Infectious bronchitis virus S1 gene sequence comparison is a better predictor of challenge of immunity in chickens than serotyping by virus neutralization.

Five strains of infectious bronchitis virus isolated from commercial chickens from the state of Pennsylvania, USA during the years 1998 and 1999 were studied. The strains were selected for cross-challenge in specific pathogen free chickens and virus neutralization in chick embryos on the basis of partial S1 sequence amino acid identity values. The partial sequences analysed spanned the hypervariable amino terminus region of S1 from amino acid residues 48 to 219, based on the Beaudette strain. Using their S1 identity values, the strains represented a continuum of genetic, and thus antigenic, relationships. When compared with strain PA/5083/99, strain PA/Wolgemuth/98 had high sequence identity (96%) followed by PA/171/99 (85%), PA/5344/98 (70%) and PA/1220/98 (34%). The method of Archetti and Horsfall was used for calculating antigenic relatedness values of virus neutralization tests. The same formula was also applied to the percentage protection values of cross-challenge tests to derive protective relatedness values among the strains. The antigenic relatedness values, protective relatedness values, and the partial S1 sequence identity values were then analysed. The findings indicated partial S1 sequence identity values were more strongly correlated with protective relatedness values and than antigenic relatedness values.