Swine Influenza > About Influenza > Evolution

Evolution

Influenza A viruses (IAV) are characterised by their wide variability, which is based partly on genetic drift and partly on reassortment. Genetic drift describes genetic mutations which have arisen over time. In IAV, nucleotide substitution occurs at a rate of 10e-5 to 10e-6 substitutions per locus per replication cycle. This means that around 1 to 10% of virus progeny have a genetic mutation. These mutations are usually "silent mutations". This means that there are no changes in the amino acid sequence or in the "fitness" of the virus progeny, i.e. their ability to replicate, adapt and infect. A change in an amino acid sequence is described as a "positive selection". An example is the gradual changes in the antigen structures of HA which enable the virus to infect a host with existing immunity to IAV.

Antigenic drift allows IAV to circumvent existing immunity and therefore increases the likelihood of infection in the host.

Antigenic drift of influenza A

Figure: Principle of antigenic drift

Reassortment, or 'antigenic shift', is a precondition for the development of new subtypes via double or multiple infections of a cell with two different subtypes or genetic lines. This can lead to the exchange and free combinability of the eight gene segments. The following diagram presents a general outline of the evolution of swine influenza viruses in North America and Europe.

Antigenic drift allows IAV to circumvent existing immunity and therefore increases the likelihood of infection in the host.

Antigenic drift of influenza A

Figure: Principle of antigenic drift (with cortesy of Prof. Roland Zell, Jena)

The most significant factor here is the reassortment of NA and HA segments. With free reassortment, the 18 HA types and 11 NA types allow a total of 198 HA/NA combinations. 110 of these are now known to occur in birds. However, it is not yet clear whether all conceivable combinations actually occur.

Besides the gene segments for the surface antigens, other segments can also be exchanged. However, such reassortments have received little attention in the past because they are serologically inconspicuous, even though resistance genes have already been demonstrated to spread in this way.

The importance of pigs

Pigs play a prominent role in the evolution of influenza viruses, as they are susceptible to avian and human influenza viruses in addition to porcine viruses. They act as a "mixing vessel" for the reassortment of new strains.

Reassortment of influenza A in pigs

Figure: This diagram illustrates the concept of “mixing vessel” - using the example of reassortant H1N2.