Swine flu usually sickens pigs alone but can cause pandemics in humans.

NIAID Scientists Trace Development of Swine Flu Viruses

This long-term study allowed them to pinpoint when specific subtypes of virus first appeared in pigs in Hong Kong.

Scientists at the National Institute of Allergy and Infectious Diseases have analyzed the genetic makeup of more than 650 flu viruses, in the process finding out how swine flu viruses might adapt most readily to infect humans. NIAID announced May 25 that the research results have been published in Nature and said swine flu usually sickens pigs alone but can cause human pandemics.

The research involved analysis of viruses isolated after pigs were slaughtered in Hong Kong between 1998 and 2010. Adding additional data stretching back 34 years, they could identify when specific subtypes of virus first appeared in pigs in Hong Kong.

"The researchers also traced the relative abundance of each of three major swine influenza virus lineages: classical, Eurasian avian-like and triple reassortant," NIAID explained in its news release. "Examples of all three of these long-established virus family lineages were found in varying proportions in samples gathered between 2002 and 2009. Before 2003, the classical lineage predominated; by 2005, the Eurasian lineage, first detected in 2001, had become most common. The most recent samples contain not only viruses from the three previously established swine lineages, but also from the new 2009 H1N1 strain. It is not yet known whether the new pandemic strain will permanently establish itself in swine. According to this analysis, the three swine influenza virus lineages have crossed geographic boundaries, including continents. Such extensive co-circulation of multiple strains facilitates gene-swapping between viruses, note the researchers, and they recommend continued surveillance of swine influenza genetic diversity to better understand how this process might give rise to variants with the potential to cause human flu epidemics."

The paper is D Vijaykrishna et al., Long-term evolution and transmission dynamics of swine influenza A virus. Nature DOI: 10.1038/nature10004 (2011). For more information, visit this website.

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