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Journal of Feline Medicine and Surgery logoLink to Journal of Feline Medicine and Surgery
. 2008 Feb 1;10(1):32–40. doi: 10.1016/j.jfms.2007.06.011

Comparison of the ability of feline calicivirus (FCV) vaccines to neutralise a panel of current UK FCV isolates

Carol J Porter 1,*, Alan D Radford 1, Rosalind M Gaskell 1, Ruth Ryvar 1, Karen P Coyne 1, Gina L Pinchbeck 1, Susan Dawson 1
PMCID: PMC10911152  PMID: 17720588

Abstract

Feline calicivirus (FCV) comprises a large number of strains which are related antigenically to varying degrees. The antigenic variability creates problems for choosing antigens to include in vaccines. Historically, these have been selected for use based on their cross-reactivity with a high proportion of field strains. However, it is important to determine the current level of cross-reactivity of vaccines and whether or not this may be decreasing owing to widespread vaccine use. In this in vitro study, we have compared the ability of antisera to two vaccine viruses (FCV strain F9 and FCV strain 255) to neutralise a panel of 40 recent UK field isolates. These 40 isolates were obtained by randomised, cross-sectional sampling of veterinary practices in different geographical regions of the UK so as to ensure they were representative of viruses circulating in the veterinary-visiting population of cats in the UK. Virus neutralisation assays showed that both vaccine strains are still broadly cross-reactive, with F9 antiserum neutralising 87.5% and 255 antiserum 75% of isolates tested with antiserum dilutions of 1 in 2 or greater. However, when antibody units were used, in order to take account of differences in homologous titres between antisera, fewer isolates were neutralised, with F9 antiserum showing a slightly higher proportion of isolates neutralised than 255. Multivariable analysis of the sample population of 1206 cats from which the 40 isolates were derived found that vaccinated cats were at a decreased risk of being positive for FCV, whereas cats from households with more than one cat, and cats with mouth ulcers were at increased risk. In addition as cats became older their risk of shedding FCV decreased.

Feline calicivirus (FCV) is a highly infectious pathogen of cats, with a worldwide distribution. Infection generally leads to acute, mild to moderate, oral and upper respiratory tract disease (URTD) (Gaskell et al 2004). More recently, virulent mutants of FCV have been identified as the cause of a severe and acute virulent systemic disease (VSD), characterised by pyrexia, jaundice, oedema and high mortality in groups of cats (Pedersen et al 2000, Hurley and Sykes 2003, Hurley et al 2004, Pesavento et al 2004, Coyne et al 2006). There are a large number of different FCV strains with slightly varying antigenicity and pathogenicity, but most appear to be reasonably closely related and comprise one serotype (Povey 1974, Kalunda et al 1975). It is on this basis, that specific FCV strains which are relatively cross-protective have been selected for vaccine use (Kahn et al 1975, Povey and Ingersoll 1975, Gaskell et al 1982).

Vaccination against FCV is relatively widespread amongst the domestic cat population and several types of vaccines have been developed, including live attenuated intranasal and systemic vaccines, and inactivated systemic vaccines (Gaskell et al 2004). FCV vaccines are generally effective in reducing the severity of FCV-associated diseases (Bittle and Rubic 1976, Kahn and Hoover 1976, Povey and Wilson 1978, Povey et al 1980). However, vaccinated cats are not protected against infection, and vaccinated cats can become carriers (Gaskell et al 1982, Dawson et al 1991, Harbour et al 1991, Binns et al 2000). The majority of attenuated vaccines have been based on a single FCV strain designated F9 (Bittle and Rubic 1976, Pedersen and Hawkins 1995), although more recently, some vaccines have been marketed which incorporate other FCV strains or a combination of strains (National Office of Animal Health compendium, Poulet et al 2005). Other types of experimental vaccines have been produced, including protein subunit vaccines (Komolafe and Jarrett 1985), virus-like particles (DeSilver et al 1997), recombinant feline herpesvirus-1 (FeHV-1) expressing an FCV capsid protein (Yokoyama et al 1996a,b, 1998), and nucleic acid vaccines (Sommerville et al 2002). However, although some of these vaccines induced a serological response post-vaccination, the majority of these potential vaccines only offered partial protection and are, therefore, not commercially available.

