Molecular virology of feline calicivirus

VP2 mediates the release of the feline calicivirus RNA genome by puncturing the endosome membrane of infected cells

Feline calicivirus (FCV) is one of the few members of the Caliciviridae family that grows well in cell lines and, therefore, serves as a surrogate to study the biology of other viruses in the family. Conley et al. (14) demonstrated that upon the receptor engagement to the capsid, FCV VP2 forms a portal-like assembly, which might provide a channel for RNA release. However, the process of calicivirus RNA release is not yet fully understood. Our findings suggest that the separation of the FCV capsid from its genome RNA (gRNA) occurs rapidly in the early endosomes of infected cells. Using a liposome model decorated with the FCV cell receptor fJAM-A, we demonstrate that FCV releases its gRNA into the liposomes by penetrating membranes under low pH conditions. Furthermore, we found that VP2, which is rich in hydrophobic residues at its N-terminus, functions as the pore-forming protein. When we substituted the VP2 N-terminal hydrophobic residues, the gRNA release efficacy of the FCV mutants decreased. In conclusion, our results suggest that in the acidic environment of early endosomes, FCV VP2 functions as the pore-forming protein to mediate gRNA release into the cytoplasm of infected cells. This provides insight into the mechanism of calicivirus genome release.IMPORTANCEResearch on the biology and pathogenicity of certain caliciviruses, such as Norovirus and Sapovirus, is hindered by the lack of easy-to-use cell culture system. Feline calicivirus (FCV), which grows effectively in cell lines, is used as a substitute. At present, there is limited understanding of the genome release mechanism in caliciviruses. Our findings suggest that FCV uses VP2 to pierce the endosome membrane for genome release and provide new insights into the calicivirus gRNA release mechanism.

Virulent systemic feline calicivirus infection: a case report and first description in Ireland

BACKGROUND

Virulent systemic feline calicivirus (VS-FCV) infection is an emerging disease. It is distinct from classic oronasal calicivirus infection as it manifests with unique systemic signs including severe cutaneous ulcerations, limb oedema, and high mortality, even in adequately vaccinated cats. Devastating epizootic outbreaks with hospital-acquired infections have been described in the United States, the United Kingdom, continental Europe and Australia with up to 54 cats affected in one outbreak and a mortality rate of up to 86%. This highly contagious and potentially fatal disease has not yet been reported in Ireland.

CASE PRESENTATION

An 11-month-old male neutered vaccinated domestic shorthair cat was presented with a 10-day history of lethargy, decreased appetite and progressively worsening pitting oedema in all four limbs. The signs were first noted after another kitten from a high-density cat shelter was introduced in to the household. Additional physical examination findings included marked pyrexia, and lingual and cutaneous ulcers. Virulent systemic feline calicivirus was diagnosed based on compatible history and clinical signs, exclusion of other causes, and calicivirus isolation by RT-PCR both in blood and oropharyngeal samples. Negative calicivirus RT-PCR in blood following resolution of the clinical signs further supported the diagnosis.

CONCLUSION

This case represents the first known case of VS-FCV infection in Ireland. Given the severity of the clinical signs, and the high risk for epizootic outbreaks, Irish veterinarians should be aware of the disease to ensure prompt diagnosis and implementation of adequate preventive measures, in order to limit the threat that this disease represents for the wider cat population and particularly given the risk of hospital-acquired VS-FCV infection. Virulent systemic calicivirus should be suspected in cats with pyrexia of unknown origin, oedema or ulceration affecting the limbs or the face, and exposure to rescue cats from high-density households.

Biological Characteristics of Feline Calicivirus Epidemic Strains in China and Screening of Broad-Spectrum Protective Vaccine Strains

Feline calicivirus (FCV) is one of the most important pathogens causing upper respiratory tract diseases in cats, posing a serious health threat to these animals. At present, FCV is mainly prevented through vaccination, but the protective efficacy of vaccines in China is limited. In this study, based on the differences in capsid proteins of isolates from different regions in China, as reported in our previous studies, seven representative FCV epidemic strains were selected and tested for their viral titers, virulence, immunogenicity, and extensive cross-protection. Subsequently, vaccine strains were selected to prepare inactivated vaccines. The whole-genome sequencing and analysis results showed that these seven representative FCV strains and 144 reference strains fell into five groups (A, B, C, D, and E). The strains isolated in China mainly fall into groups C and D, exhibiting regional characteristics. These Chinese isolates had a distant evolutionary relationship and low homology with the current FCV-255 vaccine strain. The screened FCV-HB7 and FCV-HB10 strains displayed desirable in vitro culture characteristics, with the highest virus proliferation titers (109.5 TCID50/mL) at 36 h post inoculation at a dose of 0.01 MOI. All five cats infected intranasally with FCV-HB7 or FCV-HB10 strains showed obvious clinical symptoms of FCV. The symptoms of cats infected with the FCV-HB7 strain were more severe than those infected with the FCV-HB10 strain. Both the single-strain inactivated immunization and combined bivalent inactivated vaccine immunization of FCV-HB7 and FCV-HB10 induced high neutralizing antibody titers in five cats immunized. Moreover, bivalent inactivated vaccine immunization protected cats from FCV-HB7 and FCV-HB10 strains. The cross-neutralizing antibody titer against seven representative FCV epidemic strains achieved by combined bivalent inactivated vaccine immunization was higher than that achieved by single-strain immunization, which was much higher than that achieved by commercial vaccine FCV-255 strain immunization. The above results suggest that the FCV-HB7 and FCV-HB10 strains screened in this study have great potential to become vaccine strains with broad-spectrum protective efficacy. However, their immune protective efficacy needs to be further verified by multiple methods before clinical application.

