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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