Feline calicivirus (FCV) is a highly contagious pathogen with a widespread distribution in the
feline population. It belongs to the Caliciviridae family that includes important pathogens of
man (such as the Norwalk virus, one of the commonest causes of infectious gastroenteritis in
people) and animals, including the European brown hare syndrome virus and rabbit
haemorrhagic disease virus (Green at al., 2000).
Feline calicivirus has a small single-stranded RNA genome of positive (messenger) polarity;
this allows FCV to evolve quickly. The genome is enclosed by multiple copies of the major
capsid protein. The surface of this protein contains the most variable region of the virus which
is also believed to be immunodominant (the region principally targeted by the host immune
response; Geissler et al., 2002; Radford et al., 1999; Tohya et al., 1997). Despite this
variability, there is sufficient overlap between isolates to allow classification of the viruses as
belonging to a single serotype (Povey, 1974; Povey & Ingersoll, 1975). However, antigenic
differences exist between most FCV isolates, which creates considerable difficulties when
trying to maximise vaccine cross protection. Genetically, most FCVs belong to a single
diverse genotype (Glenn et al., 1999, Geissler et al., 1997); a second genotype has recently
been described in Japan (Ohe et al., 2006).