Transcriptomic signatures of feline chronic gingivostomatitis are influenced by upregulated IL6

Feline chronic gingivostomatitis (FCGS) is a relatively common and debilitating disease characterized by bilateral inflammation and ulceration of the caudal oral mucosa, alveolar and buccal mucosa, and varying degrees of periodontal disease. The etiopathogenesis of FCGS remains unresolved. In this study, we performed bulk RNA-seq molecular profiling of affected tissues derived from a cohort of client-owned cats with FCGS compared to tissues from unaffected animals, to identify candidate genes and pathways that can help guide future exploration of novel clinical solutions. We complemented transcriptomic findings with immunohistochemistry and in situ hybridization assays to better understand the biological significance of the results and performed RNA-seq validation of biologically relevant differentially expressed genes using qPCR assays to demonstrate technical reproducibility. Transcriptomic profiles of oral mucosal tissues in cats with FCGS are enriched with immune- and inflammation-related genes and pathways that appear to be largely influenced by IL6, and include NFKB, JAK/STAT, IL-17 and IFN type I and II signaling, offering new opportunities to develop novel clinical applications based on a more rational understanding of the disease.

Classification of genotypes based on the VP1 gene of feline calicivirus and study of cross-protection between different genotypes

Feline calicivirus (FCV) causes upper respiratory tract diseases and even death in cats, thereby acting as a great threat to feline animals. Currently, FCV prevention is mainly achieved through vaccination, but the effectiveness of vaccination is limited. In this study, 105 FCV strain VP1 sequences with clear backgrounds were downloaded from the NCBI and subjected to a maximum likelihood method for systematic evolutionary analysis. Based on the genetic analysis results, FCV-positive sera were prepared using SPF mice and Chinese field cats as target animals, followed by a cross-neutralization assay conducted on the different genotype strains and in vivo challenge tests were carried out to further verify with the strain with best cross-protection effect. The results revealed that FCV was mainly divided into two genotypes: GI and GII. The GI genotype strains are prevalent worldwide, but all GII genotype strains were isolated from Asia, indicating a clear geographical feature. This may form resistance to FCV prevention in Asia. The in vitro neutralization assay conducted using murine serum demonstrated that the cross-protection effect varied among strains. A strain with broad-spectrum neutralization properties, DL39, was screened. This strain could produce neutralizing titers (10 × 23.08-10 × 20.25) against all strains used in this study. The antibody titers against the GI strains were 10 × 23.08-10 × 20.5 and those against the GII strains were 10 × 20.75-10 × 20.25. Preliminary evidence suggested that the antibody titer of the DL39 strain against GI was higher than that against GII. Subsequent cross-neutralization assays with cat serum prepared with the DL39 strain and each strain simultaneously yielded results similar to those described above. In vivo challenge tests revealed that the DL39 strain-immunized cats outperformed the positive controls in all measures. The results of several trials demonstrated that strain DL39 can potentially be used as a vaccine strain. The study attempted to combine the genetic diversity and phylogenetic analysis of FCV with the discovery of potential vaccines, which is crucial for developing highly effective FCV vaccines.

Transcriptomic signatures of feline chronic gingivostomatitis are influenced by upregulated IL6

Feline chronic gingivostomatitis (FCGS) is a relatively common and debilitating disease characterized by bilateral inflammation and ulceration of the caudal oral mucosa, alveolar and buccal mucosa, and varying degrees of periodontal disease. The etiopathogenesis of FCGS remains unresolved. In this study, we performed bulk RNA-seq molecular profiling of affected tissues derived from a cohort of client-owned cats with FCGS compared to tissues from unaffected animals, to identify candidate genes and pathways that can help guide future exploration of novel clinical solutions. We complemented transcriptomic findings with immunohistochemistry and in situ hybridization assays to better understand the biological significance of the results and performed RNA-seq validation of selected differentially expressed genes using qPCR assays to demonstrate technical reproducibility. Transcriptomic profiles of oral mucosal tissues in cats with FCGS are enriched with immune- and inflammation-related genes and pathways that appear to be largely influenced by IL6 , and include NFKB, JAK/STAT, IL-17 and IFN type I and II signaling, offering new opportunities to develop novel clinical applications based on a more rational understanding of the disease.

Isolation and phylogenetic analysis of feline calicivirus strains from various region of China

Feline calicivirus (FCV) is an important feline pathogen mainly causing upper respiratory tract disease, conjunctivitis, and stomatitis, and it is classified into genotype I and genotype II. To investigate the prevalence and molecular characteristics of FCV, this study collected 337 cat swab samples from animal hospitals in different regions of China from 2019 to 2021. The positive detection rate of FCV was 29.9% (101/337) by RT-PCR. Statistical analysis showed that FCV prevalence was significantly associated with living environment (p = 0.0004), age (p = 0.031) and clinical symptoms (p = 0.00), but not with sex (p = 0.092) and breed (p = 0.171). The 26 strains of FCV were isolated using F81 cells. Phylogenetic analysis showed that 10 isolates belonged to genotype I, and 16 isolates belonged to genotype II. These 26 isolates were highly genetically diverse, of which HB7 isolate had three same virulence-related amino acid loci with VSD strains. Potential loci distinguishing different genotypes were identified from 26 isolates, suggesting the genetic relationship between different genotypes. In addition, selection pressure analysis based on capsid protein of 26 isolates revealed that the protein is under diversifying selection. This study reveals the genetic diversity of FCV and provides a reference for the screening of vaccine candidate strains and the development of vaccines with better cross-protection effects.

