SHORT COMMUNICATION Sequence analysis of feline caliciviruses isolated from the oral cavity of clinically normal domestic cats (Felis catus) in Florida M. L. WEEKS, A. GALLAGHER, C. H. ROMERO* Department of Pathobiology, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610, USA SUMMARY Four isolates (73 per cent) of feline calicivirus (FCV), from oropharyngeal swabs taken from 55 unvaccinated apparently healthy cats, were identified by electron microscopy and reverse transcription-polymerase chain reaction (RT-PCR). A 671-bp fragment, comprising part of region B, all of regions C, D, and E and part of region F of the FCV capsid protein gene, was amplified from each isolate by RT-PCR and cloned for sequence analysis. Amino acid sequence comparison of these regions revealed significant sequence divergence from the F9 vaccine strain within regions C, E, and F. The hypervariable region E of the four Florida isolates and the NADC isolate contained three fewer amino acids than the commonly used F9 vaccine strain. This work provides support to the idea that currently circulating FCV strains may differ substantially from presently used vaccine strains. # 2001 Harcourt Publishers Ltd FELINE calicivirus (FCV) is a small, non-enveloped, single-stranded, positive-sense RNA virus with a poly- adenylated genome of about 77 kb (Carter et al 1992). Although known to play an important role in the Feline Respiratory Disease Complex (Gaskell and Dawson 1998), FCV can be isolated from the respiratory tract of apparently healthy cats. Only one FCV serotype is pre- sently recognised, despite the existence of numerous isolates with slight antigenic differences (Bittle et al 1960, Dawson et al 1993, Glenn et al 1999). Although partial cross-protection occurs among FCV strains, the lack of complete cross-protection may contribute to the failure of the vaccine to protect against some FCV isolates. Variations in the FCV capsid protein appear to be responsible for antigenic diversity (Carter et al 1992), seen in sequenced capsid protein genes from geo- graphically distinct isolates (Neill et al 1991, Tohya et al 1991, Seal et al 1993, Glenn et al 1999). Feline calcivirus capsid protein sequences have been divided into six regions: AÐamino acid residues 1 to 120; BÐresidues 121 to 396; CÐresidues 397 to 401; DÐresidues 402 to 425; EÐresidues 426 to 520 and FÐresidues 521 to 668 (Neill 1992, Seal et al 1993). In the present study, oropharyngeal swabs were obtained randomly from 55 cats brought to a local veterinary clinic between December 1997 and June 1998. Swab fluids were inoculated onto Crandell feline kidney (CRFK) cell cultures and cytopathogenic changes, characteristic of FCV infection (Fastier 1957), were observed in four (73 per cent) of the cultures (FCV/C01, FCV/C28, FCV/C46, FCV/C58). Cats that yielded FCV had not been vaccinated against calicivirus and had no ocular or respiratory signs suggestive of FCV infection. Ultra-thin sections were prepared from the four FCV- infected cultures and evaluated by electron microscopy (EM). Total RNA was obtained from infected cultures (S.N.A.P., Invitrogen, Carlsbad, CA, USA) and 5 mg were reverse transcribed (Superscript II RT, Life Technolo- gies, Grand Island, NY, USA) with random hexamer Research in Veterinary Science 2001, 71, 223±225 doi:10.1053/rvsc.2001.0491, available online at http://www.idealibrary.com on 0034-5288/01/030223  03 $35.00/0 # 2001 Harcourt Publishers Ltd *Corresponding author: Tel: 00 1 352 392 4700; Fax: 00 1 352 392 1910; E-mail: romeroc@mail.vetmed.ufl.edu FIG 1: Electron micrograph of an ultra-thin section of a CRFK cell culture infected with the FCV/C01 isolate and stained with uranyl acetate-lead citrate. Numerous virus particles inside a large cytoplasmic vesicle forming crystal-like structures (89,000).