Although FCV vaccine strains used are considered to be widely cross-protective, these vaccines may not protect equally well against all field isolates (Dawson et al 1993b, Pedersen and Hawkins 1995). It has also been suggested that field isolates may be developing resistance to commonly used vaccine strains such as F9. One study reported that the proportion of isolates that cross-reacted with such vaccines appeared to have declined over time (Lauritzen et al 1997). Interestingly, outbreaks of recently described highly virulent FCV disease are largely occurring in vaccinated cats, though each of the strains causing such outbreaks appears to be genetically distinct (Pedersen et al 2000, Hurley and Sykes 2003, Hurley et al 2004, Pesavento et al 2004, Coyne et al 2006).

The objective of this study was to evaluate the cross-reactivity of two currently used vaccine strains, FCV strain F9 and FCV strain 255, against a panel of recent field isolates of FCV obtained by randomised, cross-sectional sampling of veterinary practices selected from different regions of the UK. This sampling strategy was used to ensure that the results were likely to be representative of recent field isolates present in the UK vet-visiting population of cats. Additional demographic and clinical information was obtained in order to address possible risk factors for infection with FCV and the likely protective effect of vaccination.

Materials and methods

Field isolates

Field isolates were collected by sampling cats attending veterinary practices over a 6-month period in 2001. In order to reduce the risk of bias in the isolates obtained, one veterinary practice was chosen at random from each of 75 geographic regions identified in the Royal College of Veterinary Surgeons practice register and asked to participate in this study. If a practice was unwilling to take part, a further veterinary practice from that region was contacted. Each practice was asked to take oropharyngeal swabs from the next 20 cats presented at their surgery. The veterinary surgeons were asked to complete a brief questionnaire about each cat, which included the age, breed, sex, whether or not neutered, and total number of cats in the household. In addition, the vaccination history together with type and date of vaccine, and information about previous or current respiratory disease and mouth ulcers where known were recorded.

Virus isolation

Oropharyngeal swabs were collected into 2 ml of virus transport medium and sent by prepaid envelope to our laboratory for virus isolation. FCV and FeHV-1 were isolated using standard techniques as described previously, using feline embryo fibroblast (FEF) cells and mock-infected negative control wells (Povey and Johnson 1971, Knowles et al 1990). If no cytopathic effect was evident, cells were passaged once on to fresh monolayers before being considered negative. Viruses isolated were stored at −80°C.

Antiserum

Antisera were provided by Intervet UK (Walton Manor, Walton, Milton Keynes MK7 7AJ). Briefly, four specific pathogen-free kittens aged approximately 9–16 weeks old at time of primary inoculation were used. Prior to first immunisation, oropharyngeal swabs and serum samples were taken to confirm the absence of FCV. Two cats were vaccinated and challenged with FCV-F9 and two with FCV-255 by a combination of routes (intranasal+subcutaneous) using ascending doses of virus (104.5 tissue culture infectious dose 50 (TCID50)/ml–106.0 TCID50/ml) on four separate occasions over 64 days. Antisera were collected and pooled for the two cats challenged with FCV-F9, and also for the two cats with FCV-255 at 84 days. This method of challenge was chosen as regular vaccination fails to induce sufficient titres of antisera to use in subsequent in vitro experiments (authors unpublished observations). It is possible, therefore, that the neutralising response may have been due in part to other, possibly non-specific immune mechanisms not seen in vaccinated cats.

Virus neutralisation tests

Forty geographic regions from which FCV had been isolated were randomly selected, and one FCV isolate was chosen from each practice. Virus neutralisation tests for each of the 40 viruses were performed against F9 and 255 antiserum on 96-well plates using a constant virus varying serum method. Duplicate, serial twofold dilutions of serum were made and incubated with 32–320 TCID50 titre of virus and 37°C for 1 h before the addition of FEF cells. Plates were read at 72 h and the antibody titre expressed as the 50% end point (Reed and Muench 1938). A back titration of the virus suspension was carried out in each assay, and positive controls were included for both F9 and 255 antiserum against its homologous virus. In order to compare results for F9 and 255 which had different homologous titres, antibody unit calculations were also carried out. One antibody unit of an antiserum was defined as the highest dilution of that antiserum which can neutralise 100 TCID50 titre of the homologous virus in 50% of test cultures (Povey 1974). Concentrations of 5 or 20 antibody units are therefore fivefold or 20-fold less dilute.

