MOLECULAR ANALYSIS OF THE 793 B SEROTYPE OF INFECTIOUS-BRONCHITIS VIRUS IN GREAT-BRITAIN/

Since the winter of 1990/91 respiratory disease of poultry in Great Br itain has commonly been associated with the 793/B (or 4/91) serotype o f infectious bronchitis virus (IBV). We have sequenced a variable part of the S1 region of the spike protein (S) gene. Comparison of up to 2 70 nucleotides of 12 British 793/B isolates, obtained in 1991 and 1993 , revealed 94 to 100% nucleotide identity with each other. Eleven of t hem fell into one of Mo subgroups, A and B, one isolate forming subgro up C. Identity within subgroups A and B was > 98%. The whole S1 gene s equence (1617 nucleotides) was determined for five 793/B isolates, two from each of subgroups A and B and one from subgroup C; nucleotide id entity between any two isolates was greater than or equal to 97%. A la rge proportion of the nucleotide differences corresponded to amino aci d changes. The whole S1 amino acid sequence differed by 21 to 25% or m ore from that of all other published IBV sequences. This extensive dif ference has probably contributed to the persistence of the 793/B serot ype in Britain even though heterologous vaccines have been used. The f inding that the 793/B isolates could be placed into three subgroups su ggests that either (a) they had diverged from a common progenitor pres ent, bur undetected, in Britain prior to 1990/91 or (b) at least three different strains of the 793/B serotype had entered Britain in or pri or to 1990/91.