Endemic infection of a cat colony with a feline calicivirus closely related to an isolate used in live attenuated vaccines.

We have typed three feline calicivirus (FCV) isolates obtained over a 5-month-period from an endemically infected cat colony. Sequence analysis from variable region E of the capsid gene from these isolates strongly suggests they are minor variants of a single FCV strain, and that this strain is closely related to the one used in many live-attenuated FCV vaccines. Such a vaccine was last used approximately 2 months before the first of the isolates in this study was obtained. Sequence differences between the 'colony isolate' and the vaccine virus suggest that the colony virus has evolved from the vaccine virus and was persisting in the colony. The extent to which vaccine virus may contribute to the continued high prevalence of FCV needs to be determined.