Statistical analysis

Statistical analyses were carried out using Minitab for Windows 14 (Minitab Inc. USA). Individual cat predictor variables; age, breed, sex, whether or not neutered, breed type, total number of cats in the household, whether or not vaccinated and disease status, were screened for association with FCV infection status. Univariable associations were examined using χ2 tests for categorical variables and logistic regression for continuous variables (ie, age). Variables with P<0.3 were then considered for inclusion in a multivariable model which was built by using backward elimination procedures (Hosmer and Lemeshow 2000). The critical probability was set at P≤0.05.

Results

Isolation of FCV and FeHV-1

Of the 75 practices who agreed to participate, 62 (83%) returned a total of 1206 swabs (average returned by each compliant practice was 19.5; range 13–22). Overall, FCV was isolated from 122 of 1206 swabs giving a prevalence of 10.1% (95% confidence interval (CI) 8.3–11.76). For FeHV-1, nine positive samples were identified giving a prevalence of 0.7% (95% CI 0.2–1.2).

The average number of FCV isolations made from each of the 62 responding practices was 1.9 (range 0–7), and the average number of FeHV-1 isolations made from each of the responding practices was 0.1 (range 0–2).

Five out of the nine predictor variables examined were found to be significantly associated with the presence of FCV on univariable analysis (Table 1). Of these, four variables were still significant when using multivariable analysis (Table 2). Vaccinated cats were at a decreased risk of being positive for FCV, whereas cats from households with more than one cat and cats with mouth ulcers were at increased risk. In addition, as cats became older their risk of being FCV positive decreased. Neutered status was no longer significant in the multivariable model and this is likely due to correlation or confounding by vaccination and age (data not presented).

Table 1.

Univariable analysis of factors associated with FCV isolation

FCV negative, N (%) FCV positive, N (%) χ2 P value
Categorical variables
Vaccinated 713 (91.9) 63 (8.1)
Not vaccinated 275 (85.4) 47 (14.6) 0.001
Male 538 (89.4) 64 (10.6) 0.6
Female 508 (90.4) 54 (9.6)
Neutered cats 873 (90.8) 89 (9.2) 0.02
Unneutered cats 170 (85) 30 (15)
Pedigree 112 (91.8) 10 (8.2)
Non-pedigree 834 (89.3) 100 (10.7) 0.4
Mouth ulcers 34 (65.4) 18 (34.6) <0.001
No mouth ulcers 994 (91.1) 98 (8.9)
Current URTD 80 (87) 12 (13) 0.3
No current URTD 945 (90.2) 103 (9.8)
History of URTD 92 (90.2) 10 (9.8) 0.9
No history of URTD 885 (90.4) 94 (9.6)
One cat in household 469 (93.1) 35 (6.9)
2–3 Cats in household 455 (90.5) 48 (9.5)
4–10 Cats in household 87 (79.9) 22 (20.1)
>10 Cats in household 16 (61.6) 10 (38.4) <0.001
Continuous variables Median age of FCV negative Median age of FCV positive P value
5 Years 3 Years 0.04
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URTD=upper respiratory track disease.

Table 2.

Multivariable logistic regression model of factors associated with FCV isolation

Odds ratio 95% CI P value
Age (years) 0.9 0.9–1.0 0.05
Vaccinated
No 1 – –
Yes 0.6 0.4–0.9 0.02
Mouth ulcers
No 1 – –
Yes 4.6 2.3–9.2 <0.001
Cats in household
1 1 –
2–3 1.3 0.8–2.1 0.4
4–10 2.5 1.3–4.8 0.006
>10 6.1 2.1–17.2 0.001
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Hosmer–Lemeshow goodness of fit test P value=0.7.