Vector-borne and other pathogens of potential relevance disseminated by relocated cats

Large populations of unowned cats constitute an animal welfare, ecological, societal and public health issue worldwide. Their relocation and homing are currently carried out in many parts of the world with the intention of relieving suffering and social problems, while contributing to ethical and humane population control in these cat populations. An understanding of an individual cat’s lifestyle and disease status by veterinary team professionals and those working with cat charities can help to prevent severe cat stress and the spread of feline pathogens, especially vector-borne pathogens, which can be overlooked in cats. In this article, we discuss the issue of relocation and homing of unowned cats from a global perspective. We also review zoonotic and non-zoonotic infectious agents of cats and give a list of practical recommendations for veterinary team professionals dealing with homing cats. Finally, we present a consensus statement consolidated at the 15th Symposium of the Companion Vector-Borne Diseases (CVBD) World Forum in 2020, ultimately to help veterinary team professionals understand the problem and the role they have in helping to prevent and manage vector-borne and other pathogens in relocated cats.

Graphical Abstract

Genetic and phylogenetic analysis of capsid gene of feline calicivirus in Nanjing, China

Feline calicivirus (FCV) is a common and important pathogen in cats, typically resulting in upper respiratory tract disease or ulcerative oral lesions. Although there are large number of researches on FCV and vaccines against FCV have been widely used for years, the explanation for vaccination failure and further studies on the prevalence of FCV are still necessary in China. In this study, 86 nasopharyngeal swabs from pet cats with upper respiratory symptoms from several Nanjing animal hospitals were collected in 2020. Among them, 36 (41.86%) were positive for FCV. In addition, 13 FCV capsid genes were sequenced. The comparative analysis of linear B-cell epitopes of VP1 gene indicated that there were many amino acid variations existed among FCV vaccine strains and these strains currently circulating in Nanjing, which may relate to the failure of vaccination and maybe aid for future vaccine design. Besides, phylogenetic analysis of capsid gene revealed two genotypes. Except for the F86 strain, most of the strains were clustered with FCV I genotype, which indicated that FCV I genotype was the most prevalent genotype currently circulating in Nanjing. In conclusion, this study provided useful information as to the evolution and genetic variants of FCV in Nanjing, which is urgent for the future instructions of effective disease prevention and control strategies.

Extracellular Vesicles (EVs) Are Copurified with Feline Calicivirus, yet EV-Enriched Fractions Remain Infectious

Feline calicivirus (FCV) is a major cause of upper respiratory disease in cats and is often used as a model for human norovirus, making it of great veterinary and human medical importance. However, questions remain regarding the route of entry of FCV in vivo. Increasing work has shown that extracellular vesicles (EVs) can be active in viral infectivity, yet there is no work examining the role of EVs in FCV infection. Here, we begin to address this knowledge gap by characterizing EVs produced by a feline mammary epithelial cell line (FMEC). We have confirmed that EVs are produced by infected and mock-infected FMECs and that both virions and EVs are coisolated with standard methods of virus purification. We also show that they can be enriched differentially by continuous iodixanol density gradient. EVs were enriched at a density of 1.10 g/mL confirmed by tetraspanin expression, size profile, and transmission electron microscopy (TEM). Maximum enrichment of FCV at a density of 1.18 g/mL was confirmed by titration, quantitative reverse transcriptase PCR (q-RT PCR), and TEM. However, infectious virus was recovered from nearly all samples. When used to infect in vitro epithelium, both EV-rich and virus-rich fractions had the same levels of infectiousness as determined by percentage of wells infected or titer achieved postinfection. These findings highlight the importance of coisolates during viral purification, showing that EVs may represent a parallel route of entry that has previously been overlooked. Additional experiments are necessary to explore the role of EVs in FCV infection. IMPORTANCE Feline calicivirus (FCV) is a common cause of upper respiratory infection in cats. Both healthy and infected cells produce small particles called extracellular vesicles (EVs), which are nanoparticles that act as messengers between cells and can be hijacked during viral infection. Historically, the role of EVs in viral infection has been overlooked, and subsequently no group has studied the role of EVs in FCV infection. We hypothesized that EVs may play a role in FCV infection. Here, we show that EVs are copurified with FCV when collecting virus. To study their individual effects, we successfully enrich for viral particles and EVs separately by taking advantage of their different densities. Our initial studies show that EV-enriched versus virus-enriched fractions are equally able to infect cells in culture. These findings highlight the need to both consider the purity of virus after purification and to further study EVs' role in natural FCV infection.

Natural cases of polyarthritis associated with feline calicivirus infection in cats

The limping syndrome is occasionally reported during acute feline calicivirus (FCV) infections or as consequence of vaccination. In this retrospective study, three clinical cases of lameness in household cats naturally infected by FCV were described and phylogeny of the virus were investigated by analysing the hypervariable E region of the ORF2 viral gene. Cats were diagnosed with polyarthritis and FCV RNA or antigens were detected in symptomatic joints. One cat, euthanized for ethical reasons, underwent a complete post-mortem examination and was subjected to histopathological and immunohistochemical investigations. No phylogenetic subgrouping were evident for the sequenced FCV. Histopathology of the euthanized cat revealed diffuse fibrinous synovitis and osteoarthritis eight months after the onset of lameness and the first detection of FCV RNA, supporting the hypothesis of a persistent infection. FCV was demonstrated by immunohistochemistry in synoviocytes and fibroblasts of the synovial membranes. This study provides new data on the occurrence of polyarthritis in FCV-infected cats, demonstrates by immunohistochemistry the presence of FCV in the synovial membranes of a cat with persistent polyarthritis and supports the absence of correlation between limping syndrome and phylogenetic subgrouping of viruses.

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.

