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article imageH5N1 Avian Virus Adapting to Pigs, Closer to Human Variation

By Michael Krebs     Apr 4, 2009 in Science
The deadly H5N1 virus, a scourge among avian species in Asia, appears to have adapted to operate in swine populations, a report in the Archives of Virology finds.
The H5N1 virus - a headline-producing species that regularly ravages avian populations throughout Asia - appears to have successfully mutated toward residence in pigs. While the pig variety may be less virulent than its avian-oriented cousins, it is acknowledged among virologists that the transference to the swine environment is a precursor to human infection.
The H5N1 viruses were isolated from pigs in Indonesia and were tested on mice. The pig-oriented variation was considerably less devastating to the exposed mice than the avian H5N1 species.
"The finding suggests that in growing in pigs, the virus may have become less harmful to mammals in general, the authors report. That sounds reassuring, but the authors say it may mean the virus is one step closer to turning into a human pandemic strain," writes Robert Roos in a report for the Center for Infectious Disease Research & Policy.
There is widespread concern that the H5N1 virus, also known as Bird Flu, could spread to human populations and cause a deadly global pandemic on a scale humanity has not seen since 1917. Evidence is mounting throughout Asia that the virus is already taking hold among people.
Professor Anne Kelso, director of the World Health Organization's collaborating center for Influenza in Melbourne, told ABC News, "The WHO is reporting cases and deaths at the moment in China, Egypt and Vietnam. I think particularly worrying are the number of cases in China, which are scattered throughout different provinces."
There is evidence that the virus may be replicating into weaker variations, and the less-lethal human infections and the findings in pig varieties support that evidence.
"It's interesting because the virus, or at least I should call it a family of viruses is still spreading very widely throughout the world and there are many poultry outbreaks still occurring in many countries of the world, Professor Kelso said in the ABC News report.
Less virulence allows for greater dissemination, as a living host is better at transmission than a dead one.
"We found that swine isolates were less virulent to mice than avian isolates, suggesting that the viruses became attenuated during their replication in pigs," the authors of the Archives of Virology report stated.
Pigs are a notable concern because they have cells in their trachea that allow for both avian and human influenza infections. Influenza species commonly trade information, and if pigs carry both human and avian strains at a given time valuable replication detail can be traded among the influenza species within the pig host - and new combinations can arise.
Humans are most commonly infected by avian flu varieties, but swine infections happen occasionally. The Centers for Disease Control and Prevention reported that humans were infected by swine varieties at least twice this flu season.
It is not clear whether or not the H5N1 virus has truly adapted to swine.
"If these viruses have gone into swine, I think the key is whether they become established in swine. If that happened, we'd be concerned. I think the consensus now is that pigs are like humans; they can be infected, but it's unlikely there'd be a lot of transmission," said Richard Webby, PhD, a virologist, flu researcher, and associate member of the Department of Infectious Diseases at St. Jude Children's Research Hospital in Memphis, in the Center for Infectious Disease Research & Policy report.
The certainty is that the virus is introducing new combinations into the ecology. It is a matter of time before one of these combinations crosses the threshold into human populations.
More about H5N1, Indonesia, Pigs, Virus, Disease
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