Virus neutralisation

Individual demographic and clinical details of the 40 FCV isolates used in this part of the study are shown in Table 3.

Table 3.

Demographic, clinical details and virus neutralisation results of the 40 isolates used in this study

Region Cat identity * Age Sex Breed Number of cats in household FCV vaccine Days since last vaccine Vaccine make if known § Mouth ulcers Current URTD History of URTD Anti-F9 ‡ Anti-255 ‡
Birmingham 03/16 7 y mn DSH 4 n nk nk n n n 8 128
Berkshire 05/07 10 y mn DLH 10 n nk nk n n n 3 4
Cumbria 06/01 4 m m DSH 2 y 17 Feligen n nk n 8 128
Cambridgeshire 11/11 1 y fn DSH 4 nk nk nk n n n 3 128
Dorset 13/07 4 m m DSH 1 y 0 ♠ Fevaxyn n n n <2 6
Cleveland 14/19 4 y mn DSH 1 y 377 ∥ Tricat n n n 12 64
Derbyshire 16/05 2 y mn DSH 2 y 371 ∥ Felocell n n n 32 24
Essex 17/03 2 y mn DLH 2 y 381 Katavac n n y <2 † <2 †
Leiceshire 20/13 14 y mn DSH nk y nk Tricat nk nk nk 48 32
Lancashire 22/02 7 y fn DSH 1 n nk nk n n n 64 32
Lincolnshire 25/13 nk mn DSH 4 n nk nk y n n 4 2
Nottinghamshire 26/08 1 y nk Siamese DSH 4 y 280 Katavac n n n 3 <2
Oxfordshire 27/02 14 y fn DSH 2 n nk nk y n n 4 <2
Kent 31/04 3 y mn DSH 2 n nk nk n y n 512 64
Worcestershire 32/09 11 y mn DSH 2 y nk Tricat n n n 3 64
Warwickshire 33/12 2 y fn DSH 1 y 105 Katavac n n n 4 3
Northumberland 34/17 7 m f British DSH 4 y nk nk n n n 16 12
Wiltshire 36/11 4 y fn DSH 2 y 190 Fevaxyn n n n 2 4
Suffolk 39/09 1 y mn DSH 10 y 41 Tricat y n n <2 4
Channel Island 40/09 2 y mn DSH 2 y nk Tricat n n n 3 <2
Salop 45/05 1 y mn DSH 2 y 362 ∥ nk n n y <2 † <2 †
Hampshire 4720 10 m mn DSH 10 y nk nk n n n 6 12
Middlesex 48/19 9 y mn DSH 4 n nk nk n y n 4 <2
Dumfries 50/04 2 y mn DSH 2 y 62 Tricat n n n 6 8
Edinburgh 51/20 7 m f DSH 1 n nk nk n n n 8 6
Orkney 56/04 5 m mn DSH 2 n nk nk n n n 6 4
Glamorgan 59/03 12 w f DSH 4 n nk nk n y y <2 † <2 †
Dyfed 60/18 2 y mn DLH 4 y 230 Katavac eclipse n n n 2 <2
Strathclyde 61/14 16 y mn DSH 1 y 4270 ∥ nk n n n 64 24
Shetland 62/01 12 y fn DSH 1 y 0 ♠ Quantum n n n 8 3
Powys 63/15 3 y m DSH 1 y 49 ∥ Eclipse n n n 6 <2
Co Armagh 64/01 1 y f DSH 1 n nk nk n n n 3 1064
Co Down 66/03 12 y fn DSH 1 n nk nk n n n 6 3
Gwynedd 67/21 nk nk nk nk nk nk nk nk nk nk 3 48
Isle of Man 68/10 nk mn DSH 1 y nk Katavac n n n 3 24
Clwyd 69/09 7 w f DLH 1 n nk nk n y n 6 <2
Gwent 71/07 6 m f DSH 1 y 80 Katavac n n y 16 6
Co Antrim 72/06 nk fn DSH 10 y 168 ∥ Pentofel n n n 128 128
Co Tyrone 74/02 8 m f DLH 4 n nk nk n n nk 16 1064
Co Fermanagh 75/13 1 y f DLH 1 n nk nk n n n 6 3
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y=years, m=months, w=weeks, m=male, f=female, mn=male neutered, fn=female neutered, nk=not known, DSH=domestic shorthair, DLH=domestic longhair, URTD=upper respiratory track disease.