Molecular Characterization and Cross-Reactivity of Feline Calicivirus Circulating in Southwestern China

Feline calicivirus (FCV) is an important pathogen of cats that has two genogroups (GI and GII). To investigate the prevalence and molecular characteristics of FCVs in southwestern China, 162 nasal swab samples were collected from cats in animal shelters and pet hospitals. In total, 38 of the clinical samples (23.46%) were identified as FCV positive using nested RT-PCR. Phylogenetic analyses using 10 capsid protein VP1 sequences revealed that 8 GI and 2 GII strains formed two independent clusters. Additionally, three separated FCVs that were not clustered phylogenetically (two GI and one GII strains) were successfully isolated from clinical samples and their full-length genomes were obtained. Phylogenetic and recombinant analyses of a GI FCV revealed genomic breakpoints in ORF1 and ORF2 regions with evidence for recombinant events between GI sub-genogroups, which is reported in China for the first time. Furthermore, sera obtained from mice immunized independently with the three FCV isolates and a commercial vaccine were used to evaluate the cross-reactivity of neutralizing antibodies. The three separate FCVs were neutralized by each other at a 1:19 to 1:775 titer range, whereas the triple-inactivated vaccine was at a titer of 1:16, which suggested that different genogroup/sub-genogroup FCV strains exhibit significantly different titers of neutralizing antibodies, including the commercial FCV vaccine. Thus, our study revealed the genetic diversity and complex cross-reactivity levels of FCVs in southwestern China, which provides new insights for application in vaccination strategies.

Modified-Live Feline Calicivirus Vaccination Reduces Viral RNA Loads, Duration of RNAemia, and the Severity of Clinical Signs after Heterologous Feline Calicivirus Challenge

Feline calicivirus (FCV) is a common cat virus causing clinical signs such as oral ulcerations, fever, reduced general condition, pneumonia, limping and occasionally virulent-systemic disease. Efficacious FCV vaccines protect against severe disease but not against infection. FCV is a highly mutagenic RNA virus whose high genetic diversity poses a challenge in vaccine design. The use of only one modified-live FCV strain over several decades might have driven the viral evolution towards more vaccine-resistant variants. The present study investigated the clinical signs, duration, and amount of FCV shedding, RNAemia, haematological changes and acute phase protein reaction in SPF cats after subcutaneous modified-live single strain FCV vaccination or placebo injection and two subsequent oronasal heterologous FCV challenge infections with two different field strains. Neither clinical signs nor FCV shedding from the oropharynx and FCV RNAemia were detected after vaccination. After the first experimental infection, vaccinated cats had significantly lower clinical scores, less increased body temperature and lower acute phase protein levels than control cats. The viral RNA loads from the oropharynx and duration and amount of RNAemia were significantly lower in the vaccinated animals. No clinical signs were observed in any of the cats after the second experimental infection. In conclusion, FCV vaccination was beneficial for protecting cats from severe clinical signs, reducing viral loads and inflammation after FCV challenge.

Reverse Genetics System for Rabbit vesivirus

Most caliciviruses are refractory to replication in cell culture and only a few members of the family propagate in vitro. Rabbit vesivirus (RaV) is unique due to its ability to grow to high titers in several animal and human cell lines. This outstanding feature makes RaV an ideal candidate for reverse genetics studies, an invaluable tool to understand the molecular basis of virus replication, the biological functions of viral genes and their roles in pathogenesis. The recovery of viruses from a cDNA clone is a prerequisite for reverse genetics studies. In this work, we constructed a RaV infectious cDNA clone using a plasmid expression vector, under the control of bacteriophage T7 RNA-polymerase promoter. The transfection of permissive cells with this plasmid DNA in the presence of T7 RNA-polymerase, provided in trans by a helper recombinant poxvirus, led to de novo synthesis of RNA transcripts that emulated the viral genome. The RaV progeny virus produced the typical virus-induced cytopathic effect after several passages of cell culture supernatants. Similarly, infectious RaV was recovered when the transcription step was performed in vitro, prior to transfection, provided that a 5′-cap structure was added to the 5′ end of synthetic genome-length RNAs. In this work, we report an efficient and consistent RaV rescue system based on a cDNA transcription vector, as a tool to investigate calicivirus biology through reverse genetics.

Precise location of linear epitopes on the capsid surface of feline calicivirus recognized by neutralizing and non-neutralizing monoclonal antibodies

We report the generation, characterization and epitope mapping of a panel of 26 monoclonal antibodies (MAbs) against the VP1 capsid protein of feline calicivirus (FCV). Two close but distinct linear epitopes were identified at the capsid outermost surface (P2 subdomain) of VP1, within the E5′HVR antigenic hypervariable region: one spanning amino acids 431-435 (PAGDY), highly conserved and recognized by non-neutralizing MAbs; and a second epitope spanning amino acids 445-451 (ITTANQY), highly variable and recognized by neutralizing MAbs. These antibodies might be valuable for diagnostic applications, as well as for further research in different aspects of the biology of FCV.

Calicivirus VP2 forms a portal-like assembly following receptor engagement

To initiate infection, many viruses enter their host cells by triggering endocytosis following receptor engagement. However, the mechanisms by which non-enveloped viruses escape the endosome are poorly understood. Here we present near-atomic-resolution cryo-electron microscopy structures for feline calicivirus both undecorated and labelled with a soluble fragment of its cellular receptor, feline junctional adhesion molecule A. We show that VP2, a minor capsid protein encoded by all caliciviruses^1,2, forms a large portal-like assembly at a unique three-fold axis of symmetry, following receptor engagement. This assembly-which was not detected in undecorated virions-is formed of twelve copies of VP2, arranged with their hydrophobic N termini pointing away from the virion surface. Local rearrangement at the portal site leads to the opening of a pore in the capsid shell. We hypothesize that the portal-like assembly functions as a channel for the delivery of the calicivirus genome, through the endosomal membrane, into the cytoplasm of a host cell, thereby initiating infection. VP2 was previously known to be critical for the production of infectious virus³; our findings provide insights into its structure and function that advance our understanding of the Caliciviridae.

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.

