Feline calicivirus (FCV) is a ubiquitous RNA virus which has been isolated from cats with upper respiratory disease (cat flu), feline chronic gingivostomatitis, limping syndrome, fading kittens and acute systemic virulent disease. The ability of FCV to mutate rapidly allows it to evade the host's immune response, resulting in strain diversity such that the FCV strains used in vaccines are not likely to protect equally well against all field strains, especially where vaccines are used widely. Recently, a new vaccine was introduced in the UK comprising two new FCV vaccine strains, namely FCVG1 and FCV431. The aim of this study was to determine the cross-reactivity between these new strains and a panel of UK field isolates of FCV.

We collected 102 FCV isolates from naturally infected cats using samples submitted for FCV diagnosis from veterinary surgeons in practice across the UK and from 8 cats suffering from feline chronic gingivostomatitis being treated at the University of Glasgow Veterinary School. These isolates were tested for susceptibility to cross-neutralisation by a panel of 8 antisera raised in four pairs of specific pathogen free cats infected once by the oro-nasal route with FCV strains F9, 255, FCVG1 and FCV431. The pairs of antisera raised against F9 or 255, neutralised 20 and 21% or 37 and 56% of field strains of virus respectively, with titres of 5 or greater. In contrast, the pairs of antisera raised against the newer vaccine strains FCVG1 or FCV431 neutralised 29 and 70% or 67 and 87% of field strains respectively. Furthermore, the pairs of sera raised against the novel strains FCVG1 or FCV431 displayed high neutralising antibody titres (45 or greater) against 7-43% of the UK field strains examined. In contrast, sera against strains F9 or 255 achieved titres greater than 45 against only 3-31% of the field strains.

In conclusion, antisera raised against two novel strains of FCV, namely FCVG1 and FCV431, neutralised a greater proportion of field strains of calicivirus than did antisera raised against existing FCV vaccine strains F9 and 255.

Acknowledgements

We thank Merial for funding this study.