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. 1. Introduction Feline calicivirus (FCV) is a common pathogen of cats normally infecting the oral cavity and upper respiratory tract. The initial infection generally results in acute clinical signs such as fever and lingual or oral ulceration, in addition to sneezing, rhinitis, and conjunctivitis [1]. However, cats infected with FCV may not exhibit overt clinical disease when they are persistently infected or when they are infected with FCV isolates that are only mildly pathogenic. Other nontypical FCV infections have also been observed that produce various clinical symptoms such as lameness or diarrhea, and hypervirulent strains causing virulent systemic disease (or VSD) are diagnosed sporadically [2, 3]. FCV has a single-stranded positive-sense RNA genome of ~7.7？kb. The replication of FCV, like other RNA viruses in general, results in a high proportion of genomic as well as antigenic variants. Indeed the overall identity of FCV isolates collected worldwide was reported to be approximately 80% for the variable and immunodominant regions C to E of the capsid gene [4, 5]. This genomic diversity gives rise to antigenic variants