Feline calicivirus- and murine norovirus-induced COX-2/PGE2 signaling pathway has proviral effects

Cyclooxygenases (COXs)/prostaglandin E₂ (PGE₂) signaling pathways are known to modulate a variety of homeostatic processes and are involved in various pathophysiological conditions. COXs/PGE₂ signaling pathways have also been demonstrated to have proviral or antiviral effects, which appeared different even in the same virus family. A porcine sapovirus Cowden strain, a member of genus Sapovirus within the Caliciviridae family, induces strong COX-2/PGE₂ but transient COX-1/PGE₂ signaling to enhance virus replication. However, whether infections of other viruses in the different genera activate COXs/PGE₂ signaling, and thus affect the replication of viruses, remains unknown. In the present study, infections of cells with the feline calicivirus (FCV) F9 strain in the genus Vesivirus and murine norovirus (MNV) CW-1 strain in the genus Norovirus only activated the COX-2/PGE₂ signaling in a time-dependent manner. Treatment with pharmacological inhibitors or transfection of small interfering RNAs (siRNAs) against COX-2 enzyme significantly reduced the production of PGE₂ as well as FCV and MNV replications. The inhibitory effects of these pharmacological inhibitors against COX-2 enzyme on the replication of both viruses were restored by the addition of PGE₂. Silencing of COX-1 via siRNAs and inhibition of COX-1 via an inhibitor also decrease the production of PGE₂ and replication of both viruses, which can be attributed to the inhibition COX-1/PGE₂ signaling pathway. These data indicate that the COX-2/PGE₂ signaling pathway has proviral effects for the replication of FCV and MNV, and pharmacological inhibitors against these enzymes serve as potential therapeutic candidates for treating FCV and MNV infections.

Characterization of a Vesivirus Associated with an Outbreak of Acute Hemorrhagic Gastroenteritis in Domestic Dogs

Four of eleven affected dogs died despite aggressive treatment during a 2015 focal outbreak of hemorrhagic gastroenteritis following a stay in a pet housing facility. Routine diagnostic investigations failed to identify a specific cause. Virus isolation from fresh necropsy tissues yielded a calicivirus with sequence homology to a vesivirus within the group colloquially known as the vesivirus 2117 strains that were originally identified as contaminants in CHO cell bioreactors. In situ hybridization and reverse transcription-PCR assays of tissues from the four deceased dogs confirmed the presence of canine vesivirus (CaVV) nucleic acids that localized to endothelial cells of arterial and capillary blood vessels. CaVV nucleic acid corresponded to areas of necrosis and hemorrhage primarily in the intestinal tract, but also in the brain of one dog with nonsuppurative meningoencephalitis. This is the first report of an atypical disease association with a putative hypervirulent vesivirus strain in dogs, as all other known strains of CaVV appear to cause nonclinical infections or relatively mild disease. After identification of the CU-296 vesivirus strain from this outbreak, four additional CaVV strains were amplified from unrelated fecal specimens and archived stocks provided by other laboratories. Broader questions include the origins, reservoir(s), and potential for reemergence and spread of these related CaVVs.

Conserved Surface Residues on the Feline Calicivirus Capsid Are Essential for Interaction with Its Receptor Feline Junctional Adhesion Molecule A (fJAM-A)

Host cell surface receptors are required for attachment, binding, entry, and infection by nonenveloped viruses. Receptor binding can induce conformational changes in the viral capsid and/or the receptor that couple binding with downstream events in the virus life cycle (intracellular signaling, endocytosis and trafficking, and membrane penetration). Virus-receptor interactions also influence viral spread and pathogenicity. The interaction between feline calicivirus (FCV) and its receptor, feline junctional adhesion molecule A (fJAM-A), on host cells is required for infection and induces irreversible, inactivating conformational changes in the capsid of some viral strains. Cryoelectron microscopy (cryo-EM) structures of FCV bound to fJAM-A showed several possible virus-receptor interactions. However, the specific residues on the viral capsid required for binding are not known. Capsid residues that may be involved in postbinding events have been implicated by isolation of soluble receptor-resistant (srr) mutants in which changes in the capsid protein sequence change the capacity of such srr mutants to be inactivated upon incubation with soluble fJAM-A. To clarify which residues on the surface of FCV are required for its interaction with fJAM-A and to potentially identify residues required for postreceptor binding events, we used the existing atomic-resolution structures of FCV and the FCV-fJAM-A cryo-EM structures to select 14 capsid residues for mutation and preparation of recombinant viral capsids. Using this approach, we identified residues on the FCV capsid that are required for fJAM-A binding and other residues that are not required for binding but are required for infection that are likely important for subsequent postbinding events.IMPORTANCE Feline calicivirus (FCV) is a common cause of mild upper respiratory disease in cats. Some FCV isolates can cause virulent systemic disease. The genetic determinants of virulence for FCV are unknown. We previously found that virulent FCV isolates have faster in vitro growth kinetics than less virulent isolates. Differences in viral growth in vitro may correlate with differences in virulence. Here, we investigated the roles of specific FCV capsid residues on the receptor-virus interaction and viral growth in vitro We show that the capsid protein genes of the virulent FCV-5 isolate determine its faster in vitro growth kinetics compared to those of the nonvirulent FCV-Urbana infectious clone. We also identified residues on the capsid VP1 protein that are important for receptor binding or for steps subsequent to receptor binding. Our data provide further insight into the specific molecular interactions between fJAM-A and the FCV capsid that regulate binding and infectious entry.

Fexaramine as an entry blocker for feline caliciviruses

Feline calicivirus (FCV) is a small non-enveloped virus containing a single-stranded, positive-sense RNA genome of approximately 7.7 kb. FCV is a highly infectious pathogen of cats and typically causes moderate, self-limiting acute oral and upper respiratory tract diseases or chronic oral diseases. In addition, in recent years, virulent, systemic FCV (vs-FCV) strains causing severe systemic diseases with a high mortality rate of up to 67% have been reported in cats. Although FCV vaccines are commercially available, their efficacy is limited due to antigenic diversity of FCV strains and short duration of immunity. In this study, we identified fexaramine as a potent inhibitor of FCV including vs-FCV strains in cell culture and demonstrated that fexaramine is a entry blocker for FCV by using various experiments including time-of-addition studies, generation of resistant viruses in cell culture and the reverse genetics system. A fexaramine resistant FCV mutant has a single amino acid change in the P2 domain of VP1 (the major capsid), and the importance of this mutation for conferring resistance was confirmed using the reverse genetics system. A comparative analysis of viral resistance was also performed using a peptidyl inhibitor (NPI52) targeting FCV 3C-like protease. Finally, the effects of combination treatment of fexaramine and NPI52 against FCV replication and emergence of resistant viruses were investigated in cell culture.

