determined and analyzed. Through immunofluorescence assays (IFA) and western blotting assays, we found that FCV SH strain, like other isolates, can stably proliferate in Crandell feline kidney (CRFK) cells. Moreover, the typical morphology of FCV particles, with a diameter of about 35 nm, was observed using electron microscopy. The full-length genome of FCV strain SH was sequenced and determined to be 7704 nucleotides (nt) in length with a 5'-terminal untranslated region (UTR) of 19 nt and a 3'-terminal UTR of 67 nt. Three open reading frames (ORF1, ORF2, and ORF 3) were found within the genome, coding for a polypeptide, a capsid precursor (VP1) and a minor structural protein (VP2), respectively. Amino acid sequence comparison revealed diversity (from 82.2% to 88.5% homology) between the VP1 protein sequences of the SH/14 isolate and those of 33 reference isolates from different regions. Phylogenetic analyses using alignments of VP1 protein sequences showed that the SH/14 isolate shares the highest sequence homology with the reported VS-FCV George strain (88.5%), and is located in the same clade as other reported VS-FCV isolates, indicating that the FCV SH/14 strain is a VS-FCV isolate. However, the SH/14 strain does not belong to the same lineage as most other Chinese FCV isolates, suggesting that, in China, a very large geographical entity, the virulent systemic FCV might has emerged.

Potential Therapeutic Agents for Feline Calicivirus Infection

Feline calicivirus (FCV) is a major cause of upper respiratory tract disease in cats, with widespread distribution in the feline population. Recently, virulent systemic diseases caused by FCV infection has been associated with mortality rates up to 50%. Currently, there are no direct-acting antivirals approved for the treatment of FCV infection. Here, we tested 15 compounds from different antiviral classes against FCV using in vitro protein and cell culture assays. After the expression of FCV protease-polymerase protein, we established two in vitro assays to assess the inhibitory activity of compounds directly against the FCV protease or polymerase. Using this recombinant enzyme, we identified quercetagetin and PPNDS as inhibitors of FCV polymerase activity (IC₅₀ values of 2.8 μM and 2.7 μM, respectively). We also demonstrate the inhibition of FCV protease activity by GC376 (IC₅₀ of 18 µM). Using cell culture assays, PPNDS, quercetagetin and GC376 did not display antivirals effects, however, we identified nitazoxanide and 2′-C-methylcytidine (2CMC) as potent inhibitors of FCV replication, with EC₅₀ values in the low micromolar range (0.6 μM and 2.5 μM, respectively). In conclusion, we established two in vitro assays that will accelerate the research for FCV antivirals and can be used for the high-throughput screening of direct-acting antivirals.

Molecular characterization of feline calicivirus variants from multicat household and public animal shelter in Rio de Janeiro, Brazil

The aim of this study was to perform the molecular characterization of conserved and variable regions of feline calicivirus capsid genome in order to investigate the molecular diversity of variants in Brazilian cat population. Twenty-six conjunctival samples from cats living in five public short-term animal shelters and three multicat life-long households were analyzed. Fifteen cats had conjunctivitis, three had oral ulceration, eight had respiratory signs (cough, sneeze and nasal discharge) and nine were asymptomatic. Feline calicivirus were isolated in CRFK cells and characterized by reverse transcription PCR target to both conserved and variable regions of open reading frame 2. The amplicons obtained were sequenced. A phylogenetic analysis along with most of the prototypes available in GenBank database and an amino acid analysis were performed. Phylogenetic analysis based on both conserved and variable region revealed two clusters with an aLTR value of 1.00 and 0.98 respectively and the variants from this study belong to feline calicivirus genogroup I. No association between geographical distribution and/or clinical signs and clustering in phylogenetic tree was observed. The variants circulating in public short-term animal shelter demonstrated a high variability because of the relatively rapid turnover of carrier cats constantly introduced of multiple viruses into this location over time.

[Isolation and phylogenetic analysis of feline calicivirus in Siberia.]

