Sub-types that the flu A virus can have
At least 131 different types of strains can exist with influenza causing flu A virus, but only two of its kind can infect birds, says Centers for Disease Control and Prevention (CDC) in the US.
The influenza A virus has two surface proteins; neuraminidase (N) and hemagglutinin (H) and again these proteins have 11 and 18 different subtypes respectively calling for different combinations like the H7N9 virus or the H3N2 virus etc. Some of these strains only infect birds, while there are other strains that can be transmitted to mammals as well like pigs, horses, dogs or even humans.
Wild aquatic birds are largely the hosts of these viruses but they necessarily do not fall sick, Poultry birds like chicken are the most badly affected by the virus strains.
Virus strains of influenza that can affect humans
Three sub-types of the Hy type protein in a virus strain and two subtypes of the N-type protein strain and their two kinds of combination can cause infection in humans. H1N1 and H3N2, are the two strains that circulate among humans, causing the seasonal flu epidemics. As these strain types can easily get transmitted to human and are well adapted in residing in them they are also referred to as human flue than bird flu.
Whenever new flu A virus adapts itself well to the human body environment, pandemics are caused and four such kinds of pandemics from the H and N protein combination has happened since 1918 that is the Spanish flu (H1N1), the Hong Kong flu (H3N2), Asian flu (H2N2) and the 2009 swine flu caused by a new version of the H1N1 virus.
Some of the flu strains affect wild birds can also occasionally infects humans as well as the H5N1 strain that was not known to transmit from humans to humans but killed hundreds in various countries in 1997. The subtypes of bird flu which commonly cause infections are H5, H7 and H9 but none of the strains could establish a stable human to human transmission history, suggest finds by CDC.
The H5N8 strain of avian influenza that has been detected in most of the states has not affected humans so far. The H5N1 strain detected at Himachal Pradesh’s Pong Lake that can infect humans now appears to be contained according to wildlife authorities.
Why are there so many strains of flu A virus
The Influenza A virus is an RNA based virus with a segmented genome i.e eight separated strands that make it more prone to mutation or changes in nature. Because of the flu ‘antigenic drift’ and tendency of mutations of surface proteins, the vaccines need to be updated regularly.
When two different subtypes of flu infect a cell, genes can get mixed up. According to Carl Zimmer in his book ‘A planet of Viruses’, this mixing also called reassortment is a viral version of sex. When genes from human virus and avian creatures combine in a cell, it can be disastrous, says Zimmer. The 2009 swine flu was caused by the mixing up of four strains including one which infected pigs in 1918 pandemic.