Comparison of serological and sequence-based methods for typing feline calicivirus isolates from vaccine failures

Feline calicivirus (FCV) can be typed by exploiting antigenic differences b etween isolates or, more recently, by the sequence analysis of a hypervaria ble region of the virus's capsid gene. These two methods were used to chara cterise FCV isolates from 20 vaccine failures which occurred after the use of a commercial, live-attenuated vaccine. Using virus neutralisation, the i solates showed a spectrum of relatedness to the vaccine; depending on the c riterion adopted for identity, In to 40 per cent of them appeared to be sim ilar to the vaccine virus. Using sequence analysis, the isolates fell into one of two categories; 20 per cent had a similar sequence to the vaccine (0 .67 to 2.67 per cent distant), and the remainder had a dissimilar sequence (21.3 to 36.0 per cent distant). Sequence analysis identified one cat that appeared to be infected with two distinct FCVs. The serological and sequenc e-based typing methods gave the same result in 80 to 95 per cent of individ ual cases, depending on the criterion adopted for serological identity. It is suggested that molecular typing is a more definitive method for charader ising the relatedness of FCV isolates.