Influenza Viruses

Influenza Type A:

Type A viruses infect a range of hosts and is the one responsible for pandemics. The subtype of the Influenza A virus is determined by the two glycoproteins on the virus coat: Haemagglutinin (HA) Neuraminidase (NA).

Negative staining of influenza virus (a) and its fusion protein, haemagglutinin (b) Picture sourced from: http://www.nimr.mrc.ac.uk

There are 16 different types of HA (H1-H16) and 9 different types of NA (N1-N9), hence many different sub types are possible (theoretically 144).

How HA and NA function:

The HA molecule locks onto a sialic acid part of a receptor of the host cell. The viral HA locks on in a way that leads to a fusion with the host cell that allows the viral contents to invade the host cell where the virus RNA takes over the cell's machinery to reproduce itself. The NA enzyme is then used for getting the virus copies out of the cell by destroying the sialic acid receptor. If it did not do so, the virus copies would clump around the host cell's sialic receptor to which it would lock, as their parent entered the cell.

The Haemagglutinin is so named because it was observed that virus samples would cause clumping of red blood cells. This is still the basis for the serological test (haemagglutination inhibition). If serum is placed with virus and blood cells and inhibits agglutination, that serum (at that dilution) contains sufficient antibody to inactivate the virus.

The Neuraminidase destroys the molecule that has the sialic acid.

All the HA and NA types have been found in avian influenza viruses in aquatic birds, where they are often present without causing symptoms. Human influenza viruses that have circulated over the past 100 years have only had the H1, H2, or H3 and N1 or N2 types. Until 1977 there was usually only one subtype of Influenza A causing human disease. With each pandemic the previous subtype was "displaced" by the new Influenza A subtype.

Changing sub types in human Influenza A viruses:

In 1918, the H1N1 sub type emerged; it was displaced in 1957 by the H2N2 sub type; which in turn was displaced by the H3N2 sub type in 1968. In 1977, the H1N1 sub type re-emerged, but did not replace H3N2. Since 1977, there has been co-circulation of H1N1 and H3N2. In 1983, H1N2 emerged as a result of a combination between the two sub types.

Ref: Xu X et al Reassortment and evolution of current human influenza A and B viruses Virus Res 2004;103:55-60

For further information of types, subtypes and strains refer http://www.cdc.gov/flu/avian/gen-info/flu-viruses.htm