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

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.