Influenza Epidemics Due To Hybrid Viruses

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Main Category: Flu / Cold / SARS
Also Included In: Bird Flu / Avian Flu;  Public Health;  Genetics
Article Date: 28 Feb 2008 - 17:00 PDT

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A recent article published in the open-access journal PLoS Pathogens maintains that over its evolutionary history, reassortment of the influenza A virus happens often.

Genetic reassortment is when genetic material mixes from two similar viruses that are infecting the same cell. Influenza virus A is a species of virus that causes influenza in birds, humans, pigs, and horses; it has often given rise to human influenza pandemics.

A team of researchers from both Pennsylvania State University and the National Institutes of Health (NIH) looked at influenza viruses from 1918 to 2005. They focused on viruses that cause seasonal epidemics in humans, especially ones that were associated with high mortality.

The severe influenza epidemics of 1947 and 1951, according to the researchers, were the result of genetic reassortment events. Two human influenza viruses from the same H1N1 strained switched genetic material, and thus produced two new hybrid viruses.

It has been unknown as to exactly why very severe influenza epidemics occur periodically and lead to unusually high illness and mortality levels - like the ones in 1947 and 1951. The normal model of human influenza virus evolution argues that major pandemics (of which 1918 was the largest) are due to genetic reassortment of human and avian (bird) influenza viruses. However, the seasonal influenza epidemics that occur each winter in the United States are thought to arise without genetic reassortment.

The new research findings add a layer of complexity to the evolution of seasonal influenza than was previously believed. That is, within a single population, multiple forms of the same strain co-circulate and re-assort. These quickly-generating, novel viruses are capable of starting major epidemics.

The authors believe that vaccine design can be helped if intensive surveillance can capture the full extent of how genetically diverse the influenza virus is that is co-circulating at a given time.

Multiple Reassortment Events in the Evolutionary History of H1N1 Influenza A Virus Since 1918
Nelson MI, Viboud C, Simonsen L, Bennett RT, Griesemer SB, et al.
PLoS Pathogens 4(2): e1000012. (2008).
doi:10.1371/journal.ppat.1000012
Click Here to View Journal Website

Written by: Peter M Crosta
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