0 ♠

=cat vaccinated the same time as swab was taken.

*

Practice number/swab number.

†

Isolates 17/03, 45/05 and 59/03 were not neutralised by either of the antisera.

‡

Virus neutralising antibody titre of antisera raised to FCV-F9 and FCV-255 against the virus isolated from each cat.

§

Vaccine/manufacturer: Felocell/Pfizer; Katavac Eclipse, Fevaxyn/Fort Dodge; Feligen/Virbac.; Nobivac Tricat/Intervet UK; Quantam/Schering Plough Animal Health.

∥

Where precise day of vaccination not known, the interval was calculated assuming vaccination occurred on the first day of the month specified.

Initial comparisons for the virus neutralisation tests showed that both F9 and 255 antiserum appeared to neutralise the majority of isolates tested at a dilution of 1 in 2 or greater (Table 3). For F9 antiserum 35 (87.5%) of 40 isolates were neutralised and for 255 antiserum 30 (75%) of 40 isolates were neutralised, and there was no significant difference between these.

However, the mean homologous titres of F9 and 255 antisera, respectively, were 1 in 240±40 standard error (SE) and 1 in 2196±431 SE based on nine virus neutralisation tests. As a result, the comparison of the quantity of isolates neutralised by each antiserum may have been affected by the level of the homologous titre. Therefore, to make a more valid comparison between the cross-reactivity of each antiserum, calculations based on antibody units were carried out. For F9 antiserum 10 (25%), six (15%) and five (12.5%) isolates were neutralised by 20, 10 and 5 antibody units respectively, and for 255 antiserum six (15%), two (5%) and two (5%) isolates were neutralised. These cut-off points gave lower percentages of isolates neutralised, although F9 antiserum showed a slightly higher proportion of isolates neutralised compared to 255 antiserum.

Three isolates (59/03, 45/05 and 17/03) were not neutralised by either of the antisera. These were confirmed as FCV by indirect immunofluoresent staining, using a rabbit polyclonal antisera to a conserved antigenic site (ags 4) of region E of the FCV capsid (Radford et al 1999, Porter 2003). Specific cytoplasmic fluorescence was clearly observed for each of these three isolates. All three isolates were from cats that had a history of URTD. One of the cats (59/03) had not been vaccinated, and was reported to have current upper respiratory disease.

Discussion

In this study, FCV was isolated from 117 of 1206 (10.1%) cats sampled at veterinary practices from different geographical regions of the UK. These figures for FCV isolation are lower than those found in previous practice-based, diagnostic sample or other general population surveys, where approximately 20% or more of cats were positive for FCV (Knowles et al 1989, Harbour et al 1991, Binns et al 2000, Helps et al 2005). However, sample populations vary, and factors such as household size, which was found to be significantly associated with FCV infection in the current work, may account for our slightly lower prevalence. For example, Wardley et al (1974) found a lower prevalence (8%) in individual household cats, and the majority (83%) of our samples came from households with less than three cats. Both our study and that of Wardley et al (1974) also found that younger animals were more likely to be positive for FCV.

The proportion of cats within the sample which had URTD or mouth ulcers may also impact on FCV prevalence: in our work, the presence of mouth ulcers was significantly associated with FCV infection although respiratory disease was not associated with FCV. Although, the presence of respiratory disease has been associated with FCV infection in some previous studies, others have found a lack of association similar to this study (Binns et al 2000, Bannasch and Foley 2005, Helps et al 2005). Vaccination also appeared to have a significant protective effect on FCV infection in our study: this has also been shown in some previous work but not in others (Harbour et al 1991, Binns et al 2000). It may be that any possible associations with FCV might be difficult to show, because of the high prevalence of carriers in both clinically healthy cats and in vaccinated animals.

