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Overview — ProMED Summary of Strains
The influenza viruses are segmented genome RNA viruses classified in the
family Orthomyxoviridae. They constitute 3 of the 5 genera of the family
and are designated the genera Influenzavirus A, Influenzavirus B, and
Influenzavirus C. Each is represented by single virus species: Influenza
A virus, Influenza B virus, and Influenza C virus.
The influenza A viruses exist as several distinct subtypes, defined by the
hemagglutinin (HA) and neuraminidase (N) surface antigens, which infect
humans and a range of avian and mammalian species. In contrast, the
influenza B viruses do not exhibit subtype variation and appear to infect
humans only, causing epidemics but not pandemics, presumably because there
is no reservoir of novel antigenic variation in non-human hosts. The
influenza C viruses infect humans and have also been isolated from pigs in
China, but these viruses are not associated with either epidemic or
pandemic disease in the human population.
The influenza A viruses exist in greatest profusion in waterfowl. Until
recently, 15 distinct HA and 9 distinct N antigenic types have been
recognized. It is generally accepted that feral aquatic birds are the
reservoir for influenza A viruses and that influenza viruses in aquatic
birds has achieved "evolutionary stasis," meaning that the internal genes
of the viruses show little genetic variation. In feral aquatic birds
virtually all combinations of HA and N subtypes exist as a result of
apparently unrestricted reassortment of the HA- and N-encoding genome
subunits, whereas only a few combinations have been found in terrestrial
and marine mammals (humans, horses, some carnivores, seals and whales) and
domestic fowl. The avian influenza viruses causing disease in domestic
poultry are predominantly viruses of HA5 and HA7 subtypes. Strains of low-(LPAI) and high-(HPAI) pathogenicity avian influenza virus of each subtype
exist. Pathogenicity is determined in part by the presence of multiple
basic amino acids (arginine and lysine) at the cleavage site of the H
protein. Cleavage of the H molecule is necessary for infectivity of the
virus, and the susceptibility of the H molecule to specific cellular
proteases determines the tissue tropism and virulence of the virus.
A significant new finding has been the identification of a 16th
hemagglutinin subtype, which is described in a paper in the current issue
of the Journal of Virology. The paper is entitled: "Characterization of a
Novel Influenza A Virus Hemagglutinin Subtype (H16) Obtained from
Black-Headed Gulls," authored by Ron A. M. Fouchier and 8 others.
The Abstract of the paper states that: "In wild aquatic birds and poultry
around the world, influenza A viruses carrying 15 antigenic subtypes of
hemagglutinin (HA) and 9 antigenic subtypes of neuraminidase (NA) have been
described. Here, we describe a previously unidentified antigenic subtype of
HA (H16) detected in viruses circulating in black-headed gulls in Sweden.
In agreement with established criteria for the definition of antigenic
subtypes, hemagglutination inhibition assays and immunodiffusion assays
failed to detect specific reactivity between H16 and the previously
described subtypes H1 to H15. Genetically, H16 HA was found to be distantly
related to H13 HA, a subtype also detected exclusively in shorebirds, and
the amino acid composition of the putative receptor-binding site of H13 and
H16 HAs was found to be distinct from that in HA subtypes circulating in
ducks and geese. The H16 viruses contained NA genes that were similar to
those of other Eurasian shorebirds but genetically distinct from N3 genes
detected in other birds and geographical locations. The European gull
viruses were further distinguishable from other influenza A viruses based
on their PB2, NP, and NS genes. Gaining information on the full spectrum of
avian influenza A viruses and creating reagents for their detection and
identification will remain an important task for influenza surveillance,
outbreak control, and animal and public health. We propose that sequence
analyses of HA and NA genes of influenza A viruses be used for the rapid
identification of existing and novel HA and NA subtypes.
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