Because of the asymptomatic carrier phase, caution should be taken when interpreting any
FCV positive result because of the poor correlation between the presence of virus and clinical
signs (Sykes et al., 1998).
The diagnosis of VS-FCV relies on clinical signs, high contagiousness and high mortality rate
and isolation of the same strain from blood of several diseased cats, assessed by sequencing of
hypervariable regions of the capsid gene.
Conventional, nested and real-time reverse-transcriptase PCR (RT-PCR) assays have been
developed to detect FCV RNA in conjunctival and oral swabs, blood, cutaneous scrapings or
lung tissue, depending on the clinical form and the outcome of the disease. Diagnostic
sensitivity of RT-PCR may depend on both the primers used and the detected strain, because
of the high variability of the viral genome (Helps et al., 2002; Marsilio et al., 2005; Scansen et
al., 2004; Sykes et al. 1998; Wilhelm & Truyen, 2006). Therefore, molecular assays should be
optimised using a large panel of strains to minimize false negative results. Multiplex PCR
have also been developed in order to detect at the same time both FHV-1 and FCV (Sykes et
al., 2001), but such assays may be less sensitive.
As well as having the potential to diagnose FCV infection, RT-PCR provides the means of
identifying uniquely the virus strain and has proven useful in molecular epidemiology and
outbreak investigations. However, consistent genetic markers associated with virulence,
specifically hypervirulent strains are as yet unavailable (Foley et al., 2006; Abd-Eldaim et al.,
2005; Ossiboff et al., 2007).
Virus isolation (VI) is a useful method for detecting FCV infection; it indicates the presence
of replicating virus and has the advantage of being less sensitive to the effect of strain
variation than RT-PCR. FCV replicates in cell lines of feline origin; its rapid growth in tissue
culture may compromise identification of concurrent herpesvirus (Pedersen, 1987).
Virus can be isolated from nasal, conjunctival or oro-pharyngeal swabs (Gaskell & Dawson,
1998), but VI may fail due to small numbers of virions in the sample, virus inactivation
during transit, or to the presence of antibodies in extracellular fluids that prevent virus
replication in vitro. The chance of successful VI can be maximised if swabs from both
conjunctiva and oropharynx are collected (Marsilio et al., 2005).
FCV antibodies can be detected by virus neutralization or ELISA (Lappin et al., 2002). The
seroprevalence is generally high in cat populations due to natural infection and vaccination.
Consequently, the presence of specific antibodies is not useful to diagnose infection (Gaskell
& Dawson, 1998).
Levels of VNA can be used to predict whether a cat is protected or not, but must be
interpreted properly, as false negative results may be obtained if VNA do not cross-react with
the laboratory strains used in the test. In addition, titres may appear higher when homologous
rather than heterologous virus-antibody pairs are used. When the strain used is not defined, it
makes interpretation of the results difficult (Scott & Geissinger 1997, 1999; Dawson et al.,
2001; Gore et al., 2006).