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Fexaramine inhibits the replication of feline calicivirus in cells as an entry blocker.

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A single mutation in the capsid protein of FCV confers resistance to fexaramine.

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Combined treatment of fexaramine and a viral protease inhibitor substantially delays the emergence of virus resistance.

A Proposal for a Structural Model of the Feline Calicivirus Protease Bound to the Substrate Peptide under Physiological Conditions

Feline calicivirus (FCV) protease functions to cleave viral precursor proteins during productive infection. Previous studies have mapped a protease-coding region and six cleavage sites in viral precursor proteins. However, how the FCV protease interacts with its substrates remains unknown. To gain insights into the interactions, we constructed a molecular model of the FCV protease bound with the octapeptide containing a cleavage site of the capsid precursor protein by homology modeling and docking simulation. The complex model was used to screen for the substrate mimic from a chemical library by pharmacophore-based in silico screening. With this structure-based approach, we identified a compound that has physicochemical features and arrangement of the P3 and P4 sites of the substrate in the protease, is predicted to bind to FCV proteases in a mode similar to that of the authentic substrate, and has the ability to inhibit viral protease activity in vitro and in the cells, and to suppress viral replication in FCV-infected cells. The complex model was further subjected to molecular dynamics simulation to refine the enzyme-substrate interactions in solution. The simulation along with a variation study predicted that the authentic substrate and anti-FCV compound share a highly conserved binding site. These results suggest the validity of our in silico model for elucidating protease-substrate interactions during FCV replication and for developing antivirals.

Occurence of Bordetella bronchiseptica in domestic cats with upper respiratory tract infections

Bordetella bronchiseptica is a widespread Gram-negative pathogen occurring in different mammal species. It is known to play a role in the etiology of infectious atrophic rhinitis of swine, canine kennel cough, respiratory syndromes of cats, rabbits and guinea pigs, and sporadic human cases have also been reported. The aim of this article is to present the occurrence of infections caused by these bacteria in domestic cats with respiratory symptoms, as well as to conduct a molecular analysis of the flaA gene B. bronchiseptica for the purpose of ascertaining whether cats become infected with one or more bacteria strains. B. bronchiseptica was isolated from the respiratory system of 16 out of 35 domestic cats with symptoms of respiratory tract infections. Polymorphism analysis of polymerase chain reaction products of B. bronchiseptica flaA was performed to reveal the possible differences in nucleotide sequences of the flagellin gene. The phylogenetic analysis of nucleotide sequences obtained during PCR indicated that the isolates of bacteria from our own studies are characterised by 100% homology of the analysed fragment of the flaA gene, which suggests maintenance of a single genotype of these microorganisms in the cat population. Moreover, the bacteria revealed full homology with reference strain B. bronchiseptica ATCC 4617, and 99.4% homology with strain B. parapertussis ATCC 15311. This indicates that the PCR optimised for the Bordetella spp. flaA gene, combined with sequencing of amplicons obtained in PCR, is an effective diagnostic method allowing differentiation of Bordetella spp. type microorganisms.

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.

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.

Feline calicivirus and other respiratory pathogens in cats with Feline calicivirus-related symptoms and in clinically healthy cats in Switzerland

Background

Cats with feline calicivirus (FCV)-related symptoms are commonly presented to veterinary practitioners. Various clinical manifestations have been attributed to FCV, i.e. upper respiratory tract disease (URTD), oral ulcerations, gingivostomatitis, limping syndrome and virulent systemic disease. Additionally, healthy cats can shed FCV. The aims of this study were 1) to investigate the frequency of FCV in cats with FCV-related symptoms and in healthy cats in Switzerland, 2) to assess risk and protective factors for infection, such as signalment, housing conditions, vaccination, and co-infection with URTD-associated pathogens, and 3) to address the association between clinical symptoms and FCV infection.

Results

Oropharyngeal, nasal and conjunctival swabs were collected in 24 veterinary practices from 200 FCV-suspect and 100 healthy cats originating from 19 cantons of Switzerland. The samples were tested for FCV using virus isolation and reverse-transcription real-time quantitative polymerase chain reaction (qPCR) and for feline herpesvirus-1 (FHV-1), Mycoplasma felis, Chlamydophila felis, Bordetella bronchiseptica using real-time qPCR. Within the two populations (FCV-suspect/healthy), the observed PCR prevalences were: FCV 45 %/8 %, FHV-1 20 %/9 %, C. felis 8 %/1 %, B. bronchiseptica 4 %/2 %, M. felis 47 %/31 % and any co-infections thereof 40 %/14 %. Based on multivariable regression models amongst FCV-suspect cats (odds ratio [95 % confidence interval]), co-infection with M. felis (1.75 [0.97; 3.14]), group housing (2.11 [1.02; 4.34]) and intact reproductive status (1.80 [0.99; 3.28]) were found to be risk factors for FCV infection. In healthy cats, intact reproductive status (22.2 [1.85; 266.7]) and group housing (46.4 [5.70; 377.7]) were found to be associated with FCV infection. Based on an univariable approach, FCV-suspect cats were found to be significantly less often FCV-positive when vaccinated (0.48 [0.24; 0.94]). Oral ulcerations, salivation, gingivitis and stomatitis, but not classical signs of URTD were significantly associated with FCV infection (all p < 0.001).

