Swine breeding operations in southern China have bred more than tastier pigs for people to eat: it helped create hundreds of new influenza viruses, which spread around the world, researchers reported on Wednesday.
They analyzed the genetic makeup of more than 650 influenza viruses taken during the systematic surveillance of pigs slaughtered in Hong Kong between 1998 and 2010. They used more than 800 other samples dating back 34 years and painted a clear pattern of flu viruses emerging, reemerging, and spreading among pigs.
David Spiro, who heads the section influenza and related respiratory viruses at the U.S. National Institute of Allergies and Infectious Diseases, says the study shows the importance of testing pigs for flu viruses.
“I think that this really does show that if you put money into surveillance you can learn a lot from it,” Spiro said in a telephone interview. “Maybe we can change agricultural practices to avoid this in the future.”
NIAID, one of the National Institutes of Health, helped pay for the study, published in the journal Nature.
Vijaykrishna Dhanasekaran of Duke University, who worked on the study, said pig breeders in southern China transported live pigs from other parts of the country and the world; and they all brought new flu viruses with them.
These can pass easily to people, although these strains of true swine influenza rarely pass from person to person to person. "The majority of reported human infections have been people with close contact to farm animals," Dhanasekaran said in a statement.
"I think the risk of swine-to-human transmission has not increased greatly, but the diversity of swine viruses has increased as shown in our study," Dhanasekaran said. "This means that the repertoire of viruses that humans are in contact with every day has increased and this may lead to a higher likelihood of swine-to-human transmission, although the risk remains unquantified."
Influenza viruses can mix and mutate constantly, and it is difficult to predict when one will acquire just the right genetic makeup to easily infect humans and cause a new pandemic.
Spiro said the study supports the long-held theory that pigs are mixing vessels for flu viruses.
“With the bird flu in Hong Kong, the H5N1 outbreak, our natural response was to look at the avian population,” Spiro said. H5N1 bird flu has killed or forced the destruction of more than 300 million birds since it reemerged in Hong Kong in 2003, according to the world animal health organization OIE. It has killed about 320 people out of 550 infected in that time.
Scientists still worry that H5N1 could mutate slightly into a form easily passed from person to person, sparking a deadly pandemic.
The United States and some other countries set up big networks to test wild and domestic birds for avian influenza. “But we forgot that before them the major theory was that swine were the mixing vessel. We looked one way, and the pandemic came from the other direction,” Spiro said.
When the next pandemic did come, it was the new form of H1N1 flu that emerged in Mexico. It flashed around the world in 2009, luckily not killing more people than an average influenza virus, although it did attack younger, healthier adults and children rather than the elderly, as usually seasonal flu does.
“We were swabbing birds and we should have been swabbing pig noses,” Spiro said. Agricultural and disease experts have since called for wider surveillance of pigs, although nothing has been set up systematically.
One thing the Nature study did not find: It did not find the source of the 2009 H1N1 swine flu strain.
“The direct ancestor of the H1N1 pandemic 2009 has not been found in swine yet,” Spiro said. “There’s no evidence it came from southeast Asia in this paper.”
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