Evolution of Influenza

The virus causing influenza is one of the best known pathogens found in various species. In particular, the virus is found in birds as well as mammals including horses, pigs, and humans. The phylogeny, or the evolutionary history of a particular species, is an important component when analyzing the evolution of influenza. Phylogenetic trees are graphical models of the relationships between various species. They can be used to trace the virus back to particular species and show how organisms that look so different may be so closely related.

Two common mechanisms by which viruses evolve are reassortment and genetic drift.

Reassortment, also known as antigenic shift, allows new viruses to evolve under both natural conditions and in artificial cultures. Reassortment occurs in similar fashion as chromosome crossover events, as two different viral strains may come in contact and transfer some of their genetic information. This crossing-over event creates a mixture of the two viral strains, which may replicate as one hybrid virus that expresses traits from both original viruses. The mechanism of the evolutionary force of antigenic shift allows influenza viruses to exchange genes with strains that infect different species. Under this mechanism, a human influenza virus could exchange genes with an avian strain, and that is how pandemic strains arise. There have been three occurrences of pandemics caused by antigenic shift since 1900, and it could just as easily happen again. In fact, the 1957 evolution of the H2N2 virus is thought to be a result of reassortment. In this case, human H1N1 strains and avian influenza A genes were mixed. Infecting tissue cultures can demonstrate how pathogenic qualities can evolve for a particular species even though the reassorted virus may be nonpathogenic for another species. A prime example of evolution under natural conditions is the reassortment of two avian influenza strains that were discovered in dead seals back in 1979.

New viruses can also emerge by drift. Drift can refer to genetic drift or antigenic drift.Mutation and selection for the most advantageous variation of the virus takes place during this form of evolution. Antigenic mutants can evolve quickly due to the high mutation rate in viruses. The cause of the antigenic drift lies in the mechanisms of RNA synthesis itself. Mutations arise very easily simply due to the error prone RNA polymerase and its lack of proofreading mechanisms. These mutations lead to subtle changes in the HA and NA genes which completely changes the infectious capabilities of the virus. These changes allow for almost endless possibilities for new viral strains to arise and it is the antigenic drift of the HA and NA genes that allow for the virus to infect humans that receive vaccines for other strains of the virus. This evolution occurs under the pressure of antibodies or immune system responses.

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