Conclusions

FCV was detected in less than half of the cats that were judged FCV-suspect by veterinary practitioners. For a clinical diagnosis, FCV-related symptoms should be revisited. FCV infection was present in some healthy cats, underlining the importance of asymptomatic carriers in FCV epidemiology. To reduce FCV-related problems in multi-cat environments, reduction of group size in addition to the generally recommended vaccination are advocated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0595-2) contains supplementary material, which is available to authorized users.

Characterization of a novel calicivirus causing systemic infection in atlantic salmon (Salmo salar L.): proposal for a new genus of caliciviridae

The Caliciviridae is a family of viruses infecting humans, a wide range of animals, birds and marine fish and mammals, resulting in a wide spectrum of diseases. We describe the identification and genetic characterization of a novel calicivirus replicating in Atlantic salmon. The virus has a high prevalence in farmed salmon and is found in fish suffering from several diseases and conditions and also in presumable healthy fish. A challenge and vaccination trial shows that the calicivirus replicates in Atlantic salmon and establishes a systemic infection, which can be reduced by vaccination with formalin-inactivated virus preparation. The virus, named Atlantic salmon calicivirus (ASCV), is found in two genetically distinct variants, a cell culture isolated and a variant sequenced directly from field material. The genomes are 7,4 kb and contain two open reading frames where typical conserved amino acid motifs and domains predict a gene order reminiscent of calicivirus genomes. Phylogenetic analysis performed on extracted capsid amino acid sequences segregated the two ASCV variants in a unique cluster sharing root with the branch of noroviruses infecting humans and the unassigned Tulane virus and St-Valérien like viruses, infecting rhesus monkey and pig, respectively, with relatively large distance to the marine calicivirus subgroup of vesiviruses. Based on the analyses presented, the ASCV is predicted to represent a new genus of Caliciviridae for which we propose the name Salovirus.

Common and emerging infectious diseases in the animal shelter

The beneficial role that animal shelters play is unquestionable. An estimated 3 to 4 million animals are cared for or placed in homes each year, and most shelters promote public health and support responsible pet ownership. It is, nonetheless, inevitable that shelters are prime examples of anthropogenic biological instability: even well-run shelters often house transient, displaced, and mixed populations of animals. Many of these animals have received minimal to no prior health care, and some have a history of scavenging or predation to survive. Overcrowding and poor shelter conditions further magnify these inherent risks to create individual, intraspecies, and interspecies stress and provide an environment conducive to exposure to numerous potentially collaborative pathogens. All of these factors can contribute to the evolution and emergence of new pathogens or to alterations in virulence of endemic pathogens. While it is not possible to effectively anticipate the timing or the pathogen type in emergence events, their sites of origin are less enigmatic, and pathologists and diagnosticians who work with sheltered animal populations have recognized several such events in the past decade. This article first considers the contribution of the shelter environment to canine and feline disease. This is followed by summaries of recent research on the pathogenesis of common shelter pathogens, as well as research that has led to the discovery of novel or emerging diseases and the methods that are used for their diagnosis and discovery. For the infectious agents that commonly affect sheltered dogs and cats, including canine distemper virus, canine influenza virus, Streptococcus spp, parvoviruses, feline herpesvirus, feline caliciviruses, and feline infectious peritonitis virus, we present familiar as well as newly recognized lesions associated with infection. Preliminary studies on recently discovered viruses like canine circovirus, canine bocavirus, and feline norovirus indicate that these pathogens can cause or contribute to canine and feline disease.

Protective Efficacy of the Calicivirus Valency of the Leucofeligen Vaccine against a Virulent Heterologous Challenge in Kittens

Feline calicivirus (FCV) is a common feline pathogen with a potential for antigenic diversity. This study aimed to evaluate and characterize the protective efficacy of the FCV-F9 valency of a tetravalent vaccine, Leucofeligen, against challenge with an unrelated strain. Ten 9-week-old kittens were vaccinated while 10 remained as unvaccinated controls. The vaccinated cats received Leucofeligen twice subcutaneously with a 3-week interval. Four weeks after the second vaccination, all cats were challenged with virulent heterologous FCV and followed up for 21 days, monitoring their general condition, clinical signs, and immunological responses. During the vaccination phase, rectal temperatures and body weights were indistinguishable between the two groups. Only vaccinated cats showed FCV-specific seroconversion (both total and neutralizing antibodies). In the first week after challenge, the vaccinated cats had an 82.6% reduction in median clinical score compared to controls. Leucofeligen was thus shown to provide a significant clinical protection to kittens challenged with heterologous virulent FCV. This protection was similar whether the cats had neutralizing antibody or not, indicating a key role for cellular immunity in the overall protection. This also suggests that previously reported seroneutralisation studies may underestimate the level of cross-protection against field strains obtained with this modified live FCV-F9 vaccine.

Complete genome sequence of the feline calicivirus 2280 strain from the american tissue culture collection

Feline calicivirus (FCV) is a highly contagious pathogen of cats that can be grown in cultured cells. FCV is used as a model to study nonculturable caliciviruses, such as noroviruses. We determined the complete genome sequence of the FCV 2280 strain from the American Tissue Culture Collection.

Prevalence of viral infections in Norwegian cats with and without feline lower urinary tract disease

The prevalence of various viral infections was examined in primary accession cases of feline lower urinary tract disease (FLUTD) and healthy control cats in Norway. Urine samples from 102 cats with clinical signs of FLUTD and 73 healthy control cats were tested for the presence of feline calicivirus (FCV), feline coronavirus (FCoV) and feline herpesvirus-1 (FHV-1) by polymerase chain reaction. All urinary samples were negative for FCV and FCoV. One (1%) of the FLUTD cats was found to be positive for FHV-1. The results did not indicate an association between the viral infections examined and signs of FLUTD in the study sample.

