During 2000, 2001, and January 2002, avian influenza virus was isolated from chickens from 12 different locations in California. All the isolates were typed as H6N2 and determined to be of low pathogenicity for chickens. Nine of the isolates came from commercial layer flocks; one from a backyard flock; one from a mixed age flock, where ducks and squabs were also present; and one from a primary broiler breeder. Although a drop in egg production and increased mortality were among the disease signs reported in the layer flocks, the pathological changes observed in the early cases were primarily associated with mild respiratory infections. It was not until August 2001 that yolk peritonitis was observed; this has been a feature of all the remaining cases through 2001 and 2002.

All the isolates clustered as a unique group separate from other influenza viruses based upon sequence data of the H6, neuraminidase (N2), and matrix (MA) genes, indicating a common ancestor for these three gene segments. However, sequencing of the nonstructural (NS) gene indicates introductions from two separate origins. With the first isolate CK/CA/431/00 as the index case, the N2, MA, and NS genes are more closely related to North American isolates, as is the NP gene of CK/CA/650/00. In contrast, the H6 gene is more closely related to a Eurasian influenza isolate. Comparison of amino acid sequences of the N2 and MA genes of these isolates with available type A influenza viruses identified two unique changes in the MA gene and nine in the N2 gene, as well as four progressive changes. These results are discussed in relation to available clinical and epidemiological data.