Kittens are protected against disease by maternally derived antibodies (MDA) during the first weeks of their lives but in general levels of MDA for FHV infection are low. It has been demonstrated that MDA may persist for 2-10 weeks (Johnson & Povey, 1985) although in a more recent study levels of MDA were shown to be low, with approximately 25% of kittens appearing negative for MDA as early as 6 weeks of age (Dawson et al., 2001).
Glycoproteins embedded in the membrane of the herpesviruses are important in the induction
of immunity; following infection the detection of virus neutralizing antibodies (VNA)
correlates with the recognition of FHV glycoproteins (Burgener and Maes, 1988).
Furthermore, immunisation of rabbits with FHV-gD led to the production of high titres of
VNAs, indicating a role of these proteins in the induction of VNA (Spatz et al., 1994).
Solid immunity is not induced after natural infection; in general the immune response protects
against disease but not against infection and mild clinical signs have been observed following
reinfection, only 150 days after primary infection (Gaskell and Povey, 1979). The titres of
VNA induced by natural infection are often low and rise slowly. Indeed, VNA may still be
absent 40 days post infection (Gaskell and Povey, 1979). VNA most likely contribute to theprotection against acute infection. Other antibody-mediated mechanisms e.g. antibody
mediated cellular cytotoxicity (ADCC) and antibody-induced complement lysis have been
demonstrated (Wardley, 1976). However, (as with other alpha-herpesviruses) cell-mediated
cellular immunity plays an important role in protection, since the absence of detectable serum
antibody levels in vaccinated cats does not necessarily indicate that cats are susceptible to
disease (Lappin et al., 2002). On the other hand, seroconversion did correlate with protection
against virulent FHV challenge (Lappin et al., 2002). It is important to take into consideration
that presence of antibodies against any infectious agent may provide an indirect indication of
cellular immune responses, since T-lymphocytes are required for maintenance of Blymphocyte
function.
Although a general correlation between presence of antibodies to FHV and protection against
clinical signs has been demonstrated for FHV infection, there is currently no reliable test
available that predicts the degree of protection in individual cats.
Since FHV is a pathogen of the respiratory tract, mucosal cellular and humoral responses are
important. Several studies with intranasal vaccines have shown clinical benefits as early as 2-
6 days after vaccination (Lappin et al., 2006; Weigler et al., 1997, Slater & York, 1976).