Efficacy of intranasal administration of a modified live feline herpesvirus 1 and feline calicivirus vaccine against disease caused by Bordetella bronchiseptica after experimental challenge

BACKGROUND

Studies suggest that intranasal vaccination can stimulate nonspecific immunity against agents not contained within the vaccine, but this effect is not reported for cats.

HYPOTHESIS

A modified live feline herpesvirus-1 (FHV-1) and feline calicivirus (FCV) intranasal vaccine will reduce clinical signs of disease caused by experimental infection with Bordetella bronchiseptica.

ANIMALS

Twenty specific pathogen-free 12-week-old kittens.

METHODS

Experimental study. Cats were randomized into 2 groups of 10 cats each. The vaccinated group was administered a single intranasal dose of a commercially available vaccine containing modified live strains of FHV-1 and FCV, and the control group remained unvaccinated. All 20 cats were administered B. bronchiseptica by nasal inoculation 7 days later and were observed daily for clinical signs of illness for 20 days.

RESULTS

In the first 10 days after B. bronchiseptica challenge, vaccinated cats were less likely to be clinically ill than control cats with a median clinical score of 0/180 (range 0-5) versus 2/180 (range 0-8) (P = .01). Nine of 10 control cats and 2 of 10 vaccinated cats were recorded as sneezing during days 1-10 after challenge (P = .006).

CONCLUSIONS AND CLINICAL IMPORTANCE

Intranasal vaccination against FHV-1 and FCV decreased signs of illness due to an infectious agent not contained in the vaccine. This nonspecific immunity could be beneficial for protection against organisms for which vaccines are not available and as protection before development of vaccine-induced humoral immunity.

Feline calicivirus: a neglected cause of feline ocular surface infections?

OBJECTIVE

To investigate the prevalence of feline calicivirus (FCV) infection in relation to ocular surface lesions in cats with upper respiratory tract diseases (URTD).

ANIMALS STUDIED

Ninety-nine cats with ocular surface infection and symptoms or recent history of URTD were examined at various rescue shelters and hospitals.

PROCEDURE

A complete general and ophthalmic examination was performed including Schirmer tear test, slit-lamp biomicroscopy, fluorescein and lissamine green staining. Clinical and ocular symptoms were scored and recorded. Conjunctival samples were collected using a cytobrush, and nucleic acid extraction using RT-PCR was carried out to analyze for the presence of various infectious agents.

RESULTS

RT-PCR detected either FCV, feline herpes virus type 1 (FHV-1), Chlamydophila felis or Mycoplasma spp. in 63/99 samples. 30/63 samples were positive for FCV, 23/63 for C. felis, 21/63 for Mycoplasma spp., and 16/63 for FHV-1. Out of the 30 FCV-positive samples, 11 were positive only for FCV and in 19 samples FCV was seen in combination with other agents. FCV infection was highest in animals examined at the rescue centers and in the age group of 0-2 months. Erosive conjunctivitis was an important ocular finding. Oral ulcers were detected in all FCV-infected cats.

CONCLUSION

Results indicate that FCV is highly prevalent in cats with URTD either as a sole infectious agent or in combination with other pathogens and therefore is a potential cause for ocular surface lesions during the URTD.

Genetic characterization of a novel calicivirus from a chicken

We describe the identification and genetic characterization of a novel enteric calicivirus, detected by transmission electron microscopy and RT-PCR in two clinically normal chickens and in a chicken with runting and stunting syndrome from different flocks in southern Germany. Positive findings were confirmed by sequencing. The complete nucleotide sequence and genome organization of one strain (Bavaria/04V0021) was determined. The genome of the Bavaria virus is 7,908 nt long and contains two coding open reading frames. Phylogenetic analysis of the deduced partial 2C helicase/NTPase, 3C cysteine protease, RNA-dependent RNA polymerase and complete VP1 capsid protein amino acid sequences showed that the virus is genetically related to but distinct from sapoviruses and lagoviruses. Morphologically, the Bavaria virus particles are 37-42 nm in diameter and exhibit characteristic cup-shaped surface depressions.

Visualization of feline calicivirus replication in real-time with recombinant viruses engineered to express fluorescent reporter proteins

Caliciviruses are non-enveloped, icosahedral viruses with a single-stranded, positive sense RNA genome. Transposon-mediated insertional mutagenesis was used to insert a transprimer sequence into random sites of an infectious full-length cDNA clone of the feline calicivirus (FCV) genome. A site in the LC gene (encoding the capsid leader protein) of the FCV genome was identified that could tolerate foreign insertions, and two viable recombinant FCV variants expressing LC fused either to AcGFP, or DsRedFP were recovered. The effects of the insertions on LC processing, RNA replication, and stability of the viral genome were analyzed, and the progression of a calicivirus single infection and co-infection were captured by real-time imaging fluorescent microscopy. The ability to engineer viable recombinant caliciviruses expressing foreign markers enables new approaches to investigate virus and host cell interactions, as well as studies of viral recombination, one of the driving forces of calicivirus evolution.

Characterization of a continuous feline mammary epithelial cell line susceptible to feline epitheliotropic viruses

Mucosal epithelial cells are the primary targets for many common viral pathogens of cats. Viral infection of epithelia can damage or disrupt the epithelial barrier that protects underlying tissues. In vitro cell culture systems are an effective means to study how viruses infect and disrupt epithelial barriers, however no true continuous or immortalized feline epithelial cell culture lines are available. A continuous cell culture of feline mammary epithelial cells (FMEC UCD-04-2) that forms tight junctions with high transepithelial electrical resistance (>2000 Ω cm⁻¹) 3–4 days after reaching confluence was characterized. In addition, it was shown that FMECs are susceptible to infection with feline calicivirus (FCV), feline herpesvirus (FHV-1), feline coronavirus (FeCoV), and feline panleukopenia virus (FPV). These cells will be useful for studies of feline viral disease and for in vitro studies of feline epithelia.