Characterization of an avirulent FCV strain with a broad serum cross-neutralization profile and protection against challenge of a highly virulent vs feline calicivirus

Highly virulent, systemic strains of Feline calicivirus (vs FCV) have been described in recent years. These vs FCV isolates cause severe edema, cutaneous ulcers, lameness and other upper respiratory and oral clinical signs typically associated with FCV infection in cats. Vs FCV isolates can cause high mortality even in cats vaccinated with currently available commercial vaccines. This study reports identification and characterization of an avirulent FCV strain (FCV 21). This strain offers a broader serum cross-neutralization profile in comparison with the commonly used vaccine strain (FCV F9), as tested with two separate viral panels of FCV isolates. The first viral panel consists of 45 FCV strains isolated around 1993. The second viral panel consists of 26 FCV strains with most isolated around 2003. The potential of using this strain as a vaccine, in a 3-way (FCV+FHV+FPV) or 4-way (FCV+FHV+FPV+FCp) format, was tested by using a highly virulent vs FCV strain (FCV-33585) as a challenge virus. The mortality induced by this vs FCV in unvaccinated control cats was 78% (7 out of 9 cats). The mortality decreased to 44% (4 out of 9 cats) in cats vaccinated with a 4-way vaccine containing FCV F9. However, when this novel FCV vaccine strain (FCV 21) was used, either in combination with FCV F9 or by itself, the mortality decreased to 0% (0 out of 10 cats). The 3-way vaccine (FCV+FHV+FPV) that contained both FCV 21 and FCV F9 also had mortality of 0% (0 out of 10 cats). The clinical scores, as calculated taking into consideration the frequency and severity of various clinical signs, correlated with mortality data. The results suggested this FCV vaccine has the potential to be broadly protective against newly emergent FCV isolates, including complete protection against challenge with a highly virulent vs FCV 33585.