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.