The avian influenza virus is a type A influenza virus which is normally found in birds. Wild birds are the natural hosts for all known influenza type A viruses. This includes waterfowl, gulls and shorebirds. Ironically wild birds do not normally show symptoms of the influenza virus however when avian influenza type A viruses are passed onto domesticated birds, they are extremely susceptible to highly pathogenic avian influenza (HPAI) with a mortality rate of 90% to 100%1. Avian H2, H5, H6, H7, H8, H9 and H10 are the subtypes that are the most likely to be transferred to humans.
The subtypes of the avian influenza type A virus that routinely cause human influenza are H3N2, H2N2, H1N1 and H1N2. H1N1 was the virus that caused the Spanish influenza pandemic in 1918 that killed 20 million people as well as the Russian pandemic of 1977 which was less lethal1.
Currently, only two influenza type A virus subtypes are in general circulation among humans, H1N1 and H3N2. So far, the spread of H5N1 from person to person has been rare, very limited and not sustainable. However, of the 271 cases of H5N1 in humans reported to the World Health Organization (WHO) 59.1% were lethal cases. The continued cases of human H5N1 outbreaks, although restricted, cause concerns of a new and highly lethal influenza pandemic possibly in the future for humans2.
Most recently an H7N9 outbreak was reported in April 2013 in China by the World Health Organization. While it’s been documented that some cases of H7N7 have seen mild illness, most cases have seen patients with severe respiratory illness and approximately one third of the severe cases leading to death3.
At this point, no indication has been made that a genetic reassortment has occurred, nor has it occurred with any other HPAI virus. Direct transmission of the viruses still seems to be unsustainable. This is important because past pandemics were either through direct transmission with sustainability or by genetic reassortment. As indicated, the influenza viruses are ever changing, which means we have yet to see direct transmission or reassortment now but that’s not to say it won’t happen in the future. Once these strains of the HPAI do experience change, especially if incorporating traits from a human influenza virus, this reassortment will be highly contagious, incredibly lethal and spread easily from person to person causing a large scale influenza pandemic.
Microbiology traces the influenza viruses as enveloped RNA viruses from the Orthomyxoviridae family. Their genome has been identified as having a high mutation rate and they show increased antigenic diversity. Their core protein can be classified into three distinct types: A, B and C. Influenza viruses have two major antigenic surface glycoproteins embedded into their membrane: the hemagglutinin (HA) and neuraminidase (NA) which create the antibody response in humans4. Influenza types A, B and C can infect various...