Sequence analysis of infectious bronchitis virus isolates from the 1960s in the United States.

To better understand the molecular epidemiology of infectious bronchitis virus (IBV) in the United States following the introduction of commercial IBV vaccines, we sequenced the S1 and N structural protein genes of thirteen IBV field isolates collected in the 1960s. Analysis of the S1 sequence showed that seven isolates were of the Massachusetts (Mass) genotype, five were SE17, and one was of the Connecticut (Conn) genotype, suggesting that these three IBVs were circulating in commercial poultry raised in different regions in the United States during the 1960s. The S1 genes of Mass-type isolates had high levels of sequence variation, representing 81.3-81.9 % nucleotide (nt) and 77.3-78.7 % amino acid (aa) identity when compared to those of the SE17-type isolates. In contrast, the N genes from the same isolates were less variable (>92 % nt and >93 % aa identity) when compared to those of the SE17-type isolates. Phylogenetic analysis based on the S1 gene indicated that one isolate (L748) was more closely related to the Mass type. In contrast, phylogenetic analysis based on the N gene showed that L748 was more closely related to the SE17 type, indicating that there had been exchange of S1 genetic materials between Mass- and SE17-like viruses. In addition, the Mass-type isolates had high levels of sequence identity in the S1 gene compared with widely used modified live vaccines (Mass41, Ma5 and H120) and modern field strains from the USA and other countries, suggesting a common ancestor.