The results of the study of the distribution of calicivirus infection in a population of domestic cats of different breeds, contained individually or the group method, the virus isolation in the cell culture and a comparative phylogenetic analysis of their nucleotide sequences with published sequences of reference feld and vaccine strains of Feline calicivirus (FCV) from other countries: USA, Germany, Japan, China and Korea are presented. Clinical signs of infection were found in 14.3% of the animals examined. After several passages in the primary kidney cells of the kitten embryo, seven cytopathogenic isolates FCV were isolated: 1 - from a cat with an acute infection, 5 - subclinical infection, 1 - systemic infection. They were adapted to continuous FK-81 cells in which they reached a maximum infectious activity of 10.0 ± 1.15 lg TCD 50 / cm3. Based on the sequence analysis of the open reading frame 2 region of the viral genome Eshli strain showed a close relationship with strain KM016908 from China with the identity of the nucleotide sequences between them of 81.0%. The results of the investigations showed that FCV isolates obtained from animals on the territory of Siberia are genetically different from strains included to imported vaccines used to prevent disease in Russian Federation and also among themselves. This causes a decrease in the effectiveness of preventive measures. In nurseries that do not have contacts and connections between themselves but located in the same geographic region FCV populations may have some genetic differences. A close relationship of some feld isolates with strains from other countries geographically located so far from the Siberian region has been revealed. Studies on the molecular epizootology of caliciviruses are important in the development of test systems and the monitoring of the spread of strains in Russia.

Analysis of the codon usage of the ORF2 gene of feline calicivirus

Feline calicivirus (FCV) is a highly prevalent pathogen of the domestic cat that causes acute infections of the oral and upper respiratory tract. The E region of the ORF2 protein is responsible for the induction of virus-neutralizing antibodies, thus it is important to understand the codon usage of this gene. Here, analysed 90 coding sequences of ORF2 and show that it undergoes a low codon usage bias. In addition, although mutational bias is one of the factors shaping the codon usage bias of this gene, natural selection plays a more significant role. Our results reveal part of the mechanisms driving FCV evolution, which will lay foundation for the further research of FCV.

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The codon usage of the ORF2 gene of FCV was first analysed in this study.

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The ORF2 gene of FCV shows a low codon usage bias.

Isolation and phylogenetic analysis of three feline calicivirus strains from domestic cats in Jilin Province, China

Feline calicivirus (FCV) is a highly prevalent pathogen that can cause infectious felid upper respiratory tract disease. The majority of complete genome sequences of FCV strains reported to date are from the USA. In this study, three FCV strains, CH-JL1, CH-JL2 and CH-JL3, were isolated from domestic cats in Jilin Province, China. Sequence analysis revealed that except for strains HRB-SS, WZ-1, XH, 12Q087-1 and 12Q087-5, the 3' untranslated regions (UTRs) of CH-JL2 and CH-JL3 are more than 20 nucleotides longer than those of all other reference isolates. The complete sequences of the three CH-JLs were compared with other reference strains, with nucleotide sequence identity values in the range of 76.2%-82.2%, 76.8%-96.4 and 76.8%-96.4%. Phylogenetic analysis showed that CH-JL1 forms a branch with FB-NJ-13, GD, 12Q087-1 and 12Q087-5. CH-JL2 was found to be most closely related to CH-JL3, forming another branch together with the other isolates. CH-JL1 shares a long nucleotide span with CH-JL2 and CH-JL3. It can be inferred that many FCV strains are co-circulating in Jilin Province. The availability of complete genome sequences will serve as a reference for future epidemiological studies of FCV.

Genetic diversity and phenotypic associations of feline caliciviruses from cats in Switzerland

Feline calicivirus (FCV) is a common viral pathogen in domestic cats worldwide. The variable regions of the capsid (VP1) gene of FCV have one of the highest recorded rates of molecular evolution. Understanding the genetic diversity and phylogeny of FCV is a prerequisite to exploring the epidemiology and pathogenesis of this virus and to the development of efficacious vaccine strategies. In this study, we undertook a nationwide molecular characterization of FCV using for the first time nearly complete capsid (VP1) gene sequences. Sequences from 66 FCV samples were used to investigate the correlation between viral phylogeny and several traits, including geographic origin, signalment, husbandry, FCV vaccination and co-infections. Codon-based nucleotide alignment showed that individual nucleotides and their corresponding amino acid sites were either invariant or highly variable. Using a threshold of 20 % genetic distance in variable region E, FCV samples were grouped into 52 strains, 10 of which comprised two to three samples. Significant associations between FCV phylogeny and host characteristics were found, specifically the pedigree status of the cats, and two well-supported lineages were identified in which the current FCV strain definition was confounded. No correlation between viral genetic distances and geographic distances was evident. The greater resolution of the FCV phylogeny in this study compared to previous studies can be attributed to our use of more conserved regions of the capsid (VP1) gene; nonetheless, our results were still hampered by sequence saturation. The study highlights the need for whole-genome sequences for FCV phylogeny studies.

