The California (Cal) serotype of infectious bronchitis virus (IBV) was isolated from layer flocks in southern California in the early 1980s. Since then, it has spread to the broiler-producing regions of central California, where it has been implicated in respiratory disease outbreaks in vaccinated flocks. Lack of a procedure for quickly identifying IBV serotypes in commercial chicken flocks has prevented the causal association of the IBV Cal serotype with respiratory disease outbreaks. A protein polymorphism has been identified in the matrix protein of the Cal serotype; it appears to be unique among other common serotypes of infectious bronchitis virus found in California. This polymorphism can be identified on western blots using raw or concentrated infectious allantoic fluid as the source material. Identification of the Cal serotype and of serotypes in the Mass and Conn groups can be performed rapidly using field samples from suspect flocks. The identification of this polymorphism provides an alternative method for the rapid identification of the Cal serotype of IBV.
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