Genogrouping of vaccine breakdown strains (VBS) of feline calicivirus in Japan.

Although prevention of feline calcivirus (FCV) infection by vaccination has been attempted, and isolation of FCV, development of the disease, and a few fatal cases in vaccinated cats have been reported. Fifteen FCV strains isolated from cats that had been vaccinated with commercially available FCV vaccines (F9, FCV-255, and FC-7) were genogrouped. Molecular analysis of viral genomes involved the construction of a phylogenetic tree of capsid genes using the NJ method. Cat anti-F9 serum and rabbit anti-FCV-255 serum were used for virus neutralization tests. Molecular phylogenetic analysis of the amino acid sequences of 15 virus isolates and those of the previously published and GenBank-deposited 9 global and 14 Japanese strains showed that 8 (53%) of the 15 virus isolates as well as the vaccine strains F9 and FCV-255 belonged to genogroup I (G(A)I), and 7 (47%) belonged to genogroup II (G(A)II). Of the 8 G(A)I strains, 2 were isolated from cats that had been vaccinated with an F9 strain live vaccine, 5 from cats vaccinated with an FCV-255-derived vaccine, and 1 from a cat vaccinated with an FC-7-derived vaccine. Of the 7 GAll strains, 5 were isolated from cats that had been vaccinated with the F9 strain live vaccine, 1 from a cat vaccinated with the FCV-255-derived vaccine, and 1 from a cat vaccinated with the FC-7-derived vaccine. These results indicate that more vaccine breakdown strains isolated from the cats vaccinated with the F9 strain-derived vaccine belong to G(A)II than to G(A)I, whereas more vaccine breakdown strains isolated from the cats vaccinated with the FCV-255 strain-derived vaccine belong to G(A)I than to G(A)II, and that when the FC-7 strain-derived vaccine is used, the vaccine breakdown strains belong almost equally to G(A)I and G(A)II. Thus, the genogroups of virus isolates varied with the vaccine strain used (p < 0.05). On the other hand, the neutralizing titres of feline anti-F9 serum and rabbit anti-FCV-255 serum against the 15 isolates were very low, showing no relationships between neutralizing antibody titres and genogroups. The DNA sequence identities between the virus isolates and the vaccine strains were low, at 70.6-82.9%, and no strains were found to have sequences derived from the vaccine strains. Alignment of amino acid sequences showed that the G(A)I or G(A)II virus isolates from the F9-vaccinated cats differed at position 428 of the 5' hypervariable region (HVR) of capsid region of the F9 strain, whereas those from the FCV-255-vaccinated cats differed at positions 438, 453, and 460 of the 5'HVR of capsid region E of the F9 strain. We speculate that these differences influence genogrouping. The amino acid changes within the F9 linear epitopes common to G(A)I and G(A)II were noted at positions 450, 451, 457 of 5'HVR of the capsid region E in the isolates from F9-derived vaccine-treated cats, and 449, 450, and 451 of 5'HVR of capsid region E in the isolates from FCV-255-derived vaccine-treated cats, suggesting that these amino acid changes are involved in escapes. These results suggest that alternate vaccination with the F9 and FCV-255 strains or the use of a polyvalent vaccine containing GAll strains serves to inhibit development.

Detection of feline calicivirus (FCV) from vaccinated cats and phylogenetic analysis of its capsid genes.

We analysed genogroups of four feline calcivirus (FCV) isolates (FCV-S, H10, Ao198-1 and ML89) obtained from cats that experienced FCV infection after having been vaccinated against FCV. New PCR primer sets (8F/8R, Ao-S/Ao-A, cp-S/cp-A) were also designed, since the conventional Seal primer failed to amplify the target sequences in two samples. The genogroups of the four isolates as well as eight global and 17 domestic strains were determined by phylogenetic analysis of their amino acid sequences. One out of the four strains (25%) isolated in this study, H10, was grouped into genogroup I, along with the vaccine strains F9 and FCV-255. The other three isolates (75%) belonged to genogroup II. Thus, there were more isolates in genogroup II than in genogroup I. However, the antibody values of the four isolates against cat anti-F9 antisera were significantly decreased. There may be no relationship between the neutralizing antibody titre and genogroup. Amino acid sequence alignment of the four isolates showed that only a single amino acid in region C, which is involved in neutralization epitopes, was different in ML89 strain from that of F9. The other three strains, H10, Ao198-1 and FCV-B, shared the same amino acid sequence with F9. Alignment of amino acids for linear epitopes in the F9 strain, which are located at regions D and E, showed variations in 5' hypervariable region (HVR) of E, whereas D and conE had only synonymous substitutions i.e. no change in the amino acid sequence. This mutation in 5' HVR of region E suggested a vaccine breakdown, as the region is known to be essential for antigenicity. The genogroup II FCV is likely to be the cause of the FCV infection in this study, while the vaccine strains belong to genogroup I. Thus, the existing vaccine may need reevaluation for its effectiveness.