Complete Genome Sequence of Feline Calicivirus Strain GX01-2013 Isolated from Household Cats in Guangxi, Southern China

Here, we report the complete genome of a feline calicivirus (FCV) originating from household cats in Guangxi, southern China, in September 2013. To understand its genetic characteristics, we isolated FCV strain GX01-2013 from MDCK cells and determined its complete genome sequence.

In vitro antiviral effect of germacrone on feline calicivirus

Feline calicivirus (FCV) often causes respiratory tract and oral disease in cats and is a highly contagious virus. Widespread vaccination does not prevent the spread of FCV. Furthermore, the low fidelity of the RNA-dependent RNA polymerase of FCV leads to the emergence of new variants, some of which show increased virulence. Currently, few effective anti-FCV drugs are available. Here, we found that germacrone, one of the main constituents of volatile oil from rhizoma curcuma, was able to effectively reduce the growth of FCV strain F9 in vitro. This compound exhibited a strong anti-FCV effect mainly in the early phase of the viral life cycle. The antiviral effect depended on the concentration of the drug. In addition, germacrone treatment had a significant inhibitory effect against two other reference strains, 2280 and Bolin, and resulted in a significant reduction in the replication of strains WZ-1 and HRB-SS, which were recently isolated in China. This is the first report of antiviral effects of germacrone against a calicivirus, and extensive in vivo research is needed to evaluate this drug as an antiviral therapeutic agent for FCV.

European molecular epidemiology and strain diversity of feline calicivirus

Feline calicivirus (FCV) causes a variable syndrome of upper respiratory tract disease, mouth ulcers and lameness. A convenience-based prospective sample of oropharyngeal swabs (n=426) was obtained from five countries (France, Germany, Greece, Portugal and the UK). The prevalence of FCV by virus isolation was 22.2 per cent. Multivariable analysis found that animals presenting with lymphoplasmacytic gingivitis stomatitis complex were more likely to test positive for FCV infection. Furthermore, vaccinated cats up to 48 months of age were significantly less likely to be infected with FCV than unvaccinated animals of similar ages. Phylogenetic analysis based on consensus sequences for the immunodominant region of the capsid gene from 72 FCV isolates identified 46 strains. Thirteen of the 14 strains with more than one sequence were restricted to individual regions or sites in individual countries; the exception was a strain present in two sites close to each other in France. Four strains were present in more than one household. Five colonies, four of which were rescue shelters, had multiple strains within them. Polymerase sequence suggested possible rare recombination events. These locally, nationally and internationally diverse FCV populations maintain a continuous challenge to the control of FCV infection and disease.

Assessment of the IFN-β response to four feline caliciviruses: Infection in CRFK cells

Feline calicivirus (FCV) is a highly contagious pathogen with a widespread distribution. Although the cat genome has been sequenced, little is known about innate immunity in cats, which limits the understanding of FCV pathogenesis. To investigate the IFN-β response during FCV infection in CRFK cells, we first cloned and identified the feline IFN-β promoter sequence and the positive regulatory domain (PRD) motifs, which shared a high similarity with human and porcine IFN-β promoters. Next, we found that infections with FCV strains F9, Bolin and HRB-SS at the 100 or 1000 TCID50 doses could not activate the IFN-β promoter at 12 and 24h post-infection. Only strain 2280 infection at a 1000 TCID50 dose could induce the IFN-β promoter mainly through IRF3 and partially through NF-κB, at 24h post-infection. However, the IFN response occurred much later and was smaller in magnitude compared with that following Sendai virus (SeV) infection. Further, we found that induction of the IFN-β promoter by FCV 2280 infection depended on dsRNA and not on viral proteins. Finally, we examined whether the IFN-β response had an antiviral effect against FCV replication. The over-expression of IFN-β before exposure to the virus reduced viral yields by a range of 2.2-3.2 log10TCID50, but its over-expression at 12h post-infection did not inhibit FCV replication. Our results indicate that some FCV strains cannot induce IFN-β expression in vitro, which may be a potential factor for FCV survival in cats. Whether this is important in evading the host interferon response in vivo must be investigated.