The low prevalence of FeHV-1 (0.7%) compared to FCV is most likely due to the differences between the two carrier states for the viruses. FCV is shed more or less continuously by persistently infected cats, whereas FeHV-1 carriers are latently infected and only shed infectious virus intermittently (Gaskell et al 2004). These low figures for FeHV-1 shedding are consistent with other surveys of cat populations (Harbour et al 1991, Binns et al 2000, Holst et al 2005), although higher prevalences may be found using detection by polymerase chain reaction (PCR) (Helps et al 2005).

There are several types of FCV vaccines currently available, and the FCV strains used in such vaccines have generally been selected for their broad cross-reactivity largely on the basis of in vitro, and in some cases also in vivo testing (Kahn et al 1975, Kalunda et al 1975, Povey and Ingersoll 1975, Gaskell et al 1982, Dawson et al 1993a). Most vaccines are based on single strains, such as FCV-F9 or FCV-255, though other strains are used. Another vaccine more recently marketed in the EU is based on two strains (FCV 431 and FCV G1) (Poulet et al 2005). The cross-reactivity of strains incorporated in vaccines is likely to be important in the efficacy of such vaccines in inducing protection in cats.

Another important issue is whether the vaccines that have been used for a number of years are still cross-protective in the field, or whether widespread use of such vaccines has led to the evolution of vaccine resistant strains. It is important, however, in such assessments to ensure that a representative sample of viruses is used. In this study, we tested 40 relatively recent UK FCV isolates in order to compare their cross-reactivity with antisera to two commonly used vaccine strains (F9 and 255). For F9 antiserum, 87.5% of isolates were neutralised at a dilution of 1 in 2 or greater. This is compared to 75% for 255 antiserum. There was no significant difference between these two proportions. However, as the homologous titres differed between the two antisera, calculations based on antibody units were also carried out. These cut-off points gave lower percentages of isolates neutralised, although F9 antiserum showed a slightly higher proportion of isolates neutralised compared to 255 antiserum.

These results for F9 antiserum are broadly consistent with earlier studies, where a similar proportion of field isolates was neutralised (Knowles et al 1990, Dawson et al 1993b, Pedersen and Hawkins 1995). Lauritzen et al (1997) found that the proportion of USA isolates neutralised by F9 appeared to be decreasing over time. However, our present study showed an increase in the proportion of UK isolates neutralised by 10 antibody units of F9 antiserum when compared to the Lauritzen study. There are a number of factors which may influence the outcome of such studies, including the origin of the isolates, the way in which the isolates are selected, the titre of the antisera and its method of production, and the laboratory protocols and cut-off points used. It is also important to obtain a representative sample of viruses from the general population. In the present study the field virus samples were obtained using randomly selected veterinary practices from different geographic regions of the UK, thereby ensuring that they were likely to be representative of FCV isolates present in the vet-visiting cat population. As only a small number of cats in this study were showing signs of URTD, it is possible that cat isolates from a diseased population may have a slightly different neutralising profile. It is also possible that some of the isolates tested may have originated from vaccine strains, however, previous studies have found that it is uncommon to find vaccine strains circulating in the population (Glenn et al 1999, Radford et al 2000).

Although different cut-off points were assessed, it is currently unknown what level of neutralisation in vitro reflects an effective degree of protection in the cat, and such data can, therefore, be difficult to interpret. Povey and Ingersoll (1975) suggested that a virus neutralising titre of 1:7 or less may correlate with susceptibility to challenge with heterologous FCV, whereas cats with titres of 1:16 or greater are likely to be protected. The situation is likely to be complicated, however, by the degree of relatedness of the strains, the homologous titre of antisera, and other host and viral factors such as dose and route of infection.

The identification of isolates that were not neutralised by either or both antisera may explain why vaccine breakdowns still occur (Dawson et al 1993a, Gaskell et al 2004). Further studies using detailed genome and antigenic analysis may reveal subtle differences within such strains that could be used to rationally improve strain coverage.

The data presented in this study confirm that antiserum to F9 is still broadly cross-reactive and neutralises a comparable number of FCV isolates to those collected in previous years. FCV-255 also neutralised a similar proportion of isolates, although when different cut-off points are used, the proportion was slightly lower.

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