Arch Virol (2000) 145: 1047–1058
Phylogenetic analysis of H7 haemagglutinin subtype
inﬂuenza A viruses
, E. C. Speidel
, J. W. McCauley
, and D. J. Alexander
Avian Virology, Veterinary Laboratories Agency Weybridge, Addlestone, Surrey U.K.
Institute for Animal Health, Compton Laboratory, Compton, Newbury, Berkshire U.K.
Accepted November 18, 1999
Summary. A 945 nucleotide region (bases 76–1020) of the HA1 part of the HA
gene was obtained for 31 inﬂuenza viruses of H7 subtype isolated primarily from
Europe, Asia and Australia over the last 20 years. These were analysed phyloge-
netically and compared with sequences of the same region from 23 H7 subtype
viruses available in Genbank. The overall results showed two geographically dis-
tinct lineages of North American and Eurasian viruses with major sublineages
of Australian, historical European and equine viruses. Genetically related sublin-
eages and clades within these major groups appeared to reﬂect geographical and
temporal parameters rather than being deﬁned by host avian species. Viruses of
high and low virulence shared the same phylogenetic branches, supporting the
theory that virulent viruses are not maintained as a separate entity in waterfowl.
Wild aquatic birds are considered the primordial reservoir of all subtypes of avian
inﬂuenza A viruses [15, 29, 30] and there is strong evidence for transmission
from wild birds to domestic poultry . A severe disease of domestic poultry
formerly known as “fowl plague”, but now termed highly pathogenic avian in-
ﬂuenza (HPAI), is caused by certain strains of inﬂuenza virus which have the
potential to spread readily to neighbouring premises. Outbreaks of HPAI are as-
sociated with the H5 and H7 subtypes of inﬂuenza A viruses and a prerequisite
for pathogenicity is the presence of multiple basic amino acids at the cleavage site
of the viral haemagglutinin precursor protein, rather than just two as seen in non-
pathogenic inﬂuenza viruses . The process by which HPAI viruses emerge
is poorly understood. It is not possible to predict when introductions of HPAI to
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