Variation in the spike protein of the 793/B type of infectious bronchitis virus, in the field and during alternate passage in chickens and embryonated eggs

The degree of variation exhibited within the 793/B serotype (also known as 4/91 and CR88 serotypes) was investigated with nine French and 10 British isolates, collected between 1985 and 1994. The S1 part (1644 nucleotides) of the spike protein gene of the first known isolate of this serotype, FR/CR85131/85, had 95.9% to 97% nucleotide identity with the other isolates. Partial sequencing of isolates from Iran and Saudi Arabia, isolated in 2000, revealed approximately 95% nucleotide identity with European isolates, including the two live 793/B vaccinal strains, showing that they were not re-isolations of vaccinal virus. The data indicates that strains within the 793/B serotype have ≥96% nucleotide identity within the whole S1 gene and ≥93% nucleotide identity within the first 560 nucleotides, and ≥92% and ≥86% amino acid identities in the corresponding protein regions. This is similar to the identities exhibited within the Massachusetts serotype. Sequence analysis of a 793/B field isolate after passage in embryonated eggs, then in chickens and then again in eggs revealed selection for a serine and alanine at S1 amino acid position 95 in chicken-passaged and egg-passaged virus, respectively. There was no change in pathogenicity. This is the first demonstration at gene sequence level of host-driven selection for infectious bronchitis virus.