Maternally derived antibodies (MDA) are important for protection during the first weeks of life and may interfere with vaccination. There are very few data on the extent and longevity for FCV MDA in cats. In general, their levels are higher and persist for longer than for feline herpesvirus (FHV-1). In an experimental study, the average half-life of MDA was determined to be 15 days and their persistence as 10-14 weeks (Johnson & Povey, 1983). However, in a field study, 20% of kittens at only six weeks of age had no detectable antibodies against a widely used vaccine strain (Dawson et al. 2001).
Virus neutralising antibodies (VNA) appear by approximately seven days post infection
(Kahn et al., 1975). In general, antibody titres are higher than for FHV infection and their
levels correlate well with protection against homologous challenge (Povey & Ingersoll, 1975).
There is a considerable degree of antigenic variability amongst FCV strains, but it was
concluded from studies of in vitro cross-reactivity that FCVs belong to a single serotype
(Povey, 1974). Prior infection with one strain can significantly reduce the acute clinical signs
upon exposure to a heterologous strain, and in some cases oral shedding may be reduced
(Povey & Ingersoll, 1975; Knowles et al., 1991). In general, the level of heterologous
protection will depend on the virus strains involved.
Cats may be protected also in the absence of detectable VNA (Knowles et al., 1991; Poulet et
al., 2005), suggesting a role for other immune mechanisms: indeed, cellular responses have
been demonstrated in vaccinated cats (Tham & Studdert, 1987). Also, FCV-specific IgG and IgA antibodies have been demonstrated in the saliva during the course of infection (Knowles
et al., 1991), although their significance in protection is unknown.